Structure and succession of the surf-zone phytoplankton in Cassino beach, southern Brazil

Abstract

Cassino Beach, in Southern Brazil, is an exposed, intermediate to dissipative sandy beach, which presents conspicuous accumulations of the diatom Asterionellopsis glacialis in the surf-zone. Despite this phenomenon represents a very high input of organic matter to the beach trophic chains, it is only occasional and during most of the time, surf-zone phytoplankton is influenced by coastal and oceanic assemblages. The present work attempted to identify the phytoplankton assemblages structure (“community structure”) and the environmental factors that control its succession. An one year weekly sampling was carried out at a fixed point in the surf-zone, for determination of phytoplankton (qualitative and quantitative analysis), physical-chemical variables of surf-zone water, beach morphodynamics features and local meteorological conditions. Multivariate analyses were conducted(correspondence analysis and canonic correspondence analysis) and the results discriminated four distinct groups of species: group (1) benthic species, group (2) benthic and epibenthic species associated with A. glacialis patches, group (3) neritic planktonic chain forming diatoms and group (4) neritic planktonic species. A. glacialis dominated in biomass, but was always accompanied by other benthic species. The A. glacialis accumulations have been identified as local interferences (at the surf-zone) in the succession patterns of regional neritic phytoplankton, increasing biomass while decreasing diversity and the relative importance of neritic species. The most important environmental variables controlling this complex succession were dissolved inorganic nitrogen, dissolved silicate, water temperature and surf-zone width.