Estuarine zooplankton communities shift but are not depleted during recurrent harmful algal blooms of Heterosigma akashiwo in the Sundays Estuary, South Africa

Abstract

Anthropogenically derived eutrophication and harmful algal blooms (HABs) are both a direct and indirect threat to estuarine ecosystems globally. Micro- and mesozooplankton play a crucial intermediary role in the functioning of estuarine systems. Therefore, the impacts of HABs on zooplankton communities and estuarine productivity are of interest worldwide. This study aimed to determine the changes in the zooplankton community structure in the anthropogenically altered Sundays Estuary, South Africa, which is affected by predictable and recurrent HABs of the raphidophyte Heterosigma akashiwo. Monthly sampling was conducted over 24 mo between 2018 and 2021. This was coupled with twice weekly sampling of 2 complete HAB cycles comprising no bloom, accumulation, and hypereutrophic bloom phases (during the austral spring in 2018). Generalised linear models found that salinity (31.3%) and temperature (12.1%) were positively related to zooplankton in Sundays Estuary. There was no major zooplankton mortality resulting from hypoxia or HABs during the sampling interval adopted for this study. The ubiquitous calanoid copepod Pseudodiaptomus hessei occurred at similar abundances among the HAB phases and responded to similar triggers (temperature and salinity) as the HAB species. However, predatory or scavenging invertebrates such as the isopod Cirolana fluviatilis and the polychaete Perinereis falsovariegata were recorded in higher abundances, suggesting a shift in the community structure due to HABs. The possible negative effects of the H. akashiwo HABs on the zooplankton species are likely dependent on their concentration and duration, which may fluctuate spatially and temporally in estuaries based on the river inflow and temperature.