The carrying capacity of a tidal flat area for suspension-feeding bivalves

To investigate the relationship between stock size and production of an entire feeding guild, and in particular to find out whether this relationship is dome-shaped (showing an optimal abundance for production), we used a 40 yr data set of the 3 most important suspension-feeding bivalves (Cerastoderma edule, Mytilus edulis, and Mya arenaria) in a Wadden Sea tidal flat area (about 20 km2). The data set contained data on numerical density of individuals, annual rates of weight growth, recruitment, survival, and secondary production. At higher densities (>400 ind. m-2), we found reductions of growth rate and recruitment. At the highest densities, the reduction in weight growth was so strong that production was lower than its maximal values at intermediate densities. This optimal density of around 400 ind. m-2 was considered to represent the carrying capacity of the system for suspension-feeding bivalves. High densities resulting in reduced production, however, rarely (in only 5% of the years) occurred over the 40 yr monitoring period. Clear bottom-up limitation of bivalve production was thus very unusual in the studied area. Year-to-year variation in growth and production of suspension-feeding bivalves were not related to chlorophyll concentrations in the main tidal stream and did not follow the declining long-term trends of primary production and chlorophyll concentrations. The main conclusion of the paper is that production increases with stock size, but only to a certain threshold value that is rarely reached as a consequence of recruitment being curtailed by a top-down process (predation on young stages).