Assessing the effects of multiple environmental stressors on zooplankton assemblages in Boreal Shield lakes since pre-industrial times

Abstract

Zooplankton communities in Boreal Shield lakes of south-central Ontario, Canada, have become increasingly exposed to the effects of multiple anthropogenic stressors, such as declines in calcium (Ca) and total phosphorus (TP) concentrations, shifts in predation regimes, and climate warming. The paleolimnological approach provides an effective means of examining cladoceran zooplankton communities prior to the onset of these major environmental stressors and assessing how the increasing impacts of these stressors have affected zooplankton community composition. We examined the chitinized remains of cladocerans from recent and pre-industrial (pre- ~1850s) sediments in 42 oligotrophic lakes from south-central Ontario and compared these assemblages using ANOSIM and SIMPER. Differences in cladoceran assemblages since pre-industrial times were related to five environmental variables that significantly influence cladoceran community composition in surface sediments. These included measured physical (depth), chemical [Ca, pH, sulphate, dissolved organic carbon (DOC)], and biological (fish community biomass) limnological variables. Two changes were recorded in the cladoceran species assemblages of the study lakes. The first was a significant increase in the ratio of pelagic species compared to littoral species over time in most lakes which we cannot attribute to any measured environmental variable with certainty but it likely represents a multiple stressor effect. The second was changes in relative abundances of daphniid cladocerans in several lakes since pre-industrial times, which we attribute to the interactive effects of several environmental stressors, including: differences in Ca availability within our lakes, resulting in decreased abundances of daphniids over time in lakes with lower Ca levels; long-term increases in DOC concentrations, which may provide refuge for daphniids from visual predators; and long-term declines in TP concentrations which may contribute to the dominance of species that are more efficient grazers, such as daphniids. Overall, this study provides field-based evidence that the modern-day cladoceran communities in south-central Ontario lakes are different than they were prior to human settlement in the region, and therefore these paleolimnological data provide a long-term, historical component to contemporary cladoceran datasets and an extended perspective on how multiple environmental stressors have impacted aquatic organisms from Boreal Shield lakes

Crustacean zooplankton sedimentary remains from calcium-poor lakes: complex responses to threshold concentrations

“…The potential of daphniids as paleo-indicators of lake calcium decline has been demonstrated in several paleolimnological studies (DeSellas et al , 2011Jeziorski et al 2008), due in part to the daphniids' relatively high Ca demands (Alstad et al 1999) and Ca content (Jeziorski and Yan 2006). However, contrary to our initial hypothesis that the 1.5 mg L -1 laboratory threshold would be apparent in a Crustacean zooplankton remains and lakewater calcium 125 spatial survey of zooplankton sedimentary remains, higher relative abundances of daphniids were found in lakes currently below 1.5 mg L -1 , particularly among the Daphnia pulex species complex.…”

Modern and pre-Industrial Age distributions of Cladocera in Italian and Swiss Alpine lakes

“…Knowledge of reference conditions is very important to the characterization of ecosystem responses to stress and to establishing realistic restoration targets. Paleolimnology has become an important tool for determining baseline conditions (Rühland et al, 2003;Sweetman et al, 2008;DeSellas et al, 2011), but single samples from bottom sediments must be seen only as snapshots from the past; they can represent also the naturally changed assemblages e.g., during the cold Little Ice Age (Eggermont et al, 2010;. Community dynamics are influenced by natural as well as anthropogenic variables, and this phenomenon dictates that more continuous analysis of cores over a period of decades to centuries should be undertaken to reveal long-term averages and degree of fluctuation.…”

“…Cladocerans in Alpine lakes have been subject to many anthropogenic stresses, the most severe of which are believed to be air-borne industrial pollutants (including acid deposition) and fish introductions (Cammarano & Manca, 1997;Manca & Armiraglio, 2002;Guilizzoni et al, 2006a). In the current study, we use the ''top-bottom'' approach of paleolimnology (Smol, 2002;Sweetman et al, 2008;Eggermont et al, 2010;DeSellas et al, 2011) to assess modern and pre-Industrial Age cladoceran communities in the Italian and Swiss Alpine lakes. This approach involves comparison of sediments deposited shortly before the Industrial Age (pre *1850 AD) with those deposited at the sediment surface and representing modern conditions.…”

Anomalous rise in algal production linked to lakewater calcium decline through food web interactions

“…, Daphnia relative abundance increased owing to factors such as increasing DOC and the spread of smallmouth bass [43], changes that have also occurred in Lake George.…”