Phosphate Uptake Rates of Phytoplankton Assemblages Grown at Different Dilution Rates in Semicontinuous Culture

Abstract

We studied the effects of pulsed nutrient supplies on the physiology of natural phytoplankton assemblages. A natural phytoplankton community, from an oligotrophic lake, was grown at several dilution rates from 0.1 to 1.5∙d⁻¹in P-limited semicontinuous cultures. Particulate C:P and N:P ratios (by atoms) were approximately 300:1 and 50:1, respectively, at the lowest dilution rate and 40:1 and 7:1, respectively, at the highest dilution rate. This demonstrates that a range of P-limited conditions can be established using the semicontinuous culturing technique. The time-course of uptake, in response to a saturating addition of phosphate, revealed that there was a short lag before maximum rates of uptake were observed at all but the highest dilution rate, and that maximum uptake rates over the first 30 min of incubation were higher at intermediate dilution rates. Subsequent to the maximum uptake rate occurring, the relationship between uptake rate and time was species dependent. In Oscillatoria sp. dominated cultures the uptake rates declined with increasing cellular P, and the rates of these declines increased with dilution rate; in Synechococcus sp. dominated cultures the uptake rates remained constant and were independent of cellular P.