Oyster Reefs – The Ocean’s Cleaning Muscle

Oyster Reefs

Originally published by ReefWatch SA in January 2015.

Water quality is a constant issue in today’s media. We talk about the quality of freshwater, drinking water, the ocean. But what can be done about it?

If you look at the water along the coast of South Australia, there are areas where the water is really murky. But, after looking at the journals of some of Australia’s first settlers, it’s clear that our oceans weren’t always like this. When the settlers first came to Adelaide, the waters were beautiful, pristine and clear. They also happened to be filled with a heap of small animals stuck to the bottom of the ocean: oysters. More specifically – oyster reefs.

Oysters are part of a group of molluscs called bivalves. Bivalves are shellfish that have two parts to their shell, which can be opened and closed. Other examples of bivalves include cockles, clams and scallops. Some of these are mobile but oysters are unable to move. This means that when the juvenile oysters settle on a surface, they attach and remain there for the rest of their lives. In many parts of southern Australia young oysters settled on top of old oysters creating massive oyster reefs but this made oysters especially vulnerable to overfishing by humans.

In the last 200 years, Australia has seen a large and drastic change in its environment and climate. This isn’t just limited to the land, the ocean has also experienced massive shifts and changes. In the 1800s oyster farming took off and became a realistic way for people to earn a living. Within the next twenty to fifty years oysters were removed from the ocean bed and oyster reefs were fished to local extinction.

Not only did this fishing remove an important species from the ecosystem but it also physically altered the underwater landscape. Removing the oysters meant that other competing species could move into their habitats, which means that in the last 150 – 200 years, the ecosystems and structure of the ocean floor has been completely changed and reconfigured. What was once a rocky oyster reef ecosystem, is now covered in seagrass and soft sands. While there are still plenty of rocky areas and other bivalves present along the southern coast, the estimated decrease in coverage of oyster reefs is about 75% according to some historical records (however, more research needs to be done to be certain).

Native flat oysters glued to ropes for deployment. Image: VIC DEPI
Native flat oysters glued to ropes for deployment. Image: VIC DEPI

But the changes in the ecosystems don’t stop there. Oysters and other bivalves are filter feeders. This means that they draw water in, remove the microscopic particles that they eat, and then release the water back into the ocean. Studies have been done to see whether this can have a positive impact on water quality and it turns out that it can! Tanks with oysters in them have drastically clearer water in them.

This is an area of research that is currently getting more attention. Some projects have even been started to reintroduce native oysters to coastal regions where they used to be.

It will be very interesting to see how successful these projects are in helping to improve our water quality and restore some of our amazing oceanic ecosystems!

Find out more about the restoration projects here.

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