Effects of forest fertilization on phytoplankton in a boreal brown-water lake

The quantity and composition of phytoplankton in a brown-water lake was studied from 1988 to 1994 after forest fertilization in its catchment area. The following spring after fertilization, phosphorus concentrations were high (in average 18 ug l–1), and an evident vernal maximum of phytoplankton (chrysophytes and diatoms) was observed, but no further signs of trophicity changes could be observed. The extra nutrients must have been quickly utilized by bacteria or transported out of the lake through the out-flowing brook during the spring high-flow. Mixotrophic algae, like e.g. Cryptomonas spp. and Gonyostomum semen, were the ones to mostly benefit from the increased amounts of organic compounds and nutrient concentrations. The typical brown-water lake phytoplankton composition, in general did not change during the study period.