Melbourne’s coral reefs. The best are under threat

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Where are Melbourne’s coral reefs?

Did you know that a marine sanctuary in northern Port Phillip Bay is home to some of the best examples of our region’s coral reefs? Melbourne’s coral reefs are at Ricketts Point Marine Sanctuary but despite their importance, this biotope hasn’t yet been recognised as significant. In actual fact, the existence of these reefs is barely recognised.

Our reefs are already threatened by rise in sea temperatures and nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More input into the bay. But they also face a new and unexpected threat. Scientists are proposing to establish kelp beds in the marine sanctuary. The first stage of this work is a huge sea urchin cull. But sea urchins are a naturally-occurring and conspicuous component of this coral reef biotope.

When pressed, Parks Victoria scientists have admitted that they forgot to include the reefs in their assessment. They do not even know where the coral reefs are located, and that introducing kelp would threaten these important and rare habitatsWhat is habitat for animals and people? Habitat, hence the word "habitable" describes the natural surroundings in which any animal (or human) lives, that houses basic needs, such as food and shelter. Vegetation, for example, is habitat for animals. On its own, habitat is not necessarily stable or sustainable, which is why it differs from an ecosystem. Habitat in disrepair More. Furthermore, kelp has never occurred in abundance at this site.

If we really want to protect our marine ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More we first need to understand and value what we have. So, before we take drastic action, here is a little introduction about the temperate coral reefs in our marine parks.

What do Melbourne’s coral reefs look like?

Temperate coral reef in northern Port Phillip Bay looks barren. It’s slow growing compared to many of its tropical counterparts [1]. The reef is not a lush coral reef like we’re used to seeing in the tropics. Instead, it is boulder-like with lots of bare rock and, importantly, sea urchins.

Here are two examples of sea urchins living in association with temperate coral reef in different parts of south-east Australia:

The dominant corals we see in Melbourne’s coral reefs are green coral (Plesiastrea versipora) and octocoral (Erythropodium hicksoni).

Our sea urchin is the purple urchin, a native species. Please note, it is NOT INTRODUCED, as some people might think. Green coral varies in colour from white to cream and bright green. Octocoral is grey but can appear orange when the polyps are extended.

Photo Gallery: Coral Reef off Beaumaris

Here are some photos of coral reef taken in about 50cm of water all along the shore in Beaumaris.

Green coral biotope description for the Melbourne region

Parks Victoria has a long-term Subtidal Reef Monitoring Program in Victoria [4]. The primary objective is to provide information on the status of Victorian reef flora and fauna. It has this to say about Ricketts Point:

Ricketts Point (Site 5) and Halfmoon Bay (Site 6) were predominantly Sargassum-Caulerpa assemblages. At both sites, the dominant species have been the green mat forming species … The habitats at both sites include sea urchin barrens, in which encrusting coralline algae and the stony coral Plesiastrea versipora are common. The sites differ in the presence of Caulerpa brownii, Caulerpa longifolia and Cladophora prolifera at Halfmoon Bay and higher abundances of P. versipora at Ricketts Point.

Biotope description

Grazed barren with crustose coralline algae and Plesiastrea

Grazed barrens with crustose coralline algae, stony coral Plesiastrea and small thallose algal patches. Soft coral Erythropodium hicksoni may be present. Grazed by sea urchins.

The relationship between green coral reef and sea urchins

Sea urchin abundance in northern Port Phillip Bay is not strongly associated with so-called ‘barrens’. The diagram below is adapted from the Port Phillip Bay Sea Urchin Survey 2019/2020. These show three locations from northern Port Phillip Bay. The left hand grids represent urchin abundance and the ‘high’ densities are highlighted in red. The corresponding ‘vegetated’ or ‘non-vegetated’ cover is represented in the right-hand grids. In ten out of 12 instances of ‘high’ sea urchin abundance, the reef was still vegetated.

Bare-looking ground with urchins does not mean barren

Calling areas with lots of sea urchins ‘unvegetated’ might not mean they are barren at all. They can be temperate coral reef. In Williamstown ‘The [golden] kelp Ecklonia radiata is only sparse and not a dominant habitat forming algae in [Jawbone Marine Sanctuary, Williamstown]. The green algae Enteromorpha and the encrusting coral Plesiastrea can be important habitat forming biota on the reef.’ [2].

Also in Williamstown, scientists found that octocoral presented a barrier to grazing sea urchins ‘which allowed localised growth of seaweeds on barren ground despite sea urchin (Heliocidaris erythrogramma) densities locally exceeding 40 individuals per m².’ It’s unlikely to have been a barrier, so much as a functional component of the ecosystem. Two animals (or plants) occurring side-by-side have a relationship that balances out.

Similarly, in Ricketts Point Marine Sanctuary, Caulerpa can be dominant (the green fluffy weed in the foreground of Image B). This is poisonous to sea urchins so when it dominates the urchins go into a kind of torpor and bury themselves under the reef. The system is so dynamic that ‘barrens’ can form in a matter of months as the urchins return to action, then disappear again later (Image A was only 8 months later).

A new threat to green coral reefs in northern Port Phillip Bay

Marine ecosystems in Port Phillip Bay are threatened by the legacy nutrient input (from the Yarra River catchment and Western Treatment Plant) and an almost total loss of resident reef fish. Coral reefs are under the same pressures but they also rely on sea urchins. Sea urchins and coral reefs go hand-in-glove, regulating each others’ abundance.

