Microalgal dynamics in a shallow estuarine lake: Transition from drought to wet conditions

• High microalgal biomass levels were maintained throughout all climatic phases.
• Regime shifts were detected, coinciding with the end of the drought and the beginning of the wet phase.
• Despite hypersaline condition, cyanobacteria biomass was high in the lake basins.
• Communities shifted from cyanobacteria to chlorophyte-diatom dominance during the transition.
• Lake St. Lucia may therefore be relatively resilient to extreme drought events.

Shallow coastal lakes are under increasing pressure from climate change. Low rainfall and reduced run-off contributed to an unprecedented drought in Lake St. Lucia since 2002. Physico-chemical variables and microalgal biomass are analysed, tracking the transition from drought (2009) to wet conditions (2014). Despite low water levels and habitat loss due to desiccation, microalgal biomass remained high mainly due to cyanobacterial contribution. The system exhibited distinct spatio-temporal patterns in terms of salinity, water level, DIN, microalgal biomass and class composition associated with the drought, transition and wet climatic phases. Regime shifts were detected, coinciding with the end of the drought and the beginning of the wet phase. The St. Lucia ecosystem responds rapidly to changes in climatic phases while sustaining microalgal stocks; it may therefore be relatively resilient to extreme drought events.