39. Cunada, CL; Lesack, LFW; Tank, SE. (2021) Methane emission dynamics among CO2-absorbing and thermokarst lakes of a great Arctic delta.Biogeochemistry 156: 375-399 Methane emission dynamics among CO2-absorbing and thermokarst lakes of a great Arctic delta
Arctic delta lakes; Methane emissions; CO2-absorbing lakes; Thermokarst lakes; Methane oxidation; Lake-river hydrology
Lake-rich Arctic deltas differ biogeochemically from tundra lakes, and their role as sources and sinks of greenhouse gases remains poorly understood. Under-ice and open-water changes in methane (CH4) storage (43 lakes, 2014), floating chamber measurements of total and diffusive CH4 evasion to the atmosphere (6 lakes, 2014-2015), and water-column CH4 oxidation (MOX) (6 lakes, 2014-2015) permitted evaluation of how CH4 emissions vary among lakes with differing river-to-lake connection times within the Mackenzie Delta. CH4 emissions during ice-out were considerable, followed by substantial declines as open-water progressed. Water-column MOX rates were highest after ice-out, and declined throughout open-water. After accounting for a strong effect of CH4 substrate levels, MOX rates were inversely related to pH, which can increase to high levels during open-water because of high macrophyte production. Comparisons of water-column CH4 storage versus open-water fluxes (6 lakes) showed that diffusive evasion plus MOX removed most CH4 in the water columns every 1-2 days with only modest changes in storage, suggesting that counter-balancing water-column replenishment is substantial. Lakes with short river-connection times (i.e. most strongly autotrophic and strongly CO2-absorbing in this delta) and thermokarst lakes contribute disproportionately to CH4 flux, relative to lakes with long river-connection times. Thus, this great Arctic delta represents an important system of greenhouse-gas emitting lakes despite prior work showing their net absorption of CO2 during open-water, and having a low landscape area of CO2-saturated thermokarst lakes. Autotrophically absorbed CO2 becomes labile carbon substrate, and is microbially shunted back to the atmosphere as the more potent greenhouse gas CH4. DOI
38. Marcek, HAM; Lesack, LFW; Orcutt, BN; Wheat, CG; Dallimore, SR; Geeves, K; Lapham, LL. (2021) Continuous Dynamics of Dissolved Methane Over 2 Years and its Carbon Isotopes (delta C-13, Delta C-14) in a Small Arctic Lake in the Mackenzie Delta.J. Geophys. Res.-Biogeosci. 126 Continuous Dynamics of Dissolved Methane Over 2 Years and its Carbon Isotopes (delta C-13, Delta C-14) in a Small Arctic Lake in the Mackenzie Delta
Arctic Lake; diffusion; Mackenzie River Delta; methane; methane oxidation
Seasonally ice-covered permafrost lakes in the Arctic emit methane to the atmosphere during periods of open-water. However, processes contributing to methane cycling under-ice have not been thoroughly addressed despite the potential for significant methane emission to the atmosphere at ice-out. We studied annual dissolved methane dynamics within a small (0.2 ha) Mackenzie River Delta lake using sensor and water sampling packages that autonomously and continuously collected lake water samples, respectively, for two years at multiple water column depths. Lake physical and biogeochemical properties (temperature; light; concentrations of dissolved oxygen, manganese, iron, and dissolved methane, including stable carbon, and radiocarbon isotopes) revealed annual patterns. Dissolved methane concentrations increase under-ice after electron acceptors (oxygen, manganese, and iron oxides) are depleted or inaccessible from the water column. The radiocarbon age of dissolved methane suggests a source from recently decomposed carbon as opposed to thawed ancient permafrost. Sources of dissolved methane under-ice include a diffusive flux from the sediments and may include water column methanogenesis and/or under-ice hydrodynamic controls. Following ice-out, the water column only partially mixes allowing half of the winter-derived dissolved methane to be microbially oxidized. Despite oxidation at depth, surface water was a source of methane to the atmosphere. The greatest diffusive fluxes to the atmosphere occurred following ice-out (75 mmol CH4 m(-2) d(-1)) and during a mixing episode in mid-July, likely driven by a storm event. This study demonstrates the importance of fine-scale temporal sampling to understand dissolved methane processes in seasonally ice-covered lakes. DOI
37. Nzotungicimpaye, CM; Zickfeld, K; MacDougall, AH; Melton, JR; Treat, CC; Eby, M; Lesack, LFW. (2021) WETMETH 1.0: a new wetland methane model for implementation in Earth system models.Geosci. Model Dev. 14: 6215-6240 WETMETH 1.0: a new wetland methane model for implementation in Earth system models
Wetlands are the single largest natural source of methane (CH4), a powerful greenhouse gas affecting the global climate. In turn, wetland CH4 emissions are sensitive to changes in climate conditions such as temperature and precipitation shifts. However, biogeochemical processes regulating wetland CH4 emissions (namely microbial production and oxidation of CH4) are not routinely included in fully coupled Earth system models that simulate feedbacks between the physical climate, the carbon cycle, and other biogeochemical cycles. This paper introduces a process-based wetland CH4 model (WETMETH) developed for implementation in Earth system models and currently embedded in an Earth system model of intermediate complexity. Here, we (i) describe the wetland CH4 model, (ii) evaluate the model performance against available datasets and estimates from the literature, and (iii) analyze the model sensitivity to perturbations of poorly constrained parameters. Historical simulations show that WETMETH is capable of reproducing mean annual emissions consistent with present-day estimates across spatial scales. For the 2008-2017 decade, the model simulates global mean wetland emissions of 158.6 TgCH(4) yr (-1), of which 33.1 TgCH(4) yr (-1) is from wetlands north of 45 degrees N. WETMETH is highly sensitive to parameters for the microbial oxidation of CH4, which is the least constrained process in the literature. DOI
36. Gareis, JAL; Lesack, LFW. (2020) Ice-out and freshet fluxes of CO2 and CH4 across the air-water interface of the channel network of a great Arctic delta, the Mackenzie.Polar Res. 39 Ice-out and freshet fluxes of CO2 and CH4 across the air-water interface of the channel network of a great Arctic delta, the Mackenzie
Carbon dioxide; methane; greenhouse gas; Mackenzie River Delta; circumpolar delta; floodplain
Carbon dioxide (CO2) and methane (CH4) were monitored at five sites spanning the upstream-downstream extent of the Mackenzie Delta channel network during May 2010, capturing the historically under-sampled ice-out period that includes the rising freshet, peak water levels and the early falling freshet (flood recession). Unexpectedly, partial pressures of CO2 in the Mackenzie River were undersaturated during the rising freshet before water levels peaked, indicating net CO2 invasion at instantaneous CO2 flux rates (F-CO2) ranging from -112 to -258 mg-C m(-2) d(-1). Net CO2 invasion was also observed around the time of peak water levels at sites in the middle and outer delta. Following peak water levels, the Mackenzie River switched to saturation and net CO2 evasion (F-CO2 from 74 to 177 mg-C m-2 d-1). Although the Peel River (which flows into the west side of the Mackenzie Delta) was a strong emitter of CO2 (F-CO2 from 373 to 871 mg-C m(-2) d(-1)), overall, the Mackenzie River and Delta were weak emitters of CO2 during the 2010 ice-out period. All sites were strong emitters of CH4 during ice-out, however, with the highest evasive fluxes observed in the outer delta when the extent of flooded delta landscape was greatest. Estimated aerial fluxes from Mackenzie Delta channel surfaces during May 2010 ranged from 2.1 to 4.8 Gg-C as CO2, and 186 to 433 Mg-C as CH4. These results provide critical information that can be used to refine gas flux estimates in high-latitude circumpolar river deltas during the relatively under-studied ice-out period. DOI
35. Ji, XY; Lesack, LFW; Melack, JM; Wang, SL; Riley, WJ; Shen, CP. (2019) Seasonal and Interannual Patterns and Controls of Hydrological Fluxes in an Amazon Floodplain Lake With a Surface-Subsurface Process Model.Water Resour. Res. 55: 3056-3075 Seasonal and Interannual Patterns and Controls of Hydrological Fluxes in an Amazon Floodplain Lake With a Surface-Subsurface Process Model
Floodplain lakes represent important aquatic ecosystems, and field-based estimates of their water budgets are difficult to obtain, especially over multiple years. We examine the hydrological fluxes for an Amazon floodplain lake connected to the Solimoes River using a process-based hydrologic model. Water exchanges between the river and lake agree well with field estimates, including the timing of different hydrological phases. However, beyond available field data, modeling results show that the seven simulated years all differed from each other. These interannual differences were caused by the interplay between phases when water levels were rising with river-water flowing into the lake (RWRI), versus rising with lake-water flowing out to the river (RWLO). This exchange determines the river-water content in the lake (C-L). Maximum C-L occurred before river levels peaked because local catchment contributions can be sufficient to push lake-water out to the river, even as river levels rise. Numerical experiments show that the seasonal distribution of local rainfall, local catchment size, and interannual variability in both climate and river stage can contribute to differing dynamics of C-L, in a floodplain lake. Their impacts vary among phases: river-rise dominates the RWRI, whereas local hydrological processes dominate the RWLO and receding-water phases. Intermediate-to-long-term rainfall accumulation controls C-L during the RWLO phase, whereas annual precipitation accumulation is important for C-L, during low water. Our model generalizes beyond limited available field studies and offers potential to better understand floodplain lakes in other areas and how regional versus local changes in climate may affect their hydrological dynamics. DOI
34. Cunada, CL; Lesack, LFW; Tank, SE. (2018) Seasonal Dynamics of Dissolved Methane in Lakes of the Mackenzie Delta and the Role of Carbon Substrate Quality.J. Geophys. Res.-Biogeosci. 123 Seasonal Dynamics of Dissolved Methane in Lakes of the Mackenzie Delta and the Role of Carbon Substrate Quality
Arctic delta lakes; methane dynamics; thermokarst lakes; dissolved organic matter; carbon quality; lake-river hydrology
