Saving our Underwater Forests

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John Turnbull, Member, National Parks Association of NSW

Deforestation. A drive through the forests around Port Macquarie or Eden is all it takes to see the impact of the clearing of our native terrestrial forests. We can see the bulldozers and logging trucks.  On a grander scale, with the help of satellite imagery, we can see the loss of native forests over time. In the years since European colonisation, for example, Australia has lost almost 40% of its native terrestrial forests[i].

But what is the story underwater? We have forests there too, made up of kelp and other large brown algae. The surface area of oceans on earth is more than twice the area of land, so the processes of photosynthesis, oxygen generation and CO2 capture in our oceans are as important as those on land. Yet the state of our marine forests is far harder to assess – we can’t easily see them with the naked eye or satellites.

In recent years we have had extensive coverage of the impact of climate change on our coral reefs in particular[ii] – and rightly so; recent bleaching events have resulted in the death of up to two thirds of corals in the northern Great Barrier Reef[iii]. Corals are the foundation of marine communities in these tropical areas, so loss of corals impacts on the entire ecosystem. In cooler, temperate waters such as those in southern Australia, our algal forests are the foundation – so a loss of these underwater forests can be just as devastating.

So, what’s the story with our southern underwater forests? Recent work by scientists has highlighted the importance of our Great Southern Reef, spanning 70,000 km2 from northern New South Wales to the southern half of Western Australia[iv]. These forests produce up to 65 tonnes of biomass per hectare per year. They are home to unique Australian marine communities, with up to 80% of species found no-where else in the world. Yet these reefs receive less than one tenth the research funding of our coral reefs[v]. There is much we don’t know about them.

For those few people who look beneath the waves, however, the changes are there to see. We see declines in kelp densities, overgrowth of kelp by other algae, damage by storms and the expansion of urchin barrens[vi]. Whilst robust data sets are scarce, we do have some quantified evidence of loss of kelp forests; up to 95% decline of the grand Macrocystis forests of Tasmania and 90% loss of some kelp forests in Western Australia[vii]. Studies have found that urchin barrens are more abundant than kelp in some NSW sites[viii], and at the northern edge of NSW kelp forests are declining as fish assemblages become tropicalised[ix].

These changes are the result of a range of threats. Runoff from land, and sewage, bring pollutants which can wipe out large areas of forest before we even notice[x]. As our climate changes, warm, nutrient-poor water travels down from the tropics, placing our kelp forests under stress, allowing other organisms to out-compete them. Herbivores such as urchins and some tropical fishes that can overgraze kelp also travel further south on these strengthening currents.

The growth in urchin barrens is of particular concern. Urchin barrens form when sea urchins – avid consumers of algae – become over-abundant and strip an area bare. Barrens represent an alternate state – once they are established, it is difficult for kelp to reclaim the area, even if the original cause of the barren is removed. Barren-forming urchins, such as the black urchin Centrostephanus rodgersii, are so adept as scraping the rock surface that even a small number of individuals can maintain the barren. The conversion of a kelp forest to urchin barrens is therefore often a one-way street; in this case, prevention is better than cure.

It’s no easy task to prevent the spread of urchin barrens. Urchin larvae benefit from strengthening, warming currents, so tackling climate change is important, but on current trajectories this will take many years. Unsustainable fishing practices also give urchins a free kick, as humans have removed most urchin predators, namely large carnivorous fish and lobsters. Re-establishing urchin predator populations, through stricter fishing controls and effective Marine Protected Areas, is increasingly important. The job is just too big to be tackled without natural ecological controls, such as predation, being in place.

As these challenges become increasingly clear, scientists are turning their attention to restoration, but this is particularly difficult in the ocean. We can’t set up and maintain large fences or enclosures in the ocean, so actively controlling urchin numbers is difficult. Algae have no root system, nor seeds, so we can’t plant forests on a large scale like on land. Despite these challenges, successful projects like Operation Crayweed bring hope (www.operationcrayweed.com). Crayweed forests are being re-established, with a lot of time and care, by planting small patches that become self-sustaining growing populations; but restoring other species like kelp is more difficult.

So next time you go to the ocean, take a snorkel and mask and take a look for yourself. Make note of urchin barrens versus kelp forests – which are the most abundant in your area? Is algal growth a large healthy canopy, or the lesser fine, fuzzy turfing algae? How many black, long-spine urchins can you find per square metre? If we are to tackle the changes that are taking place in our oceans, a great place to start is on our home reef or beach; developing awareness, perhaps even keeping a photographic record of what we see. Programs like Reef Explorers (www.reefexplorers.org) provide a focal point for such citizen science information, including projects on kelp monitoring, Crayweed restoration and tracking urchin barrens. In a vast ocean, projects such as these will be increasingly important in maintaining and restoring healthy ecosystems and ultimately life on earth.

References

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