The objective of this laboratory flume study is to quantify settlement rate as a function of larval availability for species of benthic invertebrates that vary in reproductive mode (i.e., with planktotrophic or lecithotrophic larvae) and taxonomic affinity (polychaetes and bivalves). Larval availability, here defined as the supply of larvae to the bed, is a product of a larval concentration and a velocity term. Thus, experiments are designed specifically to test the relative influence of these two factors on settlement rate. Three hypotheses will be tested. Hl: Settlement rate varies as a function of reproductive mode; lower settlement rates and higher settlement variability are associated with planktotrophic as compared with lecithotrophic larvae. H2: Settlement rate increases with increasing planktonic larval concentration until it reaches an asymptotic value, at which point the bed is saturated. H3: Larval flux to the bed can be described as an eddy diffusion process, where the key hydrodynamic parameter is the boundary shear velocity. In the course of testing these hypotheses, this study will also determine (1) the relationship between bulk concentration of larvae in the flow and flux of larvae to the bed, (2) if larval settlement is directly proportional to larval availability, and (3) the probability of settlement (i.e., of a larva remaining on the bed once deposited). A long term goal of this research is the prediction of settlement rate in the field from data on larval availability and flow, and thus experiments will be conducted so that a general, quantitative expression can be developed to predict settlement from larval flux to the bed.
