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Peninsula Lake, Ontario, Canada, is a Precambrian Shield lake that has experienced many environmental stressors since European settlement of the watershed in the mid-1800s, including forest clearance, water-level management, sewage inputs, and land-use changes. The deterioration of water quality by the 1970s prompted mitigation efforts intended to reduce inputs of nutrients and untreated sewage to the lake. Paleolimnological analysis of a sediment core from Peninsula Lake, collected in 1995, revealed that efforts to address eutrophication had been somewhat successful, in that the lake had returned to oligotrophic status by the early 1990s. Nevertheless, there had been little to no improvement in inferred deepwater oxygen conditions. We used similar paleolimnological techniques on a core collected in 2014 to examine ecological responses to environmental stressors and remediation efforts in the intervening years, with a particular focus on changes in the amount of oxygenated habitat available to lake trout. Specifically, we inferred trends in volume-weighted hypolimnetic oxygen (VWHO), through analysis of sedimentary chironomid assemblages, to assess long-term changes in coldwater fisheries habitat. Potential drivers of VWHO were also investigated, and included diatom-inferred total phosphorus, spectrally derived trends in sedimentary chlorophyll a and its main diagenetic products, and lake-water dissolved organic carbon. The sedimentary chironomid assemblages indicate a long period of VWHO depletion, concurrent with known anthropogenic activities from the early twentieth century to the 1990s, followed by recent recovery to near pre-impact levels. The sedimentary diatom assemblages indicate nutrient enrichment throughout most of the twentieth century, which was also followed by a return to pre-impact conditions, albeit with recent indications of renewed nutrient enrichment. In contrast, strikingly different trajectories were observed in two other regional waterbodies that support lake trout, but experienced relatively little nutrient enrichment over the past ~ 150 years. Peninsula Lake appears to be recovering from nutrient enrichment and hypolimnetic oxygen depletion, but water quality improvements have nevertheless been accompanied by increased primary production, perhaps because climate warming has extended the ice-free season and stratification period. Therefore, instead of returning to pre-impact conditions, Peninsula Lake appears to be developing a novel ecological state. Any future improvements in lake trout habitat that result from nutrient abatement may be offset by rising temperatures.

Simmatis B, Nelligan C, Ruhland KM, Jeziorski A, Castro V, Paterson AM, Smol JP. 2020. Tracking ∼200 years of water quality in Muskrat Lake, a eutrophic lake trout lake in Ontario (Canada) with cya...

Temperature–oxygen profiles, collected biweekly to monthly for ∼40 years, were used to calculate end-of-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in six small lakes located in south-central Ontario, Canada. Coherent decreases in thermocline depth and increases in hypolimnetic volume, mean hypolimnetic dissolved oxygen (DO) concentration, and VWHO were observed in five of the six study lakes. All lakes underwent an abrupt increase in VWHO and mean hypolimnetic DO after 2010. In four of the six study lakes, the highest hypolimnetic DO concentrations were observed in years where chlorophyll a concentrations were low, whereas at five of the six study lakes the highest hypolimnetic volumes were observed when dissolved organic carbon concentrations were relatively high. Warmer spring or winter air temperatures were associated with higher hypolimnetic DO concentrations at two sites, and longer ice-free periods were associated with smaller hypolimnetic volumes at two sites. These results suggest that the recent VWHO increases in the studied south-central Ontario lakes may be a function of multiple drivers that include changes in primary production, lake water transparency, and regional climatic factors.

