Every year large quantity of wastewater is produced around the world, but its influence on CO₂ uptake by the coastal ocean is not well understood. Here we examine the influence of wastewater discharge on sea surface CO₂ partial pressure (pCO₂) and air-sea CO₂ flux in the Jiaozhou Bay (JZB), a temperate coastal bay strongly disturbed by wastewater discharge. Based on monthly surveys from April 2014 through March 2015, we find that surface pCO₂ in the JZB substantially varied both temporally and spatially between 163 μatm and 1222 μatm, with an annual average of 573 μatm. During April-December, it was oversaturated with respect to the atmosphere, with high values exceeding 1000 µatm in the northeastern part of the bay, where salinity was low due to wastewater discharge. During January-March, surface pCO₂ was undersaturated with the lowest value of less than 200 µatm mainly in the northeastern part due to low temperature and high biological production. We conclude that over an annual cycle sea surface temperature dominated the monthly variation of surface pCO₂ in this shallow bay, while wastewater discharge and related biological production/respiration dominated its spatial variability. Overall, the JZB was a net CO₂ source to the atmosphere, emitting 7.3 ± 11.3 mmol C m^-2 d^-1, unlike its adjacent western part of the Yellow Sea and most of the temperate coastal oceans which are a net CO₂ sink. We infer that the strong CO₂ release from the JZB was associated with wastewater discharge that causes high surface pCO₂ via direct inputs of CO₂ and degradation of organic matter. Thus, to enhance CO₂ uptake by the coastal ocean in the future, we have to reduce wastewater discharge via saving freshwater or to neutralize the high CO₂ in wastewater, e.g., via addition of alkaline materials.
 

Carbon dioxide; Air-sea CO2 flux; Wastewater discharge; Coastal ocean; Jiaozhou Bay