Seaweed turn to coral

Our temperate waters are becoming tropical, but it’s not all beer and BBQ’s Ocean temperatures off Sydney are just decades away from becoming “tropical”. Modelled winter sea surface temperatures will consistently exceed 18 degrees C between 2020 and 2030. And summer sea surface temperatures will consistently exceed 25 degrees C between 2040 and 2060. Source UNSW Eastern Australia waters represent a climate change hotspot, with warming rates occurring twice as fast as the global average. A key reason for this is a strengthening of the East Australian Current. Other oceanic hotspots around the world include southern Japan, south east USA, south east Africa and eastern South America. All these regions have in common the influence of strong ocean currents running close to the shore bringing warm tropical water. Tropical fishes are becoming increasingly abundant and are already a common feature in Sydney during the late summer months. Algae-eating Surgeonfish such as the convict fish are now found off Sydney in the late summer. The biggest losers are underwater algal forests. They provide food and shelter to hundreds of species, but they need cool water to survive. High temperatures can directly stress algae by damaging the machinery that supports their survival. Warmer tropical water also carries fewer nutrients, which the algae need to grow. The harmful effect of warm-water fish on temperate reefs is most evident in southern Japan and the eastern Mediterranean, where algal forests have dramatically declined. The invasion of tropical rabbitfish in the eastern Mediterranean has created eerie barren areas extending over hundreds of kilometres. These new habitats support less than half the species found in the nearest algal forests. Divers Dr Fiona Tomas and Dr Adriana Vergés set up feeding experiments in barren Adrasan, southern Turkey, where algal forests are nowhere to be found. Murat Draman Similarly, off southern Japan over 40% of algal forests have disappeared since the 1990s. Increased annual grazing rates by tropical rabbitfish and parrotfish appear to be the culprit. Now corals dominate at many sites. It has resulted in the collapse of abalone fisheries, and 12 the cultivation of other commercially important species such as the Japanese amberjack is becoming increasingly difficult. So what can we do to stop temperate algal forests turning tropical? To prevent further dramatic changes we ultimately need to reduce our CO2 emissions. Algal beds have already been replaced by corals in Tosa Bay (Southern Japan). a) Abundant algal forests in the early 1990s; b) overgrazed algal beds in October 1997; c) Rocky barren area in January 2000; d) Coral communities present in January 2013. Eureka