Nitrogen mass balances and denitrification rates in central Ontario Lakes

Nitrogen mass balances for seven unproductive lakes and 20 forested catchments in central Ontario were measured between 1977 and 1989. Average annual lake denitrification rates calculated with the N/P ratio method were strongly correlated with summer anoxic factor (extent of surficial sediment anoxia) whereas denitrification rates calculated with a²¹⁰Pb sediment N accumulation method were poorly correlated with the anoxic factor suggesting that the N/P method is superior. Substantial denitrification occurred in all lakes — an average of 36% of TN inputs or 75% of the net gain. On a regional area-weighted basis, 67% of bulk atmospheric TN deposition was stored or denitrified terrestrially, 12% was denitrified in lakes, 4% was stored in lake sediments, and 17% was exported from lakes. N/P ratios were generally less in streams than in precipitation suggesting preferential N retention in catchments, whereas the N/P ratios in lake outputs were slightly higher than lake input ratios, suggesting preferential P retention in lakes. This is consistent with the notion that P-limited lakes can exist adjacent to N-limited forests.     

Temperature, growth and dietary effects on fish mercury dynamics in two Ontario lakes

A bioenergetics-based model was used to investigate the effects of temperature, growth and dietary exposure on methylmercury dynamics in walleye (Stizostedion vitreum) and yellow perch (Perca flavescens) from two lakes sampled in northwestern Ontario. Orange Lake was smaller, warmer, had slower fish growth and higher mercury concentrations in yearling yellow perch and walleye (three fold difference in 40 cm walleye) than Trout Lake. The model was applied to test the hypothesis that higher water temperatures in Orange Lake increased metabolic needs, food consumption and mercury uptake in fish. The effects of different growths rates in the lakes were also considered. Temperature/metabolic effects and growth effects on internal methylmercury dynamics in walleye and perch were predicted to occur but be of secondary importance. Different dietary exposure to methylmercury was likely the dominant source of variation in fish mercury concentrations between the two lakes.     

The inventories of anthropogenic Ph,Zn, As,Cd, and the radionuclides 137Cs and excess 210Pb in lake sediments of the Adirondack region,USA

We determined the inventories of four anthropogenic trace elements, Pb, Zn, As, and Cd, and two radionuclides, ¹³⁷Cs and excess ²¹⁰Pb, in sediment cores collected from eight remote lakes in the Adirondack region of the northeastern United States. The inventories of all six substances vary considerably among the sediment cores, although the lakes and their associated catchments must have received similar cumulative per unit area atmospheric inputs of these substances. These variations are highly correlated, indicating that the trace elements and radionuclides are affected in a coherent way by the processes controlling their deposition to the sediments of these lakes. Assuming that the anthropogenic trace element inventories in each sediment core are enhanced or depleted relative to cumulative atmospheric deposition to the extent indicated by the sediment core inventories of either ¹³⁷Cs or excess ²¹⁰Pb, we produced estimates of the cumulative atmospheric inputs of the four anthropogenic trace elements to this region. Comparison of the excess ²¹⁰Pb normalized anthropogenic Pb, Zn, Cd inventories of the Adirondack sediment cores with excess ²¹⁰Pb normalized inventories of the same substances in a group of South Central Ontario lakes suggests that the Adirondack region has received greater cumulative anthropogenic inputs of Pb, Zn and Cd by a factor of 1.6, 4.5, and 2.9, respectively.     

