A MODEL FOR THE LIGHT-LIMITED GROWTH OF BUOYANT PHYTOPLANKTON IN A SHALLOW, TURBID WATERBODY

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 functio n of flotation speed, using field observations of photosynthetically a ctive radiation, wind speed, surface-layer thickness (from water-colum n temperature data), and light attenuation made at Rushy Billabong on the River Murray from 28 November 1991 to 26 March 1992. A unique feat ure of the model is the simulation of the development and dispersal of surface scums as a function of wind speed. Under nutrient-replete con ditions, the model predicted that phytoplankton with a flotation speed of 1-10 m day(-1) (typical of Anabaena flos-aquae and Microcystis aer uginosa) would grow up to four times faster than would neutrally buoya nt phytoplankton with the same maximum specific growth rate. In the sh allow system modelled, high flotation speeds allowed a large proportio n of the total population to rise into the euphotic zone shortly after the onset of stratification each day. Surface scums played an importa nt role in maintaining the more buoyant phytoplankton populations clos e to the water surface. Under the Very turbid conditions in the billab ong (100 nephelometric turbidity units), self-shading became significa nt only when the mean chlorophyll concentration in the water column ap proached 100 mg chla m(-3)