Fishing trawlers could harm water quality by disrupting seafloor microbes | Science

When trawl nets stir up sediments, microbes remove less nitrogen. Too much of this nutrient can harm water quality.

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Fishing boats that drag nets along the sea floor to catch seafood can indiscriminately harm marine life and destroy habitat. Now, a new study suggests “bottom trawling” can also disrupt the ability of microbes in sediment to remove excess nutrients in coastal waters, potentially increasing that pollution.

“This is one of the first papers to look at actual biogeochemical effects of bottom trawling,” says Sebastiaan van de Velde, a marine biogeochemist at the University of California, Riverside, who was not involved. “The whole angle is very novel.” 

Nitrogen is a vital nutrient for aquatic plants such as kelp and tiny marine algae known as phytoplankton. But too much—as comes with poorly treated sewage or fertilizer washing off farm fields—stimulates so-called algal blooms. The excess algae can be a nuisance, tangling boat propellers and rotting on beaches. And when the algae die in the water, things get worse: Microbes that break it down gobble up oxygen and can create a dead zone that suffocates fish and other marine creatures.

Microbes in seafloor sediment can help prevent this problem by converting excess nitrogen into an inert gas that escapes into the atmosphere. Could bottom trawling interfere?

To find out, Bradley Eyre, a biogeochemist at Southern Cross University,  and his colleagues conducted an experiment in Moreton Bay, Australia. The team selected three sites near a river that delivers nitrogen into the bay. Then, several times in 1 year, they measured the nitrogen gas coming from the sediment. This gas is the end-product of a process, called denitrification, in which microbes in the top few centimeters of sediment decompose nitrogen-rich organic matter. The process requires a special set of conditions because some of the biochemical reactions require oxygen, whereas some demand its absence. On the sea floor, this patchwork of conditions is created by the burrowing of many kinds of marine animals, such as crustaceans, clams, and worms.

Next, Eyre and his colleagues hired a shrimp trawler. They got permission for the boat to drag its net across several sites in an area protected from trawling. Immediately afterward, divers jumped into the water to study the sediment and measure the nitrogen gas emitted by microbes. As expected, the trawling mixed the sediment on the sea floor. This erased the fine structures created by burrowing animals, hindering the microbes and cutting the nitrogen gas they emitted by up to 50% compared with nearby untrawled sites, the team reported last month in Limnology and Oceanography Letters. “It’s actually a pretty big effect,” Eyre says.

Van de Velde agrees. “You’re completely changing how these bottom sediments function,” he says. “That is a major problem with bottom trawling.” 

Eyre and colleagues repeated the experiment three times, once every 3 months, and saw the same effect. The good news is that after each trawl, burrowing animals returned and re-created the conditions for denitrification. One worrying sign is that each trawl decreased denitrification more than the previous trawl. This suggests lasting damage, but the trend was not statistically significant. 

The results could be conservative. Because the water was only 4 meters deep, strong waves regularly disturbed the sediments, which would regularly decrease denitrification. Eyre thinks trawling might have a relatively greater impact on denitrification in deeper waters where animal burrows are more stable. The overall amount of denitrification is probably greater in shallow water, however, because more organic matter is present there.

It’s difficult to say how large an impact trawling has on denitrification and water quality. Eyre and his colleagues made some rough calculations. Assuming trawling occurs in half of Moreton Bay, for example, and has the maximum impact measured in the experiment, it could prevent 5477 tons of nitrogen from escaping from the water and sediment. That’s equivalent to 80% of the nitrogen that enters the bay from air and land each year.

“It’s just to show how potentially important this could be,” Eyre says.

The new study “certainly provides an important piece of the puzzle,” says Marija Sciberras, a marine ecologist who will soon join Heriot-Watt University, Edinburgh. Given the broad extent of trawling, she adds, figuring out its impact on the nutrient cycle is an urgent task.

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