MEPS 538:157-167 (2015)  -  DOI: https://doi.org/10.3354/meps11485

ABSTRACT: In suspension-feeding bivalves, growth rates generally remain at a high level over a broad range of numerical densities. However, data series on growth and production that cover a wide density range in a natural marine benthic system appear to be scarce. A long-term monitoring study (1973-2013) of zoobenthos on tidal flats in the western Wadden Sea yielded twice-annual data on numbers and weights of (among others) cockles Cerastoderma edule, allowing estimates of their annual and cohort production by the weight-increment summation method. In nearly all years, cockle densities did not exceed a few 100s m^-2, and their individual weight increments showed no significant relationship with density. Only the appearance of one exceptionally strong year class in 2011 caused higher densities over an extensive area for >1 yr. Growth rates in all cockles were significantly reduced in the second growing season of this strong cohort, when their densities amounted to >500 m^-2. Seasonal and annual survival did not show a significant relationship with density. As a consequence of the reduction in growth of its members, production of the single strong cohort was substantially less than expected on the basis of its initial abundance. In all smaller cohorts, the positive relationship between recruit numbers and cohort production was close to linear. However, when the higher density of the one strong year class was included, an inverse-U shaped line would better characterize this relationship. Appearances of very strong bivalve cohorts are rare events in the Wadden Sea: bivalve stocks on the tidal flats usually appear to be far below the carrying capacity of the area for suspension-feeding animals.

KEY WORDS: Density dependence · Seasonal weight growth · Annual production · Carrying capacity · Wadden Sea