Anthropogenic stressors affect lakes around the world, ranging in scale from catchment-specific pollutants to the global impacts of climate change. Canada has a large number and diversity of lakes, yet it is not well understood how, where, and when human impacts have affected these lakes at a national scale. The NSERC Canadian Lake Pulse Network sought to create the first nationwide database of Canadian lake health, undertaking a multi-year survey of 664 lakes spanning 12 ecozones across Canada. A key objective of the network is to determine where, by how much, and why have Canadian lakes changed during the Anthropocene. To address this objective, we compared sedimentary chlorophyll a and diatoms from modern and pre-industrial sediment intervals of ~200 lakes. The lakes spanned a range of sizes, ecozones, and degrees of within-catchment land use change. We inferred the quantity of chlorophyll a, its isomers and main diagenetic products using visible reflectance spectroscopy. We found widespread increases in primary production since pre-industrial times. Primary production increased, on average, across all ecozones, human impact classes, and stratification classes. Likewise, an increase in planktonic diatom taxa over time was detected in the majority of sampled lakes, likely due to recent climate warming. However, regional factors (ecozones) explained the most variation in modern diatom species assemblages as well as their temporal turnover. Furthermore, lakes with high human impact (i.e., higher weighted proportions of human land use in the catchment) exhibited greater taxonomic turnover than lakes with a low human impact class. The greatest diatom turnover was found in the agriculture-rich Prairies and the lowest in the sparsely populated Boreal Shield and Taiga Cordillera ecozones. Overall, our study highlights that drivers operating at different geographic scales (i.e., climatic and land-use changes) have led to significant alterations in algal indicators since pre-industrial times across the country.

Multiple stressors affect water quality and biodiversity in lakes worldwide. However, our understanding of which combinations of stressors are of greatest impact and how lakes have shifted from their pre-industrial baselines is fragmented. Questions remain regarding how multiple trophic groups are affected by global change stressors and whether region-specific differences need to be considered. Here, we apply multiple factor analyses together with latent variable modeling to quantify potential interactions within and among taxa that span multiple trophic levels (i.e. diatoms, cladocerans and chironomids) to improve our understanding of paleo-environmental dynamics from a suite of Canadian lakes spanning four ecozones: the Boreal Shield, the Mixedwood Plains, the Atlantic Highlands, and the Atlantic Maritimes. Across all ecozones, multi-trophic assemblages were distinguished in a multiple factor analysis along a land-use gradient, with mesotrophic/eutrophic diatoms and profundal chironomids tolerant of low bottom-water oxygen concentrations recorded in more disturbed sites. Functional units across three indicator groups formed distinct networks of co-responses to key environmental and land-use gradients, although cladocerans seemed to be driven by additional (i.e., residual) gradients. Applying a temporal beta-diversity approach between modern and pre-industrial assemblages, we detected significantly greater turnover of diatom functional groups in high versus low human impact sites (p = 0.03, n = 57), with similar but non-significant trends apparent with the chironomid and cladoceran groups. Ecozonal differences in temporal turnover were also evident in diatom (p = 0.003, n = 57) and chironomid functional groups (p = 0.04, n = 41). The ecozonal differences may be partially due to differential sensitivities of the pre-industrial assemblages and may also be driven by historical disturbances. Our work highlights how the magnitude and direction of ecological change vary across ecozones and can modify responses to human impacts, with a general trend of higher species turnover in sites with greater human disturbance.

Effluent from diamond mining operations rich in calcium (Ca) has transformed softwater tundra lakes in the Northwest Territories, Canada. Lakes downstream of the Dominion Diamond Corporation Ekati Mine have experienced marked changes in water chemistry and cladoceran community composition since establishment of the mine in 1998. The greatest changes have occurred at the sites closest to the effluent discharge, with [Ca] increasing from 30 mg·L−1 and corresponding increases in pH from 8. A split was identified in the cladoceran communities with Holopedium glacialis (a jelly-clad cladoceran tolerant of low [Ca]) generally dominating the cladoceran community at [Ca] < 2.5 mg·L−1, while in impacted lakes with [Ca] ≥ 2.5 mg·L−1, Daphnia longiremis and Daphnia middendorffiana (taxa with higher [Ca] requirements) were often dominant. In contrast, the three study lakes that did not receive mining effluent maintained stable and low [Ca] (mean [Ca] = 0.66 ± 0.06 mg·L−1 (SD)) throughout the monitoring period and have not experienced directional shifts in their cladoceran communities.