Dissolved CH4 among lake waters of the Mackenzie River Delta was tracked in 2014 to assess how river-to-lake connection times, plus carbon substrate quantity and quality, affects the patterns and dynamics of CH4. An under-ice survey of 29 lakes, three open-water surveys of 43 lakes, and weekly surveys of 6 lakes revealed that CH4 among lake-waters ranged from very high concentrations at the end of winter, with highest concentrations linked to shortest annual river connection times, to considerably lower concentrations as open water progressed, with a limited concentration range among lakes by late summer. Lakes most strongly affected by thermokarst varied irregularly from this pattern and did not have the highest CH4 concentrations. CH4 among lake waters was generally related to measures of carbon substrate quantity, where relations with macrophyte biomass and dissolved organic carbon in lake water were statistically stronger than % organic matter within the lake sediments. CH4 was also directly related to the molecular weight (a[250]:a[365]) of dissolved organic matter at the end of winter, but was inversely related to this measure during open water. Carbon quality per se, after accounting for differences in carbon quantity (macrophyte biomass, dissolved organic carbon concentrations, or organic content of lake sediments), appears to play a significant role in controlling CH4 concentrations among the lake waters, particularly during winter ice cover. Carbon quality in lake sediments and of dissolved organic matter in winter lake waters appears to be as important as thermokarst augmentation of carbon quantity for enhancing methanogenesis in this lake-rich Arctic system. DOI
33. Gareis, JAL; Lesack, LFW. (2017) Fluxes of particulates and nutrients during hydrologically defined seasonal periods in an ice-affected great Arctic river, the Mackenzie.Water Resour. Res. 53: 6109-6132 Fluxes of particulates and nutrients during hydrologically defined seasonal periods in an ice-affected great Arctic river, the Mackenzie
Large circumpolar rivers influence the biogeochemistry of coastal shelf ecosystems and Arctic Ocean circulation. From 2007 to 2010, Mackenzie River nutrients were measured immediately upstream of the Mackenzie Delta. Total suspended sediment (TSS) and the dissolved and particulate fractions of carbon, nitrogen, and phosphorus were measured throughout the water year, stratified by hydrologically defined seasonal periods. Sampling was most frequent during the ice breakup and freshet periods to capture changes in sediment and nutrient concentrations that occurred with rapidly changing discharge. Our 4 year sampling coverage yielded constituent-specific concentration-discharge relationships that differed among seasonal hydrological periods for 6 of 12 constituents. Results revealed that having data from the rising-water freshet, when ice effects drive much higher water levels for a given discharge than during open water, had a surprisingly modest effect on annual flux calculations for most constituents. The rising freshet, however, was dominated by high relative levels of C-rich dissolved organic matter, P-rich particles, and N-rich inorganic nutrients. Comparisons with fluxes based on volume-weighted means of sample concentrations revealed that most, though not all, river fluxes were strongly driven by short-term concentration-discharge dynamics rather than by total annual discharge. Comparisons with fluxes derived for the Mackenzie discharge record, where results from other years are available, suggest that concentration-discharge relations may not be robust beyond the observation period in this system. Constituent fluxes herein differ from previously published fluxes for the Mackenzie (e.g., lower TSS, higher dissolved organic carbon), likely reflecting more frequent measurements, more representative characterization of differing hydrological periods, and possible changes in concentration-discharge relations during the longer-term Mackenzie record. DOI
32. Crasto, N; Hopkinson, C; Forbes, DL; Lesack, L; Marsh, P; Spooner, I; van der Sanden, JJ. (2015) A LiDAR-based decision-tree classification of open water surfaces in an Arctic delta.Remote Sensing of Environment 164: 90-102 A LiDAR-based decision-tree classification of open water surfaces in an Arctic delta
LiDAR; Open water; River channel; Lake; Floodplain; Decision tree; Classification; Mackenzie Delta; Arctic
In the Mackenzie Delta, western Arctic Canada, decisions relating to navigation, socio-economics, infrastructure stability, wildlife, vegetation and emergency preparedness are closely related to the delta hydrology. Presented here is a remote sensing decision-tree approach to delineate open-water hydrological features using high-resolution LiDAR terrain, intensity and derivative data. The proposed classification scheme exploits the propensity of LiDAR point attributes and data metrics such as point density and standard deviation (of intensity and elevation) to cluster around characteristic response values over water and non-water surfaces. Due to the impracticability of validating an Arctic water surface classification over such a huge and remote area, results of the hierarchical classification were compared to alternative classifications derived from Radarsat-2 and a manually intensive digitisation technique. Open-water features were identified with >95% accuracy when compared to manually interpreted data. The spatially extensive but temporally distinct information on the hydrological setting of the delta thus extracted forms the basis for calculation of time-invariant parameters such as off-channel storage capacity and hydraulic gradients. In situations where LiDAR data are primarily collected in support of terrain-based watershed hydrologic or floodplain hydraulic assessments, contemporaneous water extent and associated level data are valuable in further characterizing terrain hydrological characteristics. (C) 2015 Elsevier Inc. All rights reserved. DOI
31. Lesack, LFW; Marsh, P; Hicks, FE; Forbes, DL. (2014) Local spring warming drives earlier river- ice breakup in a large Arctic delta.Geophysical Research Letters 41: 1560-1566 Local spring warming drives earlier river- ice breakup in a large Arctic delta
Arctic rivers; climatic change; Arctic snowfall; river biogeochemistry; river ice; river-ocean interface
Pan-Arctic rivers strongly affect the Arctic Ocean and their vast lake-rich deltas. Their discharges may be increasing because of an intensifying hydrological cycle driven by warming climate. We show that a previously unexplained trend toward earlier ice breakup in the Mackenzie River Delta is little affected by winter warming during the period of river-ice growth and is unaffected by river discharge, but unexpectedly is strongly related to local spring warming during the period of river-ice melt. These results are statistically linked to declining winter snowfall that was not expected because of an intensifying Arctic hydrological cycle. Earlier ice breakup is expected to cause declining water level peaks that will reduce off-channel flows through the lake-rich delta before river waters enter the ocean. Thus, local spring warming with unexpected snowfall declines, rather than warmer winters, can drive earlier ice breakup in large Arctic rivers and biogeochemical changes in their river-ocean interface. Key Points <list list-type="bulleted" id="grl51464-list-0001"> <list-item id="grl51464-li-0001">Earlier ice breakup in a large Arctic river is driven by spring warming trend <list-item id="grl51464-li-0002">Earlier breakup is not affected by winter warming or changes in river discharge <list-item id="grl51464-li-0003">Spring warming effect on river breakup is amplified by declining winter snowfall DOI
30. Emmerton, CA; Graydon, JA; Gareis, JAL; St Louis, VL; Lesack, LFW; Banack, JKA; Hicks, F; Nafziger, J. (2013) Mercury Export to the Arctic Ocean from the Mackenzie River, Canada.Environmental Science & Technology 47: 7644-7654 Mercury Export to the Arctic Ocean from the Mackenzie River, Canada
Circumpolar rivers, including the Mackenzie River in Canada, are sources of the contaminant mercury (Hg) to the Arctic Ocean, but few Hg export studies exist for these rivers. During the 2007-2010 freshet and open water seasons, we collected river water upstream and downstream of the Mackenzie River delta to quantify total mercury (THg) and methylmercury (MeHg) concentrations and export. Upstream of the delta, flow-weighted mean concentrations of bulk THg and MeHg were 14.6 +/- 6.2 ng L-1 and 0.081 +/- 0.045 ng L-1, respectively. Only 11-13% and 44-51% of bulk THg and MeHg export was in the dissolved form. Using concentration-discharge relationships, we calculated bulk THg and MeHg export into the delta of 2300-4200 kg yr(-1) and 15-23 kg yr(-1) over the course of the study. Discharge is not presently known in channels exiting the delta, so we assessed differences in river Hg concentrations upstream and downstream of the delta to estimate its influence on Hg export to the ocean. Bulk THg and MeHg concentrations decreased 19% and 11% through the delta, likely because of particle settling and other processes in the floodplain. These results suggest that northern deltas may be important accumulators of river Hg in their floodplains before export to the Arctic Ocean. DOI
29. Chateauvert CA, Lesack LFW, Bothwell ML. (2012) Abundance and patterns of transparent exopolymer particles (TEP) in Arctic floodplain lakes of the Mackenzie River Delta.Journal of Geophysical Research-Biogeosciences 117, G04013, 14 PP., 2012 Abundance and patterns of transparent exopolymer particles (TEP) in Arctic floodplain lakes of the Mackenzie River Delta
First study of transparent exopolymer particles (TEP) in arctic floodplain lakes
TEP is important but abundances differ from expectations based on marine systems
TEP needs further study in lakes with high primary production and complex DOC
The Mackenzie River Delta is a lake-rich arctic floodplain that receives high inputs of dissolved organic matter (DOM) and suspended particulates from allochthonous and autochthonous sources, and may transfer carbon from dissolved to particulate phase via in situ formation of transparent exopolymer particles (TEP). TEP provides food for grazers, surfaces for bacteria, and increased potential for aggregation and sedimentation of organic matter. During open water 2006, we tracked TEP abundances in three Delta lakes representing gradients that include declining river-to-lake connection times, increasing levels of dissolved organic carbon (DOC), and declining chromophoric-DOM (CDOM). Unexpectedly, TEP abundances were highest immediately after the flood, when autochthonous autotrophic production was at a seasonal low and CDOM a seasonal high. Moreover, the lake with the strongest riverine influence and lowest levels of autochthonous autotrophic production had the highest mean TEP-carbon (TEP-C) concentrations among the lakes. The mean proportion of particulate organic carbon (POC) represented by TEP-C increased with increasing river connection time, and appears to represent a substantial proportion of POC in Mackenzie Delta Lakes. Unexpectedly, the TEP gradient was most strongly related to CDOM (river water source) rather than overall DOC. Variations in CDOM accounted for 53% of TEP-C variation among the lakes, indicating allochthonous matter was the most important source of TEP. DOC release from in situ macrophytes during periods of high photosynthesis may contribute to TEP formation in the lake with lowest riverine influence, but pH levels >9.5 driven by the high photosynthetic rates complicate the interpretation of results from this lake.
