Fish biomass 10 times greater than thought (and fish “fix” carbon dioxide from sea water)

A new paper suggests that the biomass of mesopelagic fish which dominate the total biomass of fish in the ocean is 10 times higher than previously assumed. Instead of being about 1,000 million tens the researchers suggest it could be 10,000 million tons or even more.

Fish are a critical link in the Carbon cycle and especially the removal – by “fixing” as carbonates – of the carbon dioxide in sea water. They act to neutralise acidity and increase alkilinity. The level of carbon dioxide dissolved in sea water itself affects the capacity of the ocean surface waters to absorb more carbon dioxide. A change – by a factor of 10 – in the fish biomass is a not insignificant change to the carbon fluxes through the ocean and to the carbon cycle.

Xabier Irigoien et al, Large mesopelagic fishes biomass and trophic efficiency in the open oceanNature Communications, 2014; 5 DOI:10.1038/ncomms4271

EurekAlert: With a stock estimated at 1,000 million tons so far, mesopelagic fish dominate the total biomass of fish in the ocean. However a team of researchers ….. has found that their abundance could be at least 10 times higher. The results, published in Nature Communications journal, are based on the acoustic observations conducted during the circumnavigation of the Malaspina Expedition. … Mesopelagic fishes, such as lantern fishes (Myctophidae) and cyclothonids (Gonostomatidae), live in the twilight zone of the ocean, between 200 and 1,000 meters deep. They are the most numerous vertebrates of the biosphere, but also the great unknowns of the open ocean, since there are gaps in the knowledge of their biology, ecology, adaptation and global biomass. 

… Xabier Irigoien, researcher from AZTI-Tecnalia and KAUST (Saudi Arabia) and head of this research, states: “The fact that the biomass of mesopelagic fish (and therefore also the total biomass of fishes) is at least 10 times higher than previously thought, has significant implications in the understanding of carbon fluxes in the ocean and the operation of which, so far, we considered ocean deserts”.

Mesopelagic fish come up at night to the upper layers of the ocean to feed, whereas they go back down during the day in order to avoid being detected by their predators. This behaviour speeds up the transport of organic matter into the ocean, the engine of the biological pump that removes CO2 from the atmosphere, because instead of slowly sinking from the surface, it is rapidly transported to 500 and 700 meters deep and released in the form of feces.

Irigoien adds: “Mesopelagic fish accelerate the flux for actively transporting organic matter from the upper layers of the water column, where most of the organic carbon coming from the flow of sedimentary particles is lost. Their role in the biogeochemical cycles of ocean ecosystems and global ocean has to be reconsidered, as it is likely that they are breathing between 1% and 10% of the primary production in deep waters”.

According to researchers, the excretion of material from the surface could partly explain the unexpected microbial respiration registered in these deep layers of the ocean. Mesopelagic fishes would act therefore as a link between plankton and top predators, and they would have a key role in reducing the oxygen from the depths of the open ocean.

The mechanisms by which fish create carbonates and contribute to the “fixing” of carbon dioxide is through feces.

Fish feces reduce ocean CO2 levels

 .. when fish drink seawater they excrete calcium as calcium carbonate — a chalky substance that can make seawater more alkaline and diminish the carbon dioxide in the water. ….. the bulk of the world’s fish species, excluding sharks and rays, produced the carbonate to counter the salt they ingested in seawater. The carbonate binds to the salt and is expelled as pellets, which dissolve in the ocean. … (We) knew before that something in the water was producing carbonate, but believed it came from other sources, such as microscopic marine plankton near the bottom of the food chain. But (we) didn’t understand why they were seeing so much of the carbonate at shallower depths. ……. most conservative estimates suggest three to 15 per cent of the oceans’ carbonates come from fish, but this range could be up to three times higher.

File:Oceanic divisions.svg

Oceanic divisions (Wikipedia)

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