Canadian Shield Lakes Research Articles

The dissolved oxygen budget of a small Canadian Shield lake during winter

AbstractDeep‐water hypoxia is an environmental concern in temperate lakes. Seasonal turnover events provide a mechanism for deep‐water oxygenation; however, the lake oxygen budget and mixing dynamics during turnovers are poorly understood. In the present study, the oxygen cycle in a small dimictic lake was investigated from long‐term field measurements supplemented with output from a three‐dimensional numerical model. Photosynthetic production and atmospheric exchange were modeled to predominate during spring and fall turnovers, respectively, contributing 92% (surprisingly) and 8% of the net dissolved oxygen (DO) input to the lake. Of the DO production, 41% occurred under‐ice, with a potential to supply 17% ± 11% of the hypolimnetic DO saturation deficit at spring turnover. The corresponding DO sinks were sediment oxygen demand (− 54%), mineralization (− 33%), and nitrification (− 13%). The watercolumn circulation and stratification during pre‐winter and spring turnover controlled the inter‐annual variability in hypoxia during the following summer. Warm winters (~ 4°C watercolumn leading to rapid summer stratification) and severely cold winters (strong winter stratification with shallow convective mixing in spring) were followed by incomplete spring turnover, whereas, following cold winters (moderate winter stratification and extended convection) turnover was complete. This underscores the connection between winter hydrodynamics and summer water quality in dimictic lakes.

Parallel evolution of morphological traits and body shape in littoral and pelagic brook charr, Salvelinus fontinalis, along a gradient of interspecific competition.

The parallel evolution of similar ecotypes in response to comparable environmental conditions is believed to reveal the importance of divergent selection in phenotypic diversifying processes. Systems characterized by the presence of multiple replicate populations expressing resource polymorphism thus provide an ideal opportunity to address the occurrence and factors affecting the parallel evolution of ecotypes. Previous studies have shown that brook charr (Salvelinus fontinalis) exhibit resource polymorphism in some Canadian Shield lakes, where a littoral ecotype feeds mainly on zoobenthos and a pelagic ecotype feeds mostly on zooplankton. Using morphological traits and geometric morphometric analyses on 18 native brook charr populations, we explicitly tested (i) whether brook charr ecotypes show parallel evolution across populations (i.e. the same morphological traits discriminate ecotypes among lakes) and (ii) whether interspecific competition decreases the amplitude of morphological differentiation between ecotypes, if any, because brook charr experience some level of competitive exclusion from the littoral habitat in the presence of creek chub or white sucker. We observed a low level of parallel evolution, where the littoral ecotype was overall stouter with longer fins and smaller eyes than the pelagic ecotype. Interspecific competition had no clear impacts on the amplitude of morphological differentiation. We also observed that inter-lake morphological differences are greater than between ecotypes within lakes, suggesting an important effect of local environmental factors on population morphology. Early-stage of diversification as well as phenotypic plasticity and morphological integration could explain why resource polymorphism is still subtle in brook charr populations.

Turnover in a small Canadian shield lake

AbstractIn dimictic lakes, the stable density stratification during summer and winter inhibits vertical mixing of nutrients and oxygen. This favors the development of hypolimnetic hypoxia, which degrades cool‐water fish habitat and enhances nutrient mineralization from the sediments. Fall and spring turnover events, therefore, provide a crucial biannual link between surface and bottom waters. However, the physical processes occurring to mix lakes during turnover events remain comparatively uninvestigated. In this study, long‐term field observations were supplemented with output from a three‐dimensional numerical model, to better understand turnover events within a small temperate lake during 2011–2017. Mid‐basin penetrative convection, sidearm convection, and wind‐induced mixing were identified, with mid‐basin convection likely contributing the most to turnover events; a typical side‐arm convective plume had 2% (spring) to 4% (fall) of the mass flux compared to those at mid‐basin. During fall turnover, wind shear only mixed the upper 35% of the surface mixed layer, with convection acting to deepen below. During spring turnover, ice‐cover sheltered the lake from wind, causing convection to be the only process occurring and lengthening the turnover duration (~ 51 d) compared to fall (~ 13 d).

