We usually think of the ocean's inorganic carbon cycle being dominated by tiny critters such as foraminifera, but a recent study suggests fish poop may also be an important contributor.
You see, fish constantly drink calcium- and magnesium-rich sea water; to successfully use the water, they must first get rid of some of those ions. They do this by increasing the pH in their gut to around 8.4 to 9.2. This causes the Ca2+ and Mg2+ ions to precipitate out as CaCO3, with Mg occasionally substituted for Ca, making it a high-magnesium calcite.
These calcite crystals can be seen forming in successive x-rays of fish, and previous studies have measured carbonate production rates around 18-40 µmols per hour per kilogram of fish. The variability from 18-40 is a result of the fish species' metabolism and the water temperature. Increasing water temperature by 10°C almost doubles carbonate production, while decreasing fish size by an order of magnitude, with a resulting increase in metabolism, increases production ~1.6-fold.
By combining these relationships with estimates of global fish biomass and ocean temperatures, a study in Science showed that global fish CaCO3 production ranged from 3.2*1012 to 8.9*1012 mol/year. This corresponds to 2.7 and 15.4 percent of the total surface marine CaCO3 production. The estimate includes some fairly conservative assumptions; making slightly different assumptions could triple those numbers.
So what happens to all that fish poop? Well, high-magnesium calcite is much more soluble than the low-magnesium calcite created by many other ocean dwellers. As a result, it likely dissolves as it sinks through the water column, but it does sink. Some of it will make it to the ocean floor, and the remainder will be returned to the water column at a deeper level than it started. Meanwhile, the excess H+ ions that the fish pumped out as a byproduct are returned to the same level of the water column as the fish.
As the authors point out, regular vertical migrations of fish populations will complicate this vertical redistribution, but it should still lead to an increase in surface ocean acidification. Add that to our own influence on ocean acidification, and it is clear that we should be concerned about predicting future fish feces production (try saying that ten times fast).
Unfortunately, predicting future production is tricky. Increasing ocean temperatures will increase individual fish metabolisms, but may also decrease total fish populations. Separately, increasing the partial pressure of CO2 has been shown to increase fish excretions in vitro. All of this says nothing about the impacts of overfishing or lack thereof.
As an aside, does this mean that much of the carbonate rocks we find today are just a (not so) steaming pile of coprolites? No, it probably dissolves too fast for most of the fish droppings to make it into the rock record, but there may be more in there than we imagined.
Science, 2009. DOI: 10.1126/science.1157972
Listing image by Flickr CC