Primary production and light dynamics in an upper Mississippi River backwater

Abstract

Abstract In nutrient‐rich systems, phytoplankton production is frequently light‐limited and light attenuation can become a critical factor controlling the rates of production, especially in shallow systems subject to wind resuspension of sediments. This study examined the relation between light availability and primary production in Lake Onalaska, a shallow, nutrient‐rich impoundment of the Upper Mississippi River. Continuous water quality monitors recorded dissolved oxygen, temperature and light flux at a network of sites from July to September 1990 and these data were used to estimate the primary production, light availability and light attenuation coefficients. The gross primary production averaged 2·2gCm −2 day −1 and was due almost entirely to phytoplankton. Production was strongly light‐limited, with most of the light attenuation due to non‐algal components, possibly related to wind resuspension in this shallow system. Regression analyses revealed significant effects of wind speed and direction on light attenuation.