Recent anthropogenic climate change and the exponential increase over the past few decades of Saharan dust deposition, containing ecologically important inputs of phosphorus (P) and calcium (Ca), are potentially affecting remote aquatic ecosystems. In this study, we examine changes in cladoceran assemblage composition and chlorophyll-a concentrations over the past ~150 years from high-resolution, welldated sediment cores retrieved from six remote high mountain lakes in the Sierra Nevada Mountains of Southern Spain, a region affected by Saharan dust deposition. In each lake, marked shifts in cladoceran assemblages and chlorophyll-a concentrations in recent decades indicate a regional-scale response to climate and Saharan dust deposition. Chlorophyll-a concentrations have increased since the 1970s, consistent with a response to rising air temperatures and the intensification of atmospheric deposition of Saharan P. Similar shifts in cladoceran taxa across lakes began over a century ago, but have intensified over the past ~50 years, concurrent with trends in regional air temperature, precipitation, and increased Saharan dust deposition. An abrupt increase in the relative abundance of the benthic cladoceran Alona quadrangularis at the expense of Chydorus sphaericus, and a significant increase in Daphnia pulex gr. was a common trend in these softwater lakes. Differences in the magnitude and timing of these changes are likely due to catchment and lake-specific differences. In contrast with other alpine lakes that are often affected by acid deposition, atmospheric Ca deposition appears to be a significant explanatory factor, among others, for the changes in the lake biota of Sierra Nevada that has not been previously considered. The effects observed in Sierra Nevada are likely occurring in other Mediterranean lake districts, especially in softwater, oligotrophic lakes. The predicted increases in global temperature and Saharan dust deposition in the future will further impact the ecological condition of these ecosystems, Ministerio de Educación y Ciencia (MEC), Grant/Award Number: AP2007-00352; Programa Nacional de Movilidad de Recursos Humanos de Investigaci on (MICINN); Ministerio de Medio Ambiente (MMA), Grant/Award Number: 87/2007; Ministerio de Econom ıa, Industria y Competitividad (MINECO), Grant/Award Number: CGL2011-23483; Natural Sciences and Engineering Research Council of Canada

In recent decades, marked declines in calcium (Ca) concentrations have been noted in many softwater boreal lakes, and are believed to be a long-term consequence of acid deposition as well as other stressors (such as timber harvesting). Reduced Ca availability may act as a potent environmental stressor. Investigations of the direct ecological impacts of lower Ca concentrations in freshwater systems have largely focused on Ca-rich members of the Cladocera; however, a growing body of work, spanning several scientific fields, suggests Ca decline will have profound direct and indirect consequences for aquatic ecosystems. Here, we synthesize recent laboratory analyses and field surveys to provide an overview of these consequences, while highlighting paleolimnological investigations that provide some long-term perspective on the phenomenon. However, considerable questions remain regarding ‘baseline’ or pre-impact conditions, due to the accelerated leaching of Ca from watershed soils during the period of anthropogenic influence. Furthermore, catchment-specific differences in both leaching rates and the initial size of the Ca pool in watershed soils complicate determination of the eventual endpoints of the declines. Despite these uncertainties, persistent low Ca concentrations are anticipated to impede biological recovery from lake acidification, and that ongoing declines will have cascading effects throughout aquatic ecosystems due to the loss of vulnerable taxa. To better understand how reduced Ca availability will continue to change affected surface waters and how these changes will interact with other environmental stressors will require continued investigation of the declines from multiple scientific perspectives.

Nelligan C, Jeziorski A, Ruhland KM, Paterson AM, Smol JP. 2016. Managing lake trout lakes in a warming world: A paleolimnological assessment of nutrients and lake production at three Ontario sites. Lake Reserve Manage. 32:315−328.Recent declines in hypolimnetic dissolved oxygen (DO) concentrations in many lakes throughout Ontario have prompted concern regarding the health of lake trout (Salvelinus namaycush) populations. We used diatom assemblages and spectrally inferred chlorophyll a from the dated sediments of 3 lake trout lakes to track trends in total phosphorus (TP) and whole-lake production over the past ∼150 years. Eagle Lake is considered a sensitive lake trout lake because DO concentrations have been lower than the provincial standard (>7 mg/L) since the early-2000s; however, a lack of monitoring data impedes the evaluation of current conditions within a historical context. For a regional perspective, we compared trends in Eagle Lake with those of 2 additional lake trout lakes ∼200 km aw...

