Ocean warming causes physiological stress in cool water species and facilitates the expansion of warm water species onto temperate reefs. This is leading to range contractions of important cool water organisms such as canopy seaweeds, which fulfil a role similar to trees in forests. These underwater forests provide food and shelter to hundreds of species, and support Australia's most economically important fisheries, such as rock lobster and abalone.
Increased temperature impacts seaweeds directly and indirectly, e.g. via effects on species that eat them. In Tasmania, warming has been directly linked to a decline in giant kelp forest canopy cover of over 90% since the 1940s. In Western Australia, an extreme heat wave caused the disappearance of several dominant seaweeds over ~ 100 km.
The greatest indirect impacts of warming in seaweed forests are caused by herbivore consumers. As the distribution of warm water herbivores expands, cool water seaweeds that have evolved under low levels of herbivory face increasing herbivore diversity and grazing intensity. In Tasmania, warming has led to the range expansion of a sea urchin that has overgrazed kelp forests. Similarly, increased feeding by tropical and subtropical fishes is increasing herbivore pressure on seaweeds near the warm edge of their distribution.
The climate-mediated loss of seaweeds has reduced the biodiversity of temperate reefs in Tasmania and the Mediterranean, and has been linked to the collapse of commercial fisheries in Japan. Elsewhere in Australia, climate change is emerging as a key threat to algal forests and the species they support.