Simultaneous measurement of phosphorus and carbon uptake in Lake Kinneret by multiple isotopic labeling and differential filtration.

Differential filtration and multiple isotopic labeling were combined to study the uptake of [(14)C]bicarbonate, [(14)C]glucose, and [(32)P]orthophosphate by microplakton in Lake Kinneret, Israel. Short-term (4 hr) uptake experiments showed seasonal changes in the size distributions of organisms taking up inorganic carbon, glucose carbon, and orthophosphate in the lake water. In a time-course experiment of 48 hr (Jan 1976) most, but not all, of the photosynthetic activity (average 72%) and a similar fraction of chlorophyll (72%) were associated with organisms retained on 3-μm Nuclepore filters (retention on 0.4-μm filters was 100%). About 90% of the organisms that assimilated glucose passed through 3-μm filters. Photosynthetic carbon fixation, dark carbon uptake, and heterotrophic uptake of glucose carbon accounted for 99%, 1%, and 1%, respectively, of the total net carbon assimilated during the first 6 hours. Radioactive phosphorus showed an initial rapid uptake into particles, which was not affected by light or dark. We suggest that this methodology has a wide potential for elucidating the flux of nutrients into various components of the microplankton and in characterizing different aquatic environments.