Changes in abundance, composition and controls within the plankton of a fertilised arctic lake

1. An oligotrophic arctic lake was fertilised with inorganic nitrogen and phosphorus as (NH₄)₂ NO₃ and H₃PO₄ for five summers. The loading rate was 1.7–2.5 mmol N m^–2 day^–1 and 0.136–0.20 mmol P m^–2 day^–1 which is two to three times the annual loading of lakes in the area. The heterotrophic microzooplankton community was enumerated during the experiment as well as 1 year pre- and post-treatment.
2. The structure of the microplankton community changed from a nutrient limited system, dominated by oligotrich protozoans and small-particle feeding rotifers, to a system dominated by a succession of peritrich protozoans and predatory rotifers. These peritrich protozoans and predatory rotifers were not present prior to fertilisation and never constituted more than a small fraction of the biomass in other lakes at the research site. The average biomass of the rotifers and protozoans was more than seven and a half times larger by the end of fertilisation than it was initially.
3. Because of the increases in numbers of individuals in these new taxa, the structure of the microbial food web changed. When fertilisation stopped, most parameters returned to prefertilisation levels within 1 year.