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Article

Effects of an Episodic Storm-Induced Flooding Event on the Biogeochemistry of a Shallow, Highly Turbid, Semi-Enclosed Embayment (Laizhou Bay, Bohai Sea)

by
Ling Meng
1,2,
Qianguo **ng
1,2,*,
Xuelu Gao
1,2,
Diansheng Ji
3,
Fanzhu Qu
4,
**aoqing Wang
3 and
Ling Ji
3
1
CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China
2
University of Chinese Academy of Sciences, Bei**g 100049, China
3
Yantai Marine Environmental Monitoring Central Station, State Oceanic Administration (SOA), Yantai 264006, China
4
Institute for Advanced Study of Coastal Ecology, Ludong University, Yantai 264025, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(1), 563; https://doi.org/10.3390/su15010563
Submission received: 8 November 2022 / Revised: 20 December 2022 / Accepted: 21 December 2022 / Published: 28 December 2022
(This article belongs to the Special Issue Environmental Impact Assessment in the Marine Realm: Nutrient Loadings, Eutrophication, Acidification and Pollution)
Browse Figures
Versions Notes

Abstract

:
Episodic storm-induced flooding is becoming more frequent with a warming climate, which may alter the biogeochemical properties and conditions of estuaries. However, the effects of such extreme events on semi-enclosed bay ecosystems have not been fully investigated because of the difficulty in collecting in situ samples. To address this issue, a comparative study was carried out to understand the biogeochemical changes in Laizhou Bay, a shallow, highly turbid, semi-enclosed bay, by coupling satellite data and surface water samplings collected during an episodic flooding event (August 2018) and during a non-flooding period (August 2017). The results showed that the 2018 Shouguang flood delivered large amounts of suspended solids, phosphorus, and organic matter-enriched terrigenous materials into Laizhou Bay and enhanced the offshore expansion of the low-salinity seawater plume and associated nutrient fronts. Water total suspended solid (TSS) particle and chlorophyll a (Chl-a) concentrations increased by 23.79 g/m3 and 0.63 mg/m3, respectively, on average in the freshwater mixing water plume around the Mi River. Episodic flooding is a crucial driver which temporally dominates the spatial patterns of water biogeochemistry. These results are essential to anticipate the ecosystem response of estuarine regions to the high episodic freshwater flow associated with the increasing storms.

1. Introduction

Extreme weather and climate events such as precipitation extremes are expected to intensify over most of the globe because of the altered water cycle under a warmer background climate [1,2,3]. Eastern China, a populous monsoon region, frequently and severely suffers from precipitation extremes [4,5,6]. Large quantities of freshwater and land-sourced substance injections resulting from extreme precipitation events have great potential to alter the hydrological and biogeochemical dynamics of estuaries and the semi-closed bay ecosystems to which they connect [7,8,9]. However, those changes vary among individual systems, and it is difficult to generalize how a semi-closed bay ecosystem responds to extreme climate events.
Laizhou Bay, a typical semi-enclosed marginal sea with a relatively long water exchange period [10,11], is one of the three major bays in the Bohai Sea. It is a shallow, bowl-shaped bay formed by the accumulation of riverine suspended matter [12], and its average water depth is less than 10 m, with a maximum of approximately 18 m [12,13]. This bay has been subject to long-term contamination and eutrophication due to terrestrial runoff input from more than a dozen rivers, including two large rivers: the Yellow and ** the TSS temporary spatial structure than a resuspension process.
In contrast to the high TSS increase in the nearshore zones, the TSS level was reduced by 8.65 g/m3 on average inside the bay (Figure 7c), which might be attributed to the intensive rainfall produced by Typhoon Yagi. The rainfall increased the fresh water supply and diluted the nearshore water. Our results suggest that the occurrence of extreme flooding causes the flooding plume-affected zone to become more turbid. However, for other water bodies in Laizhou Bay, the preceding rainstorm can be a stressor that is sufficiently high for the transition from a turbid to a relatively clear state.

