Environmental drivers of cladoceran assemblages at a continental scale: A synthesis of Alaskan and Canadian datasets

Abstract

Cladocera serve as important bio‐ and paleo‐indicators of lake food webs and environmental conditions. The ecological optima of cladocerans are often established by regional‐scale calibration sets, with subsequent comparisons to limnological variables. However, due to logistical constraints when sampling large numbers of lakes, this approach often limits the length of the environmental gradients that can be examined. To extend spatial and limnological gradients, we combined 20 datasets (388 lakes) containing both cladoceran and environmental data, spanning multiple ecoregions across Canada and Alaska. These data were collected over c. 20 years using similar techniques in a single laboratory. Our continental‐scale analysis examined the main chemical and physical variables that influenced cladoceran assemblages, and identified critical environmental thresholds structuring assemblages. Multivariate analyses were used to examine the influences of six environmental variables (depth [either maximum depth or coring depth, which are very similar], surface area, pH, calcium [Ca], total phosphorus [TP], and dissolved organic carbon) and ecoregion classifications on cladoceran assemblages. Gradients of pH and Ca were strongly related. Daphnia longispina spp. and Chydorus brevilabris/biovatus were associated with higher pH and Ca concentrations, while Bosmina spp. and Holopedium spp. were more common in lakes with lower pH and Ca. Dissolved organic carbon was highly correlated with TP. Chydorus brevilabris/biovatus was associated with higher‐nutrient systems. Littoral chydorids, D. longispina spp., Holopedium spp., and Daphnia pulex spp. were associated with intermediate TP and dissolved organic carbon concentrations, and Bosmina spp. was more closely associated with oligotrophic systems. Physical limnological variables influenced taxonomic composition on a continental scale. For example, D. longispina spp., D. pulex spp., Bosmina spp., and Holopedium spp. were associated with larger and/or deeper systems, and littoral chydorids were associated with smaller and/or shallower lakes. A multivariate regression tree identified two thresholds important for structuring assemblages based on pH (<7 and ≥7, probably closely tied to lakewater Ca concentrations) and depth (<6.45 m and ≥6.45 m at pH < 7, and <4.75 m and ≥4.75 m at pH ≥ 7). Our results highlight the sensitivity of several cladoceran taxa to multiple chemical and physical gradients. We demonstrate that the ecological responses of cladocerans to environmental variables—often established through regional‐scale surveys—are applicable across ecoregions and broad limnological gradients, reinforcing the value of cladocerans as bioindicators of key chemical and physical variables in freshwaters and as paleoindicators for assessing human impacts on aquatic systems through time.

Congruence of water column, contemporary and pre‐industrial sediment cladoceran assemblages in 85 Canadian lakes of contrasting human impact levels

“…Lake depth and pH were identified as major environmental drivers in our study, which echoes observations reported by Labaj et al. (2021) for cladoceran assemblages in surface sediments from 388 lakes across Canada and Alaska. Our RDA analyses identified lake depth as a key factor influencing cladoceran composition in all three sample types in the all‐species data set.…”

“…Across all sample types (water column, contemporary (top) sediments and pre‐industrial (bottom) sediments), we identified 51 cladoceran taxa in the 85 study lakes (see Table S2 for the full list of taxa; Figure 3 for summary). Similarly, 51 cladoceran taxa were identified in a synthesis study across Alaskan and Canadian lakes (Labaj et al., 2021), while total richness was of 53 taxa in the larger 101 LakePulse lake dataset where subfossil cladocerans were enumerated (Paquette, Griffiths, et al., 2022). Across our 85 lakes, 16 taxa were common to all three sample types, 33 were common to both sediment samples, one taxon was uniquely shared between the water column and the top sediment sample, and one taxon was found only in the bottom sediment sample.…”

“…However, the importance of lake morphometry was seemingly intermingled with the predominant effect of human catchment impacts in modern times (see next section). Indeed, factors such as calcium (Griffiths et al., 2019), pH (Labaj et al., 2021), or sulfate (DeSellas et al., 2008), which are all baseline indicators of lake geology, have consistently been demonstrated to be more important than morphometric variables in structuring cladoceran assemblages in contemporary sediment samples and when such factors are extended to extreme levels, they are likely to overwhelm the structuring effect of lake morphometry in highly impacted catchments. The effect of lake morphometry on cladocerans could also emerge from a regional pattern.…”

“…Thus Holopedium and Eubosmina could be favoured over daphnids, which have higher calcium requirements (Jeziorski et al., 2012; Jeziorski & Yan, 2006; Labaj et al., 2021). E. longispina was also associated with low pH in the top sediment samples, while C. brevilabris was associated with elevated pH and total nitrogen values, which is consistent with other findings that those taxa are tolerant to harsh water quality conditions (Bos & Cumming, 2003; Jeziorski et al., 2013; Labaj et al., 2021). Our findings support previous studies demonstrating that cladoceran zooplankton are good bioindicators of lake environmental variables (e.g., Bos & Cumming, 2003; Davidson et al., 2007).…”

Similar zooplankton responses to lowpHand calcium may impair long‐term recovery from acidification

“…The species‐specific effects of calcium and pH that we detected in our experiment were generally in agreement with long‐term changes across the Killarney lakescape (Table 3), suggesting that declining calcium concentrations are likely to affect zooplankton communities in acid‐recovering lakes. While the co‐gradient of pH and calcium is the primary environmental driver of cladoceran assemblages in northern regions of continental North America (Labaj et al, 2021), it is likely that regional variation in abiotic conditions and food web structure will also modulate zooplankton responses. For example, in historically acidified Adirondack lakes (New York, USA), the effect of declining calcium on zooplankton was modified by changes in lake “browning”/dissolved organic carbon concentrations (Leach et al, 2019).…”