Fish guts can alter ocean's chemistry

The ocean’s surface waters contain many more microorganisms than fish — and so for years, the carbonate-shelled microorganisms were thought to be the main contributor to the carbonate chemistry in deep ocean waters. Now, new research suggests that the tiny pellets that most bony fish produce in their guts can affect the chemistry of the oceans.

Dissolved carbonate can influence the acidity of the ocean. Carbonate dissolution levels change throughout the water column, with more dissolved carbonate at the ocean’s depths than at its surface. However, the ocean’s ability to neutralize acidity — its alkalinity, linked to its concentration of dissolved carbonate — is actually greater at shallower depths, a quirk that has puzzled marine chemists for some time.

Fish form carbonate in their guts
Rod W. Wilson, Exeter University

Rod Wilson, a marine biologist at the University of Exeter in the United Kingdom, previously noted that fish produced a lot of carbonate in their feces. So, building on Wilson’s work, Frank Millero, a marine chemist at the University of Miami in Florida, had an idea as to how to solve this mystery. “When fish drink seawater, they excrete bicarbonates. Those react with the magnesium and calcium in seawater and precipitates into a very fine [and soluble] material,” Millero says. He thought that, by combining his research with that of Wilson and several other scientists, they could finally lay the issue to rest.

By estimating the amount of fish excreta, the team determined, they could see if there was enough that, if dissolved, it would impact the pH of surface water. But there was a big problem: How do you calculate how many fish are on the planet? “We did a top-down estimate,” Millero says. “From satellites one can measure the chlorophyll in certain places, so we can tell how many plants are producing chlorophyll. From there, we can guess how many fish are feeding off of these plants.” Although there’s no telling as to exactly how accurate these fish population numbers are, Millero says theirs is as accurate an estimate as is available.

The results of the model, published last week in Science, were surprising, Millero says. The team estimated that fish produced 15 percent of all marine calcium carbonate, “but could as much as double that.”

“Ocean waters have been measured to be more acidic than in the past and will be more acidic” as greenhouse gases add more carbon dioxide into the ocean, Millero says. “By putting in more alkalinity in the form of dissolved calcium carbonate, fish may compensate for some of the acidification caused by fossil fuel carbon dioxide dissolving in the oceans,” he says.

The overall effect of global warming on the fish, however, has yet to be determined. At this stage, models for warming oceans have yet to agree on production numbers of carbonate that vary between regions and latitudes. There are two ways this could play out, says Richard Feely of the Pacific Marine Environmental Laboratory in Seattle, Wash., who first discovered the alkalinity anomaly: As the oceans warm, fish metabolism would increase, causing more production of the calcium carbonate and making fish play a larger role in the cycle. However, “the other side is that in more stratified oceans, primary production would decrease,” he says. If oceans became more stratified, fewer nutrients from deep water would reach the surface, causing less plant growth and, thus, giving the fish less to eat — similar to what happens during an El Niño event. And, although no consensus has been reached, researchers have some idea of what might happen: “A lot [of models] have suggested there would be decreased production overall,” he adds.

But more research still needs to be done. Millero and his colleagues are first trying to grasp the big picture before they can concentrate on individual factors. “We’re going to try to see how fast calcium carbonate dissolves from fish in seawater. This will be useful in trying to model the system in oceans,” he says. Now that fish have been shown to have a substantial role in the inorganic carbon cycle, only the future will tell how climate change will affect this volatile system.

Alexandra Ossola
Thursday, January 22, 2009 - 12:30