LOCAL-DIFFERENCES IN MACROZOOBENTHIC RESPONSE TO ENHANCED FOOD-SUPPLYCAUSED BY MILD EUTROPHICATION IN A WADDEN SEA AREA - FOOD IS ONLY LOCALLY A LIMITING FACTOR

In the late 1970s food supply to herbivores suddenly doubled in the we stern half of the Dutch Wadden Sea probably as a consequence of increa sed nutrient concentrations. The response of zoobenthos was studied du ring 1970-1990 as part of a long-term monitoring program, including an nual sampling at 15 fixed stations with very different environmental c onditions (elevation, sediment grain size) on Balgzand, a 50-km(2) tid al flat area. The total zoobenthic stock in the affected area roughly doubled within 2 yr after the doubling of the stock of planktonic alga e. However, the magnitude of the response differed from station to sta tion. We tested the hypothesis that a strong positive response would o ccur particularly at stations where food could be Limiting, i.e. where zoobenthic food demand was high relative to food supply. Biomass incr eases of zoobenthos in the order of a doubling were indeed restricted to the part of Balgzand characterized already in the 1970s by high bio mass values. The zoobenthic biomass hardly increased in two areas of B algzand with relatively low values for zoobenthic biomass and species richness. In these impoverished areas environmental conditions are har sh and the benthic communities are dominated by small opportunistic sp ecies. One of these areas is characterized by a high intertidal elevat ion (short daily inundation periods) combined with a sheltered locatio n near the coast (resulting in muddy sediments), the other by severe e xposure to currents and wind (resulting in mobile sands with low silt and organic-matter contents) due to its offshore position. We conclude that food limitation only existed in the midshore communities living under relatively mild abiotic conditions. Apparently, the benthic cons umer populations benefitted from eutrophication-induced enhancement of their food supply by enlargement of their stocks only where they were not too stressed by unfavorable abiotic conditions.