CLIMATE CHANGE IS AN insidious entity, creeping up on the world and largely occurring so gradually that naysayers claim it to be an elaborate hoax by the world’s scientists. Whilst many effects of climate change happen so slowly they’re almost imperceptible, its impacts can be more concentrated in certain parts of the world. The most dramatic effects of climate change on the world’s oceans have been the three global coral bleaching events, where huge swathes of coral reefs have been decimated. In 1997–9, 16 percent of the planet’s hard corals died; worryingly the impacts of the world’s longest and most widespread event that began in 2015 are yet to be fully tallied.
Being a marine biologist, and having dived the world’s oceans for over 20 years, I have seen firsthand the effects of human-induced changes on our planet’s oceans. In 1998, I saw the Maldives at the pinnacle of their bleaching, and returned 15 years later to see how little some of these reefs have recovered.
Whilst the tropics are often the media’s focus, climate change isn’t only happening in tropical seas. In March of 2011, I dived the cold waters of the Tasman Peninsula in southeast Tasmania. At that time we dived an area called Waterfall Bay, which was home to one of the region’s last giant kelp forests. Previously, Tasmania’s kelp forests were so dense as to allow commercial harvesting, but in 2011 they were restricted to just a few small areas in the very south of the island.
In February of this year I returned to dive these giant kelp forests again, only to learn that they are no more. Warming waters, uncharacteristically vicious storms and the arrival of new species have all contributed to the kelp’s demise. But why has Tasmania been so disproportionately affected?
WHAT’S HAPPENING IN TASMANIA?
Although the population of Tasmania is very small, it’s suffering a huge amount of marine degradation. The southeast of Australia is a hotspot of marine climate change, and among the top 10 percent of areas where the oceans are warming fastest. Australia’s southeastern waters have warmed at almost four times the average global rate and indications are that this will continue. Due to the huge amount of research interest in the area, it is acting as a living laboratory for how other areas are likely to change in the future.
When researchers began to notice the arrival of new species in Tasmanian waters some decades ago, they started to sit up and take note. Warming waters can have various different influences on the native organisms that live there. Some species move towards the poles or into the deep, where waters are cooler; some already exist in such a narrow window of biological tolerance that their numbers dwindle. Warmer waters can also change the timing of an organism’s life cycle as well as impacting their growth. Tasmania has seen many of these influences on its marine ecosystems.
TASMANIA’S WARMING WATERS
Why are Tasmania’s waters warming so much faster than other areas of the ocean? Thankfully there has been a long history of research off Tasmania and it’s been possible to collate all this data to give a clear picture of the situation.
One of the biggest influences in oceanic currents around the Australian mainland is the East Australia Current (EAC), which flows down the east coast, carrying tropical water south. Nemo, from Finding Nemo, was caught up in this current and it carried him from the Great Barrier Reef to Sydney, although this current also continues southwards beyond Sydney. Generally, the cool waters of the sub-Antarctic zone abut the EAC at its southern reaches and the Antarctic waters move northwards in winter when the EAC retracts.
However, over recent decades the EAC has penetrated some 350 kilometres further south, bringing its warm waters into areas that were previously dominated by cold. This change in the power of the EAC is believed to be a result of greenhouse warming and local ozone depletion.
GOOD FOR SOME, BAD FOR OTHERS
Whilst I was diving in Tasmania, I met a group of researchers and citizen scientists who were searching for handfishes. This elusive and interesting group of prototype frogfishes are only found in southeastern Australia, and most are in Tasmania. Of the 14 species, several are known from only a single specimen. Meanwhile, many of those that have been historically more common have suffered dramatic population crashes and these are some of the species that the researchers were hoping to find. Some species haven’t been seen alive for a decade.
After diving the very south of Tasmania, we went in search of the spotted handfish, which is critically endangered and now only known from a small area within the Derwent Estuary outside Hobart. The introduction of the invasive North Pacific sea star has had a huge impact on its populations. These voracious invaders eat the tiny stalked sea squirts that the handfish lays its eggs on. The tiny planktonic larvae of these sea stars probably arrived here in the ballast water of huge tankers from the north.
Tasmania’s giant kelp, on the other hand, has been impacted by another Australian resident, not one introduced by man. Longspine urchins are common along the east coast of mainland Australia, but the cold waters in Tasmania have always hindered their extension southwards. As climate change has warmed the waters in Tasmania, the urchins have taken up residence in Tasmania, extending their range southwards by 640 kilometres over the past 40 years. They prevent young kelp from becoming established and create “urchin barrens” where they eat every living thing down to the rock.
For the rest of this article and other stories from this issue, see Asian Diver 2017 Issue 1 Volume 144