Mechanisms that regulate growth rates of phytoplankton in Shagawa Lake, Minnesota1

Abstract

Mean concentrations of chlorophyll a at six localities in Shagawa Lake ranged from 15 to 25 mg m−3 during June and July, increased to a maximum of 150 mg·m−3 during a bloom of Aphanizomenon that occurred in August, and decreased rapidly in early September. Theoretical limits for integral photosynthesis and the total annual integral photosynthesis wore calculated from estimates of attenuation coefficients for illumination, specific photosynthetic rates, and chlorophyll concentrations.Mechanisms responsible for the phytoplankton bloom are inferred from the changes of specific rates of production and specific rates of loss of particulate organic carbon in the mixed layer. Both rates decreased as the bloom developed, the latter decreasing faster than the former. The bloom ended when the specific loss rate suddenly increased. The specific production rate may have decreased during the bloom as nutrient utilization was inhibited by self‐shading; buoyancy changes may have been primarily responsible for the changes of specific loss rates, acting as a critical mechanism of population regulation.The effects on phytoplankton populations of changing specific production and loss rates are demonstrated by comparing the integral photosynthetic rates and chlorophyll concentrations that would prevail under steady state conditions.