Influence of phytoplankton on the response of bacterioplankton growth to nutrient enrichment

Abstract

SUMMARY 1. Laboratory experiments were conducted to test the effect of nutrient enrichment on bacterioplankton growth in the presence and absence of phytoplankton. 2. In one series of experiments, bacterioplankton growth in terms of specific activity [3H-thymidine incorporation (cell number)−1] was greater in whole lake water samples than in samples from which phytoplankton had been removed by filtration (1.0 μm), regardless of the nutrient enrichments (control, NH+4 plus PO3-4 and mannitol). Organic C enhanced bacterioplankton growth in both whole and filtered lake water. 3. In another series of experiments (with the same nutrient enrichments as in the first experiment except that glucose replaced mannitol), bacterioplankton growth in whole lake water enriched with PO3-4 plus NH+4 and incubated in the light was greater than in two treatments designed to inhibit photosynthetic activity (+DCMU and dark). Bacterioplankton response to nutrient addition was greatest in the PO3-4 plus NH+4 enrichment under all three conditions (light +DCMU, and dark). 4. These results indicate that bacterioplankton growth could be directly limited by inorganic P and N when these elements are in short supply. Enhancement of bacterioplankton growth by phytoplankton occurs only under PO3-4 and NH+4 replete environments.