Louise Cloutier

Total mercury and methylmercury fluxes via emerging insects in recently flooded hydroelectric reservoirs and a natural lake

Abstract Total mercury (total Hg) concentrations in emerging aquatic insects ranged from 140 to 1500 ng g −1 dry wt. in two hydroelectric reservoirs of northern Quebec compared with 50–160 ng g −1 dry wt. in a natural lake. Methylmercury (MeHg) concentrations were somewhat lower, ranging from 35 to 800 ng Hg g −1 dry wt. in reservoirs and from 29 to 90 ng g −1 dry wt. in the natural lake. Contamination of insect taxa of reservoirs was on average 2–3 times higher than their counterparts in the natural lake. There was no difference between total Hg and MeHg concentrations of insects sampled from flooded forest soils and flooded peatland, although total Hg and MeHg concentrations differed between flooded peatland and flooded forest soils themselves. Insect biomasses were approx. two times higher in the reservoirs than in the natural lake (580–2200 mg m 2 year −1 dry wt., 950 mg m 2 year −1 dry wt., respectively); chironomids dominated in the reservoirs and trichopterans dominated in the natural lake. Similarly, total MeHg fluxes via emerging insects were approx. 2–4 times higher in reservoirs than that of the natural lake (55–224 ng MeHg m 2 year −1 dry wt., 74 ng MeHg m 2 year −1 dry wt., respectively). Our results show the importance of the insect community in the transfer of MeHg from flooded soils and flooded peatlands to fish, and that this pathway probably makes a significant contribution to the rapid rise of Hg levels in the fish community after flooding.

Resource partitioning in a grazer guild feeding on a multilayer diatom mat

Abstract The gut contents of a guild of invertebrate grazers inhabiting the moss Fontinalis and feeding on epiphytic diatoms in a small Québec stream were analyzed to characterize resource partitioning and food selection. A multivariate approach (RLQ analysis coupled with a revised version of 4th-corner analysis) identified distinct diet patterns among co-occurring grazers. These patterns were mainly explained by differential ingestion of diatoms that differed in their spatial positions within the multilayered periphyton mat. When the size range of available diatoms was large, diet differences were partly explained by diatom size. Comparison of diatoms in grazer guts with diatoms available in the environment indicated selective feeding in different levels of the periphyton mat by grazers. Some grazers (scrapers) fed preferentially on tightly attached diatoms, whereas others (surfers) favored overstory diatoms. Spatial segregation of feeding within the periphyton mat by members of the grazer guild was more evident in a period of potential resource limitation (July) than when food was abundant (May). Our results suggest that all layers/growth forms in the diatom mat are used, resulting in spatial partitioning of the resource when considering the entire grazer community. Therefore, foraging theories already established for other ecosystems are confirmed in the unique context of stream benthos.

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (~74–76°N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68–95% to chironomid diet at a polar desert site and 70–78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0–32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not support chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study provides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.

The response of invertebrates in moss and in gravel to water level fluctuations in a Québec stream

To assess the response of stream communities to water level fluctuations, we measured the biomass and taxonomic composition of invertebrates colonizing substrata at varying depths, and thus more or less frequently exposed as flow declined. We compared the response of invertebrate communities in the moss Fontinalis dalecarlica and in gravel throughout the growing season in a small Quebec stream. Depth explained a large proportion of the variation in invertebrate biomass in moss for all dates (from 50-80%). Invertebrate biomass was lower on shallow mosses frequently exposed than on deeper ones, which remained submerged. In contrast, depth was never significantly related to invertebrate biomass in gravel. A significant fraction of the variation in invertebrate taxonomic composition was explained by substratum type (moss versus gravel) and exposure (estimated through depth) for all sampling dates (Canonical Correspondence Analysis). In particular, grazers were more common and abundant in moss than in gravel, suggesting a stronger reliance on periphyton on the part of moss-dwelling communities, and consequently greater vulnerability to periphyton reduction upon exposure. Invertebrates inhabiting gravel appear less affected by reductions in water level as they can burrow in the substratum at low flow. Gravel habitats tend to be lower on the streambed than moss, and consequently emerge less often. In view of forecasts of increased variability in stream water levels arising from climate changes, our study stresses the need to consider the differential vulnerability of communities on different substrata to hydrological disturbances.

An investigation of enhanced mercury bioaccumulation in fish from offshore feeding

We investigated the dietary pathways of mercury transfer in the food web of Morency Lake (Canada) to determine the influence of carbon source and habitat use on mercury bioaccumulation in fish. Whole-body concentrations of methylmercury (MeHg) were significantly different in four fish species (white sucker, brown bullhead, pumpkinseed and smallmouth bass) and increased with both trophic position and greater feeding on offshore (versus littoral) carbon. An examination of fish gut contents and the depth distribution of invertebrates in Morency Lake showed that smallmouth bass and brown bullhead were supplementing their littoral diet with the consumption of either opossum shrimp (Mysis diluviana) or profundal amphipods in offshore waters. The zooplanktivore Mysis had significantly higher MeHg concentrations than zooplankton and benthic invertebrates, and it was an elevated source of MeHg to smallmouth bass. In contrast, profundal amphipods consumed by brown bullhead did not have higher MeHg concentrations than littoral amphipods. Instead, partitioning of benthic invertebrate resources likely explains the greater MeHg bioaccumulation in brown bullhead, associated with offshore feeding of amphipods. White sucker and brown bullhead had a similar trophic position but white sucker consumed more chironomids, which had one-third the MeHg concentration of amphipods. Our findings suggest that offshore feeding in a lake can affect fish MeHg bioaccumulation via two different processes: (1) the consumption of MeHg-enriched pelagic prey, or (2) resource partitioning of benthic primary consumers with different MeHg concentrations. These observations on the mechanisms of habitat-specific bioaccumulation highlight the complexity of MeHg transfer through lake food webs.

Ecological determinants of methylmercury bioaccumulation in benthic invertebrates of polar desert lakes

We investigated concentrations of monomethylmercury (MMHg) at the base of benthic food webs in six lakes from polar desert (biologically poor and low annual precipitation) on Cornwallis Island (Nunavut, Canada, ~75°N latitude). Anthropogenic mercury emissions reach the Arctic by long-range atmospheric transport, and information is lacking on processes controlling MMHg entry into these simple lake food webs, despite their importance in determining transfer to lake-dwelling Arctic char. We examined the influences of diet (using carbon and nitrogen stable isotopes), water depth, and taxonomic composition on MMHg bioaccumulation in benthic invertebrates (Chironomidae and Trichoptera). We also estimated MMHg biomagnification between benthic algae and invertebrates. Similar MMHg concentrations of chironomid larvae in nearshore and offshore zones suggest that benthic MMHg exposure was homogeneous within the lakes. Chironomid δ13C values were also similar in both depth zones, suggesting that diet items with highly negative δ13C, specifically methanogenic bacteria and planktonic organic matter, were not important food (and therefore mercury) sources for profundal larvae. MMHg concentrations were significantly different among two subfamilies of chironomids (Diamesinae, Chironominae) and Trichoptera. Higher MMHg concentrations in Diamesinae were likely related to predation on other chironomids. We found high MMHg biomagnification between benthic algae and chironomid larvae compared with literature estimates for aquatic ecosystems at lower latitudes; thus, benthic processes may affect the sensitivity of polar desert lakes to mercury. Information on benthic MMHg exposure is important for evaluating and tracking impacts of atmospheric mercury deposition and environmental change in this remote High Arctic environment.