Role of phosphorus in the seasonal deoxygenation of the East China Sea shelf

[摘要] The Changjiang is the largest river in Asia and the mainterrestrial source of freshwater and nutrients to the East China Sea (ECS).Nutrient concentrations have long been increasing in the Changjiang,especially after 1960 with urbanization, the development of industrialanimal production, and fertilizer application in agriculture, resulting incoastal eutrophication and recurring summer hypoxia. The supply ofanthropogenic nitrogen (N) exceeds that of phosphorus (P) relative to theRedfield ratio. This results in seasonal P limitation in the Changjiangplume. P limitation and its effects on primary production, respiration, andhypoxia in the ECS have not been studied systematically, although suchknowledge is needed to understand bloom dynamics in the region, to assessthe consequences of altered nutrient loads, and to implement nutrientreduction strategies that mitigate hypoxia. Using a coupledphysical–biogeochemical model of the ECS that was run with and without Plimitation, we quantify the distribution and effects of P limitation. Themodel shows that P limitation develops eastward of the Changjiang Estuaryand on the Yangtze Bank but rarely southward along the Zhejiang coast. Plimitation modifies oxygen sinks over a large area of the shelf by partlyrelocating primary production and respiration offshore, away from thelocations prone to hypoxia near the Changjiang Estuary. This relocationdrastically reduces sediment oxygen consumption nearshore and dilutes theriverine-driven primary production and respiration over a large areaoffshore. Our results suggest that the hypoxic zone would be 48 % largerin its horizontal extent, on average, if P limitation was not occurring.Results are summarized in a conceptual model of P limitation on the ECSshelf that is also applicable to other systems. Then we carried out nutrientreduction simulations which indicate that, despite the effect of Plimitation on hypoxia, reducing only P inputs as a nutrient reductionstrategy would not be effective. A dual N  +  P nutrient reduction strategywould best mitigate hypoxia. The model results suggest that decreasing thesize of the hypoxic zone by 50 % and 80 % would require reductions inN  +  P load of 28 % and 44 %, respectively.

[学科分类] 大气科学