TRANSFORMATIONS BETWEEN ORGANIC AND INORGANIC SEDIMENT PHOSPHORUS IN LAKE BALATON

In order to estimate microbial P content and biological P uptake in se diments, the tungstate precipitation method of Orrett & Karl (1987) wa s used in sediment extracts. This method allows a simple and rapid sep aration of organic and inorganic P-32 radioactivity. Either inorganic P-32 (as carrierfree H-3 (PO4)-P-32) or organic P-32 (as P-32-labelled algal material) was added to surface sediment suspensions of shallow Lake Balaton. InorganiC P-32 was rapidly transformed into organic P-32 , and this process was completely inhibited by formaline. P content of living benthic microorganisms was estimated from steady state distrib ution of the radioactivity. Transformation of algal organic P into ino rganic P could also be detected. In extremely P limited Lake Balaton b enthic microorganisms were shown to supplement their high P requiremen ts by inorganic P uptake. The velocity of the inorganic into organic P transformation, i.e. the rate of microbial P uptake, was comparable t o P uptake in the water column. Microbial P uptake contributed signifi cantly to total P fixation by sediments, particularly at low (less-tha n-or-equal-to 100 mug P l-1) phosphate additions.