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

[摘要] Model studies suggested that human-induced increase in nutrient load mayhave stimulated primary production and thus enhanced the CO₂ uptakecapacity in the coastal ocean. In this study, we investigated the seasonalvariations of the surface water's partial pressure of CO₂(pCO₂^sw) in the highly human-impacted Changjiang–East China Seasystem between 2008 and 2011. The seasonality of pCO₂^sw has largespatial variations, with the largest extreme of 170 ± 75 μatm onthe inner shelf near the Changjiang Estuary (from 271 ± 55 μatmin summer to 441 ± 51 μatm in autumn) and the weakest extreme of53 ± 20 μatm on the outer shelf (from 328 ± 9 μatm inwinter to 381 ± 18 μatm in summer). During the summer period,stronger stratification and biological production driven by the eutrophicChangjiang plume results in a very low dissolved inorganic carbon (DIC) insurface waters and a very high DIC in bottom waters of the inner shelf, withthe latter returning high DIC to the surface water during the mixed period.Interestingly, a comparison with historical data shows that the averagepCO₂^sw on the inner shelf near the Changjiang Estuary has decreasednotably during summer, but has increased during autumn and winter fromthe 1990s to the 2000s. We suggest that this decadal change is associatedwith recently increased eutrophication. This would increase both thephotosynthetic removal of DIC in surface waters and the respiratory releaseof DIC in bottom waters during summertime, thereby returning more DIC to thesurface during the subsequent mixing seasons and/or episodic extreme weatherevents (e.g., typhoons). Our finding demonstrates that increasinganthropogenic nutrient delivery from a large river may enhance thesequestration capacity of CO₂ in summer but may reduce it in autumn andwinter. Consequently, the coastal ocean may not necessarily take up moreatmospheric CO₂ in response to increasing eutrophication, and the neteffect largely depends on the relative timescale of air–sea gas exchangeand offshore transport of the shelf water. Finally, the case we report forthe Changjiang system may have general ramifications for other eutrophiccoastal oceans.

[学科分类] 地球化学与岩石