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

Abstract

To investigate the relationship between stock size and production of an entire feeding guild, and in particular to find out whether it is dome-shaped (showing an optimal abundance for production), we used a 40-year data set of the 3 most important suspension-feeding bivalves (Cerastoderma edule, Mytilus edulis, and Mya arenaria) in a Wadden Sea tidal flat area of about 20 km 2 . The data set contained data on numerical density of individuals, annual rates of weight growth, recruitment, survival, and secondary production. At higher densities (> 400 individuals 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 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 during the 40-y 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 longterm 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 insufficient recruitment by a top-down process (predation on young stages).

Bottom‐up as well as top‐down processes govern zoobenthic secondary production in a tidal‐flat ecosystem

“…7). The year 2012 was the only year with a reduced chlorophyll concentration that could be linked with high bivalve abundance and grazing (Beukema and Dekker 2019). In all other years, there would have been sufficient primary production and food supply to enable a higher secondary production.…”

“…Despite the production reduction at the highest spat densities (caused by growth reduction by food competition, see Beukema and Dekker 2019), the positive correlation between total (sum of five most important species) bivalve production and the sum of their spat densities was highly significant: r = 0.70, n = 28, p < 0.001 (Fig. 7, open points, dashed line).…”

“…Sampling took place at the time of the year, March and August, when macrozoobenthic biomass was at its annual minimum and maximum, respectively (Beukema 1974). Further details on the Balgzand area and on sampling the macrozoobenthos can be found in Beukema and Cadée (1997) and in Beukema and Dekker (2019). March cores were taken to a depth of 25-30 cm, August cores were taken to this depth only starting from around 1990.…”

“…Measurements of secondary production contribute to a better understanding of the functioning of an ecosystem (Dolbeth et al 2012). Generally, such estimates were found to vary a lot from year to year (Möller et al 1985; Maurer et al 1992; Dolbeth et al 2005; Beukema and Dekker 2019, 2022). However, to our knowledge, really long‐term data series on secondary production in marine benthic systems have never been published.…”

Mosselkweek en effecten op natuur

“…In de gebieden in de westelijke Waddenzee, waar de kweekpercelen liggen, is het niet waarschijnlijk dat overbegrazing optreedt; ook niet in het Marsdiep waar de (wilde) schelpdierenbestanden en de bijbehorende graasdruk sterk zijn toegenomen 71 . De bevinding dat de draagkracht in de westelijke Waddenzee niet overschreden wordt, komt overeen met een recente studie op Balgzand 73 . (2,7 kg/m touw en 24,0 kg/m 2 net) 18 .…”