Zoobenthic biomass limited by phytoplankton abundance: evidence from parallel changes in two long-term data series in the Wadden Sea

Abstract

We address the question of whether year-to-year variability in pelagic algal food supply can explain long-term variability in macrozoobenthic biomass in an estuarine area. Starting in the early 1970s, quantitative data were frequently collected in standardized ways in the western part of the Dutch Wadden Sea on (1) concentrations of phytoplankton species and chlorophyll (and rates of primary production) in the main tidal inlet (Marsdiep) and (2) numerical densities and biomass of macrozoobenthic animals (and growth rates in a few species) in a nearby extensive tidal-flat area (Balgzand). In both data series, the most distinctive feature was a sudden change that took place around 1980, viz. a rather sudden and persisting doubling of concentrations of chlorophyll and algal cells and of primary production rates, as well as of numerical densities and biomass of zoobenthos. From these parallel changes we hypothesise that algal food largely determines the abundance of zoobenthos in the Wadden Sea. The following observations substantiate this hypothesis: (1) the significant correlation between annual mean values of chlorophyll concentration and overall mean numerical density and biomass of zoobenthos (as estimated after an appropriate time lag), (2) the observed limitation of zoobenthic biomass doubling (after the doubling of food supply) to areas with already high biomass values (where food demand was high and food could therefore be in short supply), (3) the limitation of a strong response to changes in food supply to functional groups that are directly dependent on algal food, i.e. suspension and deposit feeders, as opposed to carnivores, (4) the significant correlation between annual growth rates in Macoma balthica and food supply in the growing season, particularly in areas close to the tidal inlet where food concentrations were monitored. Some other factors were identified that could decisively influence zoobenthic abundance locally and/or temporarily. Harsh environmental conditions will have limited zoobenthic biomass in extreme areas such as the upper part of the intertidal and areas exposed to strong currents and wave action. Severe winters temporarily reduced the abundance of several sensitive (southern) species, including most of the infaunal and epifaunal predators. Even stronger temporary and local reductions of zoobenthic biomass occurred as a consequence of fishery activities for such benthic species as cockles, mussels and lugworms. Recovery after temporary biomass reductions was generally rapid, but late-winter values of total-Balgzand zoobenthos biomass never exceeded an upper level of 45 g AFDW per m 2 probably set by maximal annual rates of primary production of between 400 and 500 g C per m 2.