RADIOTRACER STUDY OF PHOSPHORUS UPTAKE BY PLANKTON AND REDISTRIBUTIONIN 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 surf ace to below the epilimnion with the steep vertical stratification of the lake undisturbed. [P-32]orthophosphate was mixed into the epilimni on of the enclosure and its fate followed for 2 weeks. In the epilimni on approximately 85% of all the P in organisms was in Dophnia longispi na, which comprised almost all the zooplankton biomass. The respective proportions for bacterioplankton and phytoplankton were approximately 12 and 3%. Early in the experiment when the temperature of the epilim nion was approximately 20-degrees-C, the turnover rate of phosphate wa s 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 t his coarse time resolution, the pattern of P-32 incorporation into phy toplankton appeared similar. The specific radioactivity in D. longispi na equaled that in the bacterial and algal fractions after only 2 d, i mplying rapid and direct food-chain linkage between these P pools. An explanation for such rapid transfer of P may be that D. longispina con sumes food with a high concentration of P, such as bacteria. At the en d of the experiment, the specific radioactivity of the dissolved P poo l was considerably lower than that of the other fractions, indicating only slow exchange between part of the dissolved P pool and the plankt on.