A new method to estimate global freshwater phytoplankton carbon fixation using satellite remote sensing: initial results

ABSTRACT Freshwater phytoplankton carbon fixation is an important water quality parameter that can provide information about the health of lake ecosystems as well as their impacts on regional carbon cycling dynamics. Traditional field methods for measuring and monitoring primary production are unable to capture the necessary spatial and temporal variability at the global scale due to the sheer number and diversity of the world’s lakes. Satellite remote sensing offers a potential tool to quantify freshwater lake primary production on a global scale. A new straightforward remote sensing approach was developed to estimate global freshwater carbon fixation from satellite observable lakes using a straightforward depth-integrated model (DIM). A key component of this approach is the estimation of the light utilization index, ψ, for freshwater systems. A significant negative linear model to estimate growing season ψ as a function of latitude was developed from data acquired through an exhaustive literature review. In conjunction with a previous remote sensing generated freshwater chlorophyll-a concentration data set, the DIM was used to compute growing season carbon fixation for 80,000 freshwater lakes. While these estimates are rough and could exhibit large errors for any given lake, they provide a reasonable synoptic global estimate of freshwater carbon fixation. In general, growing season areal carbon fixation was shown to decrease with increasing latitude in both northern and southern hemispheres. Carbon fixation rates in the southern hemisphere were found to be significantly higher than in the northern hemisphere, with the African continent exhibiting the highest rates. Total daily carbon fixation (areal rate × lake area) was estimated at 1.03 teragrams of carbon per day (Tg C day−1) with approximately 71% occurring in the northern hemisphere. Total fixation was highest in North America, which was dominated by a very large number of Canadian Shield lakes. In general, total carbon fixation was well explained by total lake surface area, except in the far northern latitudes where lakes are more oligotrophic due to limited nutrient availability. This analysis resulted in a new freshwater carbon fixation product that provides new insights into the role freshwater lakes play in the global carbon budget.

Does ecological release from distantly related species affect phenotypic divergence in brook charr?

Ecological opportunity occurs when a resource becomes available through a decrease of interspecific competition and another species colonizes the vacant niche through phenotypic plasticity and intraspecific competition. Brook charr exhibit a resource polymorphism in some Canadian Shield lakes, where a littoral ecotype feeds mainly on zoobenthos and a pelagic ecotype feeds mostly on zooplankton. The objectives of this study were to test that (i) resource polymorphism is common in these brook charr populations, (ii) the presence creek chub and white sucker, two introduced species competing with brook charr for littoral resources, will decrease the phenotypic divergence between the two brook charr ecotypes, and (iii) the ecological release from introduced species will increase population and/or individual niche widths in brook charr. The study was based on 27 lakes and five indicators of resource use (stomach content, liver δ13C, muscle astaxanthin concentration, pyloric caecum length, and gill raker length). Our results indicate that within-lake differences in resource use by both ecotypes are common and stable through time. When facing interspecific competition, both littoral and pelagic brook charr incorporated more pelagic prey into their diet but maintained the amplitude of their differences in resource use, which contradicts our second prediction. Finally, we did not find any significant effect of introduced species on population and individual niche widths of brook charr. We suggest that the difference in feeding mode among distantly related competitors could prevent the complete exclusion of a species from a given niche and explain the lack of response to the ecological release.

Evaluation of the responsiveness of the crustacean zooplankton community size spectrum to environmental change and an exotic invader in a sample of Canadian Shield lakes

We evaluated the crustacean zooplankton size spectrum as an indicator of lake characteristics and ecosystem change. First, we used time series from seven Canadian Shield lakes to identify the factors associated with among-lake and among-year variability in the spectrum slope (relative abundance of small and large zooplankton) and centered height (total abundance). Second, we used time series from an invaded and three control lakes to assess change in mean and variability in slope and height due to a Bythotrephes invasion. We found that the slope and height reflected among-lake predictors related to morphometry. The slope was responsive to long-term declining lake phosphorus levels, whereas the height reflected both increases in dissolved organic carbon and decreases in ice duration. We detected a significant increase (i.e., flattening) in mean slope and substantial (up to 120%) increases in the CV of height after Bythotrephes invaded Harp Lake. Thus, the zooplankton size spectrum was responsive to long-term environmental change, and a strong top-down perturbation can be detected through regular and frequent monitoring programs.

