Some Effects of Artificial Fertilization on Enclosed Plankton Populations in Marion Lake, British Columbia

Abstract

After single applications of nutrients to enclosures in Marion Lake in each of three seasons (fall, summer, and spring), primary productivity always increased within a 2-week period whether nitrate, phosphate, or a combination of the two was used. The species that responded to the three nutrients generally differed both within and between seasons. In all seasons, there was a decrease in total plankton diversity (based on productivity) accompanied by an increase in relative productivity of a few phytoplankton species. During fall, phosphate (P) was most stimulatory, raising primary productivity 6 mg C/m3 per hr above that of the unenriched control. During spring, N + P was most stimulatory (23 mg C/m3 per hr) and nitrate (N) next (11 mg C/m3 per hr). In summer, N + P was again most stimulatory (44 mg C/m3 per hr), primary production in the untreated lake rarely exceeding 4 mg C/m3 per hr. Of the species that increased in density, 78% were not detectable (< 1 cell/ml) at the time of fertilization. A rapid increase in density of rare species culminated in an algal bloom 2–3 weeks after fertilization.After the first phase of succession following fertilization, there was an increase in plankton diversity. At 1.5 months after fertilization, the diversity and the number of species detected were as high as or higher than before fertilization. In addition, the number of phytoplankton species per liter did not decrease significantly after enrichment though diversity decreased sharply due to the increase in abundance of a few species.