Siphonophore assemblages along the east coast of South Africa; mesoscale distribution and temporal variations

Using multivariate statistics we have examined spatio-temporal patterns in the distribution of siphonophore assemblages along the east coast of South Africa, anticipating constancy in cross-shelf and alongshore structure that would mirror the hydrographic stability. Indeed, while the surface temperature of the Agulhas Current does vary on a seasonal basis, the position of its core is thought to have a stable trajectory year round, and its other physical and chemical characteristics are considered to be relatively aseasonal. Two clear aseasonal assemblages were observed, associated either with the nearshore waters in the extreme SW (Port Alfred upwelling cell), or with the Agulhas Current and inshore waters north of East London. Assemblages in the current-driven upwelling cell were characterized by low overall diversity and dense populations of Muggiaea atlantica (up to 28 000 ind. 1000 m(-3)). Assemblages elsewhere in the region were dominated by Abylopsis eschscholtzi, A. tetragona, Bassia bassensis, Chelophyes contorta, Diphyes dispar, D. bojani, Eudoxoides spiralis, E. mitra, Lensia subtiloides and Sulculeolaria chuni. There is some evidence of both an alongshore and a cross-shelf pattern in the distribution of assemblages north of East London, but this is subordinate to a stronger seasonal signal. The impact of seasonality on regional assemblages is hypothesized to be reflected in significant changes in the upstream structure of Agulhas Current assemblages.