Calibration of the zooplankton community size spectrum as an indicator of change in Canadian Shield lakes

Developing the crustacean zooplankton community size spectrum into an indicator of change in lakes requires quantification of the natural variability in the size spectrum related to broad-scale seasonal, annual, and spatial factors. Characterizing seasonal patterns of variation in the size spectrum is necessary so that monitoring programs can be designed to minimize the masking effects that seasonal processes can have on detecting longer-term temporal change. We used a random effects model to measure monthly, annual, and interlake variability in the slope (i.e., relative abundance of small and large organisms) and centered height (i.e., total abundance) of the crustacean zooplankton normalized abundance size spectrum from 1981 to 2011 among eight Canadian Shield lakes. Consistent with theoretical predictions, the slope was a relatively stable characteristic of the zooplankton community compared with the height, which varied significantly among lakes. We identified a seasonal signal in height and slope and used a mixed effects model to characterize the linear rate of change from May to October; there was an overall decline in height and an overall increase in slope. Seasonal variance was greater than annual variance for both the height and the slope, suggesting that long-term monitoring of lakes and interlake comparisons using zooplankton size spectra should be based on temporally standardized sampling protocols that minimize the effects of seasonal processes. We recommend sampling the zooplankton community in midsummer because this results in size spectrum estimates close to seasonal mean values.

Does habitat occupancy by lake trout and lake whitefish in large lakes match published thermal habitat envelopes?

AbstractHabitat occupancy patterns of lake trout (Salvelinus namaycush) and lake whitefish (Coregonus clupeaformis) in two large Canadian Shield lakes were modelled based on detections of fish from repeated depth‐stratified surveys over several summers. Lake trout and lake whitefish consistently occupied sites outside traditional thermal envelopes and were not detected at some sites within these ranges. This included the metalimnion and shallow epilimnion for lake trout and lake whitefish in Lake Opeongo. Physical habitat covariates were not important in defining lake trout habitat in both lakes. Physical habitat as represented by the hardness/softness gradient based on acoustic substrate surveys was important for lake whitefish in Lake Opeongo but not in Smoke Lake. In addition, thermal envelopes for lake whitefish differed between the lakes possibly because of differences in substrate slope. The wash zone of lakes, where the thermocline contacts the substrate, appears to be a physical habitat feature for lake whitefish in some lakes. Lake whitefish also exhibited diurnal activity behaviour that was reflected through greater detection rates in the morning versus the afternoon. By accounting for imperfect detection, true estimated overall occupancy of lake trout and lake whitefish increased 0.15–0.30 over naïve occupancy. Thermal habitat envelopes for lake trout and lake whitefish are wider than previously thought. Lake trout occupied a consistent thermal habitat envelope while lake whitefish varied between lakes likely because of lake specific differences in basin morphology and wash zone.

Ca2+ levels in Daphnia hemolymph may explain occurrences of daphniid species along recent Ca gradients in Canadian soft-water lakes.

Calcium levels are declining in eastern North American and western European lakes. This widespread issue is affecting the composition of crustacean zooplankton communities, as the presence and abundance of several calcium-rich daphniid species are declining, while two other daphniids, D. catawba and D. ambigua, that apparently tolerate low calcium environments, are prospering. The physiological basis for low calcium tolerance of these daphniids is unknown. In this study the presence of one Ca-rich (D. pulicaria) and one Ca-poor (D. ambigua) daphniid species in Canadian Shield lakes is assessed in relation to lake water Ca levels. The occurrence of D. ambigua was independent of Ca levels in Ontario lakes, whereas D. pulicaria was more likely to occur in lakes with relatively more Ca. In the laboratory, D. ambigua maintained lower levels of hemolymph Ca2+ across a range of low Ca levels (0.7 to 7 mg l−1) compared with D. pulicaria. The hemolymph pH remained steady across this Ca gradient in D. ambigua while it was significantly more acidic in D. pulicaria in the two lowest Ca treatments. While Ca2+ uptake was observed adjacent to the surface of D. ambigua individuals, Ca2+ loss was observed for D. pulicaria assayed under moderately high Ca levels. Based on these observations we propose that D. ambigua is able to survive in low Ca lakes by maintaining low free ionic Ca2+ levels in the hemolymph which minimizes the Ca gradient across the body wall in low Ca water thus limiting overall Ca loss and facilitating Ca2+ uptake.

