Missing Underwater Forests of Australia
Large, canopy-forming macroalgae provide structural complexity, food and habitat for coastal marine ecosystems and other marine organisms. When these habitat-formers decline or disappear, the ecosystem loses its complexity, biodiversity decreases and many ecosystem services are also lost. Losing large seaweeds from temperate reefs has analogous ecosystem-level implications to losing corals from tropical reefs. Dr. Alexandra Campbell from the University of New South Wales elaborates on her research and the impact of these ‘missing underwater forests’.
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Changes on the Coast
Although seaweed is the dominant habitat-forming organism along temperate coastlines, one of the major macroalgae of Australia, Phyllospora comosa, has disappeared over the last forty years from the urban shores around Sydney, Australia.
Human activity is likely related to the degradation of these habitats in urbanized areas: During the 1970s and 1980s, humans discharged large amounts of sewage from nearby cities along surrounding coasts. Unfortunately, despite significant improvements in water quality around Sydney since, Phyllospora has not returned. To test whether Phyllospora can ever be restored in reefs where it was once abundant, authors of a recent PLOS ONE paper transplanted Phyllospora into two reefs in the Sydney area.
Interview with an Expert
In this interview, corresponding author Dr. Alexandra Campbell from the University of New South Wales elaborates on the group’s research and the impact of these ‘missing underwater forests’:
You’ve said that “seaweeds are the ‘trees’ of the ocean”. Can you tell us a little more about your study organism, Phyllospora, and explain its importance for coastal ecosystems around Australia?
Phyllospora comosa (known locally as ‘crayweed’) grows up to 2.5 m in length and forms dense, shallow forests along the south-eastern coastline of Australia, from near Port Macquarie in New South Wales, around Tasmania to Robe in South Australia. Individuals appear to persist on reefs for around 2 years and are reproductive year round.
Why Seaweed is Important?
How do these ecosystems change with the reduction of seaweed forests?
Large, canopy-forming macroalgae provide structural complexity, food and habitat for coastal marine ecosystems and other marine organisms. When these habitat-formers decline or disappear, the ecosystem loses its complexity, biodiversity decreases and many ecosystem services are also lost. Losing large seaweeds from temperate reefs has analogous ecosystem-level implications to losing corals from tropical reefs.
Kelp in Hazard Bay, Australia
Photo by Bjørn Christian Tørrissen
We’re interested in learning more about how you got involved in this research. Can you tell us how you became interested in studying Phyllospora?
For my doctorate, I studied how changing environmental conditions may disrupt relationships between seaweeds and microorganisms – which are abundant and ubiquitous in marine environments – potentially leading to climate-mediated diseases.
During my PhD, my colleagues (Coleman et al.) published a paper describing the disappearance of crayweed from the urbanised coastline of Sydney and hypothesised that the cause was the high volume, low treatment, near shore sewage outfalls that used to flow directly on to some beaches and bays in the city.
I wondered whether this pollution may have disrupted the relationship between Crayweed and its microbial associates and that’s how I got involved in the project.
Looking For Improvement
Why is the loss of canopy-forming macroalgae difficult to study retrospectively and how has this informed your current study?
Once an organism has disappeared from an ecosystem, it can be difficult to piece together the processes that caused its demise, particularly if the disappearance occurred several decades ago and the ecosystem state shifted dramatically as a consequence.
In our study, we hypothesized that poor water quality might have caused the decline of Phyllospora. There have been significant improvements in water quality in the region since the decline, but the species and ecosystems they used to support have failed to recover.
To test whether the water quality has improved enough to allow recolonisation of this seaweed, we transplanted the seaweed back onto reefs where it was once abundant.
South Cape Bay, Australia
Photo by JJ Harrison
What were some of the difficulties you faced while conducting your research?
The survival rates of transplanted seaweed were very good, suggesting that with a little help, this species may be able to recolonize Sydney’s reefs. Moving hundreds of large seaweeds many kilometres from donor populations to the restoration sites was a big job.
Thankfully, we received a great deal of help from many volunteers from the local community–mostly divers, with an interest in conserving and restoring the marine ecosystems they visit recreationally and value as a natural resource.
You’ve talked about Phyllospora ‘recruitment’ at one recipient site. Can you explain in greater detail what a ‘recruit’ is and how this is important for the success of a restoration site?
Phyllospora reproduces sexually, with gametes from male individuals fertilizing gametes from females, forming zygotes, which then attach themselves to the bottom (usually not very far from their parents) and grow into juvenile algae which we call ‘recruits’. In the context of restoration, the high level of recruitment (i.e. successful reproduction) we observed at our transplant site is very encouraging because it creates the possibility for the establishment of a self-sustaining population of Phyllospora at this site for the first time in many decades.
Courtesy of Norman Einstein
Why do seaweed forests receive less attention than other marine ecosystems, for example mangroves or coral reefs?
Most people don’t think about seaweeds very often. When they do, it’s usually because the sight, touch or smell of seaweed on the beach is annoying or offensive. Even the name “seaweed” conjures negative imagery so perhaps it’s a PR issue!
Arguably, macroalgae have traditionally received less attention from marine ecologists than other marine ecosystems as well, with much more attention and funding going to coral reef research.
With global patterns of declines of temperate, habitat-forming macroalgae, this needs to change and our understanding of the processes that affect seaweed populations needs to grow.
What would a successful restoration of underwater kelp forests mean for the ecosystem and for the local population?
It’s our hope that, by restoring habitat-forming macroalgae like Phyllospora, we will also enhance populations of other organisms that rely on this species for food or shelter. Detecting such follow-on benefits of our seaweed restoration program is the focus of ongoing research and our initial results are very encouraging.
A Forest of Variety
Seaweeds cover this rocky seabed on the east coast of Australia.
Photo by Toby Hudson
More Testing to Come
You’ve mentioned that larger scale restoration would be a sound way of combating the grazing (herbivory) you saw. What is the next step forward for you?
Enhanced grazing may be another mechanism by which Phyllospora disappeared from these reefs (or perhaps why it’s failed to recover). The impacts of grazing we observed were site-specific, so further investigations in to why one place was so severely impacted by herbivores while the other was not, are needed.
Our first step towards resolving this is to establish more numerous restoration patches of different sizes to see whether we can satiate the herbivores and whether smaller patches are more susceptible to grazing than larger patches.
This article originally appeared on PLOS.org.