The relative importance of bacterioplankton and phytoplankton in phosphorus uptake in freshwater1

Abstract

In several studies of orthophosphate cycling in lakes, most of the phosphorus uptake has been associated with the smallest limnetic particles. The hypothesis that bacteria rather than phytoplankton are responsible for most P uptake in situ was tested in Lake Memphremagog by means of size fractionation of [ 32 P]orthophosphate uptake, [ 14 C]glucose uptake (as an indicator of bacterial activity), and both [ 14 C]bicarbonate uptake and chlorophyll a (as indicators of algal activity). In biweekly samples from May to October, particles <3.0 µ m accounted for 72–98% (median 94%) of the uptake of orthophosphate. There was no diel variation in the fraction of uptake by particles <3.0 µ m. The bacterial contribution to the total community uptake of orthophosphate was estimated to range between 97 and 100% over all samples collected. However, the phosphorus released (shown not to be orthophosphate) was taken up almost entirely by the phytoplankton. The bacterioplankton apparently strongly dominates orthophosphate cycling in situ, while the phytoplankton seems to obtain most of its phosphorus from excreted organic P compounds.