Response of amphipoda and trichoptera to lake fertilization in the Canadian Arctic

Small oligotrophic lakes at Saqvaqjuac, Northwest Territories, were fertilized with phosphorus and nitrogen after 2 yr of study and the response of macroinvertebrates to increased primary production was followed for 3 yr. The amphipod Gammarus lacustris lacustris (G. O. Sars) had a 2-yr life cycle, with three cohorts present in August. Biomass under natural conditions was approximately 0.1–0.2 g dry wt∙m−2. Gammarus responded immediately to a doubling of phytoplankton production with increased survival of young-of-year. Gammarus biomass increased steadily to 0.9 g dry wt∙m−2 and had not stabilized after 3 yr of fertilization. Trichoptera were represented by three species, with Grensia praeterita composing the bulk of the biomass, followed by Apatania zonella and an uncommon Hesperophylax species. Trichoptera biomass ranged from 0.04 to 0.3 g dry wt∙m−2 before fertilization. Response to increased primary production was slow, beginning in year 2 of fertilization. Trichoptera biomass had doubled by the third...