The impact of calcium decline on population growth rates of crustacean zooplankton in Canadian Shield lakes

AbstractWithin the last decade, calcium decline has emerged as a stressor for many soft water lakes. A legacy of long‐term acid deposition and logging, calcium decline has been implicated in the loss of large herbivores, such as Daphnia, and changes in community structure as taxa with presumed low calcium demand also increase. Although lake surveys, paleolimnological studies, and laboratory experiments have provided considerable evidence that declining Daphnia abundances are associated with low calcium concentration, causal relationships for zooplankton response to calcium decline have not been assessed using field experiments with natural zooplankton communities. We conducted a six‐week field experiment under low food conditions typical of Canadian Shield lakes, using four calcium concentrations (0.6 mg/L, 1.0 mg/L, 1.4 mg/L, and 2.4 mg/L) and zooplankton communities originating from eight lakes along a gradient from 1.78 mg Ca/L to 24.8 mg Ca/L, to examine the effect of calcium on growth rates and reproduction of individual and groups of taxa. Population growth rates for daphniids, Bosmina spp., and three cyclopoids declined as calcium concentration decreased. At low calcium, growth rates of Daphnia pulex, Daphnia catawba, and Mesocyclops edax were negative indicating declining populations. There was no effect of calcium on reproduction of the most common cladocerans. Although we detected some variation in growth rates amongst source lakes, zooplankton communities did not differ in their response to low calcium. As calcium concentrations continue to decline in soft water lakes, reduced zooplankton growth rates may result in shifts in zooplankton community structure and overall declines in total zooplankton production.

Depth distribution of the native freshwater mussel (Echyridella menziesii) in warm monomictic lakes: Towards a general model for mussels in lakes

Abstract Freshwater mussels are large, long‐lived and can be important contributors to benthic biomass and processes. They are currently one of the most endangered groups of organisms and it is urgent to develop tools to predict their distribution and the potential effect of their decline or disappearance on these ecosystems. Cyr () showed that the distribution of Elliptio complanata (Unionidae) in Canadian Shield lakes is constrained by physical forces. Here we test Cyr's model in a very different group of mussels (Hyriidae), in different types of lakes (warm monomictic lakes) that cover a wider range of sizes and morphologies. We use data on the depth distribution of Echyridella menziesii along 38 depth transects in 11 warm monomictic lakes located in New Zealand to test three hypotheses: (1) wave‐mixed depth and bottom slope are good predictors of the depth of maximum mussel density, (2) thermocline depth does not limit the distribution of mussels in warm monomictic lakes, and (3) the lower boundary of mussel distribution is determined by the mud deposition boundary. Mussels in New Zealand lakes reach their maximum density within the epilimnion, at increasing depths with increasing lake size and increasing site exposure. The only exceptions were found in large lakes or parts of large lakes with shallow bathymetric slopes, where high mussel densities were found in relatively shallow waters. Mussel density peaks are found much deeper in the volcanic New Zealand lakes than in (glacial) Canadian Shield lakes, a discrepancy we hypothesise could be due to lower sediment stability along the steep slopes of volcanic lakes. Mussels appear to have a very broad range of depth distribution in these warm monomictic lakes. The deepest samples collected in deep oligotrophic lakes (down to 12–30 m) all contained mussels, often in substantial numbers (up to 2–186 mussels/m2), and can only offer a minimal estimate of the lower boundary of their distribution. However, the distribution of mussels in highly productive lakes is limited by hypolimnetic anoxia, and therefore by position of the thermocline. Echyridella menziesii are found in a wide range of substrates. Mussels were found below the mud deposition boundary in many lakes, suggesting that the presence of fine flocculent organic sediments does not prevent them from living in deep areas. The distribution of E. menziesii in warm monomictic lakes appears to be governed by relatively simple physical processes. This supports findings for a very different group of mussels in Canadian Shield lakes and suggests that general models could be developed to predict the distribution of mussels in lakes.

