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There is hope that increased development of aquaculture will help meet future global needs for protein. The growth of the freshwater aquaculture industry in Canada, however, has been hampered by insufficient information on environmental impacts. We examined the effects of an experimental cage aquaculture operation for rainbow trout ( Oncorhynchus mykiss ) on planktonic Crustacea and rotifers using 6 years of precage and 7 years of postcage data. Following the initiation of aquaculture, total crustacean and rotifer biomass, egg production, and depth distributions did not change significantly when compared with data from three nearby unimpacted lakes. We found statistically detectable increases in densities of Bosmina cf. longirostris and Diacyclops thomasi and an overall change in crustacean zooplankton community structure using correspondence analysis. The response of zooplankton was less than we expected because comparatively large changes were observed in phytoplankton biomass, hypolimnetic O2, and densities of invertebrate ( Mysis diluviana ) and fish predators. Our study emphasizes the need to monitor multiple variables when assessing the ecosystem impacts of potential stressors such as aquaculture.

The spread of nonindigenous species (NIS) over land and via interconnecting water bodies is threatening aquatic ecosystems worldwide. This study examines the invasion of the first known NIS zooplankter, Eubosmina coregoni , into Lake Winnipeg, Manitoba, Canada. Analyses of cladoceran microfossils from a sediment core collected in the North Basin of the lake indicate this species first appeared in sediments dated to the late 1980s. An increase in total cladoceran accumulation rates coupled with increasing N, C, P, and chlorophyll a over the last 40 years provides evidence of eutrophication. Extant samples from fall 2002-2005 indicate that E. coregoni is mainly restricted to the North Basin while Bosmina longirostris is present throughout the lake. Results from this study provide baseline data regarding the invasion and establishment of E. coregoni , a precursor to future NIS that may have substantial ecological and economic impacts on the Lake Winnipeg ecosystem.

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m−2 d−1 before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.