Upwelling intensity and wave exposure determine recruitment of intertidal mussels and barnacles in the southern Benguela upwelling region

Abstract

Marine larval dispersal and recruitment dynamics are fundamentally linked with nearshore circulation. In coastal upwelling systems, shoreline topography induces predictable circu- lation patterns, spanning a range of spatial and temporal scales. Based on a 5 yr time series of monthly recruitment of intertidal mussels and barnacles at 8 sites along 400 km of the southern Benguela upwelling region, which extends around the Cape of Good Hope, South Africa, we addressed the hypothesis that recruitment will exhibit spatial and temporal patterns reflecting the influence of topography and season on upwelling. Both taxa showed strongly seasonal recruitment peaks: mussels during the summer upwelling season and barnacles during spring. Spatially, mussels and barnacles differed in their regional-scale recruitment trends: recruitment of mussels increased northwards while that of barnacles peaked in the south. However, their mesoscale recruitment pat- terns were alike, with lower recruitment rates at headland upwelling centers than in adjacent bays. On a smaller scale of hundreds of meters, wave exposure had a positive effect on the recruitment of mussels and (to a lesser extent) barnacles. Spatial patterns were remarkably persistent over time and, for barnacles, strongly negatively correlated with upwelling, as quantified by a site-specific coastal upwelling index. The effect of temporal upwelling dynamics on recruitment was site dependent: mussel recruitment was strongly and positively correlated with upwelling at all sites, while barnacle recruitment showed a weak and spatially inconsistent link to upwelling fluctuations. The persistence of the spatial structure of recruitment and its coherence between unrelated taxa emphasizes the role of shoreline topography, thereby increasing our capacity to include regional-scale processes in the management and conservation of coastal ecosystems.