Calcium decline reduces population growth rates of zooplankton in field mesocosms

Regional calcium (Ca) decline, a legacy of acid deposition and logging, is a potential threat to aquatic organisms. Lake surveys and laboratory studies indicate that Ca-rich daphniids are likely most susceptible, allowing for competitive release of other taxa with low Ca demand. Indeed, dramatic shifts in zooplankton community structure have been documented in lakes where Ca has declined, amid multiple other stressors. Given the perceived threat of this large-scale stressor, manipulative studies are needed to evaluate causal relationships between Ca decline and zooplankton community structure. We analysed per capita growth rates of zooplankton from three independent mesocosm experiments where we manipulated aqueous Ca concentrations to reflect current and future Ca concentrations. In two experiments where Ca concentration was reduced to 0.6 or 0.9 mg/L, we observed reduced growth rates for several taxa, including daphniids, bosminids, and copepods. No effect of Ca was detected in the experiment where Ca concentrations ranged from 1.2 to 2.5 mg/L, a gradient representing 68% of lakes in south-central Ontario. These results suggest that future Ca decline in soft-water Canadian Shield lakes may be accompanied by shifts in community structure and overall declines in zooplankton production.

The effects of Bythotrephes longimanus and calcium decline on crustacean zooplankton communities in Canadian Shield lakes

Declining calcium concentrations and invasion by Bythotrephes longimanus are two important, often co-occurring, stressors affecting Canadian Shield lakes. However, there has been no experimental examination of how they might jointly influence zooplankton communities. We conducted a 6-week field mesocosm experiment in Havelock Lake, Haliburton, Ontario, Canada to examine the individual and joint effects of Bythotrephes and calcium along a gradient ranging from 1.2 to 2.6 mg/l on zooplankton communities. Although densities of Bythotrephes in our study are unknown, it significantly reduced total zooplankton abundance in invaded compared to uninvaded treatments by 46%, with the greatest impacts on small cladocerans and daphniids. Low calcium reduced total zooplankton and cladoceran abundances. Although Havelock Lake has the lowest calcium concentration among invaded lakes in the Muskoka–Haliburton region (1.2 mg Ca/l), an effect of calcium on individual species abundances was not detected. Additionally, we did not detect an interactive effect of both stressors. Our results suggest that lake calcium concentration may not yet be low enough to effect a strong response. However, as Bythotrephes continue to invade low calcium lakes, and as calcium concentrations further decline, we may see larger impacts on cladocerans as calcium thresholds for reproduction and growth are reached.

A review of the effects ofBythotrephes longimanusand calcium decline on zooplankton communities — can interactive effects be predicted?

Anthropogenic stressors including acid deposition, invasive species, and calcium (Ca) decline have produced widespread damage to Canadian Shield lakes, especially to their zooplankton communities. Here, we review current knowledge on the individual effects on zooplankton by the non-indigenous predator Bythotrephes longimanus and Ca decline; we identify knowledge gaps in this literature and examine the likely interactive impacts of Bythotrephes invasions and Ca decline on zooplankton. The negative impacts of Bythotrephes longimanus on zooplankton communities are well known, whereas current understanding of the effects of declining Ca on zooplankton is restricted to Daphnia spp.; hence, there is a large knowledge gap on how declining Ca may affect zooplankton communities in general. The co-occurring impacts of Bythotrephes and declining Ca have rarely been studied at the species level, and we expect daphniids, particularly Daphnia retrocurva and Daphnia pulicaria, to be the most sensitive to both stressors. We also expect a synergistic negative interaction on cladocerans in lakes with both stressors, leaving a community dominated by Holopedium glacialis and (or) copepods. Our predictions form testable hypotheses but since species and ecosystem response to multiple stressors are difficult to predict, we may actually see ecological surprises in Canadian Shield lakes as Bythotrephes continues to spread and Ca levels continue to fall.

Phosphorus Availability Alters the Effects of Silver Nanoparticles on Periphyton Growth and Stoichiometry.

