PRIMARY PRODUCTION AND LIGHT DYNAMICS IN AN UPPER MISSISSIPPI RIVER BACKWATER

In nutrient-rich systems, phytoplankton production is frequently light -limited and light attenuation can become a critical factor controllin g the rates of production, especially in shallow systems subject to wi nd 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 lig ht flux at a network of sites from July to September 1990 and these da ta were used to estimate the primary production, light availability an d light attenuation coefficients. The gross primary production average d 2.2 g C m(-2) day(-1) and was due almost entirely to phytoplankton. Production was strongly light-limited, with most of the light attenuat ion due to non-algal components, possibly related to wind resuspension in this shallow system. Regression analyses revealed significant effe cts of wind speed and direction on light attenuation.