Accumulation and stranding process of bryozoans and benthic microalgae on a beach in southern Brazil: Eutrophication...

Strandings of bryozoans and benthic microalgae (Arribadas), with biomasses >200 tons, have been monitored in Balneáriu Camboriú, southern Brazil since 2003. Starting in 2021 after beach nourishment, the occurrences of strandings were interrupted for five months, and then restarted in 2022. To study these events and evaluate a secondary succession process, physical, chemical, and plankton data were sampled in the bay's surf zone, surface, and bottom water for a period of one year (2022 to 2023). The results showed that the organisms that make up the strandings are dominated by bryozoans in the warm months, and benthic microalgae in the cold months. However, the succession of species occurs due to other forcings such as substrate availability (the biomass of organisms itself) and physical and chemical changes in the environment. The Camboriú River, located in the south of the bay, is responsible for the entry of nutrients into the system, adding suspended particulate matter to the sediment in the northern part of the bay. The accumulation of bryozoans and benthic microalgae also occurs in the northern region, with the action of waves and tidal currents being determining factors. The resuspension of sediment in the north of the bay can also be considered a source of nutrients (ammonium) and organic matter serving as a food item for bryozoans. The bay is considered eutrophic due to the high densities of phytoplankton and zooplankton.