RELATIONSHIP BETWEEN SEDIMENT AND PLANKTON ISOTOPE RATIOS (DELTA-C-13AND DELTA-N-15) AND PRIMARY PRODUCTIVITY IN FLORIDA LAKES

Surface sediment and planktonic organic matter from Florida lakes were analyzed for delta(13)C and delta(15)N to study the relationship betw een stable isotope ratios and lake productivity inferred from measurem ents of Chi a. Stepwise regression indicates that sediment delta(13)C is a function of primary productivity, which is also suggested by a st rong correlation between water column Chi a and planktonic delta(13)C (r = 0.82, P = 0.002). However, Chi a explains only 38% of the variabi lity of sediment delta(13)C. This weak correlation is mainly attribute d to the presence of nonplanktonic organic matter bearing a distinct i sotopic signature in the lake sediments. Sediment and planktonic delta (13)C are also significantly correlated with water-column DIC, CO2(aq) , delta(13)C of DIC, TN, and TP. Sediment and planktonic delta(15)N in crease from oligotrophic to eutrophic lakes but decrease in hypereutro phic lakes. The nonlinearity of delta(15)N across the trophic gradient of study lakes may be a consequence of the relative importance of cya nobacterial N-2 fixation, but this hypothesis needs further testing.