Two major floods affect fish, invertebrates and marine habitats within a temperate estuary

Abstract

Context Severe flooding can cause physiological stress and mortality for marine organisms within estuaries through reduced salinity, increased turbidity and sedimentation, and the delivery of nutrients, organic matter and contaminants. Aims The Port Stephens estuary in Australia was used as a case study to investigate the impacts that severe flooding can have on marine ecosystems in a temperate estuary. Methods Data on habitat cover, fish assemblages, and invertebrate assemblages were collected using underwater visual census from 2008 to 2025. These data documented baseline conditions from years without major flooding and affected conditions from years with major flood events (2021 and 2025). Analyses were then conducted to compare marine ecosystems within the estuary at three sites between years with and without major flood events. Key results We found that major floods in the Port Stephens estuary were followed by significant ecosystem-wide changes, with significant declines in canopy-forming macroalgae, sessile filter feeders and invertebrates, and substantial changes to fish and invertebrate assemblages. Conclusions Flooding events such as those observed in 2021 and 2025 pose a major challenge to the resilience of estuarine reef ecosystems, because recovery can take up to 1 year after a major flood. Implications If the magnitude and frequency of flooding continue to increase, as projected under future climate-change scenarios, this could lead to a cumulative loss of habitat complexity and biodiversity.