Intra- and interannual changes in siphonophore assemblages over the Transkei shelf, South Africa

Abstract

As one of the world's strongest western boundary currents, the Agulhas Current plays a significant role in global and regional climate, yet its impact on local shelf processes is not fully understood. Recent oceanographic studies conducted over the Transkei Shelf in 2017, reveal pronounced seasonal impacts of The Current on the dynamics of shelf circulation that are reflected by phytoplankton communities. We extend that investigation here to look at mesoscale coupling with the zooplankton (using siphonophores), anticipating seasonal and cross-shelf changes that mirror the bio-physical environment. We also compare these data with historic information from the same region (collected in 1990/1991), anticipating little interannual change given the wide-ranging nature of most siphonophores. The structure of regional siphonophore assemblages in 2017 was strongly influenced by season, latitude and depth: offshore waters were characterized by high numbers of species, low abundances, and a relative homogeneity in numerical composition, perhaps typical of Tropical Surface Waters. Shelf assemblages were of higher abundance but lower richness, as well as a greater heterogeneity in numerical composition that reflected the upwelling of diverse water masses onto the shelf. Persistent upwelling off Port Alfred and between Capes Padrone and Recife were characterized by high densities of Muggiaea atlantica, regardless of water source. As expected, siphonophore assemblages between 1990/1991 and 2017 did not change in structure, and indeed the cross-shelf pattern was maintained during both surveys. Our results highlight the value of siphonophores as indicators of water masses and emphasise the role they can play in understanding the stability of marine ecosystems over time.