Improving the understanding of central Bohai Sea eutrophication based on wintertime dissolved inorganic nutrient budgets: Roles of north Yellow Sea water intrusion and atmospheric nitrogen deposition

Highlights

• Wintertime DIN/DIP ratios in the central Bohai Sea were around 16 in the 2010s.

• The adjacent Yellow Sea supplies additional phosphate to the Bohai Sea in winter.

• Eutrophication in the central Bohai Sea is connected to regional nutrient changes.

Abstract

The Bohai Sea is a shallow-water, semi-enclosed marginal sea of the Northwest Pacific. Since the late 1990s, it has suffered from nutrient over-enrichment. To better understand the eutrophication characteristics of this important coastal sea, we examined four survey datasets from summer (June 2011), late autumn (November 2011), winter (January 2016), and early spring (April 2018). Nutrient conditions in the Bohai Sea were subject to seasonal and regional variations. Survey-averaged N/P ratios in estuarine and nearshore areas were 20–133. In contrast, the central Bohai Sea had mean N/P ratios of 16.9 ± 3.4 in late autumn, 16.1 ± 3.0 in winter and 13.5 ± 5.8 in early spring, which are close to the traditional N:P Redfield ratio of 16. In summer, both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) were used up in the surface waters of the central Bohai Sea, suggesting that the biological consumption of DIN and DIP may also follow the Redfield ratio. Wintertime nutrient budgets of the central Bohai Sea water were then established based on a mass balance study. Our results suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion, balancing terrigenous excess DIN that was introduced in summer. A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans. Such changes in nutrients may have fundamentally contributed to the recent development of algal blooms and seasonal hypoxia in the central Bohai Sea.

Introduction

In the past several decades, increasing human activities—including fossil fuel combustion which emits pollutants such as NO_x and NH₃ (Conley et al., 2009; Xing et al., 2017), fertilizer use (Wang et al., 2019), and sewage discharges (Yang et al., 2018; Wang et al., 2019)—have led to a nutrient enrichment in coastal oceans. Eutrophication is usually associated with changes in nutrient concentrations and element ratios. Element ratios include molar ratios of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP), and/or DIN and dissolved silicate (DSi). Both nutrient concentrations and element ratios affect community development and elemental biogeochemistry (Belias et al., 2007; Howarth et al., 2011). For example, harmful algal blooms (HABs) frequently occurred in eutrophic coastal zones, seriously affecting marine life and ecosystem function. To improve ecosystem services in these coastal oceans, nutrient reduction and element-ratio management are required (Dodds, 2006; Conley et al., 2009).

Besides above-mentioned human activities, ocean circulation also contributes to coastal nutrient dynamics (Zhao and Wei, 2005; Cloern et al., 2007; Smith et al., 2014). The tight coupling of natural variations and human activities thus complicates both research development and remediation designs.

As a shallow semi-enclosed marginal sea with heavily populated coasts off North China, the Bohai Sea has exhibited dramatic environmental degradation and eutrophication since the 1990s (Ding, 1999; Yu et al., 2001; Wang and Li, 2006). According to Wang et al. (2019), the nitrate concentration of the Bohai Sea in winter (which usually accounts for 70%–95% of total DIN) substantially increased from 1 to 5 μmol L⁻¹ in the 1980s to 5–10 μmol L⁻¹ in the 1990s and to 8–15 μmol L⁻¹ in the 2000s. Additionally, Xin et al. (2019) showed that the DIN concentration in Bohai Sea underwent a 7-fold increase from the late 1990s to the beginning of the 2010s, inducing a basin-scale DIN/DIP ratio increase and a DSi/DIN ratio decline (e.g., Wang and Li, 2006; Liu et al., 2011; Guo et al., 2014). Correspondingly, basin-wide phytoplankton growth and HABs frequently occurred in the Bohai Sea in the 2000s (Fu et al., 2016; Song et al., 2016). To mitigate the water pollution and environmental degradation in the Bohai Sea, many pollution monitoring and control measures have been adopted in the surrounding provinces and cities, including controls related to industrial pollution, domestic sewage, terrestrial non-point pollution, marine mobile pollution, marine accident contamination, and aquaculture effluents, and sewage reuse has been encouraged (Wang and Gao, 2007). Such measures have presumably cost the Chinese central government over 100 billion RMB yuan (http://www.yzhbw.net/news/shownews-14_12037.dot).

Although sewage and pollution controls are certainly important, the Bohai Sea is connected to open oceans via circulation (Fig. 1). The open oceans supply background nutrients for coastal waters (Smith et al., 2014). We contend that eutrophication in the Bohai Sea must be regarded as a regional, rather than simply a local issue. After all, the Bohai Sea is directly connected to the Yellow Sea (Fig. 1), where DIN increased from the 1980s–2000s (Wei et al., 2015; Yang et al., 2018). Yet the nutrient changes in the Yellow Sea have not been considered in past attempts to understand eutrophication in the Bohai Sea.

Additionally, an increase in excess nitrate relative to phosphate was detected over a large segment of the ocean surface in the western North Pacific during the 1980s and 1990s, likely owing to the widespread atmospheric nitrogen deposition (Kim et al., 2014). The above-mentioned Yellow Sea DIN increases have also been partially induced by atmospheric nitrogen deposition (Yang et al., 2018). Furthermore, a modeling study has suggested atmospheric nitrogen deposition may supply approximately 54% of the total DIN in the Bohai Sea (Shou et al., 2018). Since the late 2000s, however, the atmospheric deposition of reactive nitrogen has stabilized in China (Yu et al., 2019). This regional environmental change presumably also affects eutrophication in the Bohai Sea and requires further investigation.

To better understand the nutrient supply supporting the frequent HABs in the Bohai Sea, we examined four basin-wide survey datasets from the Bohai Sea, spanning all four seasons. Moreover, we used time–series data of annual water and nutrient budgets which were applied in the central part of the Bohai Sea to examine the effects of circulation and regional environmental changes on the Bohai Sea nutrients. Note that the late-autumn data have been partially published by Guo et al. (2014), and some of the hydrological data have been described by Zhai et al. (2012, 2019). Here we focused only on the cold seasons (from November to April of the next year) to examine basin-wide eutrophication and also exclude uncertain nutrient element ratios resulting from exhaustive phytoplankton uptakes (Deutsch and Weber, 2012).