Marine biogeochemistry of particulate organic carbon (POC), as one of the most important branches in the global carbon cycle, is connected with biological bump process and ecoenvironmental variation in ocean. Marine POC study is a key link to demonstrate marine biogeochemical process. In this paper, Marine biogeochemical characteristics of POC in seawater is mainly reviewed.
The distribution of POC in seawater is affected by many factors，such as physical, chemical and biological processes. In general, the high concentrations of POC, ranged from tens to hundreds μg/L, appear in coastal water and surface water, and low in offshore water and bottom water. But in the regions of surface sediment strongly resuspension such as the East China Sea（ECS）, the POC vertical profiles show the contents of POC increase with the depth and have a high content in the bottom water. The POC is mainly derived from terrigenous material, biogenic matter and sediment resuspended. The source of POC could be deduced by estimating δ13C or C/N which should be combined to differentiate POC source. Of course, the information of POC source is also obtained from POC/PON (particulate organic nitrogen), the relationship between POC and Chl a or TSM (total suspended matter). It is estimated that the POC derived from the river input is about 0.43Gt/a and mainly derived from the detritus of grassland, cropland, forest and human discharged in the estuary. In oceans, the biogenic matter including living and nonliving POC is comprised of phytoplankton, zooplankton, detritus and fecal pellets, which play an important role in marine biological pump. Nonliving POC is very important in carbon cycling because it may be ingested by plankton and thus involved into food web and turns into living POC.
Marine organism is very important in POC transformation. Zooplankton and nekton, which can ingest in the surface waters at night and metabolize the fecal pellets below the mixed layer, are the important producers of POC. In some sea areas such as the Arabian Sea, the flux of fecal pellets is about 156mgm－2d－1 and account for 12% of the primary productivity. Zooplankton and nekton can also actively increase the export of POC from the surface to the deep layer by vertical migration which is very obvious in the open sea. Generally, the POC flux by the contribution of plankton’s vertical migration is from 4% to 34% in the total POC flux, in some sea areas it is up to 70%. Microbial communities are not only an important POC source but also a significant contributor for the transformation of POC while oxygen penetration is generally limited to the upper of the sediments. Bacteria can transform POC into dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) by extracellular enzymatic hydrolysis, and the result is that POC could take part in carbon recycling process.
The POC in recycling has good relationships with nutrient, especially nitrogen, phosphorus and silicon because the phytoplanktons absorb the nutrient according to the constant Redfield ratio, But if the nutrient ratio is out of Redfield ratio, one or several nutrients will confine phytoplankton reproduction, growth and the primary productivity, and the composing and biomass of the living POC also will change. Such as the Bohai Sea, the change of nutrient ratio has led to the replacement of diatoms by dinoflagellates, which is the main feature of phytoplankton community changes in recent years. On the other hand, the metabolism of living POC may affect nutrient contents by absorbing and excreting, such as NH4-N, which is partly released by the excretion of living POC. There is a positive correlation between nutrient and nonliving POC due to organic matter mineralization. In Jiulongjiang Estuary and Western Xiamen Harbour, the positive correlation between POC with the dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) suggested that the DIN and DIP partly come from the decomposition of the POC. When environmental situation such as temperature, redox condition etc. is appropriate in coastal sediment, NH4-N is controlled by POC decomposition.