Preprints
https://doi.org/10.5194/bg-2022-152
https://doi.org/10.5194/bg-2022-152
 
15 Jul 2022
15 Jul 2022
Status: this preprint is currently under review for the journal BG.

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

Arnaud Laurent1, Haiyan Zhang1,2, and Katja Fennel1 Arnaud Laurent et al.
  • 1Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada
  • 2School of Marine Science and Technology, Tianjin University, Tianjin, China

Abstract. The Changjiang is the largest river in Asia and the main terrestrial 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 industrial animal production, and fertilizer application in agriculture, resulting in coastal eutrophication and recurring summer hypoxia. The supply of anthropogenic nitrogen (N) exceeds that of phosphorus (P) relative to the Redfield ratio. This results in seasonal P limitation in the Changjiang plume. P limitation and its effects on primary production, respiration and hypoxia in the ECS have not been studied systematically yet although such knowledge is needed to understand bloom dynamics in the region, to assess the consequences of altered nutrient loads, and to implement nutrient reduction strategies that mitigate hypoxia. Using a coupled physical-biogeochemical model of the ECS that was run with and without P limitation, we quantify the distribution and effects of P limitation. The model shows that P limitation develops eastward of the Changjiang Estuary and on the Yangtze Bank but rarely southward along the Zhejiang coast. P limitation modifies oxygen sinks over a large area of the shelf by partly relocating primary production and respiration offshore, away from the locations prone to hypoxia near the Changjiang Estuary. This relocation drastically reduces sediment oxygen consumption nearshore and dilutes the riverine-driven primary production and respiration over a large area offshore. Our results suggest that the hypoxic zone would be 48 % larger in its horizontal extent, on average, if P limitation was not occurring. Results are summarized in a conceptual model of P limitation on the ECS shelf. Then we carried out nutrient reduction simulations which indicate that, despite the effect of P limitation on hypoxia, reducing only P inputs as a nutrient reduction strategy would not be effective. A dual N+P nutrient reduction strategy would best mitigate hypoxia. The model results suggest that decreasing the size of the hypoxic zone by 50 % and 80 % would require reductions in N+P load of 28 % and 44 %, respectively.

Arnaud Laurent et al.

Status: open (until 04 Sep 2022)

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Arnaud Laurent et al.

Arnaud Laurent et al.

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Short summary
The Changjiang is the main terrestrial source of nutrients to the East China Sea (ECS). Nutrient delivery to the ECS has been increasing since the 1960s resulting in low oxygen (hypoxia) during phytoplankton decomposition in summer. River phosphorus (P) has increased less than nitrogen and therefore, despite the large nutrient delivery, phytoplankton growth can be limited by the lack of P. Here, we investigate this link between P limitation, phytoplankton production/decomposition and hypoxia.
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