DOI
28. Chateauvert, CA; Lesack, LFW; Bothwell, ML. (2012) Bacterial community dominance of particle-attached bacteria in lakes of the Mackenzie River Delta: transparent exopolymer particle contribution.Aquatic Microbial Ecology 68: 57-76 Bacterial community dominance of particle-attached bacteria in lakes of the Mackenzie River Delta: transparent exopolymer particle contribution
DISSOLVED ORGANIC-MATTER; FREE-LIVING BACTERIA; ARCTIC RIVER; ECOSYSTEMS; ABUNDANCE; CARBON; TEP; COLONIZATION; SIZE; DYNAMICS
Arctic floodplain lakes of the Mackenzie River Delta receive large inputs of dissolved organic matter (DOM) and suspended particulates from allochthonous and autochthonous sources that may drive in situ formation of transparent exopolymer particles (TEP), and represent a complex combination of substrates for aquatic bacteria. During the open-water period of 2006, we tracked abundances of free-living bacteria versus those attached to TEP and to other non-TEP particles in 3 Delta lakes representing gradients of declining river-to-lake connection times, increasing levels of dissolved organic carbon (DOC), and declining total suspended solids (TSS). Total suspended bacteria (= free-living + TEP-attached + other-attached bacteria) were high in all lakes (similar to 10(6) to 107 cells ml(-1)) compared to tundra lakes, and most cells were particle-attached, despite the high DOC concentrations. Free-living bacteria were best related to DOC concentrations, but represented only 14% of total suspended bacteria. TEP-attached bacteria were best related to TEP mass, but represented only 9.6% of total suspended bacteria. Other-attached bacteria were best related to chromophoric DOM and TSS levels, and represented 76.5% of total suspended bacteria. Bacterial densities on TEP particles increased as TEP mass declined from lakes with strong river influence to the lakes with higher autochthonous DOC. However, bacterial density on non-TEP particles declined over the same gradient as the abundance of their substrate declined. These opposing gradients in bacterial density on particles of differing origin, combined with the high abundances of other-attached bacteria, suggest that TEP colonization is driven by the mass of particles available for colonization by free-living cells within the lakes, whereas non-TEP particles may mostly enter the lake waters pre-colonized with bacteria from terrestrial or benthic sources. DOI
27. Hopkinson, C; Crasto, N; Marsh, P; Forbes, D; Lesack, L. (2011) Investigating the spatial distribution of water levels in the Mackenzie Delta using airborne LiDAR.Hydrological Processes 25: 2995-3011 Investigating the spatial distribution of water levels in the Mackenzie Delta using airborne LiDAR
channel; hydraulic gradient; DEM; laser intensity
Airborne light detection and ranging (LiDAR) data were used to map water level (WL) and hydraulic gradients (delta H/delta x) in the Mackenzie Delta. The LiDAR WL data were validated against eight independent hydrometric gauge measurements and demonstrated mean offsets from -0.22 to +0.04 m (sigma < 0.11). LiDAR-based WL gradients could be estimated with confidence over channel lengths exceeding 5-10 km where the WL change exceeded local noise levels in the LiDAR data. For the entire Delta, the LiDAR sample coverage indicated a rate of change in longitudinal gradient (delta H-2/delta x) of 5.5 x 10(-10) m m(-2); therefore offering a potential means to estimate average flood stage hydraulic gradient for areas of the Delta not sampled or monitored. In the Outer Delta, within-channel and terrain gradient measurements all returned a consistent estimate of -1 x 10(-5) m m(-1), suggesting that this is a typical hydraulic gradient for the downstream end of the Delta. For short reaches (< 10 km) of the Peel and Middle Channels in the middle of the Delta, significant and consistent hydraulic gradient estimates of -5 x 10(-5) m m(-1) were observed. Evidence that hydraulic gradients can vary over short distances, however, was observed in the Peel Channel immediately upstream of Aklavik. A positive elevation anomaly (bulge) of >0.1 m was observed at a channel constriction entering a meander bend, suggesting a localized modification of the channel hydraulics. Furthermore, water levels in the anabranch channels of the Peel River were almost 1 m higher than in Middle Channel of the Mackenzie River. This suggests: (i) the channels are elevated and have shallower bank heights in this part of the delta, leading to increased cross-delta and along-channel hydraulic gradients; and/or (ii) a proportion of the Peel River flow is lost to Middle Channel due to drainage across the delta through anastamosing channels. This study has demonstrated that airborne LiDAR data contain valuable information describing Arctic river delta water surface and hydraulic attributes that would be challenging to acquire by other means. Copyright (C) 2011 John Wiley & Sons, Ltd. DOI
26. Tank, SE; Lesack, LFW; Gareis, JAL; Osburn, CL; Hesslein, RH. (2011) Multiple tracers demonstrate distinct sources of dissolved organic matter to lakes of the Mackenzie Delta, western Canadian Arctic.Limnology and Oceanography 56: 1297-1309 Multiple tracers demonstrate distinct sources of dissolved organic matter to lakes of the Mackenzie Delta, western Canadian Arctic
Lakes of the Mackenzie Delta occur across a gradient that contains three clear end members: those that remain connected to river-water channels throughout the summer; those that receive only brief inputs of river water during an annual spring flood but contain dense macrophyte stands; and those that experience significant permafrost thaw along their margins. We measured dissolved organic carbon (DOC) concentration, dissolved organic matter (DOM) absorption and fluorescence, and stable isotopes of DOM, DOM precursor materials, and bacteria to elucidate the importance of river water, macrophytes, and thermokarst as DOM sources to Mackenzie Delta lakes. Despite standing stocks of macrophyte C that are sevenfold to 12-fold greater than those of total DOC, stable isotopes indicated that autochthonous sources contributed less than 15% to overall DOM in macrophyte-rich lakes. Instead, fluorescence and absorption indicated that the moderate summertime increase in DOC concentration in macrophyte-rich lakes was the result of infrequent flushing, while bacterial delta(13)C indicated rapid bacterial removal of autochthonous DOC from the water column. In thermokarst lakes, summertime increases in DOC concentration were substantial, and stable isotopes indicated that much of this increase came from C released as a result of thermokarst-related processes. Our results indicate that these distinct sources of DOM to neighboring arctic Delta lakes may drive between-lake differences in C cycling and energy flow. Rapidly assimilated macrophyte DOM should be an important contributor to microbial food webs in our study lakes. In contrast, the accumulation of thermokarst-origin DOM allows for a significant role in physico-chemistry but indicates a lesser contribution of this DOM to higher trophic levels. DOI
25. Gareis, JAL; Lesack, LFW; Bothwell, ML. (2010) Attenuation of in situ UV radiation in Mackenzie Delta lakes with varying dissolved organic matter compositions.Water Resources Research 46: W09516 Attenuation of in situ UV radiation in Mackenzie Delta lakes with varying dissolved organic matter compositions
In 2004, ultraviolet-B (UVB) and ultraviolet-A (UVA) attenuation were measured in Mackenzie Delta lakes spanning gradients in water renewal rate, dissolved organic carbon (DOC) concentration, and dissolved organic matter (DOM) composition. DOM compositions (ratio of chromophoric DOM (CDOM) to non-chromophoric DOM) in Delta lakes are complex, evolving seasonally via flooding, dilution, macrophyte production, photobleaching, and bacterial metabolism. Attenuation was more strongly related to CDOM absorption coefficients (a(330); UVB r(2) = 0.69, p < 0.0001; UVA r(2) = 0.58, p < 0.0001) than to DOC concentrations due to variable DOM compositions. Attenuation in one set of lakes was well related (linear models) to a330 and total suspended solids (UVB R-2 = 0.80, p < 0.0001; UVA R-2 = 0.81, p < 0.0001). When these models were applied to other Delta lakes, however, attenuation of UVB was overestimated in 17 of 19 cases and attenuation of UVA was overestimated in all 18 cases. This bias indicates that models are not transferrable among Delta lakes, and likely cannot be applied in other circumpolar delta lakes with similarly complex DOM compositions. Although attenuation is high in Delta lakes (KdUVB 17.1-33.4 m(-1); K(d)UVA 7.7-19.2 m(-1)), plankton and photoreactive solutes may be exposed to high levels of ultraviolet radiation (UVR) because Delta lakes are shallow, UVB and UVA penetrate the top 19% and 31% of water columns, respectively, and day lengths are extended during open water. Thus, climate change effects on DOM compositions may significantly alter in situ UVR environments in circumpolar delta lakes. DOI
