Local Differences in Macrozoobenthic Response to Enhanced Food Supply Caused 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 western half of the Dutch Wadden Sea probably as a consequence of increased nutrient concentrations. The response of zoobenthos was studied during 1970-1990 as part of a long-term monitoring program, including annual sampling at 15 fixed stations with very different environmental conditions (elevation, sediment grain size) on Balgzand, a 50-km2tidal flat area. The total zoobenthic stock in the affected area rougly doubled within 2 yr after the doubling of the stock of planktonic algae. However, the magnitude of the response differed from station to station. We tested the hypothesis that a strong positive response would occur particularly at stations where food could be limiting, i.e where zoobenthic food demand was high relative to food supply. Biomass increases of zoobenthos in the order of doubling were indeed restricted to the part of Balgzand characterized already in the 1970s by high biomass values. The zoobenthic biomass hardly increased in two areas of Balgzand with relatively low values for zoobenthic biomass and species richness. In these impoverished areas environmental conditions are harsh and the benthic communities are dominated by small opportunistic species. One of these areas is characterized by a high intertidal elevation (short daily inundation periods) combined with a sheltered location near the coast (resulting in muddy sediments), the other by severe exposure 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 consumer 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.