The extensive peatlands and lakes of the Far North of Ontario warrant committed scientific attention given their status as a significant carbon sink. Economic interest in this region has recently increased due to the discovery of vast mineral deposits (mainly chromite and nickel) known as the “Ring of Fire”. Mineral exploration and infrastructure planning are underway, but environmental monitoring is only beginning. Detailed baseline ecological information is required to assess the impacts of future resource extraction within the context of multiple environmental stressors (including recent regional climate warming). Here we use sediment cores from two relatively deep lakes (Zmax ~ 10 m) and two shallow lakes (Zmax ~ 2 m), all located in the vicinity of the Ring of Fire, to examine biotic responses to warming prior to the commencement of mining activities. Our data show that, over the past ~150 years, diatom and cladoceran sedimentary assemblages have transitioned from dominance by littoral/benthic forms to greater abundances of planktonic cladoceran (an increase of ~3 to 34 %) and diatom taxa (an increase of ~3 to 22 %). Increased relative abundances of planktonic taxa are consistent with warming-induced changes in lake properties including longer ice-free periods and increased production by planktonic algae. The response of diatom assemblages in shallow lakes to warming preceded the deeper lakes by ~45 to 60 years, and substantial increases in aquatic production (~4 to 15 times higher than in sediments deposited prior to 1900) were observed in the shallow lakes, in agreement with previous analyses demonstrating the heightened sensitivity of shallow systems to climate warming. These data provide important information necessary to distinguish potential ecological impacts related to resource extraction from natural variation and the ongoing responses to regional climate warming.

Lakewater calcium (Ca) decline affects softwater lakes across the Canadian Shield. Ca decline is one consequence of acid deposition, and has impeded biological recovery in formerly acidified lakes. Reduced Ca availability may advantage taxa better adapted to low Ca waters. Crosson Lake in south-central Ontario (Canada) has an extensive monitoring record and has experienced strong Ca decline since the late 1970s, recently falling below 1.5 mg L−1, a threshold value for some large Daphnia O.F. Muller, 1785 taxa. Paleolimnological analysis of the sedimentary cladoceran assemblages revealed that changes associated with the Ca decline began in the early 1970s. Relative abundances of the jelly-clad Holopedium glacialis Rowe, 2007 and the Daphnia pulex complex increased while Ca-sensitive members of the Daphnia longispina complex decreased. Zooplankton net hauls (1981–2010) corroborate the paleolimnological analysis, revealing that increases in the D. pulex complex were due to Daphnia catawba Coker, 1926 (a taxon tolerant of low Ca). Lakewater Ca, dissolved organic carbon and total phosphorus explain a significant amount of variation within the cladoceran community; however, the relationship between Ca concentration and the daphniid community was most apparent. The Crosson Lake paleolimnological and direct monitoring data may describe ecological changes that are also occurring in many other softwater lakes across the Canadian Shield and elsewhere.

The "Ring of Fire" in the Far North of Ontario (50 - 57° N, 79 - 94° W) is a region of growing economic and environmental interest due to recent mineral discoveries and the potential for resource development. Due to the remote location of the region, little baseline ecological information is available to distinguish any environmen- tal impacts of mine development from ongoing regional warming. Here, we compare modern and pre-industrial sedimentary cladoceran assemblages from 60 lakes centered on McFaulds Lake in the "Ring of Fire" to identify directional changes through time and compare the assemblages with prior paleolimnological cladoceran studies of Ontario lakes closer to the Hudson Bay coast, and further south in the Muskoka-Haliburton region. The cladoceran assemblages from the "Ring of Fire" contain diverse littoral assemblages relative to the other two regions; however, despite the shallow depth of these lakes, there have been significant increases in the relative abundances of pelagic taxa (e.g. Bosmina spp. and Daphnia spp.) since pre-industrial times; taxonomic shifts consistent with warming- induced changes in lake properties including a longer ice-free period and increased algal plankton production. This evidence of widespread changes within the cladoceran assemblages in the "Ring of Fire" prior to the onset of resource extraction provides lake managers and stewards with information to better monitor and evaluate future development projects.

