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. $[^32 P]orthophosphate$ was mixed into the epilimnion of the enclosure and its fate followed for 2 weeks. In the epilimnion $\sim 85%$ 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 $\sim 12$ and 3%. Early in the experiment when the temperature of the epilimnion was $\sim 20^\circ C$, the turnover rate of phosphate was 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 food-chain 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.