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Abstract: In the last 10 years (2012–2021), five hypoxic events have been observed in summer in the central Bohai Sea (CBS). Frequent and persistent hypoxia will have an impact on the ecosystem of the CBS. In this paper, historical sea temperature (ST), salinity (SAL), density (Den), and dissolved oxygen (DO) concentration data from three stations in the CBS are analyzed via the linear regression method, and the correlations between the stratification factors (ST, SAL, and Den) and DO concentration are determined. The thresholds of the stratification factors at the three stations in June in the year in which hypoxia occurred were determined and applied to survey data from 29 stations in late May to early June in 2022 in the CBS; this assessment found that the data from 19 stations indicated that hypoxia was about to occur. In August, the survey data showed that 14 out of the 29 stations indicated hypoxic conditions, of which 12 were from the predicted 19 stations, meaning that the estimation accuracy reached 63%. The same approach was applied to data from June 2023. The data for August from a bottom-type online monitoring system in the CBS verified the occurrence of hypoxic events around Sta. M2. The results show that the strength of the seawater stratification plays a leading role in hypoxic events in the summer in the CBS, and the thresholds of the stratification factors can be used to predict the occurrence of hypoxic events.
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Key words:
- hypoxia /
- central Bohai Sea /
- sea temperature /
- salinity /
- sea-water density /
- stratification
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Figure 1. Locations of survey stations in the CBS (Stas M1, M2, and M3). a. The black box is the study area; b. twenty-nine stations were distributed in the study area. In a, the yellow, blue, red, and green dashed boxes denote the hypoxic zones investigated by Zhang et al. (2016), Jiang et al. (2016), Zhai et al. (2012), and Li et al. (2021), respectively. The black contours and values indicate the water depth (in unit of m). In b, the black dots indicate DO concentration values of ≥4 mg/L, the white dots indicate DO concentration values of 3–4 mg/L, and the red dots indicate DO concentration values of <3 mg/L in summer in 2022; the black numbers indicate before strong winds, and the red numbers indicate after strong winds during the summer in 2022.
Figure 2. Research methods: a. the correlation between the bottom DO concentration and water depth in the study area in spring 2022; b. the observation depth range for the 29 stations (Stas 1−29) in the study area corresponds to that of Stas M1, M2, and M3; and c. the workflow of our research.
Figure 3. Variations in the ST, SAL, Den, and DO concentration with depth in different seasons at Sta. M2 in the CBS from 2013 to 2022. a. ST from winter and spring, b. ST from summer and autumn, c. SAL from winter and spring, d. SAL from summer and autumn, e. density from winter and spring, f. density from summer and autumn, g. DO concentration from winter and spring, and h. DO concentration from summer and autumn. To achieve a better comparison, the data for December, February (dotted line), and April–June (*line) are plotted in Figs 3a, c, e, and g, and the data for August (line) and November (○line) are plotted in Figs 3b, d, f, and h.
Figure 4. HRRF at Lijin Station and the air temperature T at Longkou Station minus the SST at the buoy statistics: a. the HRRF of spring and summer at the Lijin Station from 2010 to 2023, and b. the difference between T at Longkou Station and the SST at the buoy in the CBS in spring and summer from 2011 to 2022.
Figure 5. Information about the stratification factors and DO concentration at Stas M1 and M3 and distribution of flow field in the CBS. a. ST distribution with depth in May and June in 2015, 2017, and 2019; b. ST distribution with depth in August in 2014, 2015, and 2017; c. SAL distribution with depth in May and June in 2015, 2017, and 2019; d. SAL distribution with depth in August in 2014, 2015, and 2017; e. Den distribution with depth in May and June in 2015, 2017, and 2019; f. Den distribution with depth in August in 2014, 2015, and 2017; g. DO concentration distribution with depth in August in 2014, 2015, and 2017; and h. the maximum tidal current velocity in May 2022. In h, red dots: stations; red star: Stas M1, M2 and M3; contour lines and numbers represent the velocity (m/s) of the flow.
