Model studies suggested that human-induced increase in nutrient load may have stimulated primary production and thus 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 biological production driven by the eutrophic Changjiang plume results in a very low dissolved inorganic carbon (DIC) in surface waters and a very high DIC in bottom waters of the inner shelf, with the latter returning high DIC 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 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 DIC in surface waters and the respiratory release of DIC in bottom waters during summertime, thereby returning more DIC 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 timescale of air–sea gas exchange and offshore transport of the shelf water. Finally, the case we report for the Changjiang system may have general ramifications for other eutrophic coastal oceans.

To assess the impact of rising atmospheric CO₂ and eutrophication on the carbonate chemistry of the East China Sea shelf waters, saturation states (Ω) for two important biologically relevant carbonate minerals – calcite (Ω_c) and aragonite (Ω_a) – were calculated throughout the water column from dissolved inorganic carbon (DIC) and total alkalinity (TA) data collected in spring and summer of 2009. Results show that the highest Ω_c (∼9.0) and Ω_a (∼5.8) values were found in surface water of the Changjiang plume area in summer, whereas the lowest values (Ω_c = ∼2.7 and Ω_a = ∼1.7) were concurrently observed in the bottom water of the same area. This divergent behavior of saturation states in surface and bottom waters was driven by intensive biological production and strong stratification of the water column. The high rate of phytoplankton production, stimulated by the enormous nutrient discharge from the Changjiang, acts to decrease the ratio of DIC to TA, and thereby increases Ω values. In contrast, remineralization of organic matter in the bottom water acts to increase the DIC to TA ratio, and thus decreases Ω values. The projected result shows that continued increases of atmospheric CO₂ under the IS92a emission scenario will decrease Ω values by 40–50% by the end of this century, but both the surface and bottom waters will remain supersaturated with respect to calcite and aragonite. Nevertheless, superimposed on such Ω decrease is the increasing eutrophication, which would mitigate or enhance the Ω decline caused by anthropogenic CO₂ uptake in surface and bottom waters, respectively. Our simulation reveals that, under the combined impact of eutrophication and augmentation of atmospheric CO₂, the bottom water of the Changjiang plume area will become undersaturated with respect to aragonite (Ω_a = ∼0.8) by the end of this century, which would threaten the health of the benthic ecosystem.

Bioluminescent seas caused by blooms of the dinoflagellate Noctiluca scintillans are a famous scenic attraction in the Matsu archipelago near the Chinese coast in the northern part of the Taiwan Strait. The relationship between the abundance of N. scintillans and various environmental factors was examined in the subtropical coastal waters of these islands from April to December, 2016. N. scintillans disappeared after June 20th, when the sea water temperature exceeded 27°C. A strong inverse correlation between N. scintillans and diatoms in PCA analysis indicated that diatoms suffer high grazing pressure from N. scintillans. N. scintillans appeared most abundantly during the flood season, when diatoms are most abundant. Strong inverse correlations between N. scintillans and the wind speed and wind direction in PCA further point to the effect of wind on the accumulation of cells.