The impact of calcium-rich diamond mining effluent on downstream cladoceran communities in softwater lakes of the Northwest Territories, Canada

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 <1 to >30 mg·L−1 and corresponding increases in pH from <7 to >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.

Dispersal limitation and climate-related environmental gradients structure microcrustacean composition in freshwater lakes, Ellesmere Island, Canada

Dramatic environmental change is expected in the Arctic, yet little is known about the occurrence and community composition of microcrustaceans in Arctic lakes and how this will be influenced by future environmental change. We sampled and calculated relative abundances of microcrustacean species in 54 lakes on Ellesmere Island, Canada. New species records on Ellesmere Island included Daphnia umbra , Tachidius discipes , and Artemeopsis stefanssoni . Daphnia middendorffiana/tenebrosa was the most common taxon and often dominated microcrustacean assemblages, likely a result of its pigmentation, which offers resistance to ultraviolet radiation. Species richness was positively associated with nutrients, dissolved organic carbon (DOC), temperature, calcium, and conductivity and negatively affected by elevation. In contrast to most findings in temperate systems, we detected a negative relationship between species richness and surface area. Community composition was influenced by DOC, nutrients, and elevation but was also related to spatial variables, suggesting that spatial gradients in environmental conditions and dispersal are important drivers of differences among sites. Arctic ecosystems are expected to change rapidly in the coming years because of climate change and ozone thinning, and we expect that associated changes in DOC, temperature, and nutrients will affect microcrustacean species richness and distribution throughout the landscape.

The functional response of drift-feeding Arctic grayling: the effects of prey density, water velocity, and location efficiency

An important aspect of a predator–prey system is the functional response of the predator to changing prey densities. We studied the feeding rate response of drift-feeding Arctic grayling (Thymallus arcticus) on a small invertebrate prey, Daphnia middendorffiana, at densities ranging from 0.01 L–1 to 1.8 L–1 and current velocities of 25, 32, and 40 cm·s–1. We videotaped the feeding of grayling to determine the duration of the search and pursuit components of the feeding cycle and the location efficiency of grayling feeding at different current velocities. Feeding rate increased approximately as the prey density to the 0.4 power from 0.01 to 1.25 prey·L–1, above which the feeding rate dropped. Current velocity had no significant effect on feeding rate. Search and pursuit times dropped with increasing prey density, but neither was affected by current velocity. However, current velocity reduced both maximum location distance and location efficiency. The lack of increase in feeding rate with increasing current velocity may be due to a trade-off between the increasing likelihood of encounter and decreasing location efficiency as current velocity increases. These data suggest that grayling could effectively feed in a variety of stream habitats with different current velocity.

Effects of Gammarus lacustris (Crustacea: Amphipoda) on plankton community structure in an alpine lake

The amphipod Gammarus lacustris is generally considered as a herbivore or a detritivorous scavenger, but we and others have observed it preying on pelagic zooplankton in fishless alpine lakes of the Canadian Rockies. We tested the hypothesis that G. lacustris predation alters the plankton community structure of alpine lakes by manipulating G. lacustris density in large (2800 L) in situ enclosure experiments. Compared with control enclosures (no Gammarus), Daphnia middendorffiana abundance was reduced 49, 85, and 100% in the low, medium, and high (40, 200, and 400 individuals·m-2) Gammarus density enclosures, respectively. Hesperodiaptomus arcticus density was not significantly related to G. lacustris density. Comparison of mesocosm results with lake data showed that G. lacustris predation on D. middendorffiana in the lake was lower, possibly because a pelagic deepwater refugium allows Daphnia to escape predation. The results show that G. lacustris predation can affect the plankton community structure in small fishless alpine lakes.