Ecophysiological Effects of Marine Heatwaves on the Green-lipped mussel (Perna canaliculus)

Abstract

The green-lipped mussel, Perna canaliculus, is an economically and ecologically important species found throughout New Zealand’s coastal environment. However, the increased frequency and intensity of chronic stressor events, such as marine heatwaves, are threatening population dynamics. Furthermore, acute stressor events, such as periods of intense rainfall, can amplify the physiological impacts associated with chronic stressors. In light of recent multiple stressor events such as the 2017/18 summer, the impacts associated with acute salinity reductions and temperature spikes are predicted to be particularly significant for P. canaliculus. To investigate this, effects of marine heatwaves were simulated by exposing P. canaliculus to increased water temperatures, ranging from ambient (17 °C) through to extreme (24 °C) for two-months. Additionally, the physiological impacts of salinity reductions (36 to 0 ppt) and air temperature spikes (20 to 40 °C) were determined for subtidal and intertidal populations of P. canaliculus respectively. Subtidal P. canaliculus were found to have a high tolerance to acute salinity reductions. However, when combined with chronically increased water temperatures, an increase in sensitivity to salinity reductions occurred, as P. canaliculus closed themselves off from the surrounding environment earlier. Lastly, while a shift in ontogenetic temperature tolerance of P. canaliculus was observed, this was only significant for the critical thermal maximum (LTcrit) and not the LT50. For intertidal P. canaliculus chronic exposure reduced the thermotolerance to acute air temperature spikes. Measures of LT50 were reduced from 36.8 °C in ambient conditions to 28.3 °C extreme conditions. Furthermore, exposure to elevated water temperatures increased utilisation of energy reserves for both subtidal and intertidal populations of P. canaliculus. The results from this research demonstrate the negative impacts associated with the combination of chronic and acute stressors. The persistence of P. canaliculus under future climate change will be heavily impacted by marine heatwaves, with an increase in mortality expected for intertidal populations upon exposure to extreme air temperatures. For subtidal populations marine heatwaves are proposed to increase susceptibility to additional stressors such as disease events, as energy reserve utilisation is maximised during elevated water temperatures.