Why are heatwaves a problem to marine life?
The majority of marine invertebrates and fish are what we call ectothermic where their body temperatures rely upon the temperature of the surrounding environment. These animals have a certain window of temperatures in which they can survive.
However, animals need to do more than just simply exist – they also have to perform key activities such as movement, feeding, growth and reproduction. These activities are all essential for sustaining both healthy populations and diverse marine ecosystems. Therefore, within the temperature window for survival, animals may actually have a much narrower range of temperatures that are optimal for their performance.
The high temperatures occurring during heatwave events can cause stress  and may push animals past their optimum temperature range resulting in reduced fitness and mortality in more extreme cases.
There may be some ability of organisms to partially increase their tolerance of temperature if given sufficient time to acclimatise. For example, we know that in many species thermal tolerance can increase over the months from winter to summer. However, much less work has been conducted on whether the ability of animals to adjust to temperature can keep up with the rapid, extreme temperatures typical of a heatwave.
Species that are immobile, such as many benthic invertebrates, will not have the ability to escape high temperatures and will essentially have to try to 'weather the storm'. For highly mobile species, e.g. some fish, we may see their loss from warmer areas, as they outwardly migrate seeking cooler temperatures .
This redistribution of species in our seas may also bring the increased risk of invasion of non-native species from warmer climates . Heatwaves can also cause a suite of other problems such as triggering the growth of harmful algal blooms, affecting predator-prey interactions and also reducing the oxygen available in the water for marine animals to breathe .