INFLUENCE OF PHYTOPLANKTON ON THE RESPONSE OF BACTERIOPLANKTON GROWTHTO NUTRIENT ENRICHMENT

1. Laboratory experiments were conducted to test the effect of nutrien t enrichment on bacterioplankton growth in the presence and absence of phytoplankton. 2. In one series of experiments, bacterioplankton grow th in terms of specific activity [H-3-thymidine incorporation (cell nu mber)-1] was greater in whole lake water samples than in samples from which phytoplankton had been removed by filtration (1.0 mum), regardle ss of the nutrient enrichments (control, NH4+ plus PO43-, and mannitol ). Organic C enhanced bacterioplankton growth in both whole and filter ed lake water. 3. In another series of experiments (with the same nutr ient enrichments as in the first experiment except that glucose replac ed mannitol), bacterioplankton growth in whole lake water enriched wit h PO43- plus NH4+ and incubated in the light was greater than in two t reatments designed to inhibit photosynthetic activity (+DCMU and dark) . Bacterioplankton response to nutrient addition was greatest in the P O43- plus NH4+ enrichment under all three conditions (light +DCMU, and dark). 4. These results indicate that bacterioplankton growth could b e directly limited by inorganic P and N when these elements are in sho rt supply. Enhancement of bacterioplankton growth by phytoplankton occ urs only under PO43- and NH4+ replete environments.