Molecular Weight Distribution of Dissolved Organic Phosphorus in Euphotic Zone at South China Sea

Colloidal organic phosphorus (COP) plays an important role in marine P cycle; however, studies on COP are still scarce. This study focused on the distribution and variation of dissolved and colloidal organic phosphorus in euphotic zone of South China Sea.

 Seawater samples were separated into total dissolved (≤0.4μm), truly dissolved (≤1kDa), and colloidal (1 kDa ~ 0.4 μm) fractions by in-line filtration of 0.4 μm-size capsule filter and then Cross-Flow Ultrafiltration (1kDa). After separation, each fractions were digested by a high temperature oxidation method and analyzed by phosphomolybdate-blue spectrophotometry. Therefore, different fractions of both inorganic and organic phosphorus were determined. Our results shown that the COP accounted for a significant amount (63±18%) of total dissolved P in the euphotic zone. It was also found that the depths of COP maximum concentration correlated well with the depths of chlorophyll a maximum, varies between day time and night hours. All these evidences highlight the strong biological effect on production and assimilation of COP and its vital importance in marine P cycle.