RESPONSES OF BACTERIOPLANKTON TO TIDAL INUNDATIONS OF A SALT-MARSH INA FLUME AND ADJACENT MUSSEL ENCLOSURES

Samples of tidal water flooding a cordgrass (Spartina alterniflora Loi sel) saltmarsh were taken as the water moved onto the marsh, over and back into an undisturbed bed of mussels, then back to the rim of the m arsh. Samples were also taken contemporaneously from cylindric enclosu res containing or not containing mussels. Three summer tides and one w inter tide were sampled. In summer, as water moved into the cordgrass stands, its bacterioplankton rapidly became 2-fold more productive (as determined from rates of thymidine incorporation per cell), presumabl y due to increased availability of nutrient from cordgrass shoots and/ or the marsh sediment. Movement of summer tidal-flooding water over th e mussel beds, or entrapment of the water over enclosed mussels, had v ery similar effects: net removal of about 30-35% of bacterioplankton c ells, and another increase (beyond the initial rapid 2-fold jump) in p roductivity per cell (about 20-30%, to 55 fmol thymidineh . h(-1) 10(- 6) cells). Thus net import of bacterioplankton into the marsh can prob ably be nearly entirely attributed to mussel filtrational activity (th ough in agreement with the literature, small autotrophs were collected greater than or equal to 2-fold more efficiently than bacterioplankto n). Mussel ingestive (and possibly egestive) activity resulted in shif ts in composition of the bacterioplankton: decreases in large bacteria l rods and cocci, and in the larger cells of the predominant small rod s and cocci, but no apparent change in the vibrioid (curved-cell) asse mblage. Mussel influence upon bacterioplankton appeared to be strongly seasonal: in the winter sampling, there were no changes detected in t he bacterioplankton in water moving over the mussel beds, or in mussel enclosures.