Role of the Jiaozhou Bay as a source/sink of CO₂ over a seasonal cycle

Abstract

The seasonal evolution of dissolved inorganic carbon (DIC) and CO 2 air-sea fluxes in the Jiaozhou Bay was investigated by means of a data set from four cruises covering a seasonal cycle during 2003 and 2004. The results revealed that DIC had no obvious seasonal variation, with an average concentration of 2035 µmol kg-1 C in surface water. However, the sea surface partial pressure of CO 2 changed with the season. pCO 2 was 695 µatm in July and 317 µatm in February. Using the gas exchange coefficient calculated with Wanninkhof's model, it was concluded that the Jiaozhou Bay was a source of atmospheric CO 2 in spring, summer, and autumn, whereas it was a sink in winter. The Jiaozhou Bay released 2.60 × 10 11 mmol C to the atmosphere in spring, 6.18 × 10 11 mmol C in summer, and 3.01 × 10 11 mmol C in autumn, whereas it absorbed 5.32 × 10 10 mmol C from the atmosphere in winter. A total of 1.13 × 10 12 mmol C was released to the atmosphere over one year. The behaviour as a carbon source/sink obviously varied in the different regions of the Jiaozhou Bay. In February, the inner bay was a carbon sink, while the bay mouth and the outer bay were carbon sources. In June and July, the inner and outer bay were carbon sources, but the strength was different, increasing from the inner to the outer bay. In November, the inner bay was a carbon source, but the bay mouth was a carbon sink. The outer bay was a weaker CO 2 source. These changes are controlled by many factors, the most important being temperature and phytoplankton. Water temperature in particular was the main factor controlling the carbon dioxide system and the behaviour of the Jiaozhou Bay as a carbon source/sink. The Jiaozhou Bay is a carbon dioxide source when the water temperature is higher than 6.6ºC. Otherwise, it is a carbon sink. Phytoplankton is another controlling factor that may play an important role in behaviour as a carbon source or sink in regions where the source or sink nature is weaker.