DAY 21 – MARCH FOCUS ON NORFOLK ISLAND’S REEF
March is often the warmest month in the water on Norfolk Island’s reef, so it is no surprise that the algae are very busily photosynthesising and flourishing.
While out snorkelling yesterday, I noticed that where the algae were thickest they had created a thick carpet of bubbles that glistened in the afternoon sun. As I swam in the shallower areas, my face was gently brushed by thousands of tingling bubbles, fizzing and rising to the surface before popping into the atmosphere. It was quite a sight. And a rather weird sensation.
So what is going on here?
I’ll keep it very general and very simple, because there is a lot of chemistry happening on our reefs. This is merely a brief layman’s summary, for interest’s sake.
Let’s talk about algae
For the purposes of this post, here’s a simple definition of the types of algae found on and around coral reefs:
macroalgae – seaweed that you can see growing in the marine environment
microalgae – one-celled and living freely in the water column
zooxanthellae – tiny cells living inside corals in a symbiotic relationship.
We are going to talk about macroalgae, or seaweed, referred to here as simply ‘algae’.
Some algal growth on a reef is good and an important part of our coral reef ecosystems, but when the system is out of balance and algae begins to take over, that isn’t so good. That is what is happening at the moment on Norfolk Island’s reef. We are shifting from a coral-dominated reef to an algae-dominated one as a direct result of the poor water quality, full of the nutrients that algae love, flowing into the bays.
Human-caused pollutants (including nutrients such as phosphates and nitrates), either directly from us, or from our farming and manufacturing processes, can stimulate the growth of opportunistic algae.[i] If you then remove the main algae eaters from the coral reef environment – such as, sea urchins, sea cucumbers, parrotfish and other fishy herbivores – it allows algal growth to explode.
Rampant algal growth can cause corals to die by blocking out the light; it can lead to reduced water flows to the corals and a reduction in rates of growth; and, because there’s no real estate left, algal growth can prevent new baby corals from gaining a toehold on the reef. As well, profuse algal growth can shade other organisms in addition to the coral. The overgrowth can also abrade the the corals.[ii]
Increased alkalinity beneath the algae cover, which is caused by the algae, can lead to the coral skeleton and reef substratum dissolving away.[iii] In addition to all this bad news, recent studies have shown that ‘algae can transmit pathogens to corals’.[iv] [v]
A small outcrop of algae on a coral colony photosynthesising and producing oxygen bubbles
Creating energy to grow and reproduce
Algae use photosynthesis to create their energy, just like terrestrial plants. To do this they use carbon dioxide, sunlight and water to form sugar. In the process they release oxygen. Yesterday was a bright sunny day, the water was warm, and, as I saw when I was out on my swim, the algae was churning out those oxygen bubbles at a fast rate.
While the algae are busily growing and reproducing at such a pace, we need all the algae-eating critters to do their job more than ever. The recent initiative by Australian Marine Parks to introduce no-take zones for Norfolk Island’s three lagoons is a vital step to help protect our coral reef from being smothered. I discuss the new no-take zones in an earlier post of my March focus on Norfolk Island’s reef: You don’t always know what you’ve got – til it’s gone.
Soundscapes of our reef
Recent research has looked at the sounds made by coral reefs.
As they go about their daily lives, some reef fish will communicate with each other with clicks and clacks. You will hear other species before you see them, for example, the banded scalyfin (Parma polylepsis, locally known as an aatuti) will send out a distinctive ‘bok, bok, bok’ to warn you off his patch, while schools of elegant wrasse (Anampses elegans) can be heard gnawing on corals as they seek out small molluscs and crustaceans.
But the sound of an algae-dominated reef is much quieter. Researchers recording reef sounds have discovered that the photosynthesising algae make a distinctive high-pitched ping as the bubbles release and ascend to the surface.
This knowledge can be put to use when assessing a coral reef’s health. Coral researchers can gauge a reef’s health by the sounds it is making. When there are plenty of those distinctive pings, then that is probably not good.[vi] [vii]
I wonder what noises they would have picked up on Norfolk Island’s reef yesterday.
Recommended reading
If you are interested and would like to go into the coral reef and algae relationship further, as well as the references, below, I can recommend this article:
The Nature of Macroalgae and Their Interactions on Reefs by MM Littler and DS Littler.
References
[i] Too much algae – and too many microbes – threaten coral reefs, ‘Microbialization’ destroys reef habitats
[ii] Coral-macroalgal interactions: Herbivory and substrate type influence growth of the macroalgae Eucheuma denticulatum (N.L. Burman) Collins & Hervey, 1917 on a tropical coral reef
[iii] Chemical and Physical Environmental Conditions Underneath Mat- and Canopy-Forming Macroalgae, and Their Effects on Understorey Corals
[iv] Coral-macroalgal interactions: Herbivory and substrate type influence growth of the macroalgae Eucheuma denticulatum (N.L. Burman) Collins & Hervey, 1917 on a tropical coral reef
[v] Too much algae – and too many microbes – threaten coral reefs, ‘Microbialization’ destroys reef habitats
[vi] Click, clack and pop: sounds indicate health of coral reefs, study finds, Sofia Quaglia, 22 May 2022, The Guardian
[vii] Healthy coral reefs crackle with the sounds of busy inhabitants, Alison Bosman, earth.com