Mesopelagic Ecosystem and Biological Pump Powered by Mesopelagic Fish in the Northern South China Sea

The vertical transport of organic carbon by mesopelagic fish and responses of mesopelagic communities in the northern South China Sea (NSCS) were studied using integrated satellite remote sensing, acoustic, optical, biomarker, trace element and molecular methods. The results revealed that the horizontal gradients in mesopelagic fish distributions between the northern shelf slope and basin were less pronounced than those cross-shelf slope gradients of nutrients and phytoplankton biomass. The biomass of mesopelagic fish was estimated between 30 and 10³ g (wet weight) m^-2 with various acoustic models. Even the minimum estimates are 6-8 times larger than historical estimates, indicating a substantial accumulation of biomass and carbon in mesopelagic fish. Though the NSCS is a classically defined as an oligotrophic ocean with low nutrient concentrations and low chlorophyll, the primary production was high to support an active layer of mesopelagic fish with a density approximately 30-40 g (wet weight) m^-2 living in waters 50-1000 m below the surface. The plankton and fish appeared to be tightly coupled, resulting in a surprisingly high trophic transfer efficiency (ca. 35%). Significant amount of organic carbon was pumped into the depth between 400 and 600 m through diel vertical migration and feeding behavior of mesopelagic fish. The particulate organic carbon (POC) concentration decreased as the depth increased due to scavenging reaching a minimum around 200 m, and increased reaching a maximum between 400 and 600 m where the mesopelagic fish maxima were found during the day. A specific microbial community was also found within the day-depth of mesopelagic fish maximum. The mesopelagic fish plays a key role in the global carbon cycle pumping organic carbon to the deep ocean and fueling the microbial community.