Nutrients From Fish Help Marine Organisms Thrive

Healthy fish communities contribute more amounts of nutrients to marine ecosystems, playing a significant role in the growth rate of organisms, a new study reveals.

Researchers from University of Georgia (UGA) and Florida International University carried out a study in the waters of a large bay on Abaco Island, Bahamas, in order to understand the impact of nutrients released by fish on marine organisms like seagrass and algae. These species require right proportions of phosphorous and nitrogen to grow and thrive.

For their study, the research team created artificial coral reefs, as fish are known to gather around larger reefs. They built two reefs, large and small, and also monitored some sites with no reefs at all. For two years, the researchers surveyed the sites to estimate the number, size and species of fish present.

They also measured the amount of nutrients released by the fish. Their results revealed that the seagrass located near large coral reefs received more amount of nutrients compared to those seagrass at smaller reefs and control sites where there was no reefs.

"The rate of daily seagrass growth ranged from 37 square millimeters at large reefs to 10 square millimeters at control sites-nearly a four-fold difference," lead author Jacob Allgeier, from UGA Odum School of Ecology, said in a statement.

"Fish are putting an enormous amount of nutrients into this system-it appears to be even more than all other sources, including runoff from golf courses and all other human caused impacts, combined," Allgeier said.

Allgeier and his colleague Craig Layman noticed that the fish released more amounts of nutrients than the seagrass could take in. The excess nutrients were drifted further and helped seagrass and algae located at a distance of three meters around each reef to fertilize.

Researchers describe the large coral reefs as "biogeochemical hotspots" with high rates of chemicals cycling between organisms and the environment. "The reefs are nodes within the ecosystem matrix," Allgeier said. "They're increasing productivity around the reefs by orders of magnitude. If there are enough of them (reefs), then they may be increasing productivity at the ecosystem level by orders of magnitude as well. That's something we're going to be looking at next."

The study insists on the importance of maintaining a diverse community of fish species in order to ensure that the seagrass receive various nutrients. Even if there are large numbers of fish belonging to one particular species, they will be able to supply a single nutrient to the marine ecosystem. "That's not how these systems are used to being fed nutrients," Allgeier concludes.

The findings of the study, "Consumers regulate nutrient limitation regimes and primary production in seagrass ecosystems", are published in the journal Ecology.