A model for the light-limited growth of buoyant phytoplankton in a shallow, turbid waterbody

Abstract

A computer model was used to explore the relationship between buoyancy and the light-limited growth of phytoplankton in very turbid waters. The model simulates the potential growth of phytoplankton as a function of flotation speed, using field observations of photosynthetically active radiation, wind speed, surface-layer thickness (from water-column temperature data), and light attenuation made at Rushy Billabong on the River Murray from 28 November 1991 to 26 March 1992. A unique feature of the model is the simulation of the development and dispersal of surface scums as a function of wind speed. Under nutrient-replete conditions, the model predicted that phytoplankton with a flotation speed of 1-10 m day^-1 (typical of Anabaena flos-aquae and Microcystis aeruginosa) would grow up to four times faster than would neutrally buoyant phytoplankton with the same maximum specific growth rate. In the shallow system modelled, high flotation speeds allowed a large proportion of the total population to rise into the euphotic zone shortly after the onset of stratification each day. Surface scums played an important role in maintaining the more buoyant phytoplankton populations close to the water surface. Under the very turbid conditions in the billabong (100 nephelometric turbidity units), self-shading became significant only when the mean chlorophyll concentration in the water column approached 100 mg chla m^-3.