SEASONAL AND SPATIAL VARIATION IN NUTRIENT LIMITATION IN A SHALLOW SUBTROPICAL LAKE (LAKE OKEECHOBEE, FLORIDA) AS EVIDENCED BY TROPHIC STATE INDEX DEVIATIONS

Historical data (1980-1992) from Lake Okeechobee, Florida were used to construct trophic state indices (TSIs). Deviations among chlorophyll-a, phosphorus, nitrogen and transparency-based TSIs were used to quantify seasonal variation in nutrient limitation and seston composition at four sites in the lake. The results indicated that nitrogen was the nutrient most often limiting to algal production. However, at stations in the South and West, phosphorus limitation was sometimes found. The TSI deviations also indicated a seasonal cycle of nitrogen limitation and seston composition at all sampling sites. During winter, non-algal particles dominated light attenuation, and nutrients tended not to be limiting. During mid-summer, there was a greater contribution of algae to light attenuation, and nutrient limitation was most pronounced. Nutrient limitation was more frequent at the South and West stations than at the North and East stations. At the latter, light limitation was nearly always indicated, and non-algal particles always dominated light attenuation. The results are in close agreement with those recently obtained from nutrient bioassays in Lake Okeechobee, and they confirm the great seasonal and spatial heterogeneity of algal production limitation in this large subtropical lake.