Extreme flooding and reduced salinity causes mass mortality of nearshore kelp forests

Extreme flooding due to climate-induced increases in storm frequency and intensity may negatively impact marine species and habitats. These impacts may be particularly severe in nearshore marine systems where freshwater influx and reduced salinity will be most apparent. In March 2021, severe storms associated with an intense La Niña occurred along 450 km of the east Australian coast causing widespread flooding and freshwater incursion into estuaries and nearshore marine systems. These floods were linked to widespread kelp mortality (Ecklonia radiata) in nearshore systems, particularly near and within estuaries. We quantified levels of kelp mortality with distance from the entrance and depth in one of the most impacted estuarine embayments, Port Stephens. Experiments were conducted to confirm that the flood-induced reduction in salinity was the primary cause of the observed mass mortality of kelp sporophytes and to determine the likelihood of recovery from surviving gametophytes. Floods caused a ∼50% net loss of kelp within the embayment, with almost 100% mortality at shallow depths (<4 m) and increased loss with increased distance from the embayment mouth. Experiments verified that reduced salinity, at levels comparable to those during the floods, caused severe reductions in kelp health, photosynthetic yield and tissue strength. Contrastingly, experiments on gametophytes revealed that they were resilient to short-term reduced salinity. This likely facilitated the rapid recovery of kelp that was observed seven months after the floods. Overall, we identified that future increases in climate-induced severe weather and storms events are likely to increasingly impact kelp and the ecosystems they underpin in nearshore systems.