Oysters and clams are some of nature’s most efficient feeders: these shellfish slurp up gallons of water, sieving out food and nutrients as they go, and repurposing some of those raw materials to make their shells.
Now researchers have calculated that if coastal cities and towns planted beds of these industrious bivalves along the urban edge, they could save councils several million dollars in clean-up costs associated with nitrogen pollution – all thanks to the water-purifying power of shellfish gills.
Filter-feeding shellfish have long been celebrated for their cleansing abilities, especially when it comes to nitrogen pollution, which wends its way down to the coast from sources like farms when nitrogen fertilizer is overapplied. This can cause eutrophication and dead zones in oceans and lakes. But bivalves are hungry for these nutrients and can successfully cleanse them from inflowing water – so they represent an obvious nature-based solution to this threat.
However, the economic value of their pollution-fighting appetite hasn’t been widely quantified at local scales, which is what the researchers on the new study wanted to do. To do so, they applied the reverse logic of what it would cost us to clean up nitrogen pollution, if oysters and clams weren’t there to do the dirty work.
They focused their study on the American coastal town of Greenwich in Connecticut, where oyster and clam aquaculture is part of the local economy. Using their method, called ‘transferable replacement cost’, they estimated how much it would cost to build and maintain the infrastructure that would be needed to replace the water-cleansing contribution of bivalves.
To tally that up, they first had to estimated how much of a dent shellfish aquaculture currently makes on different sources of nitrogen pollution from Greenwich. This revealed that the beds of diligent bivalves, filtering away, currently remove almost 10% of Greenwich’s overall nitrogen load – which amounts to 14,000 kilograms each year. Broken down by source, that figure went up notably. Clams and bivalves, it turns out, remove 38% of the nitrogen that originates from local fertilizer use on land, and a striking 51% of nitrogen from septic sources – which are those originating from households and wastewater flow.
The multiple sources of nitrogen on land means that tackling it with infrastructure can be a challenge, because it requires a variety of different technological approaches such as improved stormwater management, and specialised septic tanks, to intercept the pollutant on its journey to the coast. And this increased infrastructure would be costly too, the researchers proved.
They showed that if the contributions currently made by oysters and clams had to be replaced by infrastructural upgrades to tackle nitrogen pollution, it would cost Greenwich an additional $2.3 to $5.8 million a year. But by filtering away beneath the surface, shellfish quietly sink this invisible cost.
Beyond cost-savings, the researchers mention the multiple other benefits that shellfish aquaculture can also bring, and which they didn’t explore in the study – such as cleansing pollutants other than nitrogen out of the water, creating habitat for aquatic life, providing coastal buffering against weather extremes, and of course, food if those shellfish are harvested for human consumption.
It’s clear from the data, however, that shellfish – however efficient – aren’t a complete solution to nitrogen pollution. Human-made infrastructure and technologies are needed too, and ideally some of those would be devoted to capturing more nitrogen at source, before it gets into rivers, lakes, and the sea.
Yet the study does show just how surprisingly large a role these natural cleaners can play in dealing with some of our worst environmental challenges. And although Greenwich is just one case study, the researchers hope that by demonstrating the clear cost savings, it might encourage other coastal towns and cities to embrace shellfish aquaculture too. “Our hope is that the approach we developed here can help inform local discussions about aquaculture around the country,” they say.
Rose et. al. “Quantification and Valuation of Nitrogen Removal ServicesProvided by Commercial Shellfish Aquaculture at the Subwatershed Scale.” Environmental Science and Technology. 2021.
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