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

Abstract

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.