Fish Poop a big player in carbon sequestration in the ocean

TSaturated with sunlight, the surface of the sea teems with life. But the influence of a certain microbe, every plankton or fish goes far beyond this upper layer. In the form of dead organisms or excrement, organic material rains thousands of meters on the ocean floor, nourishes ecosystems, influences sensitive ocean chemistry and binds carbon in the deep sea.

But mankind’s fondness for fish may have disrupted some of that cycling, according to a new study released today (Oct. 8) in. has been published Scientific advances. A team of scientists in the USA and Canada modeled the historical changes in the biomass of commercially used fish species and their influence on the biogeochemical processes of the oceans. The researchers estimate that prior to the development of industrial fisheries around 1900, the feces of these species made up about 10 percent of the biological material that sank to the ocean floor – enough to contribute significantly to carbon sequestration, nutrient fluxes, and ocean chemistry in the deep sea. By 1990, when industrial fishing reached its peak, the biomass of the exploited species – and the influence of their droppings – had fallen by about half, with possible repercussions on the deep sea food web.

The study “does a good job of really putting the role of fish in the context of these massive global cycling processes,” says Jacob Allgeier, an ecologist at the University of Michigan who focuses on tropical coastal systems and is involved in the research. “These are certainly not exact numbers, but [that] You can only roughly estimate these roles, that is a really important contribution. ”

Scientists studying the interactions between marine life, climate, and ocean circulation have long focused on the role of photosynthetic plankton and microbes, which are responsible for much of the ocean’s nutrient and carbon cycle, notes Daniele Bianchi, an oceanographer specializing in biogeochemistry from the University of California, Los Angeles. But around 2015, he and his colleagues became curious about larger organisms like fish. They wanted to develop a global fisheries model, similar in spirit to global climate simulations, that could estimate the biomass of marine fish and its impact on the biogeochemical cycle.

The model was based on ecological theory and simulated primary production through photosynthesis of plankton, energy transfer through the food web and fish populations. Bianchi and his colleagues focused on commercially targeted fish, crustaceans, and molluscs weighing 10 grams to 100 kilograms assessments to narrow the results. They estimated that these target species were before the 19th (in a coarser calculation, non-targeted fish species represented a similar number). By 1990, the biomass of the target species had decreased by 47 percent, give or take 20 percent – a number that is in the range of earlier estimates of global fish decline in the last century.

While this decline in biomass is broadly what traditional fishing theory would consider sustainable and long-term profitable, University of Southampton’s marine ecologist Clive Trueman notes that the number hides the fact that many fish stocks are well above levels were fished where they can regenerate, causing a significant population decline when fishing restrictions were largely introduced in the 1990s and 2000s. Already today, more than a third of the fish stocks are considered to be not biologically sustainable.

Of greater interest to Bianchi was the broader effects of fish biomass. The team estimated that before industrial fishing, the target species consumed about 2 percent of global primary production – the earth. Much of the material used rushes to the sea floor in the form of fish droppings – or “fecal pellets” – where the carbon can remain inside for hundreds of years. Fish droppings are heavier and fall faster than the snow-like organic waste from smaller living things, making it “one of the most effective natural carbon sequestration mechanisms we know,” says Bianchi. Before industrial fishing, targeted fish made up about 10 percent of all biologically bound carbon in the deep sea, the team estimates.

Organic carbon and the nutrients in fish droppings also feed the seabed food web, which relies on organic matter from above. Life in the deep sea uses oxygen to devour fish droppings, thereby helping to maintain a delicate chemical balance that ecosystems are adapted to. In fact, fish droppings from exploited species once drove about 10 percent of the oxygen consumption in the deep sea and perhaps up to 20 percent in the deepest oceans, the team writes in its paper. The lesson from these estimates, says Bianchi, “is that fish is indeed important for these cycles. They may change the way carbon is stored in the ocean. ”

As the target fish’s biomass decreased, the carbon sequestration, nutrient transport, and chemical effects associated with their droppings decreased by about half by 1990, the team estimated. Bianchi notes that these numbers are background calculations because fixed numbers are difficult to determine. But overall, the changes in the biogeochemical cycle caused by fisheries are of a similar magnitude to the effects of climate change on such processes, writes the team. “We would like to point out that this is not a negligible number,” says Bianchi. “My hope is that we will get this on the radar and there will be more research into the cascading effects of. to understand [altering] the marine ecosystem. . . on carbon, nutrients and oxygen. ”

How many fish were in the ocean before industrial fishing and their contribution to global biogeochemical cycles are fundamentally important but extremely difficult questions to answer, says Trueman, who was not involved in the new research. “It’s pretty darn hard to count how many fish there are,” and the study’s main caveat is that it takes a lot of guesswork and simplification to get these kinds of numbers. “I suspect a lot of people who work in this field will read this [paper] and say, “Yes, but what if you change that number? And yes, but what if you change that number? ‘”

For example, both Allgeier and Trueman note that it is not immediately clear to what extent the team has taken into account the fact that some of the fish droppings are returned to the food web before they reach the seabed, where carbon is captured. Instead of a discrete pellet, most fish feces are loose and have plenty of surface area for microbes to attach to and digest as they sink. “I’ve seen a lot of fish droppings, and it’s rarely a pellet,” says Allgeier. He adds that at least at first glance, the team failed to take into account the different species identities of the target fish, which affects all aspects of their role in the biogeochemical cycle. “But I don’t really expect that from them on this scale,” he says. “The scale on which they operate is insane: it’s the global fish biomass.”

Allgeier and Trueman agree that these limitations and uncertainties are no reason to reject the results. “Even if you didn’t want to invest a lot of money in the absolute numbers,” says Trueman, the study provides a useful framework for examining questions about fish and their biogeochemical effects, and examining how sensitive the results are to various variables. such as how the metabolism scales depending on body size. “That is a useful contribution in and of itself,” he says.

The study also draws attention to the role of fish in global biogeochemical cycles, which has only recently been recognized, says Trueman. In fact, he suspects the study may underestimate the overall role of fish by excluding those under 10 grams, which are “by far the most abundant vertebrates on the planet,” he says. Regarding the effects of fishing, Allgeier adds: “I think studies like this show that these effects are real and that we need to understand them better because we learn how important these biogeochemical cycles are.”

Indeed, Bianchi hopes the results will stimulate further research into the true impact of humankind on the oceans. Compared to terrestrial ecosystems, “we think differently about the ocean because we don’t see it, we don’t live in the ocean,” he says. “There are as many consequences as [the ones we describe] for carbon and oxygen. . . . And there could be many, many more. “

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