Simmatis B, Jeziorski A, Zemanek A, Selbie DT, Hyatt K, Fryer JK, Cumming BF, Smol JP. 2018. Long-term reconstruction of deep-water oxygen conditions in Osoyoos Lake: implications for Okanagan River sockeye salmon. Lake Reserve Manage. 34:392–400. Osoyoos Lake is the primary nursery lake supporting sockeye salmon (Onchorhynchus nerka) originating from the Okanagan River system in south-central British Columbia, Canada. By the early 1960s, Osoyoos Lake had experienced significant cultural eutrophication related to rapid human development of the watershed, increasing total phosphorus concentrations, and frequent algal blooms. Concern exists regarding recent (post-1990s) declines in hypolimnetic oxygen concentrations and the potential degradation of salmon habitat in this multi-basin lake. However, reliable observations are largely unavailable for oxygen conditions prior to the mid-1990s. We reconstructed late-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in Osoyoos Lake from the late 1800s to 2009 by examining past chironomid (Diptera: Chironomidae) subfossil assemblages from 210Pb-dated lake sediment cores. Chironomid-inferred VWHO revealed only slight variations since the late 1800s (mean values for the north and south basins are 4.4 and 2.9 mg/L, respectively), with seemingly little response to the cultural eutrophication of the mid-20th century. The chironomid-inferred VWHO trends suggest that the current management target of 5.8 mg/L dissolved oxygen for both basins of Osoyoos Lake may require revision to better reflect reference conditions and natural seasonal anoxia.

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.

A multi-proxy paleolimnological survey was performed on 13 lakes in the Hudson Bay Lowlands (HBL) of northern Ontario in order to provide a regional analysis of recent environmental changes in this poorly studied sub-Arctic region. In contrast to the amplified warming experienced by most of the circumpolar Arctic since the mid-19th century, the climate of the Hudson Bay (HB) region has remained relatively cool and stable for hundreds of years. However, since approximately the 1990s, the HBL has experienced rapid and large increases in air temperature and declines in sea ice. Diatom, cladoceran, and chironomid remains preserved in the recent (surface) and pre-1850 sediments of 13 lakes were used to examine whether this new climate regime has resulted in species assemblage changes across multiple trophic levels. Our results indicate clear limnological responses to warming among the freshwater biota of HBL lakes; however, the magnitude of this change varied among both biological indicators and sites...

Calcium (Ca) concentrations of many softwater lakes on the Canadian Shield have been in decline for decades as a response primarily to regional acid deposition and repeated cycles of forest harvesting. These Ca declines are of ecological interest, as many lakes have fallen to Ca levels detrimental to the fitness of ecologically important Ca-rich cladoceran taxa such as Daphnia pulex. However, distinguishing the impacts of reduced Ca from acidification (i.e., decreasing pH) on Ca-demanding fauna has proven difficult due to strong correlations between pH and Ca in softwater lakes. Here, we examine cladoceran sedimentary assemblages from low Ca lakes (mean present-day Ca

In response to biotic and abiotic cues, the cladoceran genus Bosmina can undergo changes in body size and appendage length and shape over successive generations. To improve our understanding of the environmental controls on Bosmina size structure, we used paleolimnological techniques to examine Bosmina size responses to the extreme acidification and metal contamination, and then subsequent chemical recovery, of lakes in the vicinity of mining and smelting operations near Sudbury, Canada. During the acidification period, Bosmina antennule and carapace length significantly increased in three of the five study lakes, while mucro length significantly decreased in four of the five lakes. However, despite the recent return to pre-impact pH levels, the size structure of the present-day Bosmina community still differs from the pre-impact size distributions. We suggest that the continued dominance of the food webs by small invertebrate predators (e.g., cyclopoid copepods) is responsible for the persistent changes to Bosmina size structure.