The dynamics of forest tent caterpillar outbreaks across east‐central Canada

An analysis of forest tent caterpillar Malacosoma disstria defoliation records from Ontario and Quebec over the period 1938–2002 indicates that outbreaks recur periodically and somewhat synchronously among regions of the two provinces. Cluster analysis revealed that the most strongly periodic, large‐scale, synchronized fluctuations occurred within three regions: northwestern Ontario, eastern Ontario/western Quebec and southeastern Quebec. Defoliation in the vast surrounding hinterlands tended to be infrequent and sporadic, loosely tracking defoliation in the core outbreak regions. One small cluster in northeastern Ontario stood out as anomalous, as a result of an increasing trend in the duration of periodic defoliation episodes, marked by an unprecedented double‐wave of defoliation that persisted from 1992 to 1999. This is the precise area where, in the early 2000s, trembling aspen Populus tremuloides stems were mapped as being in a state of decline of unprecedented severity and extent. Our results suggest forest tent caterpillar has the potential to cause significant impacts on forest health and, hence, carbon budgets in east‐central Canada and that the forest tent caterpillar deserves more attention as a model system of forest insect disturbance ecology.     

13C and 15N depletion in components of a foodweb from an ephemeral boreal wetland compared to boreal lakes: putative evidence for microbial processes

Stable carbon and nitrogen isotope ratios were used to posit the relative importance of microbial processes on energy pathways in an ephemeral, humic boreal wetland compared to four clearwater lakes in northwestern Ontario, Canada. In addition to algae and dipteran larvae, odonate larvae were sampled as these latter organisms are known to predate indiscriminately on smaller invertebrates and are thus likely to have average isotope ratios reflective of their habitats. Similarities in ¹³C and ¹⁵N values between lake insect larvae and emerged adults suggested that littoral foodwebs in these oligotrophic lakes may rely to a considerable degree upon terrestrial carbon. Wetland insect larvae and algae were depleted in both ¹³C and ¹⁵N compared to biota in lakes. Carbon isotope analysis implied a substantial presence of microbial respiration from decomposition in the humic wetland, whereas nitrogen isotope analysis suggested the prevalence of microbially modified nitrogen dynamics, including the possibilty of N-fixation.     

Mercury distribution in sediment profiles from lakes of the high pantanal,Mato Grosso State,Brazil

Sediment cores from lakes located in the Pantanal Swamp, Central Brazil were analysed for the distribution of mercury released by the local gold mining. Atmospheric transport is the only pathway of mercury contamination of these remote lakes. Mercury concentrations were higher at the surface of sediments (62 to 80 ug.kg^–1) decreasing to values of 20 to 30 ug.kg^–1 in deeper layers. Mercury deposition rate was estimated as 90 to 120 ug Hg.m^–2yr^–1 Although mercury concentrations were much lower than in industrialized areas, mercury deposition rate for these Pantanal lakes is of the same order of magnitude of deposition rates measured in lakes in industrialized areas     

Climate tracking by freshwater fishes suggests that fish diversity in temperate lakes may be increasingly threatened by climate warming

Aim

Many freshwater fishes are migrating poleward to more thermally suitable habitats in response to warming climates. In this study, we aimed to identify which freshwater fishes are most sensitive to climatic changes and asked: (i) how fast are lakes warming? (ii) how fast are fishes moving? and (iii) are freshwater fishes tracking climate?

Location

Ontario, Canada.

Methods

We assembled a database containing time series data on climate and species occurrence data from 10,732 lakes between 1986 and 2017. We calculated the rate of lake warming and climate velocity for these lakes. Climate velocities were compared with biotic velocities, specifically the rate at which the northernmost extent of each species shifted north.

Results

Lakes in Ontario warmed by 0.2°C decade⁻¹ on average, at a climate velocity of 9.4 km decade⁻¹ between 1986 and 2017. In response, some freshwater fishes have shifted their northern range boundaries with considerable interspecific variation ranging from species moving southwards at a rate of −58.9 km decade⁻¹ to species ranges moving northwards at a rate of 83.6 km decade⁻¹ over the same time period. More freshwater fish species are moving into northern lakes in Ontario than those being lost. Generally, predators are moving their range edges northwards, whereas prey fishes are being lost from northern lakes.

Main Conclusions

The concurrent loss of cooler refugia, combined with antagonistic competitive and predatory interactions with the range expanding species, has resulted in many commercially important predators moving their range edges northwards, whereas prey species have contracted their northern range edge boundaries. Trophic partitioning of range shifts highlights a previously undocumented observation of the loss of freshwater fishes from lower trophic levels in response to climate-driven migrations.     

