Mechanisms of nearshore retention and offshore export of mussel larvae over the Agulhas Bank

Ecological connectivity is critical for population dynamics but in many benthic species it is complicated by a planktonic larval phase, whose dispersal remains poorly understood. Using a plankton pump, we examine the distribution of intertidal mussel larvae along three axes: alongshore, cross-shelf and by depth during a large scale (600 km) cruise over the Agulhas Bank off southern Africa in August/September 2010. As a general pattern, higher veliger abundances were found close to the coast. Our analyses of the nearshore flow, estimated from ADCP data and the vertical distribution of larvae, show that onshore larval retention may be mediated by active vertical swimming through the water column guided by light and wind-induced turbulence. A massive offshore export of larvae off St Francis Bay was, however, observed during an Agulhas Current meander which influenced inner shelf waters. We hypothesize that, by increasing and homogenizing flow, the Agulhas Current may erase the effects of larval vertical positioning on onshore retention and transport larvae offshore. Our study highlights the need to integrate the effects of complex, region-specific physical dynamics with the swimming behaviour of larvae in order to explain their spatial distribution, population connectivity and the consequences for population dynamics.