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

Abstract

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. [ 32 P]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 ∼20°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 32 P 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 foodchain linkage between 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.