Radiotracer study of phosphorus uptake by plankton and redistribution in I&he 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 foodchain 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 production of phytoplankton is often P limited (Schindler 1977). However, the complexity of planktonic food chains makes P cycling complicated and difficult to model. In lakes with a high allochthonous load of organic matter, the cycling of P seems to be further complicated. In clear-water lakes, algal excretion may be an important source of dissolved organic C (DOC) (Jones et al: 1983), but in the coniferous forest zone and some tropical regions, allochthonous (mostly humic) matter often completely dominates the pool of dissolved organic matter (DOM) in water. In such cases allochthonous DOM can serve as an important C source for microheterotrophs, particularly bacteria, and renders their role in sustaining food chains comparable to that of phytoplankton (Salonen et al. 1992). Acknowledgments This work was supported by the Academy of Finland and Maj and Tor Nessling Foundation. However, most substances contributing to the pool of DOM are deficient in P relative to C. The biological utilization of allochthonous organic matter in humic bodies of water might thus be expected to promote the importance of bacteria as P scavengers (Stewart and Wetzel 1982) and diminish the competitive ability of phytoplankton. Currie and Kalff (1984a) proposed that an increased supply of allochthonous organic C influences bacterial biomass positively and algal biomass negatively. Ryhanen ( 196 8) found increased decomposition of humic substances with mineral nutrient additions, but Jones (1990) found no evidence that high allochthonous DOM, in the absence of severe P deficiency, shifts the balance of plankton phosphate uptake toward bacteria at the expense of algae. He concluded that biological P transformations may be modified in humic lakes by an increased relative abundance of bacteria over algae. The availability of P to both bacteria and algae is potentially affected by abiotic inter-