Figure 6. Hypoxic validation of the August 2022 and 2023 data in the CBS. a. DO concentration distribution in the bottom layers at the 29 stations in August 2022 (black numbers: DO concentration values); b. distribution of bottom DO concentration values in June 2022 minus bottom DO concentration values (▲DO) in August at the 29 stations (black numbers:▲DO concentration values); c. comparison of ▲stratification factors (surface value minus bottom value) and SFTs (dotted line) in June 2023; and d. DO concentration distribution (the dash line is DO concentration = 4 mg/L) at the bottom at Sta. M2 during August 1–20, 2023.
Figure 7. ST and SAL distributions in June and August in 2022: a. up-ST in June; b. bottom-ST in June; c. up-SAL in June; d. bottom-SAL in June; e. up-ST in August; f. bottom-ST in August; g. up-SAL in August; and h. bottom-SAL in August. The pink dots and numbers represent the stations.
Table 1. Data sources of historical and real-time
Data classification Time Station Data Historical data 2013–2022 M1, M2, M3 ST, SAL, Den, DO concentration 2011–2022 Longkou Station T 2010–2022 Lijin Station Huanghe River runoff Real-time data June and August in 2022 Stas 1–29 ST, SAL, Den, DO concentration June–August in 2023 M2 ST, SAL, Den, DO concentration June–August in 2023 buoy SST, SAL 
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Table 2. Simulation results of the correlations between the stratification factors and DO concentration
Correlation ST/℃ and DO
concentration/(mg·L–1)SAL and DO
concentration/(mg·L–1)Density/(kg·m–3) and
DO concentration/(mg·L–1)y DO concentration DO concentration DO concentration x ST SAL Density Formula Y = 10.482 − 0.002x2.5 + 4.42 × exp(x) Y = 10.08 − 3.6/{1 + [(x − 31.54)/0.14]2} y = 5.7 × 106 − 1.67 × 104x + 16.27 × x2 − 5.3 × 10−3x3 R 0.91 0.45 0.81 RMSE/
(mg·L–1)0.97 2.06 1.34 SSE 74.62 335.06 141.52 R-squared 0.84 0.20 0.66 DC 0.82 0.20 0.66 Correlation ΔST/℃ and ΔDO
concentration/(mg·L–1)ΔSAL and ΔDO
concentration/(mg·L–1)ΔDen/(kg·m–3) and ΔDO
concentration/(mg·L–1)y ΔDO concentration ΔDO concentration ΔDO concentration x ΔST ΔSAL ΔDen Formula y = 3.36 × exp[−exp(110.06 – 47.53 x)] y = exp[−25.74 + 82.61x3+21.82exp(−x)] y = (0.69 + x)/(8.04 + 7.84x) − 1.05x R 0.82 0.78 0.81 RMSE/
(mg·L–1)0.96 1.05 0.97 SSE 17.64 20.77 17.99 R-squared 0.66 0.61 0.67 DC 0.66 0.60 0.66
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Table 3. SFTs for Stas M1, M2, and M3 in June during a hypoxia year
Item Value Sta. M1 Sta. M2 Sta. M3 ΔST/°C (assessment SFTs of hypoxia in June ) − ≥2.32 ≥3.54 ΔSAL (assessment SFTs of hypoxia in June) − ≤−0.24 ≤−0.17 ΔDen/(kg·m–3) (assessment SFTs of hypoxia in June) − ≤−0.83 ≤−1.04 Note：− indicates that there is no threshold.
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Table 4. Assessment of the stratification factors for June, 2022
Station Sta. M2 Station Sta. M3 ΔST ≥ 2.32 ℃ ΔSAL ≤ −0.24 ΔDen ≤ −0.83 kg/m3 ΔST > 3.5 ℃ SAL ≤ −0.17 ΔDen ≤ −1.04 kg/m3 2, 3 √ × √ 1 √ × √ 5 × × × 8, 26 × × × 6, 29 √ × √ 9, 10, 17 √ √ √ 7, 11, 12 √ √ √ 18, 19, 20 √ √ √ 13 √ × × 21, 23, 24, √ √ √ 14, 15 √ √ √ 25, 27, 28 √ √ √ 16, 22 √ √ √ 29 √ × × Note: √ indicates that the stratification factor thresholds are satisfied. × indicates that the stratification factor thresholds are not satisfied.
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