Marine ecosystems: Fish have big impact on nutrients, study finds

Researchers have found that fish play a much more important role in contributing nutrients to marine ecosystems than has been thought. Two studies co-authored by Jacob Allgeier, a doctoral student in the UGA Odum School of Ecology, say fish inject more nutrients into marine ecosystems than any other source.

The studies were published in the journal Ecology.

In most tropical coastal ecosystems, the primary food sources such as sea grass and algae cannot grow and thrive without enough phosphorus and nitrogen in the environment.

In marine food webs, Allgeier says, fish are usually thought of as predators, consuming microorganisms, plants and smaller animals. But through their excretions, they provide the essential fertilizer that algae and sea grass need to grow.

To examine the impact of fish poop on marine ecosystems, the team compared sites where the number of fish present differed. Fish are attracted to reefs. So the researchers built a number of artificial reefs, each of one of two sizes, large and small (large reefs attract more fish than do small ones).

Then, for two years they monitored the sites and recorded the size, number and species of fish present. They  also measured sea-grass growth rate and nutrient content.

Their finding was that sea grass around the big "reefs" grew much faster and contained far more nutrients . "Nearly a four-fold difference," Allgeier said. "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 described the large reefs as "biogeochemical hotspots"—areas where chemicals cycle between organisms and the environment at extremely high rates.

"The reefs are nodes within the ecosystem matrix," he 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."

Both papers are in press and available online here (Consumers regulate nutrient limitation regimes and primary production in seagrass ecosystems, Allgeier, Yeager and Layman) and here (Thresholds of Ecosystem Response to Nutrient Enrichment from Fish Aggregations, Layman, Allgeier, Yeager and Stoner).