Elevated planktonic rotifer biomass in acidified metal-contaminated lakes near Sudbury,Ontario

The planktonic rotifer biomass and abundance in two trace metal-contaminated, acidified lakes were compared with that of six non-acidic lakes in central Ontario, Canada, which had low levels of metals. Rotifers, especially Keratella taurocephala Myers, were an order of magnitude more abundant in the acidified lakes. They composed 16 to 51% of the total zooplankton biomass in these lakes, but <2% in the non-acidic lakes. Recent experimental work supports a hypothesis that the great abundance of rotifers in the acidic lakes is attributable to a reduction in densities of crustacean competitors.     

Richness of aquatic macrophyte floras of soft water lakes of differing pH and trace metal content in Ontario,Canada

The richness of the aquatic macrophyte floras, i.e., the total number of species, was assessed in 39 soft water lakes in central Ontario, Canada. The Cu and Ni concentrations and pH of the lakes ranged from 1 to 360 mg m^?3, 2 to 3700 mg m^?3 and 3.9 to 7.0, respectively. Two non-exclusive subsets of the data were examined to determine firstly, if floral richness was related to lake pH in lakes with low Cu and Ni levels (Data Set I) and secondly, if floral richness in acidic (pH ? 5.3) lakes was related to levels of various trace metals (Data Set II). Charophytes were not found in lakes with pH < 5.2. In Data Set I, there was no relationship between the richness of tracheophytes and pH, and there was a negative relationship between pH and bryophyte richness. Unlike phytoplankton, zooplankton, benthic macroinvertebrates and fish, there was no decrease in total species richness in lakes of pH < 5.5, as long as trace metal levels were low. Examination of Data Set II indicated tracheophyte richness of acidic lakes was negatively correlated with Cu and Ni levels. Biological surveys of metal-contaminated acidic lakes are, therefore, not of use for predicting the effects of acid deposition alone on aquatic macrophytes.     

Bioaccumulation of mercury in yellow perch (Perca flavescens) and common loons (Gavia immer) in relation to lake chemistry in Atlantic Canada

Mercury biomagnifies in aquatic foodwebs in freshwater lakes, and common loons (Gavia immer) breeding in eastern Canada can be exposed to reproductively toxic concentrations of mercury in their fish prey. We assessed the bioaccumulation and biomagnification of mercury in juvenile and adult common loons, and their preferred prey: yellow perch (Perca flavescens) in Kejimkujik National Park (KNP), Nova Scotia by measuring mercury levels and stable isotope ratios in tissues. Total mercury levels and stable-carbon (δ¹³C) and nitrogen isotope ratios (δ¹⁵N) were determined in composite whole-fish samples from lakes in KNP and blood samples from juvenile and adult loons captured on lakes in KNP and southern New Brunswick. Geometric mean mercury concentrations were 0.15 and 0.38 μg/g (wet wt.) in small (9-cm fork length) and large (17-cm fork length) yellow perch, and were 0.43 and 2.7 μg/g (wet wt.) in blood of juvenile and adult common loons, respectively. Mercury concentrations in perch and loons were positively associated with body mass and δ¹⁵N values. Juvenile loons and large yellow perch had similar mercury levels and δ¹⁵N values, indicating similar trophic status despite their 22-fold difference in body mass. Mercury concentrations were higher in yellow perch and common loons in acidic lakes. Our findings highlight the importance of both chemical and ecological factors in understanding mercury biomagnification in lakes and associated risks to fish-eating wildlife. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Metals in Northern Forest Ecosystems: Role of Vegetation in Sequestration and Cycling,and Implications for Ecological Risk Assessment

