STRUCTURING FACTORS IN A MARINE SOFT-BOTTOM COMMUNITY DURING EUTROPHICATION - AN EXPERIMENT WITH RADIOLABELED PHYTODETRITUS

The quantitative development and uptake of radio-labelled phytodetritu s in benthic macro- and meiofauna was studied in a 5-month experiment in two mesocosms, one of which received a single large nutrient (N and P) addition, while the other served as control. In response to the 12 -fold increase in phytoplankton biomass noted after 2 weeks and the re sulting enhanced accumulation of fresh phytodetritus, the abundance an d biomass of the polychaetes Mediomastus ambiseta and Polydora ligni a nd the mud anemone Cerianthiopsis americanus increased significantly i n the enriched tank. The abundances of P. ligni and M. ambiseta increa sed 37-fold and 12-fold, respectively, within the first two months of the experiment. No other macrofaunal or meiofaunal taxa showed any con sistent positive quantitative response to the increased input of phyto detritus. In the control tank no considerable change in the benthic co mmunity structure was noted. The measurements of radio-label uptake wi thin the benthic fauna showed that the quantitatively most successful species utilized fresh phytodetritus highly. However, a high degree of utilization of fresh detritus was also shown by taxa that did not res pond quantitatively within the 5 month of the experiment, and almost a ll taxa showed a preference for fresh detritus over older organic mate rial. Within the benthic meiofauna, kinorhynchs and especially foramin iferans showed a remark ably low preference for fresh detritus. A budg et calculation comparing the total amounts of labelled organic carbon bound in animal tissue and in the sediment indicated that at any time at least 75% of this carbon was available for assimilation by deposit feeders. These results suggest that factors other than the availabilit y of food, such as competition for space by a few opportunistic macrof auna species, limited the response of other species within this benthi c community to the increased input of phytodetritus.