Net heterotrophy and low carbon dioxide emissions from biological processes in the Yellow River Estuary, China

Although estimates of total CO₂ emissions from global estuaries are gradually decreasing, current numbers are based on limited data and the impacts of anthropogenic and seasonal disturbances have not been studied extensively. Our study estimates annual and seasonal CO₂ fluxes in China's Yellow River Estuary (YRE) which incorporated spatiotemporal variations and the effects of water and sediment regulation (WSR). Aquatic metabolism was estimated using Odum's open water dissolved oxygen methods and used to represent the production and assimilation of CO₂. Net ecosystem production (NEP) was used to represent the CO₂ flux from biological activities and estimate the major CO₂ emitters in the YRE. According to our measurements, the annual CO₂ release was 6.14 ± 33.63 mol C m^-2 yr^-1 from 2009 to 2013 and the annual CO₂ efflux from the 1521.3 km² of estuarine surface area was 0.11 ± 0.61 Tg C yr^-1 in the YRE. High CO₂ emissions in autumn were balanced by high CO₂ sequestration in summer, leading to a lower than expected annual net CO₂ efflux. The system is an atmospheric CO₂ source in spring and winter, near neutral in early summer, a large sink in late summer after WSR, and finally a large atmospheric CO₂ source in autumn. Discharge events and seasonality jointly affect estuarine CO₂ flux. High CO₂ sequestration in summer is due mainly to a combination of high water temperature, chlorophyll a levels, dissolved inorganic carbon, and solar radiation and low turbidity, discharge, and chemical oxygen demand (COD) after WSR. WSR supports the high gross primary productivity rate which exceeds the increase in ecosystem respiration. Although the YRE, as a whole, is a source of atmospheric CO₂, the amount of CO₂ released is lower than the average estuarine value of mid-latitude regions. Our findings therefore suggest that global CO₂ release from estuarine systems is overestimated if spatiotemporal variations and the effects of anthropogenic disturbance are excluded. The NEP method is effective for estimating the CO₂ flux, especially in estuaries where CO₂ variation is mainly due to biological processes.