Decoupling of recruitment from adult clam assemblages along an estuarine shoreline

Abstract

The maintenance of populations of benthic species requires both effective (if not regular) recruitment plus sufficient post-recruitment survival through vulnerable juvenile phases. Decoupling of abundances of settlers and adults often occurs when conditions for juvenile survival are poor. For infaunal bivalves, predation can be a key process that alters the abundance or distribution of adults relative to early recruits. We examined spatial patterns of recruitment of intertidal clams along the marine-estuarine axis in Puget Sound, Washington, USA. Abundances and species richness of clam recruits were variable in space and time, but during the period of high recruitment (summer) were equal across all sites. Juvenile and adult clams, however, were consistently rare at the inner-estuarine sites relative to the outer. We hypothesized that predation was higher at our inner sites, and manipulated predator access to newly recruited clam assemblages along the estuarine gradient. Predators >1cm were excluded with mesh net from intertidal PVC tubes where recruitment of clams was allowed to occur naturally, and abundances and species richness of clams were measured after one summer season. Tubes with mesh nets over the top had significantly higher overall abundances of clams than unprotected tubes after 4months, but showed no difference in clam species richness. More strikingly, the clam assemblage at the inner-estuarine sites comprised very few individuals relative to the outer-estuarine sites, regardless of predator exclusion. Multivariate analyses showed that clam assemblages in the tubes were better explained by position along the estuarine gradient than by predator treatment. Parallel studies at these sites suggested that growth of suspension feeders is actually higher in the inner estuary. Physical gradients are much less extreme in this well-mixed fjord than in many other estuaries, but we hypothesize that physical stresses such as higher temperatures at the inner beaches in the summer may be a major contributor to juvenile mortality, decoupling recruitment from adult populations.