Mass estimates of phytoaccumulated trace metal contaminants and transfers to soils are necessary to properly evaluate the impact of historic and continued anthropogenic metal deposition to northern forest ecosystems. An investigation of Cu, Ni, Pb, and Zn mass balances in plant communities subject to metal contamination from smelter emissions in Canada, found that accumulation of metals differed significantly among plant vegetation compartments (foliage, fine roots, bark, trunk, and branches). Analysis of plant community biological accumulation factors (BAFs), calculated using total soil metal and free soil metal ion (Me²⁺) found that free soil metal BAFs were more similar among locations than total soil metal BAFs, but that disparity still existed. Fine roots were found to dominate annual vegetation transfer of Cu, Ni, and Pb to soils, relative to foliage; fine root mortality played a smaller role than foliage for Zn plant-soil transfer. Plant-associated metal inputs were found to rival or exceed current estimates of atmospheric metal deposition, suggesting that potential benefits of future reductions in emissions to forests need to be evaluated within the context of phytocycling of metals already present.     

Microbial Decomposition of Cellulose in Acidifying Lakes of South-Central Ontario

The rate of cellulose breakdown and density of bacterial populations were measured in the epilimnetic sediments and water columns of lakes in central Ontario that differ in pH, alkalinity, and nutrient status and are particularly sensitive to acidic inputs from atmospheric decomposition. There was no significant difference in decomposition rate in either oxic or anoxic sediment when mean epilimnetic pHs were in the range 5.5 to 6.9. The importance of these findings for the breakdown of autochthonous detritus in Canadian Shield lakes is discussed. Furthermore, the results of these experiments, in which dyed strips of cellophane (regenerated cellulose) were used as substrate, were compared with results of earlier decomposition studies carried out with coarse litter (leaves, twigs). Acridine orange direct counts of bacteria in the top 1 cm of sediment ranged from 5.5 × 10⁸ to 1.0 × 10⁹ per g and in planktonic water samples from 1.1 × 10⁶ to 1.8 × 10⁶ per ml. Bacterial densities were significantly higher in both the shallow sediment (P < 0.01) and the water column (P < 0.05) of dystrophic lakes than at these sites in oligotrophic lakes.     

Changes in fish condition and mercury vary by region,not Bythotrephes invasion: a result of climate change?

We compared changes in body condition (relative weight) and mercury concentrations ([Hg]) in two species of coregonid fish (lake herring Coregonus artedi, lake whitefish C. clupeaformis) among discrete populations in Ontario between 1967 and 2006. Temporal comparisons among populations were made to determine whether 1) the establishment of Bythotrephes longimanus had affected coregonid populations, or 2) if changes in body condition or [Hg] were related to regional differences in the degree of climate change during the time period investigated. Climate data from northwestern, northeastern and southern Ontario showed a general warming trend in all regions over the period of study. However, greater temporal changes in climate were observed in the northwest where growing degree days >5°C (GDD) increased and precipitation declined over the study period compared with relatively little change in southern or northeastern Ontario. Correspondingly, northwestern Ontario coregonid populations demonstrated significantly greater declines in body condition relative to those from northeastern or southern Ontario. Declines in [Hg] of both species were also greater among northwestern populations compared with those from northeastern or southern Ontario but only significantly so for lake herring. These declines were independent of the invasion of non‐native Bythotrephes, and declines in [Hg] were opposite predictions based on the hypothesis that Bythotrephes invasion lengthened aquatic food chains. Based on our findings and further evidence from the literature, we propose that warming regional climates are capable of contributing to declines in both condition and [Hg] of fishes. Because fish condition affects both reproductive success and overwinter survival, observed condition declines of the magnitude reported here could have profound implications for the structure of future aquatic ecosystems in a warming climate.     