In the region northeast of Wawa, Ontario (Canada), many circumneutral lakes downwind of a nearby iron-sintering plant were strongly acidified (pH 3–4) in response to the emissions of large amounts of sulfur dioxide from 1939–1998. Following closure of the plant in 1998, lakewater pH has returned to circumneutral conditions due to the high buffering capacity of the local geological substrate. Prior paleolimnological analyses of dated sediment cores have detected some biological recovery among algal communities (diatoms and chrysophytes), although they have not returned to their pre-impact assemblages. Here we take a broader ecosystem approach, and build upon the algal analyses by examining cladoceran sedimentary assemblages, and spectrally-inferred chlorophyll a and dissolved organic carbon (DOC) from the same dated sediment cores. Similar to the algal communities, recent cladoceran sedimentary assemblages from three impacted lakes remain in an altered state relative to the pre-impact period (for example, increased relative abundances of Chydorus brevilabris and reduced cladoceran density in sediments). However, trends in the spectrally-inferred chlorophyll a and DOC were mixed, with long-term decreases in the study lake closest to the plant and long-term increases within the other lakes. Collectively, the multi-proxy paleolimnological analyses of these markedly acidified lakes demonstrate the delayed biological recovery from acidification (and differences in timing) across multiple trophic levels, despite the near-elimination of acid deposition almost a decade previously, which led to a striking recovery in lakewater pH and increased food availability.

Aqueous calcium (Ca) concentrations are currently decreasing in many softwater lakes on the Boreal Shield. As the onset of these declines often pre-date direct monitoring programs, indirect techniques are required to examine the impacts of reduced Ca availability on aquatic communities with relatively high Ca demands such as the Cladocera (Class: Branchiopoda). Among the Cladocera, the family Daphniidae has been identified as a taxonomic group potentially useful for inferring past Ca concentrations due to their high Ca demands and preservation in lake sediments. Here, we use a “top/bottom” paleolimnological analysis to compare present-day cladoceran communities preserved in the surface sediments of 36 softwater lakes in south-central Ontario, Canada, which are potentially vulnerable to Ca decline (i.e. small headwater systems with present-day lakewater [Ca]

Lake water calcium (Ca) decline has recently been recognized as a stressor impacting softwater lake districts that have experienced long-term patterns of acid deposition and/or timber harvesting. Declining aqueous Ca levels may impact the survival of aquatic biota, particularly Ca-rich cladoceran taxa such as daphniids. Daphnia pulex are sensitive to laboratory Ca levels below 1.5 mg l−1; however, responses of cladoceran communities to Ca decline in natural environments require further study. Dickie Lake (Ontario, Canada) is the site of an inadvertent natural experiment, providing insight into the effects of changing aqueous Ca availability upon cladoceran communities, as the lake has a history of acidification, followed by recent (1990s) Ca additions to the watershed via applications of calcium-rich road dust suppressants. Paleolimnological analyses were used to examine changes in cladoceran community structure (with a focus upon Ca-rich daphniids) from pre-industrial times to present day. Three distinct temporal stages were apparent in Dickie Lake’s daphniid community: 1870–1950, 1950–1990, and 1990–present. The daphniid community of the pre-industrial assemblages was dominated by members of the Daphnia longispina species complex, but shifted in the late 1950s to more acid- and Ca-insensitive members of the D. pulex species complex. During the most recent stage, coincident with dust suppressant applications, both daphniid complexes are well represented. Observed transitions between daphniid species complexes provide further evidence of the influence of Ca availability upon cladoceran community structure, indicating the potential importance of the controlled addition of Ca to freshwater systems (i.e., liming) as a mitigation/recovery strategy as Ca declines continue.

Aqueous calcium (Ca) concentrations are declining in softwater lakes of the Canadian Shield largely because of decades of acid deposition and afforestation following timber harvesting. Populations of pelagic cladoceran taxa with high Ca requirements, especially Daphnia spp., are declining in response to reduced aqueous Ca availability. However, the Ca content, and thus the requirements of littoral cladoceran taxa are unknown; therefore, the potential vulnerability of this major component of lake ecosystems to ongoing regional Ca decline remains uncertain. Here, we identify the body Ca concentration of nine littoral cladoceran taxa collected from three lakes in the Muskoka-Haliburton region of Ontario, Canada. Ca content differed among taxonomic groups, ranging from 3.6 mg g−1 for Acroperus harpae to 23.2 mg g−1 for Disparalona spp. Perhaps surprisingly, some littoral microcrustacean taxa have Ca burdens comparable to the Ca-rich daphniids. Therefore, there may be differential responses to ongoing lake water Ca declines among taxa within littoral communities as has been observed among open-water taxa, with potential ecological repercussions for near-shore food webs.

