Phosphorus kinetics of planktonic and benthic assemblages in a shallow subtropical lake

1. Phosphate uptake kinetics and uptake rates were calculated for planktoni c (phytoplankton and bacterioplankton) and benthic (epiphyton and epipelon) assemblages in a large, shallow, subtropical lake. Samples were taken bimo nthly over the period of 1 year at three different sites to examine spatial and temporal variability in these processes. 2. Two of the sites, located at the edge of the Littoral zone next to the o pen water (ecotone sites), had low irradiance at the sediment surface and h igh total phosphorus (TP) concentration (annual mean TP = 112 mu g L-1). Th e third site,located in the littoral marsh zone, had high irradiance at the sediment surface and Low TP concentration (annual mean TP = 7 mu g L-1). 3. Based on P-32-PO4 turnover time, P availability varied temporally and sp atially. At the two high TP ecotone sites, P concentration was lowest in Ju ly and August. At the low TP marsh site, P Limited algal production through out the year. 4. The quotient of maximum uptake rate to half saturation constant (V-m/K-s ) in the plankton increased by over two orders of magnitude during the P-li mited (summer) period at the two ecotone sites, suggesting that plankton us ed the scarce phosphorus more efficiently. The specific uptake rate of plan kton was significantly greater than that of periphyton at all sites, sugges ting that the plankton were more efficient than periphyton at taking up pho sphate. 5. Periphyton biomass, as well as absolute and percentage P uptake rate, wa s greater at the marsh site than at the ecotone sites, despite the lower P concentrations in the marsh. This was probably a result of rapid nutrient c ycling, combined with high Light availability in the marsh.