3.3.4. Satellite-Obtained Changes in Chl-a after Storm-Induced Flooding

Figure 8 illustrates the daily averaged surface Chl-a concentrations both before (Figure 8a) and after (Figure 8b) the flood. Before flooding, a high concentration of Chl-a was observed in the southwestern bay and extended to central Laizhou Bay, which was similar to the climatological Chl-a concentration in August [47].
After storm-induced flooding, a marked increase in Chl-a was observed in offshore freshwater mixing water, and the chlorophyll concentration increased by 0.63 mg/m3 on average. As phosphorus is considered a limiting nutrient in Laizhou Bay [14], the delivery of biologically available phosphorus, which was suggested to be the dominant P form of the stormwater runoff event [48,49], may stimulate an increase in phytoplankton biomass. However, the concentration of Chl-a unexpectedly decreased by 0.57 mg/m3 on average in the southern nearshore zone despite the higher availability of nutrients, possibly due to increased flushing or reduced light penetration [50].
To summarize, Laizhou Bay is one highly eutrophic water body where, under ‘normal’ summer conditions, the distribution of biogeochemical properties was dominated by the freshwater discharge. High nutrient, TSS, and Chl-a concentrations were generally observed in the inner shore of Laizhou Bay, where there are large amounts of effluent discharge, especially in the freshwater-dominated upper estuary from the **aoqing River.
The 2018 Shouguang flood caused a surge of freshwater influx and induced a rapid export of excess nutrients and TSS into nearshore regions, resulting in temporary eutrophication. As indicated by the extended shape of low-salinity water, the short-lived flood appeared spatially to impact nearly a third of the entire bay. Large inputs of TP into Laizhou Bay were responsible for a drastic increase in phytoplankton biomass. The Chl-a increase observed in Laizhou Bay, a typical shallow, highly eutrophic embayment, was comparable to the reported oligotrophic system [33,51,52]. Pulsed natural disturbances are probably an important control of nutrient and material fluxes in this bay. However, due to the lack of continuous large-area observations, we were not able to track the impacts of such climate extremes on a time scale.

4. Conclusions

Opportunistic in situ cruise observations plus satellite datasets were fortuitously available to reveal the key biogeochemical responses of a shallow, highly turbid, semi-enclosed bay to extreme precipitation and storm-induced flooding events. Our results revealed that the event brought a large amount of nutrient-rich, highly turbid freshwater into Laizhou Bay, thus increasing the water turbidity and promoting phytoplankton growth in the offshore freshwater mixing water plume. Episodic flooding temporarily replaced the **aoqing River, which otherwise dominates the spatial pattern of water biogeochemistry in Laizhou Bay, and caused significant northeastward expansions of salinity, TN, and TP levels.
Our results provide a good view of the significant biophysical responses of the coastal sea to episodic weather events, such as flooding, which are meaningful to develop effective coastal management strategies. However, routine monitoring lacks the temporal resolution to track the impacts of such climate extremes. Thus, high-intensity sampling coupled with joint physical–biogeochemical models are crucial for understanding how these events impact complex systems such as Laizhou Bay.

