Long‐term variability in secondary production of an intertidal bivalve population is primarily a matter of recruitment variability

Abstract

The importance of recruitment processes in determining benthic population dynamics has received considerable interest among marine ecologists in the last two decades. Observational demographic studies, in which recruits were followed to and through the adult state, aimed to estimate whether the variation in the numbers of recruits is dampened by density‐dependent post‐recruitment processes. These studies revealed conflicting results on the importance of post‐recruitment density dependence, but were performed over at most a few years. Based on a study of the demography and growth of the bivalve Macoma balthica (L.) on an intertidal flat in the Dutch Wadden Sea for a period of almost 30 years, we explored the extent to which the among‐cohort variability in recruitment was reflected in the secondary production (which results from the combined action of recruitment, mortality and growth) of the adult population. Since growth in length was of the Von Bertalanffy type and post‐recruitment instantaneous mortality rate was approximately constant for each cohort, production could be explicitly written in terms of the following parameters: recruit density, condition (ratio of mass to cubic length), Von Bertalanffy growth coefficient, asymptotic cubic length, and expected life span. Decomposing the among‐cohort variability in production in terms of the covariance matrix of these parameters revealed that most of the variance in production could be attributed to year‐to‐year variability in recruit density. Small negative covariances between recruit density and asymptotic size and expected life span indicated weak density dependence after recruitment. The major cause of temporal variation in abundance and production is inter‐annual variation in recruitment and competition for resources seems to play a small role in affecting post‐recruitment population density and production.