The effect of mesoscale oceanographic features on the distribution of mussel larvae along the south coast of South Africa

The coupling of benthic and pelagic life stages of marine invertebrates is central to understanding their population dynamics and is influenced by hydrodynamic processes at multiple spatial scales. Here we provide a regional scale study that identifies mesoscale oceanographic features linked to the distribution of mussel larvae. We surveyed the distribution of mussel larvae along 12 shelf to slope transects within an alongshore distance of about 700 km along the Agulhas Bank off South Africa. Oceanographic and larval abundance data were collected simultaneously to determine how larval distribution is related to physical factors. Larval abundances were most closely correlated with temperature, salinity, beam attenuation (which indicates particle concentrations), and zonal (east-west) water velocity. In most regions, the highest larval concentrations were observed within an average distance of 3.7 km from the coastline. Three mesoscale oceanographic mechanisms were identified that could explain larval distribution on separate parts of the coast. These were: a solitary meander whose leading end advects the Agulhas Current onto the continental shelf as an Agulhas Current Meander and whose lagging end advects the Agulhas Current offshore as a Natal Pulse, and the accumulation of larvae in a region with known internal wave activity. The Agulhas Current Meander swept large numbers of recently “spawned” larvae up to 30 km offshore, acting as a corridor for larval loss, or a population sink. Onshore retention seems, however, to be generally characteristic of this region, when the Agulhas Current is not on the continental shelf. This study highlights how mesoscale features are associated with larval density, transport, and coupling between the pelagic and benthic systems of the Agulhas Bank.