THE EFFECT OF NUTRIENT ADDITIONS ON BACTERIAL-ACTIVITY IN SEAGRASS (POSIDONIA-OCEANICA) SEDIMENTS

The influence of nutrient additions on benthic bacterial activity unde r seagrass meadows was tested by enriching five seagrass (Posidonia oc eanica) meadows with nutrients over one year. We found a highly signif icant response of benthic bacterial activity to nutrient additions, wh ich was reflected in greater (about two-fold) ammonification rates and , to a smaller extent, a significant tendency for a greater exoenzymat ic activity. Nutrient additions significantly raised bacterial activit y, without altering the seasonal changes in bacterial activity. As a r esult of the increased bacterial activity, the organic content of the sediments declined significantly, by about 33%, after one year of nutr ient addition. Hence, nutrient additions to the seagrass meadows enhan ce seagrass production but also accelerate bacterial decomposition of seagrass carbon, thereby reducing the capacity of the sediments to sto re organic carbon. These results demonstrate that sediment nutrient av ailability limits bacterial activity in these Posidonia oceanica meado ws, and identify bacteria as important nutrient consumers in these sys tems. (C) 1998 Elsevier Science B.V.