Impact of the migration behavior of mesopelagic fishes on the compositions of dissolved and particulate organic carbon on the northern slope of the South China Sea

The biogeochemistry of organic carbon in mesopelagic waters and the deep ocean is still not well studied. In this investigation, the interaction of organic carbon pools with vertical migrating mesopelagic species is addressed. The diel vertical migration (DVM) behavior of mesopelagic fish was observed with an echo sounder on the northern slope of the South China Sea in October 2014. Dissolved organic carbon (DOC), dissolved carbohydrates (DCHO), and particulate organic carbon (POC) were measured to determine the relationships between organic carbon and biological activity. Except shallow stations (depth < 200 m), the DOC concentrations was similar during the day and night, where the photosynthesis is absent at night in the upper layers. DOC in mesopelagic layer during daytime is about 8.78 pM higher than at night. During time series observation, the DOC concentration in surface layer is higher at night and in mesopelagic layer, however, is higher during daytime, which is coincident with the DVM behavior of mesopelagic fish. We suggest that this can be explained by the vertical migration of mesopelagic fish. Although most previous studies have focused on the impact of DVM behavior on POC, our study suggests that the biogeochemistry of DOC in the deep ocean is also linked to the vertical migration of mesopelagic species.