How fish help create seafloor sediments

Fish ingest sea water and later excrete it as fine-grained carbonates, now known to make up a substantial portion of sea floor sediments.

In February 2011, a team of scientists from the U.K. and U.S. announced that a significant component of sea floor sediment is created in the intestines of fish.

They say that fine-grained carbonates, excreted by fish at a very high rate, are created from sea water ingested by the fish and not from their food. These findings could change the way geologists seek to understand conditions during Earth’s geologic and climatic past, recorded in ancient carbonate deposits like limestone and chalk.

The work was published on February 21 in the Proceedings of the National Academy of Sciences. Lead author Chris Perry, a marine geoscientist at Manchester Metropolitan University, said in a press release:

The recognition that fish can act as major producers of carbonate in marine environments will be completely unexpected to a large section of the marine science community. Given how much carbonate these fish can produce, the findings also clearly have major implications for our understanding of different sources and sinks of carbonate sediment in the oceans, and some exciting implications for understanding where much of the mud in limestones and chalks may derive from.

Spheroidal carbonate crystal precipitates, as seen under a microscope, from the silver jenny (Eucinostomus gula). Image Credit: Chris Perry, et. al

Fine-grained carbonates found in marine sediment had been previously thought to precipitate out of sea water or result from the disintegration of marine invertebrate skeletons such as coral and shells. But scientists have also long known that marine fish waste included fine-grained carbonates. What did it look like, and how much of it was produced? The researchers of this study decided to search for the microscopic fish-secreted carbonates in the Bahamas, an area famous for its beautiful white carbonate sands and shallow tropical waters brimming with life.

First, they needed to examine the fine-grained carbonates found in the fecal pellets of eleven different fish species. Members of each fish species were collected and held in tanks for a period of time to determine the amount of fecal pellets they produced. Then the scientists analyzed carbonate crystals extracted from the freshly-deposited fecal pellets. They discovered that different fish species produced different kinds of carbonate crystals; most of the individual crystals were no larger than 30 micrometers (0.0011 inches, about 1/3rd the thickness of a piece of paper). Within the variations in shape and size of the carbonate crystals, the most commonly-found morphologies were ellipsoid- , straw-bundle-, dumbbell-, and spherical-shaped carbonate crystals.

A school of schoolmaster fish (Lutjanes apodus) in a lab tank. White carbonate pellets have settled on the floor of the tanks. Image Credit: Chris Perry, et. al

Schoolmaster fish (Lutjanus apodus) secrete densely-packed microscopic ellipsoidal carbonate crystals. Image Credit: Chris Perry, et. al

The next question was, how much of the carbonates in sea floor sediments were produced by fish? The scientists measured the amount of carbonates found in the fecal pellets for fish species of different sizes. They used those baseline measurements, along with an estimate of the total fish population based on surveys taken by other marine biologists, to conclude that fish of the Bahamian archipelago contributed about 6 million kilograms (over 13,000,000 lbs.) of carbonates each year. The distribution of these fish-derived carbonate crystals varied by habitat, with the highest concentrations found in reefs and mangrove swamps where fish populations were highest.

In terms of total carbonate mud production – all sources of carbonates including calcareous algae and inorganic calcium carbonate precipitation from saltwater – the fish contributed, on average, about 14 per cent of the annual carbonate mud production across the Bahamas. The concentrations varied with habitat, ranging from less than one per cent in seagrass and algal meadows to about 70 per cent in mangrove swamps.

A sample from yellowfin mojarra (Gerrus cinereus), as seen under a microscope, showing irregularly-shaped carbonate crystals. Image Credit: Chris Perry, et. al

Evidence that fish play an important role in replenishing carbonates in marine sediment has fascinating implications in understanding the Earth’s past. Dr. Rod Wilson, a fish biologist at the University of Exeter, said in the same press release:

An obvious area of future study in this field relates to the geological record and, in particular, to the role of this process in periods of the Earth’s history when ocean chemistry was very different and temperatures considerably warmer. For example, a preliminary study has estimated fish carbonate production under Cretaceous seawater conditions, the time (146-65 million years ago) when large masses of chalk were deposited (famously including the White Cliffs of Dover).

These studies, although in their early stages, suggest massive increases in production of this carbonate by fish during this ancient time. Perhaps fish have been a major contributor to these iconic carbonate deposits, in addition to the better known micro-fossils of shelled organisms. However, we are yet to look for direct evidence of this unusual contribution of fish, and we are currently seeking research funds to help answer this intriguing question.

It’s uncertain to what extent these fish-derived carbonate crystals will affect future climatic conditions. Rising sea temperatures could increase fish populations, thereby increasing the amount of carbonates in the ocean sediment. But increasing acidity of the ocean from carbon dioxide could cause more carbonates to dissolve, adversely affecting animals that depend on carbonates.

The discovery that fish contribute up to 14 per cent of carbonates in marine sediments in the Bahamas casts a new light on how ocean ecosystems work. The fine-grained organic crystals secreted by the fish have shapes and sizes that vary by species. Most deposits are found in areas with high-density fish populations, like coral reefs and mangrove swamps. This discovery also has implications in understanding the geologic and climatic history of our planet as recorded in deposits of limestone and chalk. And it opens up new questions about the role of fish in marine ecosystems and their influence on climate change.

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