Local dispersal of juvenile bivalves: implications for sandflat ecology

Dispersal has profound influences on population dynamics and is a key proce ss maintaining spatial and temporal patterns. For many benthic marine inver tebrates dispersal occurs primarily during the planktonic larval stages. It is now widely recognised that post-larval and juvenile stages of benthic i nvertebrate 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 dis persal rates and the spatial distribution of dispersing individuals in situ . We used a fluorescent stain for marking bivalves in vivo and a mark-and-r ecapture methodology to investigate dispersal patterns of post-larval and j uvenile bivalves on a sandflat. Wave-induced energy dissipation on the seaf loor was measured using a DOBIE wave gauge. Tracer sediment and bedload tra nsport was used as a template for bivalve movement. The experiment was cond ucted over a short-time span (60 h) and encompassed spatial scales relevant to many sampling designs and manipulative experiments. Our results show th at juvenile bivalves dispersed over scales of meters within one tidal cycle . Modelling the half-life of juvenile bivalve retention using radioactive d ecay 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-l arval (il mm) bivalves within the first 17.4 h, whereas juvenile (1-4 mm) b ivalves persist longer with a 50 % turnover after 31.5 h. Considering the v ery calm hydrodynamic conditions during the experiment, these dispersal rat es are remarkable. Bivalve dispersal was decoupled from sediment bedload tr ansport, illustrating the importance of active dispersal behaviour under th e prevailing hydrodynamic conditions. Our results suggest that dispersal is potentially more important than mortality for the population dynamics of j uvenile bivalves over small and meso spatial-time scales.