Differential filtration studies of carbon flux from living algae to microheterotrophs, microplankton size distribution and respiration in Lake Kinneret.

The flux of newly photosynthetically fixed, dissolved organic carbon (PDOC) from phototrophs to microheterotrophs in Lake Kinneret was examined by differential (3 and 0.4μm) filtration after samples were incubated with(14)C-bicarbonate for 3 hours (in the light) and subsequently for 21 hours (in darkness). Only a small proportion (average about 10%) of the carbon fixed from(14)C-bicarbonate in the light was associated with particulate matter <3μm. In 14 out of 16 experiments there was no significant decrease in the relative proportion of radioactivity associated with larger (>3μm) organisms after the dark period, suggesting that the amount of PDOC taken up by unclumped, single bacteria (<3μm) was not very great. Respiration rates, estimated by the decrease in(14)C particulate counts in the dark period, ranged from 3.4 to 21.2% of daylight net photosynthesis. In almost all cases, parallel experiments with additions of radioactive glucose or amino acids indicated that the majority of active heterotrophs passed through 3μm filters. Apparent residence times for glucose and amino acids were 20 to 168 hours and 20 to 152 hours, respectively.