Seasonality of CO<sub>2</sub> in coastal oceans altered by increasing anthropogenic nutrient delivery from large rivers: evidence from the Changjiang–East China Sea system

Abstract

Abstract. 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 (pCO2sw) in the highly human-impacted Changjiang–East China Sea system between 2008 and 2011. The seasonality of pCO2sw 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 pCO2sw 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.

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems: Concepts, emerging trends, and research challenges

“…By comparing fluorescence excitation-emission matrices (EEMs) of DOM between the branches and tributaries of the Yangtze River estuary, Guo et al (2014) found that labile DOM components delivered by the Huangpu River, a highly polluted tributary, exerted a disproportionately large influence on the biodegradability of DOM in the 365 Yangtze estuary. Direct underway measurements of pCO2 along the river and estuarine reaches of a few river systems in China also indicated a potential activation of riverine microbial processing and enhanced CO2 evasion from polluted waterways (Zhai et al, 2005;Chou et al, 2013;Wang et al, 2017).…”

“…Compared to the extensive studies conducted in polluted rivers and estuaries in Europe and North America (Frankignoulle et al, 1998;Borges et al, 2006;Hartmann et al, 2007;Borges and Abril, 2011;Griffith and Raymond, 2011;Amann et al, 2012;Joesoef et al, 2015), relatively few efforts have been made to measure pCO2 in polluted Asian rivers, except for some large rivers and estuaries in East Asia (Zhai et al, 2005;Chou et al, 2013;Ran et al, 2015b;Yoon et al, 2017). These studies, together with a small number of studies that used water chemistry data to estimate the levels of pCO2 in major Asian 355 rivers such as the Mekong (Li et al, 2013), the Yangtze (Ran et al, 2017b), the Ganges-Brahmaputra (Manaka et al, 2015), and Indian estuaries Sarma et al, 2012), underscored the importance of anthropogenic OM and nutrients for riverine CO2 dynamics, particularly along the lower river reaches and estuaries draining highly populated areas.…”

Carbonate mineral saturation states in the East China Sea: present conditions and future scenarios

“…Therefore, the ECS may represent one of the areas most impacted by worsening eutrophication worldwide over recent decades. Some recent studies have shown that the elevated nutrient discharge had led to some ecological consequences (e.g., harmful algal blooms and hypoxic events; Li et al, 2007;Rabouille et al, 2008), and might have altered biogeochemical cycles in the ECS (Chou et al, 2013 …”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…Kim et al (2014) reported the repeat observations in the Ulleung Basin. Chou et al (2013a) contrasted the different seasonal patterns of pCO 2 in the ECS in the 1990s and the 2000s. Zhai et al (2013) surveyed the entire SCS over 6 years to assess the air-sea CO 2 exchange fluxes.…”

“…The faster increasing trends in the marginal seas probably induce the decreasing capacity of CO 2 storage in the more confined water bodies of the marginal seas, which warrant further investigation. Chou et al (2013a) reported dramatic changes in the seasonal patterns of pCO 2 variation in the East China Sea, reflecting the complicated controlling factors. In contrast to the general increasing trend of pCO 2 , the observed pCO 2 Figure 4.…”