Effects of Topography on Nutrient Variations in the Western South China Sea

• Published in: Journal of geophysical research. Oceans 2024-11, Vol.129 (11), p.n/a
• Main Authors: Huang, Fangjuan, Chen, Yong, Wang, Kuo, Liang, Junjian, Liu, Qinyu, Xiong, Zhiyao, Lan, Fei, Yin, Kedong
• Format: Article
• Language: English
• Subjects: Biological effects; Biological production; Chlorophyll; Chlorophylls; Cruises; Deep water; Dipoles; Downwelling; Dynamic structural analysis; Euphotic zone; Isobaths; Marine biology; Marine ecosystems; Mixed layer; Nitrates; Nutrient cycles; Nutrient dynamics; Nutrients; Ocean circulation; Ocean floor; Oceans; oligotrophic oceans; Phosphates; Primary production; Pycnocline; Pycnoclines; Salinity; Salinity effects; Sea currents; seamount effect; Seamounts; Shallow water; Shoaling; Topographic effects; topographically induced upwelling; Topography; Upwelling; Vertical distribution; Water circulation; Water column; Water depth; western South China Sea
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021006

A topography change in the continental plain plays an important role in nutrient replenishment mechanisms in oligotrophic oceans. Effects of the topography on the nutrient distribution in the western South China Sea (WSCS) have been overlooked since most studies have focused on the dipole‐induced upwelling and downwelling processes of nutrients. We hypothesize that the seamount topography in the northwestern side of the WSCS contributes to the upward distribution of nutrients. We conducted a cruise to investigate the vertical distribution of nutrients in a large area where there is a gradient in the topography: shallow in the north to deep in the south. Our results showed that the depth contours of nutrients, temperature, and salinity shoaled upward from deep to shallow with their isolines being parallel to the bottom depth. The depth of mixed layer, pycnocline, nutricline, and deep chlorophyll maximum showed the similar topographic effect. In the deep water column of 4,308 m deep, integrated NO3− and PO43– over 0–200 m were 879.60 and 81.78 mmol m−2, but increased to 2010.17 and 143.79 mmol m−2 in the shallow water column of 930 m deep, respectively. The increased supply of nutrients enhanced 0–200 m integrated chlorophyll from 21.71 mg m−2 in the deep water column to 51.51 mg m−2 in the shallow water column. These results demonstrate that topographic elevations such as seamounts induce deep‐to‐shallow shoaling and upwelling that lead to enhanced nutrients and biological production in the euphotic zone of oligotrophic oceans. Plain Language Summary The supply of nutrients from deep water to the euphotic zone is a very important source of nutrients for maintaining primary productivity in marine ecosystems. Seamounts as elevations of the topography from seabed play a significant role in shaping currents and regulating nutrients in the oceans. However, the effects of topography on biogeochemical processes in the western South China Sea (WSCS) have been overlooked. We conducted a cruise to investigate the nutrient dynamics along with the changing of topography in the WSCS. We found that the isolines of nutrients, temperature, and salinity were parallel to the bottom depth, showing low nutrients in the deeper southeastern region and high nutrients in the shallower northwestern region. Additionally, the water column structures such as the depth of mixed layer, pycnocline, and nutricline were significantly correlated with bottom depths. The nitrate and phosphate