Author Contributions

Conceptualization, L.M.; data curation, L.M., D.J., F.Q. and X.W.; formal analysis, L.M.; investigation, L.M. and X.G.; methodology, L.M., X.G., F.Q. and X.G., project administration, Q.X. and L.J.; resources, D.J.; software, L.M.; supervision, Q.X. and X.G.; validation, Q.X.; visualization, Q.X.; writing—original draft preparation, L.M.; writing—review and editing, Q.X. and X.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Shandong Provincial Natural Science Foundation, China (grant number ZR2020QD092), and the National Natural Science Foundation of China (grant numbers 42076188 and 31870468).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors give thanks to the Yantai Marine Environmental Monitoring Central Station, State Oceanic Administration (SOA), for providing the biogeochemical datasets.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of sampling sites and Laizhou Bay, Bohai Sea.
Figure 1. Location of sampling sites and Laizhou Bay, Bohai Sea.
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Figure 2. Rainfall distribution characteristics in Shouguang City: (a) daily precipitation on 19 August 2018; (b) total precipitation from June to July in 2018; (c) daily precipitation from 6 June to 31 August 2018; (d) summer precipitation from 2000 to 2018.
Figure 2. Rainfall distribution characteristics in Shouguang City: (a) daily precipitation on 19 August 2018; (b) total precipitation from June to July in 2018; (c) daily precipitation from 6 June to 31 August 2018; (d) summer precipitation from 2000 to 2018.
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Figure 3. Sentinel 2A MSI image (RGB) of offshore regions (shown by the dashed red line in Figure 1) in Laizhou Bay on 25 August 2018 (5 days after the Shouguang flood).
Figure 3. Sentinel 2A MSI image (RGB) of offshore regions (shown by the dashed red line in Figure 1) in Laizhou Bay on 25 August 2018 (5 days after the Shouguang flood).
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Figure 4. Variations in daily water turbidity, pH, COD, DO, TN, TP, and flow rate at the Yangkou Station in August 2018.
Figure 4. Variations in daily water turbidity, pH, COD, DO, TN, TP, and flow rate at the Yangkou Station in August 2018.
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Figure 5. The interpolation map of salinity, DO, and pH in Laizhou Bay. (a–c) Observations during non-flooding, August 2017. (d–f) Observations after the 2018 Shouguang flood. (g–i) Difference between August 2018 and August 2017.
Figure 5. The interpolation map of salinity, DO, and pH in Laizhou Bay. (a–c) Observations during non-flooding, August 2017. (d–f) Observations after the 2018 Shouguang flood. (g–i) Difference between August 2018 and August 2017.
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Figure 6. Same as in Figure 5, but showing the interpolation map of TN, TP, and COD in Laizhou Bay. (a–c) Observations during non-flooding, August 2017. (d–f) Observations after the 2018 Shouguang flood. (g–i) Difference between August 2018 and August 2017.
Figure 6. Same as in Figure 5, but showing the interpolation map of TN, TP, and COD in Laizhou Bay. (a–c) Observations during non-flooding, August 2017. (d–f) Observations after the 2018 Shouguang flood. (g–i) Difference between August 2018 and August 2017.
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Figure 7. GOCI-retrieved daily TSS concentration on (a) 11 August 2018 (pre-flood) and (b) 25 August 2018 (post-flood). (c) The corresponding TSS difference (post-flood TSS subtracted by pre-flood TSS).
Figure 7. GOCI-retrieved daily TSS concentration on (a) 11 August 2018 (pre-flood) and (b) 25 August 2018 (post-flood). (c) The corresponding TSS difference (post-flood TSS subtracted by pre-flood TSS).
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Figure 8. GOCI-retrieved daily Chl-a concentration on (a) 11 August 2018 (pre-flood) and (b) 25 August 2018 (post-flood). (c) The corresponding Chl-a difference (post-flood Chl-a subtracted by pre-flood Chl-a).
Figure 8. GOCI-retrieved daily Chl-a concentration on (a) 11 August 2018 (pre-flood) and (b) 25 August 2018 (post-flood). (c) The corresponding Chl-a difference (post-flood Chl-a subtracted by pre-flood Chl-a).
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Table
Table 1. The mean ± SD values for the different variables measured in the surface waters of Laizhou Bay in August 2017 and August 2018.
Table 1. The mean ± SD values for the different variables measured in the surface waters of Laizhou Bay in August 2017 and August 2018.
VariableUnitAugust 2017 (Non-Flooding)August 2018 (Flooding)
Whole BayWithin the Freshwater PlumeWhole BayWithin the Freshwater Plume
Salinitypsu29.59 ± 1.6128.04 ± 1.6027.25 ± 5.0719.82 ± 4.31
DOmg/L7.48 ± 0.277.22 ± 0.107.43 ± 0.527.48 ± 0.20
pH 8.13 ± 0.038.12 ± 0.038.04 ± 0.048.04 ± 0.02
CODmg/L1.16 ± 0.501.61 ± 0.141.25 ± 0481.85 ± 0.20
TNmg/L0.53 ± 0.120.61 ± 0.150.93 ± 0.481.27 ± 0.32
TPmg/L0.017 ± 0.0060.021 ± 0.0050.020 ± 0.0150.041 ± 0.017
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Meng, L.; **ng, Q.; Gao, X.; Ji, D.; Qu, F.; Wang, X.; Ji, L. Effects of an Episodic Storm-Induced Flooding Event on the Biogeochemistry of a Shallow, Highly Turbid, Semi-Enclosed Embayment (Laizhou Bay, Bohai Sea). Sustainability 2023, 15, 563. https://doi.org/10.3390/su15010563

AMA Style

Meng L, **ng Q, Gao X, Ji D, Qu F, Wang X, Ji L. Effects of an Episodic Storm-Induced Flooding Event on the Biogeochemistry of a Shallow, Highly Turbid, Semi-Enclosed Embayment (Laizhou Bay, Bohai Sea). Sustainability. 2023; 15(1):563. https://doi.org/10.3390/su15010563

Chicago/Turabian Style

Meng, Ling, Qianguo **ng, Xuelu Gao, Diansheng Ji, Fanzhu Qu, **aoqing Wang, and Ling Ji. 2023. "Effects of an Episodic Storm-Induced Flooding Event on the Biogeochemistry of a Shallow, Highly Turbid, Semi-Enclosed Embayment (Laizhou Bay, Bohai Sea)" Sustainability 15, no. 1: 563. https://doi.org/10.3390/su15010563

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