Enhanced photochemical loss of organic carbon in acidic waters

Previous studies have shown that (a) a large portion of theannual total organic carbon (TOC) inputs to central Ontario lakes iseither lost to sediments or degraded and lost via evasion to theatmosphere, (b) the partitioning of organic carbon between sediments andthe atmosphere appears to be a function of acidity and (c) UVirradiation can account for observed long-term loss of TOC from thewater column. These findings were extended by examining whether acidityenhances photo-oxidative losses of TOC. Stream waters (initialalkalinities between –97 and 233 µeq l^–1)were incubated in UV-transparent containers under incident solarradiation for periods ranging from 14 to 23 days. The highestphoto-oxidation rates occurred when alkalinity was negative. Additionsof acid and base to stream waters increased and decreasedphoto-oxidation rates, respectively. The exceptional clarity ofatmospherically acidified lakes is usually attributed to increasedprecipitation of Al-organic carbon complexes but may instead be due tohigher photo-oxidation rates of allocthonous organic carbon leading tohigher evasion rates of CO₂.     

Potential risk to common loons (Gavia immer) from methylmercury exposure in acidified lakes

Piscivorous birds and mammals in areas remote from point sources of Hg contamination may be exposed to dietary methylmercury concentrations that are sufficiently high to cause reproductive impairment. Common loons (Gavia immer) were observed to show aberrant nesting behavior and low overall reproductive success when Hg concentrations in prey (small fish and crayfish) averaged > 0.3 µg g^–1 wet weight (Barr, 1986), levels known to occur in fish from many lakes in central Ontario. We used data on Hg in Ontario fish to estimate the proportion of lakes where fish small enough for loons to eat (< 250 g) had Hg concentrations that exceeded estimated thresholds for reproductive impairment. Up to 30 % of lakes exceeded thresholds for reproductive impairment, depending on the species of fish and the threshold Hg concentrations chosen. There was a significant negative correlation between fish-Hg concentration and lake pH in most fish species examined. For these species, reductions in sulfate deposition rates are predicted to result in a corresponding reduction of lakes in Ontario having fish with potentially toxic concentrations of Hg.     

Mercury concentrations in Irish headwater lake catchments

An estimated 215,000 tonnes of mercury (Hg) have been emitted to the atmosphere from anthropogenic sources since the nineteenth century, igniting widespread environmental monitoring owing to its toxicity. The environmental fate of Hg is strongly determined by catchment characteristics, especially soil organic matter. In this study, concentrations and pools of Hg were determined for lakes and soils in upland peat-dominated catchments in Ireland to assess controls of aquatic Hg and soil response to changes in emissions. Headwater lakes in upland coastal regions were surveyed for water chemistry and total Hg (THg) during spring 2008. In addition, a sub-set of lakes (n = 5) were repeatedly sampled during 2009–2011, and their surface soils collected for Hg analysis, including a short (30 cm) peat core to assess temporal Hg fluxes using radiometric ²¹⁰Pb dating. Peat cores indicated a significant decrease in Hg deposition since the 1980s, in broad agreement with other ‘background’ regions. Total Hg was correlated with total organic carbon (TOC) in the survey and intensive study lakes (r = 0.70 and 0.45), indicative of the strong affinity of Hg to organic matter. At the intensive lakes, monomethylmercury (MMHg) made up 3.3 % of mean THg and exhibited a positive correlation with total SO₄ ^2? (r = 0.55). Further, both THg and MMHg were significantly correlated with conductivity (r = 0.48 and 0.54, respectively) potentially owing to marine inputs, and negatively correlated with pH (r = ?0.59 and ?0.56 respectively). Significant differences in THg (and MMHg) were observed between the five lakes, the highest concentrations (4.45 and 0.16 ng L^?1, respectively) tended to be associated with TOC in lakes and occurred at sites in the northwest, characterized by higher levels of soil organic matter (peat) and soil moisture relative to the other sites. In contrast, surface soil pools of THg ranged between 13.6 and 20.8 μg m^?2 across study sites and did not vary significantly, but were typical of global background regions. Nonetheless, the organic rich soils that dominate Ireland are a natural sink for THg, and peat harvesting for energy production may release long-term stores of Hg from deeper soil layers.     

