Relationships between phytoplankton dynamics and the availability of light and nutrients in a shallow sub-tropical lake

Abstract

The role of light and nutrient availability in controlling the abundance and structure of phytoplankton populations was studied in Lake Okeechobee, a large eutrophic lake in south florida, USA. Measurement of selected environmental parameters at sampling sites within four ecologically distinct regions of the lake were combined with direct experimental determinations of limiting levels of light and nutrients for phytoplankton growth to determine spatial and temporal variations in the relative roles of these limiting factors. Estimated mean light availability in the mixed layer, I_m, was significantly lower in the turbid central region of the lake than in other regions. Correlations between I_m and phytoplankton standing crops led to the conclusion that low light availability in the central region of the lake, and to a lesser extent in other areas, restricts phytoplankton standing crops to levels below the potential provided by the nutrients available. The results of the irradiance-growth experiments confirmed the conclusions of the correlation analyses that phytoplankton growth is restricted by the levels of light availability experienced during the winter and spring in the central region of the lake. Bioassays indicated that nitrogen was the most frequently limiting nutrient for phytoplankton growth. High rates of nitrogen fixation were frequently observed in the lake, along with correspondingly high abundances of nitrogen-fixing cyanobacteria and nitrogen fixation activity. Elevated concentrations of soluble inorganic nitrogen appeared to suppress both nitrogen fixation and the relative abundance of nitrogen-fixing cyanobacteria.