SEASONAL SUCCESSION OF PHYTOPLANKTON IN A LARGE SUB-ARCTIC RIVER

The factors influencing the abundance of phytoplankton in the Yellowknife River, in the Canadian subarctic, were determined from collections made for 42 consecutive months from June 1975 to November 1978. The spring bloom of plankton occured during April of each year in response to changing light conditions. While Chlamydomonas lapponica was dominant during this period, it was replaced during the early part of the summer by a rapid succession of Dinobryon species in which D. cylindricum was followed by D. sociale and in turn by D. bavaricum and D. divergens. Although low nutrient levels permitted the development of Dinobryon during the summer, the abundance of diatoms was greatly limited by the concentrations of SiO2 (< 0.1 g/m3). Algal densities began to decline in August and reached low overwintering levels by November. The absence of a fall bloom in densities was due to a combination of low temperatures and nutrient levels. © 1979 Dr. W. Junk b.v. Publishers.