Exposure to silver nanoparticles (AgNPs) may alter the structure and function of freshwater ecosystems. However, there remains a paucity of studies investigating the effects of AgNP exposure on freshwater communities in the natural environment where interactions with the ambient environment may modify AgNP toxicity. We used nutrient diffusing substrates to determine the interactive effects of AgNP exposure and phosphorus (P) enrichment on natural assemblages of periphyton in three Canadian Shield lakes. The lakes were all phosphorus poor and spanned a gradient of dissolved organic carbon availability. Ag slowly accumulated in the exposed periphyton, which decreased periphyton carbon and chlorophyll a content and increased periphyton C:P and N:P in the carbon rich lakes. We found significant interactions between AgNP and P treatments on periphyton carbon, autotroph standing crop and periphyton stoichiometry in the carbon poor lake such that P enhanced the negative effects of AgNPs on chlorophyll a and lessened the impact of AgNP exposure on periphyton stoichiometry. Our results contrast with those of other studies demonstrating that P addition decreases metal toxicity for phytoplankton, suggesting that benthic and pelagic primary producers may react differently to AgNP exposure and highlighting the importance of in situ assays when assessing potential effects of AgNPs in fresh waters.

Development of an In Situ Ion-Exchange Technique for the Determination of Free Cd, Co, Ni, and Zn Concentrations in Freshwaters

It is now well established that the bioavailability of metals toward aquatic organisms varies as a function of the free metal concentration. The ion-exchange technique (IET), which consists of equilibrating a calibrated cation-exchange resin with the water sample, is one of the few existing speciation methods that provide sufficient sensitivity and specificity to measure free metals in natural waters. In the present study, we developed an in situ IET (field-IET) in which the resin was directly equilibrated on site using dialysis cassettes. The field-IET was tested in six Canadian Shield lakes and in the Athabasca River (AB, Canada) for Cd2+, Co2+ (only in the Athabasca River), Ni2+, and Zn2+. Measurements were compared with those obtained on samples collected from the same sites with in situ diffusion samplers and analyzed in the laboratory (lab-IET). IET results were also compared with predictions from the Windermere Humic Aqueous Model [WHAM VII; the Co and Ni carbonato complex formation constants were updated according to the US National Institute of Science and Technology (NIST compilation)]. Good agreement was obtained between the field-IET and the lab-IET for Co2+ and Ni2+ concentrations, but field-IET Cd2+ and Zn2+ concentrations were systematically higher than the lab-IET results (factors of ~1.5× and ~2.4×, respectively). Uncertainties in the field-IET resin calibration and a significant Zn contamination could explain these results. WHAM VII predicted the free metal concentrations reasonably well, except for Ni2+ concentrations in the lakes, probably due to inappropriate formation constants for complexation of Ni with dissolved organic matter. This study showed that the field-IET, with its relative simplicity and its low detection limits, could be a useful method for the determination of free metal concentrations in acidic to neutral freshwaters.

Reaction rates, depositional history and sources of indium in sediments from Appalachian and Canadian Shield lakes

Sediment cores were collected at the deepest site of twelve headwater lakes from the Province of Québec, Canada that receive contaminants only from atmospheric deposition, either directly to the lake surface or indirectly from the watershed. Several of the lakes are located within relatively short distance (<40km) and others at more than 200km from potential sources of contamination. The sediments were dated and analyzed for In and other elements including Fe, Mn, Al and organic C. Fe-rich authigenic material was collected on Teflon sheets inserted vertically into the sediments at the only study site whose hypolimnion remains perennially oxic. Porewater samples collected at the coring site of four of the lakes were also analyzed for In and other solutes including sulfide, sulfate, Fe, Mn, inorganic and organic C and major ions. The porewater In profiles display concentration gradients at or below the sediment–water interface. Modeling these profiles with a one-dimensional transport-reaction equation assuming steady state allows definition of depth intervals (zones) where In is either released to or removed from porewater and quantification of net In reactions rates in each zone.The position of the In consumption zones, the shape of the vertical profiles of dissolved In, sulfide and iron, as well as thermodynamic calculations of saturation states collectively suggest that In(OH)3(s) and In2S3(s) do not precipitate in the sediments and that adsorption of In onto sedimentary FeS(s) does not occur. However, similarities in the In and Fe porewater profiles, and the presence of In in the authigenic Fe-rich solids, reveal that part of the In becomes associated with authigenic Fe oxyhydroxides in the perennially oxic lake and is coupled to the Fe redox cycling. Comparison of the In/Corg and In/Fe molar ratios in the authigenic Fe-rich material and in surface sediments (0–0.5cm) of this lake suggests that most non-lithogenic In was bound to humic substances. From the magnitude of the net In reaction rates, we infer that the post-depositional redistribution of this element is quantitatively not important and that the In sedimentary record represents accurately In deposition at the sampling sites. Reconstructed chronologies of the anthropogenic In deposition and comparison of In inventories among lakes point to non-ferrous metal smelters as a past significant source of atmospheric In contamination and to a significant reduction of industrial In emissions into the North American atmosphere in recent decades.