Scientists at Montague Island in New South Wales have observed that “Preliminary observations suggest that stable populations of herbivorous sea urchins at 5–15 m depth contribute to the success of this coral in sheltered temperate sites”[3].

Are sea urchins really ‘overabundant’, like the scientists are telling us?

The evidence suggests not – at least not in marine parks like Ricketts Point [5]. Even if they are in places, Parks Victoria scientists have admitted that they do not know where the coral reefs are. They also admit, that overlooking our precious coral reefs from their work, is a serious scientific omission. Sea urchins are a principle driver of ecosystem dynamics that keep these ecosystems in balance.

If sea urchins can be over-abundant, so too can kelp or coral. For example, Black (1971) did the first ever surveys of Port Phillip Bay [6]. At that time, she reported that ‘reefs [of] the star coral Plesiastrea occurs in some cases almost to the exclusion of other species.’

Based on a definition of over-abundance used today, this would make green coral a pest.

But the reality is more nuanced. Like urchins, coral is as animal. It forms a relationship with plants and animals around it, to become part of the ecosystem. Scientific papers routinely cite how one animal excludes another, or eats another, but that’s a gross over-simplification of an ecosystem. In reality, corals, urchins, kelp and countless other life forms, create a dynamic structure. There is just as much cooperation as resistance to each other. We have to be very careful pointing the blame for imbalance at one species that we regard as being a pest, before acknowledging the role of all animals together.

Why are we introducing golden kelp?

Black (1971) found Golden Kelp in only one out of seven monitoring locations at Ricketts Point. Subsequent surveys in 2009 and also found significant cover of coral reef but scarcity of golden kelp [3]. This accords with the findings in Williamstown, that suggest octocoral suppresses urchin abundance. Though again, it’s unlikely to suppress, as much as form a balance with urchins.

So, why are we introducing Golden Kelp to Ricketts Point? Kelp are coral opposites. Kelp shades coral from receiving the sunlight it needs to survive. At what stage does regeneration of Golden Kelp become a new problem? When is it ‘over abundant’ and has be culled to restore coral reefs? Golden Kelp was never dominant at Ricketts Point.

The reasoning behind work being done in our marine parks is questionable.

Another problem is Undaria or Japanese Kelp. Recognised as being among the 100 worst invasive species in the world, it is a nationally significant marine pest, on the Australian Priority Marine Pest List (APMPL). Yet the scientists’ own evidence tells us that Undaria becomes established once urchins are culled [7].

After Undaria first got established last century, Port Phillip Bay’s seaweed forests began to decline. This was well before urchin abundance was considered a problem. Peaks in urchin abundance only started to occur since about 2012, following a break in the millennial drought. The last two years of La Niña won’t have helped but it’s part of long-term cycles. Urchins could not have been the cause of decline in the past, as the two events happened decades apart.

Let’s recognise Melbourne’s precious green coral reefs

Before we start majorly tampering with animal culls we need to recognise our coral reefs. Why not make it your mission to learn more about our temperate coral reefs this year? They are easy to see just offshore.

Meanwhile, is it wise to call ‘a ‘pause’ on the ill-founded introduction of kelp? Our precious green coral reefs might face this new threat, one of our own making. Our efforts to restore habitat by over-stating the threat of urchins too, may further imbalance the ecosystem we are trying to protect.

A leading Parks Victoria scientist has even said they would rather see Undaria beds than barrens. The logic seems to be that Undaria at least provides habitat. But that’s the point. Some habitats, including coral reefs, look barren when they are in actual fact thriving. Equally, habitats that look pretty and are thriving can be poor.

Most of all, how can we proceed to manipulate our marine park ecosystems with honesty or integrity, after admitting we have no knowledge of where the most precious of Melbourne’s coral reefs are even situated?

References

1. Burgess SN, McCulloch MT, Mortimer GE, Ward TM (2009) Structure and growth rates of the high-latitude coral: Plesiastrea versipora. Coral Reefs 28:1005–1015
2. Edmunds, Matt & Flynn, Adrian. (2016). Victorian Reef Monitoring Database and Indicators Victorian Reef Monitoring Database and Indicators Document Control Sheet. 10.13140/RG.2.2.30760.49926.
3. Barton, Jan, Pope, Adam and Howe, Steffan 2012, Marine protected areas of the Victorian embayments bioregion part 1: Port Phillip Bay Parks Victoria, Melbourne, Vic.
4. Edmunds M, Stewart K and Pritchard K (2009) Victorian subtidal reef monitoring program: the reef biota in the Port Phillip Bay Marine Sanctuaries. Parks Victoria Technical Series No. 67. Parks Victoria, Melbourne.
5. Graham, T., Carnell, P. & Warren-Myers, F. (2020) Port Phillip Bay Sea Urchin Survey 2019/2020. The University of Melbourne and Deakin University.
6. Black, H.J. (1971) Port Phillip Bay Survey 2. Memoirs of the National Museum of Victoria. 32. pp 129-170.
7. Carnell, Paul & Keough, Mick. (2014). Spatially variable synergistic effects of disturbance and additional nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More on kelp recruitment and recovery. Oecologia. 175. 10.1007/s00442-014-2907-9.