RESPONSE OF PHYTOPLANKTON AND BACTERIA TO NUTRIENTS AND ZOOPLANKTON -A MESOCOSM EXPERIMENT

Although both nutrient inputs and zooplankton grazing are important to phytoplankton and bacteria in lakes, controversy surrounds the relati ve importance of grazing pressure for these two groups of organisms. F or phytoplankton, the controversy revolves around whether zooplankton grazers, especially large cladocerans like Daphnia, can effectively re duce phytoplankton populations regardless of nutrient conditions. For bacteria, little is known about the balance between possible direct an d indirect effects of both nutrients and zooplankton grazing. However, there is evidence that bacteria may affect phytoplankton responses to nutrients or zooplankton grazing through direct or apparent competiti on. We performed a mesocosm experiment to evaluate the relative import ance of the effects of nutrients and zooplankton grazing for phytoplan kton and bacteria, and to determine whether bacteria mediate phytoplan kton responses to these factors. The factorial design crossed two zoop lankton treatments (unsieved and sieved) with four nutrient treatments (0, 0.5, 1.0 and 2.0 mu g phosphorus (P) l(-1) day(-1), together with nitrogen (N) at a N:P ratio of 20:1 by weight). Weekly sieving with 3 00 pm mesh reduced the average size of crustacean zooplankton in the m esocosms, decreased the numbers and biomass of Daphnia, and increased the biomass of adult copepods. Nutrient enrichment caused significant increases in phytoplankton chlorophyll a (4-5x), bacterial abundance a nd production (1.3x and 1.6x, respectively), Daphnia (3x) and total zo oplankton biomass (2x). Although both total phytoplankton chlorophyll a and chlorophyll a in the <35 mu m size fraction were significantly l ower in unsieved mesocosms than in sieved mesocosms, sieving had no si gnificant effect on bacterial abundance or production. There was no st atistical interaction between nutrient and zooplankton treatments for total phytoplankton biomass or bacterial abundance, although there wer e marginally significant interactions for phytoplankton biomass <35 mu m and bacterial production. Our results do not support the hypothesis that large cladocerans become less effective grazers with enrichment; rather, the difference between phytoplankton biomass in sieved versus unsieved zooplankton treatments increased across the gradient of nutr ient additions. Furthermore, there was no evidence that bacteria buffe red phytoplankton responses to enrichment by either sequestering P or affecting the growth of zooplankton.