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

1. The importance of recruitment processes in determining benthic populatio n dynamics has received considerable interest among marine ecologists in th e last two decades. Observational demographic studies, in which recruits we re followed to and through the adult state, aimed to estimate whether the v ariation in the numbers of recruits is dampened by density-dependent post-r ecruitment processes. These studies revealed conflicting results on the imp ortance of post-recruitment density dependence, but were performed over at most a few years. 2. Based on a study of the demography and growth of the bivalve Macoma balt hica (L.) on an intertidal flat in the Dutch Wadden Sea for a period of alm ost 30 years, we explored the extent to which the among-cohort variability in recruitment was reflected in the secondary production (which results fro m the combined action of recruitment, mortality and growth) of the adult po pulation. 3. Since growth in length was of the Von Bertalanffy type and post-recruitm ent instantaneous mortality rate was approximately constant for each cohort , production could be explicitly written in terms of the following paramete rs: recruit density, condition (ratio of mass to cubic length), Von Bertala nffy growth coefficient, asymptotic cubic length, and expected life span. 4. Decomposing the among-cohort variability in production in terms of the c ovariance matrix of these parameters revealed that most of the variance in production could be attributed to year-to-year variability in recruit densi ty. 5. Small negative covariances between recruit density and asymptotic size a nd expected life span indicated weak density dependence after recruitment. 6. The major cause of temporal variation in abundance and production is int er-annual variation in recruitment and competition for resources seems to p lay a small role in affecting post-recruitment population density and produ ction.