Abundance, growth and recruitment of Mytilus galloprovincialis on the west coast of South Africa in relation to upwelling

Upwelling has the potential to influence the structure of intertidal populations and communities by enhancing productivity and thereby food supplies, and by influencing the transport and abundance of propagules. We investigated alongshore variability in Mytilus galloprovincialis populations in relation to upwelling intensity at 11 sites spread across similar to 1000 km of the west coast of South Africa. Mussels grew faster at upwelling centres than at downstream sites characterized by weaker upwelling, presumably due to greater availability of food supply provided by organic material from nearshore subtidal kelp beds and/or phytoplankton. Maximum sizes reached by mussels were correlated with growth rate. Large among-site variability existed in mussel recruitment, density, percentage cover, biomass, mean size, condition and shell thickness. However, contrary to predictions of bottom-up regulation, none of these differences were consistent with differences in upwelling intensity. High growth rates did not, therefore, influence local population structure. In general, where densities were low, mean size was larger and shells were thinner. This implies that although upwelling influences growth rate and maximum size of M galloprovincialis, these effects are overridden by other factors such as recruitment and intraspecific competition so that fast growth does not translate into greater abundance, biomass, cover or improved condition at upwelling sites.