Warming temperatures and prevalence of marine heatwaves in a temperate fjord

Climate change is causing shifts to thermal regimes in coastal marine environments often expressed as alarming marine heat waves; however, such changes are not well understood or observed in fjord environments. Ongoing exposure to intensifying marine heatwaves is becoming increasingly likely and could cause significant and even catastrophic outcomes on the unusual and sensitive ecosystems that characterize fjords. Here we identify spatial and temporal changes in temperature and salinity in the surface waters (0.5–19 m) of a temperate fjord, Doubtful Sound, New Zealand, utilising high resolution data. Data were collected between 2005 and 2023 and recorded at 15-min intervals at three sites, an outer-, mid- and inner-fjord site. IIncreasing temperature anomalies, including two marine heatwave events, were superimposed on seasonal and interannual temperature and salinity patterns. We detect both increasing temperatures through time, from the surface to 19 m and a notable freshening of the near surface low salinity layer. Annual mean temperatures increased at a rate of 0.39–0.67 °C per decade, with the deeper waters (11–19 m) warming faster than shallow waters (<2 m). This is one of the first studies to document the temporal oceanic progression of a heatwave into a fjord, taking several hours for the most intense temperatures to travel from the outermost site to the innermost site, about 30 km. The documented heatwaves in this temperate fjord caused increases of 2–3 °C above mean summer temperatures. Marine heatwaves are emerging as a pervasive threat to marine communities; hence the ecological implications of these findings are considered. • Two marine heatwaves in temperate fjord with different effects on system. • Decreasing salinity in fjord surface waters, not linked with glacial ice melt. • Fjord temperatures increasing steadily over 17 years. • Temperature anomalies increasing through time in temperate fjord.