Daphnia subfossils from lake sediments are useful for exploring the impacts of environmental stressors on aquatic ecosystems. Unfortunately, taxonomic resolution of Daphnia remains is coarse, as only a small portion of the animal is preserved, and so the identification of daphniid subfossils typically relies upon postabdominal claws. Daphniid claws can be assigned to one of two species complexes: D. longispina or D. pulex. Both complexes contain species with differing environmental optima, and therefore improved taxonomic resolution of subfossil daphniid claws would aid paleolimnological analyses. To identify morphological features that may be used to help differentiate between species within complexes, we used species presence/absence data from net tows to select lakes in central Ontario (Canada) containing only a single species from a particular complex, then used remains preserved in surface sediments of these lakes to isolate four Daphnia species: D. ambigua and D. mendotae from the D. longispina complex, and D. pulicaria and D. catawba from the D. pulex complex. Our analyses demonstrate that, within the D. longispina complex, postabdominal claw length (PCL) and spinule length can be used to distinguish D. mendotae from D. ambigua. In addition, within the D. pulex complex, there are differences between D. pulicaria and D. catawba in the relative lengths of the proximal and middle combs on the postabdominal claw. However, the number of stout spines on the middle comb is an unreliable character for differentiating species. Overall, our data demonstrate that greater resolution within Daphnia species complexes is possible using postabdominal claws; however, the process is arduous, and applicability will likely decrease with the number of taxa present.

Declines in lakewater calcium (Ca) concentrations are occurring in many softwater lakes of the Canadian Shield, and likely elsewhere, and there is growing interest regarding the potential impacts that reduced Ca availability may have on aquatic ecosystems. Here, we test the hypotheses that the Ca limitations of Daphnia pulex (reduced growth/survival when Ca

The effects of low counts on assemblage inferences in paleolimnological investigations have been examined for many biological proxies, but not yet for Cladocera. Established guidelines leading to the determination of an adequate, minimum count are absent with respect to sampling cladoceran remains from lake sediments. Using simulated subsamples derived from observed assemblages of considerably higher counts, we investigated the effect of counting effort on three principal characteristics of an assemblage: richness, number of new taxa encountered, and the absolute differences in relative abundances of dominant taxa. Results from six lakes located within diverse ecological regions (i.e. Subarctic, Canadian Shield, Acadian Forest) suggest that a minimum of between 70 and 100 individuals is satisfactory to characterize most assemblages. Doubling counting effort from 100 to 200 individuals leads to only modest gains in subsample relatedness to the observed assemblage. Greater counting effort may be necessary when the primary interest is in presence-absence or distributional data, or when abundances of ecologically relevant taxa are exceptionally low.

In most Ontario lakes, phosphorus is present in trace quantities, making the precise measurement of concentrations difficult. The considerable variation that results in many datasets can be attributed to imprecise analysis. Even with precise analysis, substantial variation in ice-free whole-lake, mean mixed layer, and spring turnover total phosphorus (TP) concentrations often remain, both between years and within a given year. Lake managers have adopted many strategies to address these issues. Whole-lake TP concentrations expressed as ice-free season means are difficult to derive because they require the collection of numerous, volume-weighted samples often from multiple lake layers. As a result, spring turnover or ice-free-mean, mixed layer TP concentrations are often used to characterize the trophic status of a lake or to describe the nutrients available for primary production. These different phosphorus characterizations will vary between years, leading many lake managers to use long-term (mul...

Despite reductions in atmospheric sulphur emissions and the resulting decline in acidic deposition, many lakes on the Canadian Shield that have experienced acidification are either recovering at a rate slower than expected or not recovering at all. This lack of recovery is believed to be partly the consequence of the depletion of exchangeable base cations (principally calcium, Ca) from watershed soils. Although the implications of reduced Ca availability for biological communities remain poorly understood, the effects on crustacean zooplankton populations may be severe, as Ca is the primary structural component of the crustacean zooplankton exoskeleton. Because the daphniid resting egg is protected by a modified portion of the exoskeleton (ephippia) and because these ephippia are well preserved in lake sediments, we investigated whether inter-specific differences in Ca content are recoverable from Daphnia ephippia. However, using two methods, we did not find a recoverable Ca signal in the ephippia, indicating that future efforts should be focused upon sedimentary taxonomic assemblage differences, not differences in ephippial Ca levels.

