Fishing boats that use trawl nets to catch seafood along the seabed can indiscriminately harm marine life and destroy their habitat. Now, a new study shows that trawlers may also destroy the ability of seafloor microorganisms to eliminate excess nutrients in coastal waters, thereby increasing ocean pollution.
"This is the first paper to study the practical biogeochemical effects of trawl fishing." said Sebastiaan Van de Velde, a marine biogeochemist at the University of California, Riverside who did not participate in the study. "The perspective of the whole study is very novel."
Nitrogen is an important nutrient for aquatic plants such as seaweeds and tiny seaweeds called phytoplankton. However, as improperly disposed sewage or fertilizers from farmland are washed into the ocean, too much nitrogen can stimulate the so-called "algae blooms." Too much seaweed can cause trouble, such as being wrapped around the propeller of a boat and decayed on the beach. When the algae dies in the water, the situation becomes worse-the microorganisms that synthesize it can swallow oxygen and form a dead zone, suffocating fish and other marine life.
The microorganisms in the seabed deposits can help deal with this problem by converting excess nitrogen into an inert gas that is released into the atmosphere. But will trawlers interfere with this?
To find out, Bradley Eyre, a biogeochemist at Southern Cross University in Australia, and his colleagues stopped an experiment in Moreton Bay in the country.
The research team selected three locations near a river that transports nitrogen to the bay. In a year, they repeatedly measured nitrogen from the deposits. This gas is the final product of the denitrification process, in which microorganisms on the top of the deposit a few centimeters synthesize nitrogen-rich organic matter.
This process requires a series of special conditions, because some biochemical reactions require oxygen, and some require anaerobic. On the seafloor, this confusing condition was invented by the excavation of many species of marine animals (such as crustaceans, clams and worms).
Next, Eyre and his colleagues hired a shrimp trawler. The ship was allowed to trawl through several points in an area where trawling was stopped. Then, divers dived into the water to study the deposits and measure the nitrogen released by the microorganisms. Unsurprisingly, the trawl has mixed up the deposits on the seabed.
The research team recently published a report in the "Limnology and Oceanography Letters" that compared with nearby places where trawling was not experienced, this process erased the precise structures invented by burrowing animals, hindered the growth of microorganisms, and reduced Up to 50% of nitrogen emissions. "This is a considerable impact in practice." Eyre said.
Van de Velde agreed. "This completely changes the function of these bottom deposits." He said, "This is a major problem caused by trawling."
Once every 3 months, Eyre and his colleagues repeated the experiment 3 times, and finally saw the same effect. The good news is that after each trawl, burrowing animals will return and reinvent the conditions for denitrification. One worrying sign is that each trawl reduces denitrification more than the previous trawl. This indicates that the damage caused by trawling is durable, but this trend is not statistically significant.
The results of this study may be radical. Since the water depth is only 4 meters, intense waves will regularly disturb the deposits, which will regularly reduce denitrification. Eyre believes that in deeper waters where animal caves are more stable, trawling may have a relatively large impact on denitrification. However, the total amount of denitrification may be greater in shallow waters due to more organic matter there.
It is difficult to say how much impact trawling has on denitrification and water quality. Eyre and his colleagues did some rough calculations. It is assumed that trawl fishing occurs in half of the waters of Moreton Bay, and the largest impact measured in the experiment is to prevent 5477 tons of nitrogen from escaping from the water body and deposits. This is equivalent to 80% of the nitrogen entering the bay from the air and land each year.
Eyre said: "This only shows the potential importance of the study." Marija Sciberras, a marine ecologist at Heriot-Watt University in Edinburgh, UK, said that the new study "undidly provides an important puzzle." She added that it is an urgent task to think about the general level of trawl fishing and to understand its impact on the nutritional cycle.