Published On: Thu, Dec 7th, 2017

A nasty-tasting shellfish could be just the job for cleaning rivers

SHELLFISH thrive in waters rich in nutrients. These include the nitrogen used in fertiliser, which passes from the land into rivers and then into the sea. The shellfish grow, as do the profits of those who harvest them. The problem comes when discharges into the sea are tainted with more noxious material, such as bacteria that pose a threat to human health. Once the bugs are in the shellfish, they can be passed on to anyone who eats them.

This problem—and another, of excess nitrogen that can cause poisonous algal blooms—might be mitigated by shellfish that people don’t eat, reckon Eve Galimany and Julie Rose at the National Oceanic and Atmospheric Administration at Milford Laboratory in Connecticut. As they report in Environmental Science & Technology, their chosen candidate for the job is the ribbed mussel, more formally known as Geukensia demissa.

The ribbed mussel is edible, but it tastes terrible and so has no commercial value. This means growing the mussels in tainted waters is unlikely to tempt anyone into harvesting them. Previous studies have shown that the ribbed mussel is both hardy and adept at collecting a range of troublesome materials from its environment. Dr Galimany and Dr Rose thought it would be ideal to help clean up the Bronx River Estuary in New York. With an industrial waterfront and wastewater run-offs from a dense urban environment, the estuary has a long history of suffering from harmful bacteria and high levels of nitrogen.

With a group of colleagues they moored a six-square-metre commercial mussel-growing raft in the estuary and populated it with ribbed mussels. They closely monitored the health of the mussels over six months and, using a flow-through device, also analysed the chemistry of the water both before and after the mussels had done their filtering. The results were impressive.

The researchers found that not only did the mussels thrive in the polluted waters of the Bronx River Estuary, but they also collected a lot of pollutants. More specifically, a fully stocked raft of mussels cleared an average of 12m litres of water daily, removing 160 kilograms of particulate matter, of which 12 kilograms was absorbed by the mussels’ digestive systems and integrated into their bodies. The remainder was excreted as waste, which drops down and is ultimately buried in the river sediment. The material filtered out by the mussels included nitrogen, bacteria, relatively harmless trace metals like aluminium, copper and iron, as well as toxic metals like mercury, lead and arsenic. Five groups of organic contaminants, including the insecticides chlordane and DDT, also ended up accumulating in the mussels.

Based upon these numbers, the team estimates an annual harvest of a single raft of mussels would remove more than 62 kilograms of nitrogen waste alone by sequestering it into the tissues and shells of the animals. But what can be done with the mussels once harvested? The researchers hope they can be treated and recycled instead of ending up in landfill. Provided the levels of contaminants are not too high, the mussels could be used as animal feed or fertiliser. What they will not do, though, is end up on someone’s plate.

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A nasty-tasting shellfish could be just the job for cleaning rivers