A subsurface nitrite maximum was observed along one transection near the Xisha Islands in the South China Sea in early September 2009. Possible causes were examined using in situ observed data and remote sensing data. A cold eddy was identified near the Xisha Islands during the periods of study. The Xisha Islands' water receives copious supplies of nutrients through cold eddies. Accumulation of nitrite about 50 m deep reaches 0.49 μmol l−1, and forms and maintains the primary nitrite maximum. The relationship between NO2 and Chl-a is very significant (R2 = 0.5829, p-value(statistical significance) = 0.00). Nitrification processes would result in apparent oxygen utilization (AOU) ranging from 0.22 to 35.88 μM, around 24.60% of the total biological oxygen demand in the water column of under-saturation of dissolved oxygen. This value is very close to the Redfield stoichiometry (32/138 = 23%) of total oxygen consumption associated with organic matter diagenesis. These results may support the hypothesis that phytoplankton and the nitrification process have an important influence on the PNM or 260 maintenance in the euphotic layer. Our results indicate that physical conditions and biological activities near the Xisha Islands play a significant role in regulating biogeochemistry.

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