Ocean Biogeochemical Fingerprints of Fast‐Sinking Tunicate and Fish Detritus.

Pelagic tunicates (salps, pyrosomes) and fishes generate jelly falls and/or fecal pellets that sink roughly 10 times faster than bulk oceanic detritus, but their impacts on biogeochemical cycles in the ocean interior are poorly understood. Using a coupled physical‐biogeochemical model, we find that fast‐sinking detritus decreased global net primary production and surface export, but increased deep sequestration and transfer efficiency in much of the extratropics and upwelling zones. Fast‐sinking detritus generally decreased total suboxic and hypoxic volumes, reducing a "large oxygen minimum zone (OMZ)" bias common in global biogeochemical models. Newly aerobic regions at OMZ edges exhibited reduced transfer efficiencies in contrast with global tendencies. Reductions in water column denitrification resulting from improved OMZs improved simulated nitrate deficits relative to phosphate. The carbon flux to the benthos increased by 11% with fast‐sinking detritus from fishes and pelagic tunicates, yet simulated benthic fluxes remained on the lower end of observation‐based estimates. Plain Language Summary: Marine ecosystems play a critical role in the global carbon cycle through the food web regulation of air‐sea carbon fluxes and the transfer of particulate matter from the upper oceans to depth. Recent evidence has suggested that the detritus from fishes and gelatinous zooplankton (GZ), specifically the pelagic tunicates such as salps and pyrosomes, may have a disproportionate impact on the ocean's biological pump due to them sinking approximately 10 times faster than bulk detritus. These fluxes result in increased sequestration of particulate carbon and nutrients into the deep oceans, but their impact on biogeochemical cycles at depth is poorly understood. Here, we investigated the sensitivity of deep ocean carbon, oxygen, and nutrient cycles to fast‐sinking detritus from tunicates and fishes. We found that the fast‐sinking detritus decreased surface productivity and export, as well as the size of ocean oxygen minimum zones (OMZs). Also, we examined whether observational evidence of seafloor oxygen consumption could support the increased detrital fluxes (and respiration) at depth, and found that even with the increased oxygen consumption, the modeled values were still below the observations. This suggests that these processes could be realistically incorporated into future generations of Earth System Models. Key Points: We incorporated fast‐sinking detritus from pelagic tunicates and fishes into a modified version of the ocean biogeochemical model COBALTThe fast‐sinking detritus increased carbon sequestration and transfer efficiency to depth, but decreased surface productivity and exportFast‐sinking detritus decreased the size of oxygen minimum zones (OMZs) and water column denitrification, a common model bias [ABSTRACT FROM AUTHOR]