Effect of nearshore surface slicks on meroplankton distribution: role of larval behaviour

During a 10 d survey off the Central Cantabrian Coast, we used GPS-drifters and bongo nets to observe water circulation and meroplankton distributions associated with 4 different nearshore surface slicks or foam lines. Accumulation of larvae was observed associated with surface convergence at these slicks. Three of the slicks moved onshore at velocities ranging between 2 and 11 cm s−1 and accumulated cyprid barnacle larvae, crab zoeae, littorinid veligers, and annelid and ascidian larvae from the onshore side of the front. The predominant onshore source of larvae suggests that, in our study area, surface slicks may result in onshore retention of larvae. Accumulation at surface slicks was greatest for larvae with swimming speeds about half the speed of the cross-frontal, horizontal surface convergence. We hypothesize that this peak corresponds to an optimum slick speed for which the horizontal surface flow is strong enough to bring significant numbers of larvae to the front, but the associated downward vertical flow at the convergence line is weak enough to be countered by upward swimming. However, we estimated that the vertical flow may be stronger than the horizontal convergence, thus buoyancy and behavioural shifts in larval swimming performance may play an important role in the frontal accumulation of larvae. Given differences in swimming capabilities of different taxa and larval stages, a surface convergence can bring about different transport outcomes, accumulating or acting as a barrier for some larvae while allowing others to move through.