24. Lesack, LFW; Marsh, P. (2010) River-to-lake connectivities, water renewal, and aquatic habitat diversity in the Mackenzie River Delta.Water Resources Research 46: W12504 River-to-lake connectivities, water renewal, and aquatic habitat diversity in the Mackenzie River Delta
Past and ongoing investigations have established that lakes in the Mackenzie River Delta collectively represent gradients in water transparency, nutrient regime, and biotic communities, each strongly linked to the sill elevations of the lakes. Analysis of 40 years of water levels in East Channel of the central delta, in combination with a floodplain geometry model to estimate river water volumes added to lake waters at the annual flood peak, permitted direct estimation of annual river-to-lake connection times, lake water renewal, and interannual variabilities in nine lakes spanning the full range of sill elevations in the delta. Results have revealed a broad range of river-to-lake connectivities and river water renewals that are temporally dynamic and vary considerably among the lakes of this river delta system. Lakes with short and variable connection times plus low and variable river water renewal yield groups of lakes with high degrees of individuality because they are strongly influenced by particular sequences of antecedent years (legacy effects) that may result in lakes simultaneously containing residual waters from multiple river inundation events separated by more than a decade. Lakes with long and less varying connection times plus high river water renewal with multiple possible river water resets per year yield lakes with high degrees of similarity. The full combination of lakes arranged in an intermittently connected continuum, creating variable connectivity for aquatic organisms and water intermixing, may be an important mechanism driving the collectively distinctive habitat productivity and biodiversity of aquatic communities in this system, relative to lakes on the surrounding landscape. DOI
23. Graydon, JA; Emmerton, CA; Lesack, LFW; Kelly, EN. (2009) Mercury in the Mackenzie River delta and estuary: Concentrations and fluxes during open-water conditions.Science of the Total Environment 407: 2980-2988 Mercury in the Mackenzie River delta and estuary: Concentrations and fluxes during open-water conditions
Mercury; Methylmercury; Arctic; Mackenzie River; Delta; Estuary; Arctic Ocean
Estimates of mercury (Hg) loadings to the Arctic Ocean from circumpolar rivers have not considered biogeochemical changes that occur when river water is temporarily stored in large deltas (delta effect). There are also few data describing Hg changes across the freshwater-saltwater transition zone (FSTZ) of these rivers. We assessed temporal changes in unfiltered total mercury (THg) and methylmercury (MeHg) concentrations during open-water 2004 in the Mackenzie River upstream of the Mackenzie River delta, and in 6 floodplain lakes across an elevation gradient. These data were used to calculate Hg fluxes from the Mackenzie River and to evaluate a delta effect on Hg using an estimate of delta river water storage and a mixing analysis. Mean THg concentrations were highest in river water (9.17 +/- 5.51 ng/L) and decreased up the lake elevation gradient. Mean MeHg concentrations were highest in lakes periodically connected to the river (0.213 +/- 0.122 ng/L) and MeHg concentrations in elevated lakes showed a mid-summer peak. Results from the mixing analysis showed that the delta effect may be large enough to affect Hg loadings to the Arctic Ocean. THg concentrations exiting the delta (10.2 ng/L) were 16% lower than those entering (12.1 ng/L), whereas MeHg showed little change. We calculated 2.5-month (open-water) THg and MeHg fluxes from the Mackenzie River of 1208 and 8.4 kg. These fluxes are similar in magnitude to previous annual estimates in the arctic literature suggesting that previously published annual Hg fluxes from the Mackenzie River may be large underestimates. We also assessed changes in Mackenzie River water THg and MeHg concentrations as it crossed the FSTZ during an open-water cruise. THg decreased non-conservatively across the estuary from 3.8-0.6 ng/L, possibly due to mixing and particle settling. MeHg concentrations were variable and near detection. Our results show that the Mackenzie River estuary is a dynamic environment and may have important controls on Hg delivered to the Arctic Ocean. (C) 2009 Elsevier B.V. All rights reserved. DOI
22. Marsh, L. Lesack, F. Hicks, A. Roberts, C. Hopkinson, S. Solomon, D.L.Forbes, M. Russell, H. Haywood. (2009) Hydrology of the Mackenzie Delta: off-channel water storage and delta interaction with the Beaufort Sea.17Th International Northern Research Basins Symposium and Workshop Iqaluit-Pangnirtung-Kuujjuaq, Canada, August 12 to 18, 2009 217-226 Hydrology of the Mackenzie Delta: off-channel water storage and delta interaction with the Beaufort Sea
This paper reports on the development and application of a hydrodynamic model of river flows
and off-channel storage effects in the Mackenzie Delta, Canada, being developed in support of
the Canadian International Polar Year (IPY) project entitled “Polar Terrestrial Freshwater: State
and Flow to the Ocean”. This modelling effort is aimed at addressing questions related to
transport of fresh water and nutrients to the Arctic Ocean under open water and ice affected
conditions, and to estimate how water levels through the delta may change in response to
changing river flows, reduced or increased ice jamming, increasing sea-level, and enhanced storm
surges from the Beaufort coast.
The Mackenzie Delta Hydrodynamic Model (MDHM) is being built upon the University of
Alberta's River1D hydrodynamic and ice process model, capable of handling the highly dynamic
flows associated with events such as storm surges and ice jam formation and release. With an
area of more than 13,000 sq. km., and containing more than 45,000 lakes, the Mackenzie Delta is
a large and complex hydrodynamic system that presents unique modelling challenges. This paper
will present the details of the model application, and in particular, will focus on the unique
challenges involved in establishing a hydrodynamic model of such a large and complex northern
delta, in the face of limited topographic data on channel and lake geometry and flow connections,
sparse flow and water level monitoring, and complex river and ocean ice effects. PDF
21. Nafziger J, Hicks F, Andrishak R, Marsh R, Lesack L. (2009) Hydraulic Model of River Flow and Storage Effects in the Mackenzie Delta, Canada.17Th International Northern Research Basins Symposium and Workshop Iqaluit-Pangnirtung-Kuujjuaq, Canada, August 12 to 18, 2009 237-247 Hydraulic Model of River Flow and Storage Effects in the Mackenzie Delta, Canada
This paper reports on the development and application of a hydrodynamic model of river flows
and off-channel storage effects in the Mackenzie Delta, Canada, being developed in support of
the Canadian International Polar Year (IPY) project entitled “Polar Terrestrial Freshwater: State
and Flow to the Ocean”. This modelling effort is aimed at addressing questions related to
transport of fresh water and nutrients to the Arctic Ocean under open water and ice affected
conditions, and to estimate how water levels through the delta may change in response to
changing river flows, reduced or increased ice jamming, increasing sea-level, and enhanced storm
surges from the Beaufort coast.
The Mackenzie Delta Hydrodynamic Model (MDHM) is being built upon the University of
Alberta's River1D hydrodynamic and ice process model, capable of handling the highly dynamic
flows associated with events such as storm surges and ice jam formation and release. With an
area of more than 13,000 sq. km., and containing more than 45,000 lakes, the Mackenzie Delta is
a large and complex hydrodynamic system that presents unique modelling challenges. This paper
will present the details of the model application, and in particular, will focus on the unique
challenges involved in establishing a hydrodynamic model of such a large and complex northern
delta, in the face of limited topographic data on channel and lake geometry and flow connections,
sparse flow and water level monitoring, and complex river and ocean ice effects.
