ZOOPLANKTON GRAZING IN A POTOMAC RIVER CYANOBACTERIA BLOOM

During summer, bloom-forming cyanobacteria,including Anacystis, Aphani zomenon and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 10(8) cells l-1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing e xperiments were conducted in July and August 1987. Using size-fraction ated river phytoplankton assemblages, zooplankton grazing rates were d etermined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on C-14-labeled river assemblages, C-14-labele d river assemblages enriched with unlabeled cyanobacteria, and unlabel ed river assemblages enriched with C-14-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mi ra, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trich ocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither ro tifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl . [individual . h]-1 while copepod rates ranged from undetectable to 135 mul . [cop epod . h]-1. Grazing rates on labeled river phytoplankton assemblages were 4-100 times higher than noted for zooplankton feeding on cyanobac teria. The addition of the colonial alga to labeled river phytoplankto n assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no ad ded cyanobacteria. Total zooplankton demand for cyanobacteria and rive r phytoplankton assemblages was estimated for the study period july-Au gust 1987. Rotifer plus C. vernalis herbivory would have removed 1-5% and 49%, respectively, of the standing stock of the two autotroph pool s each day. Literature-derived clearance rates for Bosmina indicate, h owever, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungraze d and may be transported to the lower estuary for salinity-induced agg regation and sedimentation.