Zooplankton communities in Precambrian Shield lakes (Québec, Canada): responses to spatial and temporal gradients in water chemistry and climate

Zooplankton are relevant indicators of changes in lake water quality, used for monitoring the response of aquatic ecosystems to the combined effects of declining acidic deposition and rising air temperatures. First, the current landscape was defined from the recent (2017) spatial patterns of zooplankton communities in 73 Québec lakes distributed over an 800-km SW-NE gradient, spanning a wide range of water quality, climate and morphometric characteristics. At large-scale, we identified among-lake clustering of three types of zooplankton assemblages and variation in species composition at fine scale among lake pairs. Distance among zooplankton clusters calculated using lake pairs were best correlated (r > 0.400, p < 0.001) with air temperature, pH and calcium, reflecting spatial gradients in climate and lake acid-base status. Second, to examine long-term response in the zooplankton community, we compared acidification indicators and abundance of taxa for a subset of 19 lakes sampled in 1982 and 2017. Despite an average 3-fold drop in sulfate concentration, changes in calcium and pH were relatively small, and consequently, no major changes in zooplankton assemblages were detected since 1982.

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 the 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.

Long-term trends in hypolimnetic volumes and dissolved oxygen concentrations in Boreal Shield lakes of south-central Ontario, Canada

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.

Genomic potential for photoferrotrophy in a seasonally anoxic Boreal Shield lake

Photoferrotrophy, the light-induced oxidation of ferrous iron, is thought to have contributed to primary production within Earth’s early anoxic oceans yet is presumed to be of little modern environmental relevance. Here we use genome-resolved metagenomics and enrichment cultivation to explore the potential for photoferrotrophy in the anoxic water columns of globally abundant Boreal Shield lakes. We recovered four high-completeness and low-contamination draft genome bins assigned to the class Chlorobia (formerly phylum Chlorobi) from environmental metagenome data and enriched two novel sulfide-oxidizing species, also from the Chlorobia. The sequenced genomes of both enriched species, including the novel “Candidatus Chlorobium canadense”, encoded the cyc2 candidate gene marker for iron oxidation, suggesting the potential for photoferrotrophic growth. Surprisingly, one of the environmental genome bins encoded cyc2 and lacked sulfur oxidation gene pathways altogether. Despite the presence of cyc2 in the corresponding draft genome, we were unable to induce photoferrotrophy in “Ca. Chlorobium canadense”, suggesting that yet-unexplored mechanisms regulate expression of sulfide and ferrous iron oxidation gene systems, or that previously unrecognized functions for this outer membrane cytochrome exist. Doubling the known diversity of Chlorobia-associated cyc2 genes, metagenome data showed that putative photoferrotrophic populations occurred in one lake but that only sulfide-oxidizing populations were present in a neighboring lake, implying that strong ecological or geochemical controls govern the favourability of photoferrotrophy in aquatic environments. These results indicate that anoxygenic photoautotrophs in Boreal Shield lakes could have unexplored metabolic diversity that is controlled by ecological and biogeochemical drivers pertinent to understanding Earth’s early microbial communities.

Models to predict total phosphorus concentrations in coastal embayments of eastern Georgian Bay, Lake Huron

Several coastal embayments of eastern Georgian Bay show signs of water-quality impairment thought to be caused by human activities. Here, we evaluate the ability of the Lakeshore Capacity Model (LCM), developed for Precambrian Shield lakes, to assess the impact of cottage development on the trophic status of ten Georgian Bay embayments. The LCM could only be applied to eight embayments due to the large size and complexity of two watersheds and produced unacceptably high estimates of mean seasonal total phosphorus concentrations ([TP]; i.e., exceeded 20% of measured values for five of eight embayments); accuracy of [TP] estimates could not be improved by accounting for internal phosphorus loading. We developed an additional model, the Anthro-Geomorphic Model (AGM), which uses building density and basin morphometry as variables. Estimates of [TP] for the AGM were within 20% of measured values for all sites. Compared with other aquatic systems, coastal embayments of Georgian Bay have significantly higher chlorophyll a concentrations per unit [TP]; we suggest that the TP–chlorophyll relationship presented in this study be used to estimate productivity in these systems.

2. Effect of climate change on the predation rate of zooplankton in Swan Lake, Sudbury, ON

Climate change interacts with other environmental stressors (e.g., acid deposition, calcium depletion, invasive species) to alter both the chemical and biological characteristics of Boreal Shield lakes, potentially leading to changes in aquatic biodiversity. Changes in biodiversity can result in loss of sensitive species and affect dynamic interactions among species at varying trophic levels. Currently, little is known about the effect of climate warming on predator-prey relationships in aquatic ecosystems. I examine how predicted warming of Boreal Shield lakes may affect predation rate. More specifically, my research examines how temperature affects the predation rate on zooplankton by common macroinvertebrate predators. Zooplankton, Chaoborus and Notonectidae were used from Swan Lake in Sudbury, ON. I performed 24-hr laboratory feeding trials to examine the rate at which predators feed over a range of natural and predicted lake temperatures. By investigating differences in invertebrate predation occurring in Swan Lake, we will be able to predict predator -prey relationships in Boreal Shield lakes subject to warming as a result of climate change.