20. Squires, MM; Lesack, LFW; Hecky, RE; Guildford, SJ; Ramlal, P; Higgins, SN. (2009) Primary Production and Carbon Dioxide Metabolic Balance of a Lake-Rich Arctic River Floodplain: Partitioning of Phytoplankton, Epipelon, Macrophyte, and Epiphyton Production Among Lakes on the Mackenzie Delta.Ecosystems 12: 853-872 Primary Production and Carbon Dioxide Metabolic Balance of a Lake-Rich Arctic River Floodplain: Partitioning of Phytoplankton, Epipelon, Macrophyte, and Epiphyton Production Among Lakes on the Mackenzie Delta
DISSOLVED ORGANIC-CARBON; WATER TRANSPARENCY; FRESH-WATER; CHAR LAKE; NORTHWEST-TERRITORIES; NUTRIENT LIMITATION; NORTHERN WISCONSIN; HYDROGEN-PEROXIDE; CORNWALLIS-ISLAND; SURFACE WATERS
The Mackenzie River Delta, a floodplain system in the western Canadian Arctic contains approximately 45,000 lakes used by resident and migratory fish, aquatic birds, and mammal populations, which are critical resources for aboriginal peoples. Our results show the Mackenzie Delta is a remarkably productive aquatic ecosystem, not out of place relative to other large river floodplains and unusually productive for its Arctic latitude. Along with other such deltas of north flowing rivers to the Arctic Ocean, it represents a critical habitat of high productivity to support dependent consumers. Our results also showed a consistent under-saturation of CO2 in lake waters of the delta, co-occurring even with high concentrations of dissolved organic carbon, and indicating P:R greater than 1 during the open water period. In less turbid lakes, abundant macrophytes provide a considerable surface area for supporting production of epiphyton, but epiphyton production is constrained strongly by macrophyte shading, when macrophyte biomass is high. Despite this, epiphyton represented a greater supply of non-macrophyte algal carbon than phytoplankton, and thus explains why benthic algae may be a more important food source for primary consumers than phytoplankton, except in the most turbid systems. Most importantly, the high autotrophic production in the Delta lakes relative to nearby lakes on the arctic tundra suggests the extended ice-free season of the floodplain lakes and their landscape setting on, and replenishment by, nutrient-rich river sediments, is the strongest influence on aquatic production levels. DOI
19. Tank, SE; Lesack, LFW; Hesslein, RH. (2009) Northern Delta Lakes as Summertime CO2 Absorbers Within the Arctic Landscape.Ecosystems 12: 144-157 Northern Delta Lakes as Summertime CO2 Absorbers Within the Arctic Landscape
MACKENZIE DELTA; WATER TRANSPARENCY; NORTHWEST-TERRITORIES; ORGANIC-CARBON; EXCHANGE; SURFACE; CANADA; BUDGET; PERMAFROST; ATMOSPHERE
The vast majority of lakes examined worldwide emit CO2 to the overlying atmosphere, through a process by which catchment-derived subsidies of terrigenous C, often in the form of dissolved organic carbon (DOC), augment within-lake CO2 production above the level consumed via photosynthesis. We show that shallow, macrophyte-rich lakes of the Mackenzie Delta, western Canadian Arctic, do not follow this pattern. These lakes are strong summertime CO2 absorbers, despite DOC concentrations at or above levels commonly shown to produce CO2 emission. Paradoxically, CO2 levels were lowest where DOC was greatest, in lakes which appear to be annual net CO2 absorbers, and have poor hydrologic connection to the terrestrial landscape. CO2 in these lakes is depleted by high macrophyte productivity, and although catchment-derived C subsidies are low, within-lake DOC generation appears to occur as a byproduct of macrophyte photosynthesis and evapoconcentration. Additionally, after accounting for DOC and macrophytes, lakes that were least connected to the larger terrestrial landscape remained weaker CO2 absorbers, suggesting that CO2 balance may also be affected by DOC quality, foodweb structure, or inputs of pCO(2)-rich riverwater to connected lakes. In contrast, a small subset of Delta lakes that were strongly affected by permafrost melting were CO2 emitters, suggesting future permafrost degradation could engender a change in the overall CO2 balance of these lakes from near-CO2 neutral over the ice-free season, to clear CO2 emission. Our work suggests that the current paradigm of lakewater CO2 regulation may need to specifically incorporate shallow, productive lakes, and those that are poorly connected to their surrounding landscape. DOI
18. Tank, SE; Lesack, LFW; McQueen, DJ. (2009) Elevated pH regulates bacterial carbon cycling in lakes with high photosynthetic activity.Ecology 90: 1910-1922 Elevated pH regulates bacterial carbon cycling in lakes with high photosynthetic activity
alkalization; bacterial growth efficiency; bacterial production; bacterial respiration; carbon cycling; community adaptation; ecological stressors; high productivity; Mackenzie Delta; western Canadian Arctic; pH; trophic cascades GROWTH EFFICIENCY; COMMUNITY COMPOSITION; SEASONAL DYNAMICS; HYPERTROPHIC LAKE; PHYTOPLANKTON; SHALLOW; ZOOPLANKTON; METABOLISM; FISH
Bacteria are critically important for carbon (C) cycling and energy flow in aquatic environments. However, studies to date have largely focused on the role of substrate quality in the regulation of this important process. As such, we know little about the role of other ecological drivers in shaping bacterially mediated C cycling. Here we examine the manner in which planktonic bacterial abundance (BA), productivity (BP), respiration (BR), and growth efficiency (BGE), and thus C cycling are affected by elevated pH, an ecological factor that occurs commonly in highly productive aquatic systems. We undertook our study in lakes of the Mackenzie Delta region of Canada. These lakes routinely experience high pH caused by rapid macrophyte photosynthesis. Two different experiment types were employed: first, a series of short-term experiments was used to assess the direct effects of elevated pH on bacteria experiencing differing pH levels in situ. Second, long-term mesocosms were used to explore the effect of elevated pH on bacteria over longer time scales and in the presence of other trophic levels. Bacterial productivity and BR slowed dramatically with elevated pH over the short term, potentially uncoupling bacterial processing of organic matter from its in-lake production and causing a switch away from biomass creation and toward C mineralization. With longer term exposure, bacterial communities adapted to the direct stress of elevated pH, but responses at higher trophic levels caused a cascade that mediated the effect of alkalization on bacteria, in a manner that could well vary among aquatic ecosystems. Our study establishes elevated pH as a key driver of bacterial C cycling and energy flow in aquatic systems with high autotrophic productivity.Website DOI
17. Emmerton, CA; Lesack, LFW; Vincent, WF. (2008) Mackenzie River nutrient delivery to the Arctic Ocean and effects of the Mackenzie Delta during open water conditions.Global Biogeochemical Cycles 22 Mackenzie River nutrient delivery to the Arctic Ocean and effects of the Mackenzie Delta during open water conditions
Large rivers have a strong influence on the Arctic Ocean, but little attention has been given to the biogeochemical effect that lake-rich delta floodplains may have on river waters prior to marine discharge. We assessed the effect of the Mackenzie Delta on riverine fluxes of nutrients and organic matter to the Arctic Ocean during the open water period of 2004. Using a new estimate of peak off-channel water storage in the delta floodplain, a two-source mixing model was developed (channel water plus recovery of off-channel water) to estimate the volume-weighted nutrient composition of river water after the off-channel water was recovered from the delta during the hydrograph recession period. Results with the delta effect included (i.e., with recovery of off-channel water) relative to results with the effect omitted (i.e., analogous to historical monitoring upstream of the delta) show particulate levels were 10-18% lower, but enriched in organic content (POC: TSS, PN: TSS, PP: TSS) by 75-280%; dissolved inorganic nutrients were lower (NO3- 14%; SRP 14%; SRSi 5%) except for ammonium (10%); and dissolved organic matter was higher (DOC 15%; DON 62%; DOP 239%). The resulting nutrient quality (C:N:P stoichiometry) was more enriched in carbon (TOC:TP) by 79% and in nitrogen (TN:TP) by 77% relative to phosphorus. Model results were compared against nutrient measurements throughout the delta channel network taken three times over this same period, and differences from upstream to downstream matched reasonably well to the model, though they also suggested the delta effect may be more complex than represented by the model. Our results generally indicate the Mackenzie Delta has an important effect on the magnitude and quality of riverine particulates and nutrients prior to entering the sea. Such an effect has not been quantified in prior work and is likely to be important in other arctic rivers with lake-rich deltas. Our enhanced sampling of the high-discharge period during early hydrograph recession has also better captured the detailed composition of C, N, and P constituents in the river water, ultimately leading to improved estimates of nutrient levels and overall nutrient quality for the open water period that differ appreciably from prior observations on the Mackenzie River. DOI
