Local dispersal of juvenile bivalves: implications for sandflat ecology

Dispersal has profound influences on population dynamics and is a key process maintaining spatial and temporal patterns. For many benthic marine invertebrates dispersal occurs primarily during the planktonic larval stages. It is now widely recognised that post-larval and juvenile stages of benthic invertebrate species can also exhibit high rates of dispersal. In particular, post-settlement dispersal has been demonstrated for many bivalve species. Despite this appreciation, no studies to date have analysed the direct dispersal rates and the spatial distribution of dispersing individuals in situ. We used a fluorescent stain for marking bivalves in vivo and a mark-and-recapture methodology to investigate dispersal patterns of post-larval and juvenile bivalves on a sandflat. Wave-induced energy dissipation on the seafloor was measured using a DOBIE wave gauge. Tracer sediment and bedload transport was used as a template for bivalve movement. The experiment was conducted over a short-time span (60 h) and encompassed spatial scales relevant to many sampling designs and manipulative experiments. Our results show that juvenile bivalves dispersed over scales of meters within one tidal cycle. Modelling the half-life of juvenile bivalve retention using radioactive decay equations provided insight into the local persistence of individuals. These models indicate a 50% turnover within an area of 0.25 m(2) for post-larval (il mm) bivalves within the first 17.4 h, whereas juvenile (1-4 mm) bivalves persist longer with a 50 % turnover after 31.5 h. Considering the very calm hydrodynamic conditions during the experiment, these dispersal rates are remarkable. Bivalve dispersal was decoupled from sediment bedload transport, illustrating the importance of active dispersal behaviour under the prevailing hydrodynamic conditions. Our results suggest that dispersal is potentially more important than mortality for the population dynamics of juvenile bivalves over small and meso spatial-time scales.