Quantification of the Seasonal Intrusion of Water Masses and Their Impact on Nutrients in the Beibu Gulf Using Dual Water Isotopes

Abstract

A new data set of seasonal stable water isotopes (δD and δ18O) and temperature‐salinity profiles was applied to improve our understanding of water mass distributions and their impact on the environment of the Beibu Gulf (BG). Our study revealed that the coastal current (CC), West‐Guangdong coastal current (WGCC), and South China Sea water (SCSW) were the three dominant water masses in the BG, and their influence was exhibited in seasonal variations. The CC was the dominant contributor to the BG water during summer (43%) and fall (45%), while it changed to the intrusion of SCSW with higher salinity in winter (57%). The contribution of WGCC to the BG was relatively stable during the three seasons (24%–31%). In addition, the nutrients in the BG were greatly affected by different water mixing occurring in the gulf. The nutrients mainly originated from the CC in summer (52%–68%) and fall (32%–69%), while the dominant source shifted to the WGCC in winter (36%–69%). Moreover, the contribution of SCSW to the nutrients loading (15%–49%) in the BG was relatively high due to its high contribution (57%) to the BG water during winter. These indicated that the BG has a stable input of external nutrients from different water masses to sustain primary production in the BG. Our study uses dual water isotopes to quantify the seasonal intrusion of water masses and their impact on nutrients, providing a new method to study the impact of the distribution of water masses on nutrients in the gulf.

Sources of nitrate in a heavily nitrogen pollution bay in Beibu Gulf, as identified using stable isotopes

“…Higher nutrient concentrations were also observed in the other coastal bays of Beibu Gulf during the rainy seasons (Lai et al, 2014;Lao et al, 2021a andLao et al, 2021c), which were mainly related to the heavy land-source discharge induced by the increasing runoff around the coastal Beibu Gulf (Lao et al, 2020;Lao et al, 2021a). Particularly in Qinzhou Bay, because there are several rivers, such as the Qin River, Maoling River and Dafeng River, that directly input a large amount of terrestrial contaminants into the bay (Chen et al, 2019a;Zhang et al, 2019;Lao et al, 2020;Lao et al, 2021c;Lao et al, 2022b;Lyu et al, 2022). However, heavier DIN pollution was found in Xi Bay.…”

“…In addition, soil N was another important source that contributed to the heavy nitrogen pollution in the bay. With the increase in rainfall in the rainy season, a large amount of soil N on the land can be washed away and enter the coastal bay with the river (Lao et al, 2022b). Previous studies suggested that the nitrate source from the soil N in a coastal bay with riverine input was significantly higher than that in a bay without riverine input during the same period (Chen et al, 2022;Lao et al, 2022b).…”

“…With the increase in rainfall in the rainy season, a large amount of soil N on the land can be washed away and enter the coastal bay with the river (Lao et al, 2022b). Previous studies suggested that the nitrate source from the soil N in a coastal bay with riverine input was significantly higher than that in a bay without riverine input during the same period (Chen et al, 2022;Lao et al, 2022b). Moreover, nitrogen fertilizer (almost in the reduced form) is heavily applied in catchment agriculture in the Beibu Gulf during the rainy seasons (Xia et al, 2011;Kaiser et al, 2014;Lao et al, 2021a: Lao et al, 2021b, which could leak and contribute to the bay in this rainy season. )…”

“…Beibu Gulf is a semi-enclosed gulf and a newly developing industry and port area in the northwestern South China Sea (SCS). There are several rivers around the northern Beibu Gulf, and the influence of runoff input by the rivers and tidal influx from the outer gulf constitutes a well estuary-bay multi-ecosystem and has become an important mariculture area in China (Lao et al, 2020 andLiu et al, 2020;Xu et al, 2020: Xu et al, 2021Lao et al, 2021a;Lao et al, 2022a and2022b). However, with the rapid development of local urbanization and industrialization, the coastal bays in Beibu Gulf have been subjected to a high loading of contaminants from anthropogenic inputs, such as domestic, industrial, port and agricultural waste (Lao et al, 2019b;Xu et al, 2020: Lao et al, 2021aLao et al, 2021b;Lao et al, 2021c;Lao et al, 2021d: Xu et al, 2021and Lao et al, 2022a.…”

“…The Fangcheng River is approximately 98 km long with a drainage area of 843 km 2 . The annual runoff of the Fangcheng River is 542.25 × 10 8 m 3 , with the highest runoff in July (127.41 × 10 8 m 3 ), and flows into the northern end of Xi Bay (Lao et al, 2020;Lao et al, 2022b). The eastern part of Xi Bay is the urban center of Fangchenggang city.…”