16. Emmerton, CA; Lesack, LFW; Vincent, WF. (2008) Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice.Journal of Marine Systems 74: 741-755 Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice
Arctic; Estuary; Nutrients; Organic matter; Particulates, Salinity gradient
Suspended material, nutrients and organic matter in Mackenzie River water were tracked along a 300 km transect from Inuvik (Northwest Territories, Canada), across the estuarine salinity gradient in Kugmallit Bay, to offshore marine stations on the adjacent Mackenzie Shelf. All particulates measured (SPM, POC, PN, PP) declined by 87-95% across the salinity gradient and levels were generally below conservative mixing. Organic carbon content of suspended material decreased from 3.1% in the river to 1.7% in shelf surface waters while particulate C:N concurrently decreased from 17.1 to 8.6. Nitrate and silicate concentrations declined by more than 90% across the salinity gradient, with nitrate concentrations often below the conservative mixing line. Phosphate concentrations increased from 0.03 mu mol/L in the river to 0.27 mu mol/L over shelf waters, thereby changing the inorganic nutrient regime downstream from P to N limitation. Dissolved organic carbon decreased conservatively offshore while dissolved organic N and P persisted at high levels in the Mackenzie plume relative to river water, increasing 2.7 and 25.3 times respectively. A deep chlorophyll-a maximum was observed at two offshore stations and showed increases in most nutrients, particulates and organic matter relative to the rest of the water column. During river passage through the Mackenzie estuary, particulate matter, dissolved organic carbon and inorganic nutrients showed sedimentation, dilution and biological uptake patterns common to other arctic and non-arctic estuaries. Alternatively, inorganic content of particles increased offshore and dissolved organic N and P increased substantially over the shelf, reaching concentrations among the highest reported for the Arctic Ocean. These observations are consistent with the presence of a remnant ice-constrained ('stamukhi') lake from the freshet period and a slow flushing river plume constrained by sea-ice in close proximity to shore. Nutrient limitation in surface shelf waters during the ARDEX cruise contributed to the striking deep chlorophyll-a maximum at 21 m where phytoplankton communities congregated at the margin of nutrient-rich deep ocean waters. (C) 2007 Elsevier B.V. All rights reserved. DOI
15. Emmerton, CA; Lesack, LFW; Marsh, P. (2007) Lake abundance, potential water storage, and habitat distribution in the Mackenzie River Delta, western Canadian Arctic.Water Resources Research 43 Lake abundance, potential water storage, and habitat distribution in the Mackenzie River Delta, western Canadian Arctic
The complete landscape surface of the active Mackenzie River Delta ( 13,135 km(2)) was manually partitioned into discrete lakes ( 3331 km(2)), channels ( 1744 km(2)), wetlands ( 1614 km(2)), and dry floodplain area ( 6446 km(2)) via GIS analysis of digital topographic maps recently available for the system. The census total of lakes ( 49,046) is almost twice as large as prior estimates. Using this new information, total lake volume in the delta during the post river flooding period is estimated as 5.4 km(3). Total floodwater storage in the delta lakes and floodplain at peak water levels is estimated at 25.8 km(3) and thus is equivalent to about 47% of Mackenzie River flow ( 55.4 km(3) yr(-1)) during the high-discharge period of delta breakup. During this period the stored river water can be envisioned in the form of a thin layer of water ( 2.3 m thick on average) spread out over 11,200 km(2) of lakes and flooded vegetation and exposed to 24 h d(-1) solar irradiance. Consequently, this temporarily stored water has significant potential to affect the composition of river water flowing to the Beaufort Shelf as it recedes to the river channels after the flood peak. DOI
13. Febria, CM; Lesack, LFW; Gareis, JAL; Bothwell, ML. (2006) Patterns of hydrogen peroxide among lakes of the Mackenzie Delta, western Canadian Arctic.Canadian Journal of Fisheries and Aquatic Sciences 63: 2107-2118 Patterns of hydrogen peroxide among lakes of the Mackenzie Delta, western Canadian Arctic
Dissolved organic carbon (DOC) in Mackenzie Delta lakes varies in composition and concentration, ranging from low concentrations and high colour in frequently flooded lakes to high concentrations in clear lakes that are infrequently flooded. DOC is a precursor to the photochemical production of hydrogen peroxide (H2O2) in lake waters. Here we assessed the patterns of H2O2 using three approaches: (i) H2O2 levels were tracked in 40 lakes during the open water season from the Arctic summer solstice (24 h sunlight) to late summer; (ii) diurnal dynamics of in situ H2O2 were tracked in a pair of lakes with contrasting DOC regimes (coloured vs. noncoloured DOC); and (iii) buildup of H2O2 was tracked in experimental microcosms of lake water exposed to manipulated UV levels. H2O2 levels were highest at the solstice and in intermittently flooded lakes. During 24 h daylight, lakes with contrasting DOC regimes showed cumulative build up of H2O2 during multiple cloudless days. Cumulatively, H2O2 was highest in the Delta lake with a higher DOC concentration and low in colour. H2O2 buildup in lake water shielded from UVB exposure was not significantly lower from microcosms under full sunlight. UVA was the most important in coloured DOC photobleaching and H2O2 production.
12. Spears, BM; Lesack, LFW. (2006) Bacterioplankton production, abundance, and nutrient limitation among lakes of the Mackenzie Delta (western Canadian arctic).Canadian Journal of Fisheries and Aquatic Sciences 63: 845-857 Bacterioplankton production, abundance, and nutrient limitation among lakes of the Mackenzie Delta (western Canadian arctic)
The effects of nutrient availability and quality of dissolved organic carbon (DOC) on bacterioplankton production were assessed in six lakes with differing frequencies of river flooding. Bacterial productivity, dissolved nutrients, and DOC were tracked weekly throughout the open-water period of 2001. Inorganic nutrient (N and P) enrichment microcosm experiments were conducted to directly assess the effects of DOC quality (i.e., mixtures of colored and noncolored DOC) and inorganic nutrient limitation on bacterial productivity among the lakes. Averaged over the open-water season, both abundance and production of bacterioplankton increased with decreasing flood frequency (R-2 = 0.61 and R-2 = 0.78, respectively). Reduced bacterial production occurred in frequently flooded lakes, where colored DOC, light attenuation, and phosphate were high but ammonium was low. Bacterial production was greatest in infrequently flooded lakes, where noncolored DOC and ammonium were high but phosphate was low. Bacterial production was enhanced by amendments of inorganic nutrients in duplicate experiments (two-factor analyses of variance). Production was also enhanced in response to higher concentrations of either colored or noncolored DOC following release from inorganic nutrient limitation. Size fractionated (< 1 mu m versus > 1 mu m) N-debt and P-debt bioassays typically showed demand for P and release of N by bacteria in all study lakes.
11. Squires, MM; Lesack, LFW. (2003) The relation between sediment nutrient content and macrophyte biomass and community structure along a water transparency gradient among lakes of the Mackenzie Delta.Canadian Journal of Fisheries and Aquatic Sciences 60: 333-343 The relation between sediment nutrient content and macrophyte biomass and community structure along a water transparency gradient among lakes of the Mackenzie Delta
Macrophyte abundance and distribution among lakes of the Mackenzie Delta were assessed where increasing distance from the river (chain set) and increasing frequency of flooding (sill set) corresponded with increasing water transparency. Overall, sediment organic matter (OM) and total nitrogen (TN) content increased with increasing biomass of macrophytes but was higher in the sill set than in the chain set. The amount of phosphorus (P) in sediments was similar among lakes, but pore-water P was appreciably higher in the chain set. Increasing sediment OM and water clarity corresponded with increasing biomass of macrophytes in the lakes. Community structure shifted from dominance by erect Potamogeton at low and intermediate transparency and moderate sediment OM content to low-growing Chara and Ceratophyllum at high transparency and high sediment OM. Similar transparency in the chain set supported greater biomass of macrophytes than in the sill set. A high rate of inorganic sedimentation (linked with frequent flooding) and organic sedimentation (linked with high transparency and plant biomass) may result in the most suitable substrate for the growth of macrophytes among lakes of the Mackenzie Delta. Submersed plant biomass was higher in the Mackenzie Delta lakes than in temperate lakes and comparable to that in the temperate and tropical floodplains, despite the high-latitude location.
