Insight into nearshore diatom assemblages from an island ecosystem: Composition, spatial variation, and benthic contribution to the pelagic community (Bird Island, South Africa)

Abstract

Marine epilithic diatom abundance and species richness are highly variable along the South African coastline. To determine the variability in spatial diversity and benthic–pelagic​ exchanges, we conducted a baseline study of the Bird Island ecosystem in Algoa Bay. Diatom samples were collected from the rocky substrate and the adjacent water column in conjunction with physico-chemical variables at five localities around the island. Species belonging to Achnanthes, Cocconeis, Licmophora and Navicula genera were found to be widely distributed around the island. Diatom species abundance and diversity varied around the island and a dominance of typically benthic species in the surf zone, coupled with a low abundance of truly planktonic species, was evident. The low number of planktonic diatoms in the water column and their absence in benthic samples in this study suggests an intertidal system driven largely by benthic diatoms. Elevated dissolved inorganic nitrogen (DIN), indicative of the guano influence from resident seabirds, along with temperature and turbidity, influenced the observed diatom composition and distribution. This study reveals an interplay between benthic and planktonic diatoms in the nearshore environment providing some insight into their contribution to the intertidal and subtidal food web. It further provides important information for future comparisons of benthic diatoms on Bird Island in light of their ecological indicator properties under global environmental change scenarios for this region.