Radiotracer study of phosphorus uptake by plankton and redistribution in the water column of a small humic lake

The movement of P in the plankton of a humic lake was studied in late July within a 2-m-diameter tube. The tube enclosed water from the surface to below the epilimnion with the steep vertical stratification of the lake undisturbed. [“2P]orthophosphate was mixed into the epilimnion of the enclosure and its fate followed for 2 weeks. In the epilimnion-85 % of all the P in organisms was in Daphnia longispina, which comprised almost all the zooplankton biomass. The respective proportions for bacterioplankton and phytoplankton were- 12 and 3%. Early in the experiment when the temperature of the epilimnion was-2o”C, the turnover rate of phosphate was of the order of 3 h. By the first sampling, 3 h after the experiment began, bacteria showed the highest affinity for phosphate, but with this coarse time resolution, the pattern of 32P incorporation into phytoplankton appeared similar. The specific radioactivity in D. longispina equaled that in the bacterial and algal fractions after only 2 d, implying rapid and direct food-chain linkage bctwcen these P pools. An explanation for such rapid transfer of P may be that D. longispina consumes food with a high concentration of P, such as bacteria. At the end of the experiment, the specific radioactivity of the dissolved P pool was considerably lower than that of the other fractions, indicating only slow exchange between part of the dissolved P pool and the plankton. In temperate-zone lakes, primary produc