This study aims to understand the major driving force on net ecosystem production (NEP) in estuaries. Season variation in NEP was investigated over two years in the North estuary China, along with physical (water temperature, river discharge, turbidity, salinity), chemical (nutrients, chemical oxygen demand), and biological (chlorophyll a) properties. NEP varied from-5.79 mgO2L~(-1)~(-1) to 7.29 mgO2'd~(-1) with a relatively low annual average of-1.13 mgO2L~(-1)d~(-1) during our monitoring period from 2010 to 2011. NEP was significantly positively correlated with chlorophyll a, dissolved oxygen, and solar radiation, but significantly negatively correlated with turbidity, river discharge, and chemical oxygen demand. The stepwise multiple linear regression models showed that dissolved oxygen, river discharge and solar radiation were particularly important factors that influenced the variation in NEP. NEP was the lowest during peak discharge period in autumn and increased in summer. The autotrophic state prevailed on most days in summer, whereas the heterotrophic state occurred in spring and autumn. Hydrologic alteration and seasonal variation simultaneously affected the estuarine NEP. High turbidity lowered NEP much further in autumn.
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