Abstract
The global increase in frequency and severity of marine heatwaves (MHWs) is highlighting the impact these extreme climatic events can have on marine ecosystems. Throughout the summer of 2021/2022 northeastern Aotearoa/New Zealand suffered an unprecedented MHW. Worst impacted areas, such as the semi-enclosed Te Moananui-ā-Toi/Tīkapa Moana/Hauraki Gulf, experienced more than three continual months of temperatures at or above the climatological mean maximum (20.7 °C). During this period, we observed a rapid increase in the abundance and cover of the invasive, warm-affinity colonial ascidian Symplegma brakenhielmi on temperate rocky reefs . Population expansion of this species has not previously been linked to MHW events. Benthic monitoring as water temperatures cooled showed a sharp decrease in S. brakenhielmi abundance, but not complete disappearance, and the coverage of individually monitored colonies also declined. There was no observed increase in abundance or cover throughout the summer of 2022/2023, a potential consequence of cooler water temperatures and multiple cyclones. Observed impacts included the growth of S. brakenhielmi over other sessile invertebrate and macroalgal species, as well as on highly mobile spiny lobster, suggesting that this species can have a variety of impacts on temperate rocky reefs. In the future, as ocean temperatures continue to warm, these findings provide insight into what we can expect and highlight how MHW can facilitate the rapid expansion and integration of non-native, warm-affinity species into temperate reef ecosystems.