CO2 flux and seasonal variability in the turbidity maximum zone and surrounding area in the Changjiang River estuary

The turbidity maximum zone(TMZ) is one of the most important regions in an estuary.However,the high concentration of suspended material makes it difficult to measure the partial pressure of CO(pCO) in these regions.Therefore,very little data is available on the pCOlevels in TMZs.To relatively accurately evaluate the COflux in an example estuary,we studied the TMZ and surrounding area in the Changjiang(Yangtze) River estuary.From seasonal cruises during February,August,November 2010,and May 2012,the pCOin the TMZ and surrounding area was calculated from pH and total alkalinity(TA)measured in situ,from which the COflux was calculated.Overall,the TMZ and surrounding area acted as a source of atmosphere COin February and November,and as a sink in May and August.The average FCOwas-9,-16,5,and 5 mmol/(m~2·d) in May,August,November,and February,respectively.The TMZ’s role as a source or sink of atmosphere COwas quite different to the outer estuary.In the TMZ and surrounding area,suspended matter,phytoplankton,and pH were the main factors controlling the FCO,but here the influence of temperature,salinity,and total alkalinity on the FCOwas weak.Organic carbon decomposition in suspended matter was the main reason for the region acting as a COsource in winter,and phytoplankton production was the main reason the region was a COsink in summer.