Inlet mouth phase influences density, variability and standing stocks of plankton assemblages in temporarily open/closed estuaries

Temporarily open/closed estuaries (TOCEs) are systems temporarily closed off from the sea due to the development of a sandbar across the mouth during low rainfall periods. Due to their generally small size and overall higher retention times during the closed phase, these environments are highly susceptible to anthropogenic changes such as catchment degradation, climate change, pollution and changing river flow patterns. Several authors have reported marked fluctuations in biomass and density of biotic communities between the open and closed phases of specific estuaries. This study determines the responses of water column nutrients, plankton density and biomass to inlet phase changes of these systems, for which information was collated on dissolved inorganic nitrogen and phosphorus concentrations, phytoplankton biomass, zooplankton density and biomass from 16 TOCEs in South Africa. Our analyses detected higher phytoplankton chlorophyll-a concentrations, zooplankton density and biomass during the closed phase of these estuaries. The analysis of temporal dynamics of five systems showed that plankton biomass can attain pre-breaching levels 10 days after mouth closure. Estimates showed that plankton standing stocks during the closed phase can be 26 to 10 000 times higher than during the open phase. Our findings have significant implications for the management and conservation of TOCEs, especially the regulation of freshwater inflow and artificial breaching of their mouths, and highlight the vulnerability of these systems to current and expected impacts of anthropogenic and climatic changes on their productivity and functioning.