10. Squires, MM; Lesack, LFW. (2003) Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta.Freshwater Biology 48: 1-20 Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta
dissolved organic carbon; floodplain lakes; light attenuation; total suspended sediments; water colour
1. The seasonal dynamics of light attenuation, and the relative roles of total suspended solids (TSS), dissolved organic carbon (DOC) and chlorophyll as light attenuators among two sets of lakes in the Mackenzie Delta, were assessed during the open-water periods of 1998 and 1999. 2. The first set consisted of 40 spatially discrete lakes where the frequency of flooding with river water was controlled by sill height ('sill-set lakes'). The second set consisted of a chain of six lakes connected to a main river channel ( frequently flooded, all with same frequency), but where riverine influence was controlled by the distance from the channel connection point ('chain-set lakes'). 3. As the flooding frequency of lakes decreased (sill-set), and as the distance from the channel connection point increased (chain-set), lake water became increasingly transparent and the stability ( decreasing temporal variability) of underwater light increased. 4. The effect of flooding on transparency was greater in years with a high minimum summer water level. However, the effect of river flooding on lake water transparency was damped more by an increase in the frequency and duration of flooding than by an increase in distance from the channel connection point. 5. The index of scattering was linearly related to TSS over the common range of concentrations in both sets of lakes. The specific attenuation coefficient for TSS ( and scattering) increased substantially from the most turbid to the most transparent waters. 6. During the summer, DOC provided an approximate index of water colour in the sill-set lakes but not in the chain-set lakes, where the gradient of DOC ran counter to the gradient of water colour. The specific attenuation coefficient for water colour was roughly constant among both sets of lakes. 7. Calculations of partial attenuation show that, during the spring flood peak, TSS is the dominant attenuator among most lakes, other than those with high sills or positioned far from channel connection points. During the lengthy summer period of open water, however, water colour appeared to be the most important light attenuator among almost all of the lakes in the central delta, with chlorophyll a of only minor importance. 8. Lakes of the Mackenzie Delta may be quite sensitive to changes in climate and ultraviolet-b (UV-b) radiation in the circumpolar arctic because of the role of DOC as an attenuator of photosynthetically active radiation and UV-b irradiance and as an energy source for microbial foodwebs in this system.
9. Headley, JV; Marsh, P; Akre, CJ; Peru, KM; Lesack, L. (2002) Origin of polycyclic aromatic hydrocarbons in lake sediments of the Mackenzie Delta.Journal of Environmental Science and Health Part A-Toxic/Hazardous Substances & Environmental Engineering 37: 1159-1180 Origin of polycyclic aromatic hydrocarbons in lake sediments of the Mackenzie Delta
PAHs; Mackenzie River; alkylated PAHs; sediment; mass spectrometry
The concentrations and distribution of polycyclic aromatic hydrocarbons (PAHs) were assessed in sediment cores from among 14 lakes from three regions comprising a transect across the central Mackenzie Delta. PAHs were consistently found in the lake sediments, with parent concentrations in the 20-200 ng/g range. Concentrations were generally independent of depth in the sediment cores and this pattern was similar among the 3 regions of the delta. Concentrations increased in a westerly direction among the regions. For some lakes, the concentration of PAHs decreased with decreasing flooding frequency, and decreasing sedimentation rates. For the latter, maximum concentrations occurred at shallower depths within the -sediment cores as flooding frequency among the lakes decreased. The distributions of C-0-C-4 alkylated 2- and 3- ring PAHs were consistent with a petrogenic origin, while the corresponding distribution of 4-ring PAHs appears to be more consistent with a biogenic or pyrogenic origin. Based on relative contributions to the overall PAH budget, a petrogenic source appears to be dominant. However, the pyrene/fluoranthene ratio is more consistent with a source derived from peat. The alkylated PAH profiles are inconsistent with those in the Athabasca River system, and supports a previously published hypothesis that the contribution of PAHs from the Athabasca oil sands to the lower Mackenzie River is minimal. A double ratio plot of chrysene vs dibenzothiophene, diagnostic of weathering, suggests most weathering occurred before the sediments were deposited in the lakes, while a double ratio plot of dibenzothiophene vs phenanthrene suggests a common source of PAHs across the delta, despite differing water sources from east to west across the delta. PAH inputs to the delta appear to mirror sediment inputs documented in previous work, where high sediment input from the Mackenzie mainstem during high floods dominates the delta sediment influx and masks any influence of the Peel River.
8. Squires, MM; Lesack, LFW. (2002) Water transparency and nutrients as controls on phytoplankton along a flood-frequency gradient among lakes of the Mackenzie Delta, western Canadian Arctic.Canadian Journal of Fisheries and Aquatic Sciences 59: 1339-1349 Water transparency and nutrients as controls on phytoplankton along a flood-frequency gradient among lakes of the Mackenzie Delta, western Canadian Arctic
Relatively low phytoplankton production among lakes of the Mackenzie Delta has been attributed to light limitation in frequently flooded lakes and to nutrient limitation among infrequently flooded lakes; productivity peaks have been attributed to optimal light-nutrient conditions at intermediate flood frequency. We found that the distribution and abundance of phytoplankton among a large number of lakes was explained by optimal light-nutrient conditions but not by nutrient or light limitation. For a subset of lakes, seasonal dynamics of biomass were consistent with light limitation and optimal light-nutrient conditions but not nutrient limitation; photosynthetic rates were consistent with optimal light-nutrient conditions and nutrient limitation but not light limitation. Results of dilution-deletion experiments across a light-nutrient gradient indicated transition from light limitation to optimal light-nutrient conditions to nutrient limitation. Surprisingly, adding river water to lake water did not increase photosynthetic rates; this result and experimental incubations in situ during river inflow and lake outflow suggested that continuous supply of river water may be necessary to increase phytoplankton growth rates. Among infrequently flooded lakes, phytoplankton response to nutrient additions showed that phosphorus (P) limitation was no more likely than nitrogen (N) limitation, co-limitation, or no limitation by N or P.
7. Squires, MM; Lesack, LFW; Huebert, D. (2002) The influence of water transparency on the distribution and abundance of macrophytes among lakes of the Mackenzie Delta, Western Canadian Arctic.Freshwater Biology 47: 2123-2135 The influence of water transparency on the distribution and abundance of macrophytes among lakes of the Mackenzie Delta, Western Canadian Arctic
macrophyte biomass; water transparency; floodplain lakes; arctic; Mackenzie Delta
1. Macrophyte abundance and distribution was assessed in a chain of six interconnected lakes (all with the same flooding frequency) in the Arctic, where increasing distance from the Mackenzie River channel resulted in a gradient of water transparency ('chain-set' lakes), and in a group of 26 spatially discrete lakes where increasing frequency and duration of lake flooding with river water (controlled by sill height) also resulted in a transparency gradient ('sill-set' lakes). 2. Among the chain-set lakes, above-ground macrophyte biomass increased from 0 to 1000 g m(-2) with increasing water transparency. Among the sill-set lakes, the transparency gradient among the lakes was less well defined and the relations with biomass were more varied. A decrease in flooding was associated with increasing water transparency and an increasing biomass of macrophytes from about 0 to over 2000 g m(-2). For a specific flood frequency, however, the effect of flooding was much greater when lakes were directly connected to a river channel than when floodwaters flowed first through an intervening lake. Among infrequently flooded lakes the effect of flooding on water transparency and biomass was negligible. 3. Among relatively clear lakes in both sets of lakes, biomass increased with increasing water transparency and decreasing lake depth. Among relatively turbid lakes, however, biomass increased with the combined effect of increasing water colour (decreasing water transparency) and increasing lake water depth. The increases in biomass with increasing water colour (coloured dissolved organic matter) and increasing depth, which together result in reduced light at the bed, may be explained by reduced exposure to ultra violet light. 4. An average light attenuation of 1.3 m(-1) (Secchi depth about 1 m) over the growing season appears to represent a threshold water transparency which, in combination with water depths early in the growing season, is consistent with a light supply on the bed required for growth of the common macrophytes in lakes of the Mackenzie Delta. However, a comparison with other systems indicates that macrophytes among lakes of the Mackenzie Delta grow deeper, for a given level of transparency, than is reported in lakes at lower latitude, despite the lower sun angles and increased reflectivity of water surfaces in the arctic. 5. A complete accounting of water transparency (at PAR and UV wavelengths), lake depth, summer sun angle and duration of sunlight may be necessary to explain patterns of macrophyte growth among lakes across a full range of latitudes.
6. Michelutti, N; Hay, MB; Marsh, P; Lesack, L; Smol, JP. (2001) Diatom changes in Lake Sediments from the Mackenzie Delta, NWT, Canada: Paleohydrological applications.Arctic Antarctic and Alpine Research 33: 1-12 Diatom changes in Lake Sediments from the Mackenzie Delta, NWT, Canada: Paleohydrological applications
Information on hydrological fluctuations and lake dynamics in the Mackenzie Delta in Arctic Canada is provided in a paleolimnological study of eight delta lakes. Because macrophyte production within Mackenzie Delta lakes is largely governed by the degree of river influence, estimates of past production within a lake may be used to infer past Mackenzie River influence. A diatom-based predictive model, using sub-ice winter methane concentrations as an indirect estimate of macrophyte production, was applied to fossil diatom assemblages identified in Mackenzie Delta lake sediment cores. Temporal shifts in the relative abundance of fossil diatom assemblages were recorded at all sites. Benthic and epiphytic fossil diatom taxa were common in lakes with minor river influence, whereas planktonic species dominated lakes with appreciable river connection. The fossil diatom record indicated that taxa are responding to changes in the degree of river influence and lake genesis. Lakes having the least amount of river connection supported the most favorable environment for macrophyte production, resulting in the highest sediment organic matter content, and consequently, the highest estimates of methane concentrations. Tn general, the diatom-inferred winter methane concentrations appeared to be reliable, and they indirectly track the main direction of variation in the fossil diatom assemblages. The lack of consistent trends between sites in diatom assemblage changes/inferred methane values indicates that the hydrology near Inuvik, although highly variable, has not undergone a steady-state shift over the past similar to 200 yr. However, our results do indicate that diatom taxa in delta lakes are sensitive to hydrological fluctuations in the Mackenzie River and that our present approach would be ideally suited for detecting long-term (e.g., decadal) hydrological variations.
