LINKING LARVAL SUPPLY TO RECRUITMENT: FLOW-MEDIATED CONTROL OF INITIAL ADHESION OF BARNACLE LARVAE

Abstract

Dispersal of propagules may have far-reaching consequences for the local dynamics of open populations. In aquatic environments the dispersal and supply of larvae is strongly influenced by coastal-scale hydrodynamics. The role of local flow on settlement and recruitment is, however, poorly understood. In a test of the effect of local hydrodynamics on recruitment dynamics we studied the relationships between local flow speed and larval supply, contact rate, settlement, and recruitment rates for the barnacle Balanus improvisus. Planktonic cyprid larvae were sampled with pumps while suspended panels were used to measure initial contact (grease-coated panels), settlement (5-d immersion), and recruitment (30-d immersion). Larval supply varied with coastal-scale upwelling but was uncorrelated to local flow speed. Settlement rate was highly correlated to contact rate, and the proportion of settling larvae increased with established recruit density. In contrast to model predictions, contact rate was negatively correlated with local flow speed, which explained 33% of total variation. This correlation was maintained through settlement and recruitment suggesting that flow-mediated initial contact is an important link between larval supply and recruitment. No evidence from field studies indicated behavioral responses by larvae to flow although observations in a laboratory flume suggested active rejection of potential settlement sites at increasing flow speeds. It is suggested that initial adhesion is limited by flow speed through drag and lift forces acting on larvae. A conceptual model of mechanisms controlling the transfer of planktonic larvae to recruited benthic individuals is proposed.