Effects of zooplankton grazing on the bloom-forming Cyanothece sp in a subtropical estuarine lake

Modern anthropogenic modifications to aquatic environments, specifically through changes in trophic state and hydrodynamic conditions, have facilitated increasing frequency and duration of cyanobacterial blooms world-wide. This study investigated the potential for top-down grazing control by mesozooplankton on a persistent bloom-forming (18 months) cyanobacterium, Cyanothece sp., isolated from Lake St Lucia (South Africa). Seasonal surveys and in situ experiments were conducted to assess the zooplankton community structure and its grazing impact on Cyanothece sp. cells, respectively. Grazing experiments showed that zooplankton can have high grazing impacts on Cyanothece sp., but that increased salinity levels are responsible for the observed decrease in total abundances and the shift to a halotolerant community. Most typical estuarine zooplankton species at St Lucia disappear when salinity increases above 100. Additionally, the halotolerant zooplankton species, specifically Fabrea salina, occurred in low densities at the time of the study. This would result in a major drop of grazing control by zooplankton on Cyanothece sp. The results, which indicate that zooplankton can potentially play a key role in top-down grazing control of cyanobacterial bloom, are relevant in the global context where the occurrence of persistent cyanobacterial blooms is on the rise.