Calcium (Ca) is a major component of the crustacean zooplankton carapace. As crustacean zooplankton obtain most of their Ca from lake water, taxa with high Ca demands may be negatively affected if environmental Ca concentrations fall below thresholds necessary for adequate Ca uptake rates. Currently, Ca concentrations are falling in many soft-water lakes with a history of acid deposition, both within Canada and globally. Hence, there is a need to identify species with high Ca demands. We analyzed Ca concentrations of eight crustacean zooplankton taxa from nine lakes spanning a 0.4–35 mg·L–1 Ca gradient. Our objectives were to determine (i) if both species identity and lake Ca concentrations are determinants of zooplankton Ca concentration and (ii) if zooplankton Ca concentration varies seasonally. Large differences in Ca concentrations were observed among taxa. Levels in Daphnia sp. ranged from 2% to 8% of dry body weight, whereas levels in copepods and non-daphniid cladocera were much lower at 0.2%–0.4%. Seasonal variation in zooplankton Ca concentration was negligible, but positive correlations with lake Ca concentration were detected, with up to twofold differences in Ca concentration between lakes in three of the eight species. Species identity was the most important determinant of crustacean zooplankton Ca concentration.

We assess Holocene environmental change at alpine Lake Njulla(68°22′N, 18°42′E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujavri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.

Calcium (Ca) concentrations are decreasing in softwater lakes across eastern North America and western Europe. Using long-term contemporary and palaeo-environmental field data, we show that this is precipitating a dramatic change in Canadian lakes: the replacement of previously dominant pelagic herbivores (Ca-rich Daphnia species) by Holopedium glacialis , a jelly-clad, Ca-poor competitor. In some lakes, this transformation is being facilitated by increases in macro-invertebrate predation, both from native ( Chaoborus spp . ) and introduced ( Bythotrephes longimanus ) zooplanktivores, to which Holopedium , with its jelly coat, is relatively invulnerable. Greater representation by Holopedium within cladoceran zooplankton communities will reduce nutrient transfer through food webs, given their lower phosphorus content relative to daphniids, and greater absolute abundances may pose long-term problems to water users. The dominance of jelly-clad zooplankton will likely persist while lakewater Ca levels remain low.

In the mid-20th century, similar to many lakes in the vicinity of Sudbury, Canada, Middle Lake was severely acidified due to nearby smelting operations. However, this lake is of particular interest because it was limed in 1973, and later fertilized as part of a restoration effort. Here, we use paleolimnological methods to track cladoceran assemblage responses to acidification, liming, and subsequent recovery in a ∼250-year lake sediment record. Cladoceran assemblage changes, notably increases in Chydorus brevilabris, coincided with the late 1800s establishment of open-pit ore roasting in the region. As acidification progressed, the Daphnia pulex complex was replaced by the Daphnia longispina complex. At the height of acidification, and with similar timing to the liming, C. brevilabris increased abruptly in relative abundance in the sediment record, followed by a rapid decline. Invertebrate predation was investigated using Bosmina mucro length; however, no significant trends were evident. Our results suggest that complete biological recovery has not occurred. Specifically, species richness (rarefied) is ∼64 % lower after the onset of acidification, and many rare species present prior to the onset of acidification have not yet returned to pre-impact levels despite dispersal events of these rare taxa being observed during contemporary zooplankton monitoring. Factors impeding the complete biological recovery of the cladocerans in Middle Lake may include biotic resistance, ongoing metal contamination, and a warming climate.

Calcium concentrations are now commonly declining in softwater boreal lakes. Although the mechanisms leading to these declines are generally well known, the consequences for the aquatic biota have not yet been reported. By examining crustacean zooplankton remains preserved in lake sediment cores, we document near extirpations of calcium-rich Daphnia species, which are keystone herbivores in pelagic food webs, concurrent with declining lake-water calcium. A large proportion (62%, 47 to 81% by region) of the Canadian Shield lakes we examined has a calcium concentration approaching or below the threshold at which laboratory Daphnia populations suffer reduced survival and fecundity. The ecological impacts of environmental calcium loss are likely to be both widespread and pronounced.