Composition and Distributions of Nitrogen and Phosphorus and Assessment of Eutrophication Status in the Maowei Sea

Maowei Sea(MWS) is the largest oyster maricuture bay in Southwest China. Surface water from 12 sampling sites were collected during the dry season to study the composition and distribution of different forms of nitrogen(N) and phosphorus(P) beginning from the inner bay to the bay mouth in the MWS. An improved multi-parameter eutrophication index was developed and applied for the evaluation of the water quality in the MWS. Dissolved inorganic nitrogen(DIN), dissolved organic nitrogen(DON), and particulate nitrogen(PN) averagely accounted for 11.28%, 65.32%, and 23.39% of total N(TN) pool, dissolved inorganic phosphorus(DIP), dissolved organic phosphorus(DOP), and particulate phosphorus(PP) averagely accounted for 54.58%, 30.31%, and 15.12% of total P(TP) pool, respectively. DON and DIP were respectively the dominant forms of N and P in surface water of the MWS, during the study period. Concentration trends of DIN, DIP, DOP, PN, dissolved silicate(DSi), total organic carbon(TOC), chemical oxygen demand(COD) decreased from the inner bay to the bay mouth, suggesting that the MWS may be largely influenced by land-based inputs. Based on nutrient contents and stoichiometry, it likely that phytoplankton growth in the MWS is strongly limited by DIP during the dry season. Results from the improved eutrophication index revealed that the water quality in the MWS is in a state of moderate to poor eutrophication(average EI = 0.953). The eutrophication state decreased from the inner bay to the bay mouth. Nitrate(NO3-N), DON, DIP, DOP, and DSi showed significant positive correlations with eutrophication index(r2 = 0.52 – 0.78, P < 0.05), implying that these nutrients are among the significant factors, which could be used in evaluating the eutrophication status of the MWS.