1182 / 2024-09-20 16:18:52

Contribution of water column particle composition to POC in oligotrophic waters of Western Pacific Ocean

Tropical Western Pacific; Backscattering coefficient of particulate matter; Phytoplankton carbon; Particle size; POC

It highly variable that the contribution of particulate matter present in different proportions throughout the water column to POC. The investigation found that the particle size in the oligotrophic waters of the western Pacific has a higher content of relatively small particles in the mixed layer and makes a greater contribution to the primary productivity, while in the euoptical layer below the mixed layer (average water depth >50m), large particles may dominate, which has a greater impact on the scattering characteristics of the water and the POC content estimated based on these features. Therefore, we estimate the contribution of the particle components to the POC in this region, and characterize the temporal and spatial variations of these vertical variations by reevaluating in situ measurements and biogeochemical buoy (BGC-Argo) data. The results showed that phytoplankton carbon with particles smaller than 2µm in the water column above the Deep Chla Maximum (DCM) layer contributed more to POC, accounting for up to 60%. The contribution of particles larger than 200µm and smaller than 500µm to the POC of the water column below the DCM layer is the largest, and can reach 60%. The contribution of 2-200µm to the POC of the entire water column is comparable, about 10%. Based on the number concentration of phytoplankton, detrital particles, mineral particles and cross-sectional area obtained from field investigation, the contribution of particle components to backscattering is that non-living particles are greater than living particles, and living particles are the most significantly affected by space and season. Spatially, from the eutrophic region in the north to the hyperoligotrophic region in the south, the contribution of DCM to backscattering decreases by steps with increasing water depth. According to previous studies, this effect may be related to the photodomestication of phytoplankton and the distribution of maximum phytoplankton biomass. In terms of time, the contribution is the smallest in September and the largest in winter. Finally, using the negative exponential relationship between backscattering and POC in the water column, the total POC content in the water column was estimated. The study can provide reference for accurate estimation of regional geochemical processes.