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In praise of the dancing, spitting oyster, nature’s great purifier

In thus removing excess nutrients, oysters (and other shellfish) help counteract eutrophication. This is a process whereby excess nitrates and phosphates – in particular, from fertiliser-rich farm runoff – feed excess algal growth. Deprived of light, the underlayers of algae die and rot, consuming oxygen. The water becomes hypoxic, killing fish, crustaceans and other marine life and becoming toxic also to humans.

(It fascinates me that 95 to 99 per cent of the waterways in pastoral and populated areas of “pristine” Aotearoa (New Zealand) – which has legislation giving at least one river “personal” rights – are so polluted by farm runoff that many are too toxic for swimming.)

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The Australian coastline, including Sydney Harbour, was once rich in huge shellfish reefs – the less spectacular subtropical equivalent of coral reefs. These covered many hectares. Accreted over millennia, as new oysters stood on the shoulders of their ancestors, they reached considerable heights and acted as very effective breakwaters, guarding the coastline from erosion and protecting the delicate estuarine ecosystems of seagrass and saltmarsh.

We know, too, that the shores of Sydney Harbour and the Parramatta River, as well as Broken Bay, Botany Bay and much of the coast, were dotted with equally old and almost equally vast Indigenous middens. Up to 100m high and hundreds of metres long, these middens were noted, inter alia, at Cockle Bay, Barangaroo and Bennelong Point, once a tidal island known as Tubowgule.

Now that’s all gone. With white people came exploitation. We set about burning the middens – Tubowgule, where the Opera House now stands, became known as Limeburner’s Point – reducing the calcium carbonate to mortar. With the middens gone, we started on the reefs. By the 1870s they, too, were 95 per cent destroyed.

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Since then, constant dredging and silting have kept any rocky outcrops buried in soft sediment. So, although there’s a thin bathtub-ring of oysters around the intertidal mark, the reefs cannot rebuild. The rich saltmarsh and seagrass ecosystems they protected have been replaced by mangroves. Seagrass is especially critical, not only as a marine nursery but also because it sequesters carbon 40 times faster than tropical rainforest.

Now, remediation projects are under way. With $20 million from the federal government, The Nature Conservancy’s Reef Builder project will provide hard (rock or concrete) substrate for 60 such reefs and seed them with millions of oyster larvae, starting with Port Stephens.

Every hectare of oyster reef, says the Conservancy’s Chris Gillies, will produce 375 kilograms of fish a year, remove 225 kilograms of nitrogen and phosphates and filter 2.7 billion litres of seawater. The cleaner water, and the wave protection, will nurture seagrass.

“There is a massive movement toward co-engineering ecosystems,” says Associate Professor Paul Gribben of UNSW’s Centre for Marine Science and Innovation. “Seagrass, salt marsh, oysters are connected in ways that are subtle.”

Subtle yes, but also fundamental. There’s poetry in returning the calcium carbonate we removed 150 years ago. Perhaps my sweet spitting oysters are overjoyed at the prospect, or perhaps they’re just impatient to have more of their brethren closer, faster, in Port Jackson.

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