A model study on variations in larval supply:: are populations of the polychaete Owenia fusiformis in the English Channel open or closed?

The polychaete Owenia fusiformis is one of the most ecologically important species in the muddy fine sand sediments in the English Channel where it is distributed in geographically separated populations. A vertically averaged Lagrangian hydrodynamic model integrating tidal residual currents and wind-induced currents was used to drive an advection-diffusion model for investigating the variability of larval transport in order to assess the self-seeding capabilities and the degree of connectivity between local populations. Three different types of environmental forcing (i.e. tidal forcing alone, tidal forcing coupled with either NE winds or SW winds) were applied to 19 distinct populations. Without wind influence, self-seeding is the principal mechanism involved in the renewal of most populations. However, larval retention ranged from under 1% up to 81% in relation to the adult habitat size and the mean velocity of tidal residual currents. Wind forcing had a strong influence on larval dispersal patterns by modifying the origin and densities of settlers as well as the degree of connectivity between populations. As a consequence, larval supply from distant populations generally exceeded local supply and the inter-annual variability of wind forcing induced large year-to-year variations in larval settlement rates. Larval exchanges occurred mainly between neighbouring populations and three groups of interconnected local populations were thereby identified. Within each group, settlement patterns were related to inter-annual variations in the direction and magnitude of larval exchanges.