Shifts in flow and salinity shape phytoplankton communities in the Coorong, a Ramsar-listed shallow coastal lagoon (South Australia)

Coastal lagoons support high biodiversity and provide many ecosystem services but are under threat from changes in freshwater flow due to climate change and direct anthropogenic actions. Understanding how phytoplankton community structure responds to flow-induced changes in environmental conditions is critical for assessing the impact of climate change on ecosystem function. This study investigated patterns in phytoplankton abundance and diversity across a wide range of environmental conditions, from extreme dry (drought) to extreme wet (flood), by analysing a 16-year dataset (2007–2023) in the Coorong, South Australia, a Ramsar-listed shallow coastal lagoon. The Coorong stretches over 100 km long and functions as a reverse estuary, with a salinity gradient from fresh to brackish water near the entrance to the ocean to hypersaline in its southern reaches. Multiple statistical analysis techniques, including correlations, analysis of principal coordinates, biota and environmental matching, distance-based linear modelling and generalised additive modelling identified salinity and water level as the principal drivers of change in the abundance and diversity of the phytoplankton community, with distance from the ocean inlet and water temperature also significant, but to a lesser degree. Phytoplankton communities were characterised by salinity-derived habitats, from ‘freshwater’ H1 to ‘extreme hypersaline’ H6. Cyanoprokaryotes and chlorophytes dominated in H1, whereas Diatoms and Dinoflagellates increased their relative abundance with increasing salinity, ultimately dominating hypersaline H6. The most equal representation of all groups occurred in estuarine H2. This work supports the development of a habitat-based predictive framework for coastal lagoon management to adapt to climate change.