Seasonality of CO2 in coastal oceans altered by increasing anthropogenic nutrient delivery from large rivers: evidence from the Changjiang-East China Sea system.

Model studies suggested that human-induced increase in nutrient load may have stimulated primary production and thus has enhanced the CO₂ uptake capacity in the coastal ocean. In this study, we investigated the seasonal variations of the surface water's partial pressure of CO₂ (pCO₂^sw) in the highly human-impacted Changjiang-East China Sea system between 2008 and 2011. The seasonality of pCO₂^sw has large spatial variations, with the largest extreme of 170±75 µatm on the inner shelf near the Changjiang Estuary (from 271±55 µatm in summer to 441±51 µatm in autumn) and the weakest extreme of 53±20 µatm on the outer shelf (from 328±9 µatm in winter to 381±18 µatm in summer). During the summer period, stronger stratification and bio-logical production driven by the eutrophic Changjiang plume results in a very low CO₂ in surface waters and a very high CO₂ in bottom waters on the inner shelf, with the latter returning high CO₂ to the surface water during the mixed period. Interestingly, a comparison with historical data shows that the average pCO₂^sw on the inner shelf near the Changjiang Estuary has decreased notably during summer, but it has increased during autumn and winter from the 1990s to the 2000s. We suggest that this decadal change is associated with recently increased eutrophication. This would increase both the photosynthetic removal of CO₂ in surface waters and the respiratory release of CO₂ in bottom waters during summertime, thereby returning more CO₂ to the surface during the subsequent mixing seasons and/or episodic extreme weather events (e.g. typhoons). Our finding demonstrates that increasing anthropogenic nutrient delivery from a large river may enhance the sequestration capacity of CO₂ in summer but may reduce it in autumn and winter. Consequently, the coastal ocean may not necessarily take up more atmospheric CO₂ in response to increasing eutrophication, and the net effect largely depends on the relative time scale of air-sea gas exchange and offshore transport of the shelf water. Finally, the case we reported for the Changjiang system may have general ramifications for other eutrophic coastal oceans. [ABSTRACT FROM AUTHOR]