5. Squires, MM; Lesack, LFW. (2001) Benthic algal response to pulsed versus distributed inputs of sediments and nutrients in a Mackenzie Delta lake.Journal of the North American Benthological Society 20: 369-384 Benthic algal response to pulsed versus distributed inputs of sediments and nutrients in a Mackenzie Delta lake
epipelon; floodplain lakes; Mackenzie Delta; suspended sediments; light attenuation; primary productivity
The Mackenzie Delta is a lake-rich region where the distribution and abundance of phytoplankton, macrophytes, and benthic algae among lakes may be controlled by an interaction between nutrient supply and sediment in the water column, and episodic reduction in light availability associated with river flooding. To simulate the effects of episodic river inflow on benthic algae, limnocorrals (depth = 3 m) were either not manipulated (control), or received high or low weekly additions of nutrients (N and P) and sediments, delivered as a pulse (1/wk: high-pulsed, low-pulsed) or distributed incrementally through each week (1/3 of high or low load x 3 d/wk: high-distributed, low-distributed) in a balanced triplicated design (15 limnocorrals). Artificial substrates, enriched to mimic the supply of nutrients at the sediment/water interface, were suspended in the limnocorrals at 2 rn depth for 6 wk beginning in mid-July 1995. Maximum average levels of phytoplankton biomass (inferred from chlorophyll) occurred in the treatment where light attenuation was highest (high-distributed), and lowest levels occurred where light attenuation was lowest (control). By contrast, maximum average accrual of benthic algal biomass on the artificial substrates occurred in the control, whereas lowest accrual occurred in the high-distributed treatment. Chlorophyll content per unit biovolume of benthic algae increased modestly in treatments experiencing low light relative to treatments with higher light, but was insufficient to explain the 3-fold difference in inferred biomass between the control and high-distributed treatment in the case of benthic algae, or the 4-fold difference in the case of phytoplankton. Under equivalent in situ light conditions, areal net photosynthesis of benthic algae was highest in the control and lowest in the high-pulsed treatment. Net photosynthetic rate per unit benthic algal chlorophyll was not significantly different among the control and treatments. Last, biomass accrual rates among treatments showed a strong inverse relation with average light attenuation among the limnocorrals over the duration of the experiment. These results indicate that light availability was primarily responsible for maximum biomass accrual in the control and was the dominant factor affecting accrual rates among treatments. Overall, this study suggests that the abundance of benthic algae among Mackenzie Delta lakes may progressively increase as flood frequency decreases and light availability becomes greater, whereas phytoplankton abundance may peak in lakes with intermediate transparencies by responding to nutrient additions despite interruption of the light environment by suspended sediments.
4. Lesack, LFW; Marsh, P; Hecky, RE. (1998) Spatial and temporal dynamics of major solute chemistry among Mackenzie Delta lakes.Limnology and Oceanography 43: 1530-1543 Spatial and temporal dynamics of major solute chemistry among Mackenzie Delta lakes
The Mackenzie Delta of the Canadian arctic is a complex environment, containing similar to 25,000 lakes, where annual river flooding may exert a considerable degree of control over the ecological characteristics of lakes in the system. The solute composition of 92 lakes representing differing frequencies and durations of flooding was determined from among three clusters of lakes distributed over a north-south (N-S) gradient along the eastern margin of the delta. The solute compositions yield an end-member system of dominant solutes (shifting among Ca2+ plus HCO3-, Mg2+ plus HCO3-, Ca2+ plus SO42-) in correspondence with a gradient from frequently flooded lakes to infrequently flooded lakes. This end-member system is consistent between years and along the N-S elevational gradient of the delta. The solute composition of the frequently hooded lakes appears to be stable among years, while the composition among infrequently hooded lakes is not stable. The end-member system is consistent with the seasonal dynamics in solute composition observed for three lakes, representing differing flooding regimes, plus the dynamics of the nearest main distributary channel. The dominance of local hydrological and biogeochemical processes in about one-third of the lakes in the delta, where flooding occurs less frequently than every year, is not consistent with a common paradigm where flooding from river channels is thought to dominate the annual flushing, nutrient replenishment, and reinitialization of lacustrine ecosystems on the floodplains of major world rivers.
3. Hay, MB; Smol, JP; Pipke, KJ; Lesack, LFW. (1997) A diatom-based paleohydrological model for the Mackenzie Delta, Northwest Territories, Canada.Arctic and Alpine Research 29: 430-444 A diatom-based paleohydrological model for the Mackenzie Delta, Northwest Territories, Canada
Floodplain lakes are tightly coupled to their associated river systems and their sediment records should provide integrative records of this interaction. Surface sediments and selected limnological variables were collected from 77 Mackenzie Delta lakes representing three categories of river influence: lakes having continuous connection with the Mackenzie River (n = 23; no-closure), lakes that flood every spring but lose connection during the summer (rt = 26; low-closure). and lakes that flood only during an extreme spring flood stage (n = 28; high-closure). Summer lake production, using winter methane concentration as a proxy, and river influence were identified as the principal limnological gradients separating delta lakes. This river influence/primary production gradient also accounted for the greatest amount of variation within the surface sediment diatom assemblages. The diatom flora of the Mackenzie Delta lakes was dominated by benthic taxa, particularly the genera Nitzschia and Navicula, with a greater abundance of stalked, epiphytic ta?ta in the high-closure lakes. A robust predictive model was developed for inferring lake production from fossil diatom assemblages. The model provides a tool for estimating long-term changes in river influence and lake dynamics from the sediment record of Mackenzie Delta lakes. Diatom-inferred river influence changes within these records may then be linked with past river discharge variability.
2. Lesack, LFW; Melack, JM. (1996) Mass balance of major solutes in a rainforest catchment in the Central Amazon: Implications for nutrient budgets in tropical rainforests.Biogeochemistry 32: 115-142 Mass balance of major solutes in a rainforest catchment in the Central Amazon: Implications for nutrient budgets in tropical rainforests
Central Amazon; nutrient budgets; tropical rainforests
A solute mass balance for a 23.4 ha catchment of undisturbed rainforest in the central Amazon Basin was computed from detailed measurements of water and solute fluxes via rainfall, streamflow, and subsurface outflow over an annual cycle. Annual atmospheric deposition fluxes are lower than previously reported among mass balance studies conducted in the Amazon. Nutrient export fluxes are lower than previously reported for the Amazon, despite the fact that export fluxes via flow paths not previously measured were included. Given that climatic conditions were representative of a one in 10 wet year, the ecosystem was expected to show a net loss of nutrients rather than net gain. Instead, an excess of nutrient inputs via rainfall over ecosystem outflows was detected, ranging in annual quantities from 0.30 to 0.50 of the measured input. Among several mechanisms that could reconcile this budget, two are supported by the information presently available while two others cannot be evaluated without further research. Interannual variability in the amount of water available for runoff at the spatial scale of small catchments varies by a factor of two, in contrast to rainfall variability of +/-20%, and may be a critical control on the apparent changes in ecosystem storage detected by annual-scale nutrient budgets in rainforests. Entrainment of materials from the terrestrial ecosystem to the atmosphere, including particulates containing elements which do not exist as gases, may be a particularly important loss pathway in rainforests existing on deeply weathered or nutrient poor soils.
1. Marsh, P; Lesack, LFW. (1996) The hydrologic regime of perched lakes in the Mackenzie delta: Potential responses to climate change.Limnology and Oceanography 41: 849-856 The hydrologic regime of perched lakes in the Mackenzie delta: Potential responses to climate change
To illustrate potential impacts of climate change on perched or high-closure lakes in the Mackenzie Delta, we developed and tested a simulation model. Comparison of model output with measured lake level demonstrates that the model simulates the major features of the annual hydrologic regime of high-closure delta lakes. To demonstrate potential changes in lake regime, we used a 12-yr empirical data set of temperature, solar radiation, precipitation, and main-channel water level for the model runs, with the temperature and precipitation modified using results from a 2xCO(2) GCM (general circulation model) run. With these data, modeled water levels for a high-closure lake decline more rapidly between episodes of flooding, ice covers are thinner, and the ice-covered season is much shorter than at present. If precipitation were to decrease slightly instead of increasing slightly as suggested by the GCM, the decrease in lake level between flooding episodes is much faster, and a typical high-closure lake would disappear within 10 yr.