The Distribution of Mercury in the Sediments of Eight Saskatchewan Lakes

Eight lakes located in the southern half of Saskatchewan were sampled in May and June, 1987, to determine the concentration and spatial distribution of mercury in their surface sediments. Katepwa and Buffalo Pound lakes had maximum total mercury sediment concentrations of 0.158 and 0.100 μg · g⁻¹ dry weight respectively, while in the other six lakes the mercury concentration was never greater than 0.04 μg · g⁻¹ dw. Despite this difference in mercury concentration in the sediments of the lakes, walleye sampled from them have high mercury concentrations exceeding 0.5 μ · g⁻¹ (wet weight).     

Source areas of Blue-winged Teal harvested in Ontario and Prairie Canada based on stable isotopes: implications for sustainable management

Determining the source areas of harvested individuals is important for effective conservation and management of migratory game birds. Banding has provided much information about source areas, but obtaining samples of marked individuals representative of all potential breeding areas is difficult for most species. To add to previous knowledge of harvest derivation based on banding data and to assist with regulatory decisions, we used stable hydrogen isotope (δ²H) techniques to estimate natal and molt source areas of Blue-winged Teal (Spatula discors) harvested in southern Canada in 2014 and 2017. We found that most birds harvested in southern Saskatchewan, southern Manitoba, northern Ontario, and southern Ontario likely originated in the prairie and boreal plains regions of Canada and the United States, which is the core production area for the species. Based on feather δ²H values, some birds harvested in Ontario may have also originated in Ontario. Our results differ from those of a long-term analysis of band recovery data that revealed that most Blue-winged Teal harvested in Ontario originated in the eastern part of the province and areas along the lower Great Lakes and southwestern Quebec. We found that nearly all birds harvested in Ontario in our study likely originated from areas north and west of the province. Together, banding and stable isotopes likely provide the best information available on source areas of harvested birds for regulatory decision making.     

Mercury in waterfowl from a contaminated river in Virginia

Many bodies of water around the world are contaminated with mercury from historic industrial and mining activities or ongoing atmospheric deposition, resulting in numerous fish consumption advisories. However, concerns about mercury have only rarely led to consumption advisories on waterfowl. In contrast with fish, waterfowl frequently disperse long distances to new watersheds, so hunters and wildlife managers do not know whether waterfowl at a pristine site have spent time at a contaminated site elsewhere. We sampled tissue mercury concentrations of mallards (Anas platyrhynchos), wood ducks (Aix sponsa), and Canada geese (Branta canadensis) at a site contaminated with mercury, during the breeding and hunting seasons. We found that many mallards had bioaccumulated mercury to levels that had the potential to produce reproductive effects and exceeded consumption advisories set for fish by regulatory agencies, whereas this was true for only a few wood ducks and Canada geese. We also documented that mercury-exposed waterfowl from this contaminated site were harvested by hunters as far as 1,054 km away. Our results suggest the need for more proactive sampling of waterfowl for mercury, and likely other bioaccumulating contaminants, in order to allow hunters to make more informed choices about consumption of their harvest. © 2012 The Wildlife Society.     

Using empirical methods to assess the risks of mercury accumulation in fish from lakes receiving acid rain

It has been hypothesized that concentrations of mercury ([Hg]) in fish increase with increasing acidic deposition. Our investigation suggests that this hypothesis is false for three species of fish from Ontario lakes. Across Ontario, Canada, slightly lower average [Hg] in Lake Trout, Walleye and Northern Pike (controlled for fish length) were associated with regions receiving progressively greater rates of sulphate deposition. We also observed that lake concentration of dissolved organic carbon ([DOC]), not pH, alkalinity or sulphate ion concentration, was the strongest correlate of this residual fish Hg contamination. Path analysis also determined a net reduction in fish [Hg] at higher rates of sulphate deposition. However, this analysis indicated a far more complex web of interactions between sulphate deposition, water chemistry and fish [Hg] than simple correlation analysis could define. Path analysis provided empirical evidence in support of several previously described mechanisms interacting simultaneously to influence fish Hg accumulation. Based on these empirical observations, an alternate hypothesis is proposed whereby increasing sulphate deposition results in lower [DOC] levels in lakes which, in turn, cause a decline in fish Hg contamination.