Sessile invertebrate assemblages on artificial structures at a commercial kelp farm and on natural substrates at an adjacent kelp forest site, Lüderitz, Namibia: baseline patterns of spatial and temporal variation

Abstract

Kelp aquaculture is expanding globally, yet baseline information on sessile invertebrates associated with farm infrastructure remains limited in southern Africa. We characterised assemblages on artificial structures at a commercial Macrocystis pyrifera farm in Shearwater Bay and on natural substrates within an adjacent kelp forest at Diaz Point, Lüderitz, Namibia. Three sampling campaigns were conducted during 2024. Individual-count and percentage-cover data were used for within-site substrate analyses, while presence-absence data were used for an exploratory description of broader assemblage patterns. Thirty taxa from eight phyla were recorded: 24 at Diaz Point and 19 at Shearwater Bay, with 13 taxa shared. Because the sites differed in sampling effort, substrate composition, kelp species, seabed type and hydrodynamic exposure, these observed totals and between-site patterns are descriptive rather than estimates of habitat effects. Within each site, assemblage structure varied among substrates and sampling campaigns, although abundance-based PERMANOVA results were partly associated with heterogeneous multivariate dispersion. Occurrence-based ordination showed partial separation with overlap; Shearwater Bay samples were more tightly clustered, whereas Diaz Point samples occupied a broader ordination space. Farm structures were frequently occupied by mussels and barnacles, while the natural-site taxon list included a wider range of sponges, anemones, bryozoans and polychaetes. The study establishes a baseline for monitoring fouling assemblages, non-native kelp cultivation and farm operations in the Lüderitz coastal system.