Effects of Experimental Thermocline and Oxycline Deepening on Methylmercury Bioaccumulation in a Canadian Shield Lake

Environmental disturbances like deforestation or climate change may influence lake thermal and oxic stratification, thereby modifying cycles of contaminants such as mercury (Hg). In a lake naturally separated into three basins, the thermocline and oxycline of an experimental basin were deepened by 4 and 3 m, respectively, to study the effect on the methylmercury (MeHg) accumulation. This treatment decreased hypolimnetic MeHg concentration by approximately 90%, zooplankton concentrations by 30 to 50%, and in some fish by 45%. A multiple linear regression indicated that oxycline depth significantly influenced hypolimnetic MeHg concentrations, with no significant effect of thermocline depth, anoxic water volume, interface area of oxic-anoxic water, and sediment area in contact with anoxic water. Fish MeHg decline varied, with a greater response by low oxygen-tolerant bullhead. Increased pelagic primary and secondary production likely caused zooplankton and fish MeHg decreases via algal and growth dilution. Environmental changes leading to oxycline deepening are therefore predicted to cause a decrease in MeHg bioaccumulation in similar Canadian Shield lakes. If associated ecosystem impacts related to the deepening treatment are deemed acceptable, then this experiment provides a potential remediation method for small lakes confronted with MeHg accumulation.

Decadal‐scale regional changes in Canadian freshwater zooplankton: the likely consequence of complex interactions among multiple anthropogenic stressors

Summary Ecological integrity is increasingly threatened by multiple anthropogenic stressors, but the cumulative impact of stressors is poorly understood because they can interact in unexpected ways. Knowledge of these interactions and their associated impacts is needed to support the conservation of valued ecosystems. We used a large‐scale, replicated field survey and multiple regression analysis to investigate the cumulative impacts of multiple physical, chemical and biological stressors on the crustacean zooplankton assemblages of 34 Canadian Shield lakes between 1980s and 2004–2005. Zooplankton total abundance, species richness, diversity and community structure, as well as the relative abundances of prominent taxonomic orders of zooplankton, changed at a regional scale. These changes occurred in response to changes in water quality and lake thermal regime, and invasion by an exotic predator. Interactions between stressors were common and represented an important determinant of zooplankton assemblage changes over time. We provide the first evidence that the individual and combined impacts of multiple stressors cause regional ecological change over decades. Our results demonstrate the prevalence of stressor interactions in natural environments and highlight the complexity of ecosystem responses to multiple stressors. Zooplankton changes recorded here may be widespread because climate change, acidification, development and the spread of invasive species are globally pervasive. These changes could have cascading impacts because zooplankton is an essential link in aquatic food webs. Our findings highlight the need to consider the interactive effect of stressors when assessing anthropogenic impacts and will inform management and conservation of ecosystems threatened by multiple stressors.

Human shoreline development and the nutrient stoichiometry of aquatic plant communities in Canadian Shield lakes

Human activities associated with residential development potentially alter ecological processes in lake littoral zones. We determined how the nutrient stoichiometry of aquatic plant communities relates to residential density around lakes of south-central Ontario. We calculated the elemental composition of entire plant communities from multiple sites in 12 lakes using measurements of individual plant C:N:P ratios and their areal biomass. We found considerable variability in the C:N:P ratios of whole aquatic plant communities among sites and lakes, which was not accounted for by intraspecific variability in the elemental composition of aquatic plants. Instead, differences in community-level C:N:P ratios primarily resulted from high interspecific variability in the elemental composition among dominant plant taxa and variable taxonomic composition of sampled plant communities. Plant community composition differed among lakes with and without shoreline residences, and we found lower C:N and C:P ratios in communities from littoral zones in human-developed lakes. Our results thus demonstrate a link between the elemental and taxonomic composition of aquatic plant communities, which may mediate biogeochemical responses of littoral zones to development of lake shorelines.