Orthophosphate uptake of planktonic microorganisms in Lake Balaton

Abstract

Seasonal orthophosphate uptake of natural plankton has been studied in hypertrophic and mesotrophic basins of Lake Balaton. Initial uptake of carrier-free labelled phosphoric acid followed first order kinetics in accordance with a single two-compartment model. In isotope equilibrium from 1.2 to 9.7 per cent of radioactivity was found in the filtrate. Turnover times of orthophosphate in water were some minutes in summer and two orders of magnitude longer in winter and April. Turnover time of phosphorus in microorganisms has been estimated on the basis of equilibrium isotope distribution between water and particular phases and turnover time in water. It ranged from 28 to over 1 000 min. The shortest times were found in summer, and the longest in winter. Turnover time of orthophosphate in water increased in relation to phosphate additions according to Michaelis-Menten kinetics. The values of Michaelis-constant plus the ambient orthophosphate concentration (K + S_n) suggested an overestimation of real phosphate concentrations by chemical measurements, being much lower than soluble reactive phosphorus contents in most cases, as it is well-known in many other lakes. Both K + S_n values and maximum uptake velocities (V) were significantly higher in the hypertrophic basin than in the mesotrophic one, in contrast to essentially similar values of turnover times. Maximum uptake velocity per unit chlorophyll-a P _opt:V ratio showed extreme to moderate phosphorus deficiency for Lake Balaton plankton, which seemed to decrease somewhat in the course of eutrophication.