The Effect of Weir Opening on Physiochemical Properties in the Geum Estuary of Republic of Korea

Abstract

Deteriorated river water quality affects estuarine environments due to river and estuary linkages. Thus, river water quality improvement could play a pivotal role in the maintenance of estuarine ecosystem functions. The South Korean administration opened river weirs to restore river ecosystems and announced that this helped restore habitats and improve water quality. Despite these reported positive results in the rivers, little research has been conducted on the effects of weir opening on estuaries linking to rivers. We investigated the physicochemical properties of the Geum Estuary during the initial weir opening (2018) and stabilization (2021) periods. The results indicated that the NO₃⁻ inputs from rivers to estuaries decreased from 2018 to 2021 by 19~68%; however, the trends of [NO₃⁻] in estuarine water columns in nearby estuary dams fluctuated between 2018 and 2021. No change in NO₃⁻ pollutant sources was observed from 2018 to 2021. Unlike [NO₃⁻], [PO₄⁻] and [SiO₂] were higher in 2021; thus, large amounts of previously accumulated PO₄⁻ and SiO₂ within the weirs might have been released from the river into the estuary. This supply of PO₄⁻ could enhance estuarine phytoplankton growth, where PO₄⁻ is a limiting factor for primary production due to excessive inputs of NO₃⁻ from rivers. Therefore, dredging the accumulated sediment within weirs should be considered prior to opening them to prevent the release of PO₄⁻ and SiO₂ into estuaries.

1. Introduction

Estuaries are located in transition zones between river and marine environments and are subject to marine influences, such as tides, waves, and saline water, as well as terrestrial influences, such as flows of freshwater and sediments. These mixed characteristics can not only provide various habitats for marine organisms, but also ecosystem services, such as diversity maintenance, toxicity removal, flooding control, and abundant fisheries [1]. Because estuaries receive large amounts of nutrients from river discharge, freshwater quality is very important for maintaining estuarine ecosystem health. However, many estuaries are experiencing eutrophication due to excessive nutrients from sewage, fertilizer, animal waste, or artificial structures, such as estuary or river dams, weirs/dikes, levees, and embankments. In particular, the construction of these artificial barriers has blocked natural river water flow and extended the residence time of the water column, leading to eutrophication, deoxygenation, and sediment accumulation [2,3,4]. Thus, the removal of these barriers has attracted interest for restoring river and estuary ecosystems and the positive consequences of these removals have been reported [5,6,7,8].

In South Korea, the Four Major Rivers Restoration Project was initiated from 2008 to 2012 on four major rivers: the Han, Geum, Yeongsan, and Nakdong Rivers. The project implemented the construction of 16 river weirs to control flooding and secure water resources, as well as the installation of wastewater treatment, monitoring facilities, and sediment dredging to improve water quality [9,10]. However, weir construction had transformed river channels, blocked natural water flow, and increased river residence time, causing severe eutrophication, deoxygenation, and destruction of organism habitats [11,12,13,14,15,16]. The subsequent South Korean administration decided to permanently or partially open 11 of the 16 weirs in the four major rivers to resolve environmental risks. As a result, the Geum and Yeongsan Rivers, where all weirs were opened, showed an increase in the self-purification index by 8 and 9.8 times, respectively, and a significant decrease in the occurrence of hazardous green algae blooms and habitat restoration [17,18]. Recently, however, the new government has been discussing halting the gradual opening of weirs and maintaining the existing systems in place since 2023. This confusion in government policy might be attributed to a lack of long-term scientific monitoring of the weir-opening effect on various ecosystems [19]. Furthermore, despite the linkages between rivers and estuaries, there has been little research on the influence of weir opening on water quality in estuarine ecosystems. Thus, this study investigated the changes in physiochemical properties, including salinity, temperature, nutrients, and Chlrorophyll-a (Chl-a) concentrations, between 2018 and 2021 in the Geum River and Estuary. Here, we classified 2018 as a transitional period immediately after weir opening, and 2021 as a stabilization period. By monitoring the water quality, we used isotope techniques to investigate any changes in NO₃⁻ pollutant sources entering the estuary from the river after opening the weirs. Therefore, this information could help policymakers consider the environment of the estuary, as well as the river, when making decisions about opening weirs.

2. Site Description and Analytic Methods

2.1. Site Description and Water Sampling

The Geum Estuary is located on the west coast of South Korea and is part of the Yellow Sea between the Korean Peninsula and the North China Plain (Figure 1). It is a habitat for 13 internationally endangered migratory birds that travel from Siberia to Korea in winter because it provides abundant shellfish and safe spawning grounds. Owing to these natural conservation values, the Geum Estuary has been designated as a protected wetland by the Ramsar Convention since 2009 [20].

The Geum River, which connects to the Geum Estuary, has a total watershed and waterbody area of 9914 and 274 km², respectively [21], and was impounded by an estuary dam built in 1990 (Figure 1). The Geum Estuary dam, located between the river and offshore, discharges freshwater into the estuary 1–3 times per week, depending on the precipitation amount [22]. In the upper and middle reaches of the Geum River, three weirs—Sejongbo, Gongjubo, and Baekjebo—were built in 2012 and were maintained to control flooding and secure water resources, shown in Figure 1b. Next administrations decided that the weirs were gradually opened in 2018 to improve river water quality [17]. However, the new government have recently been discussing on stopping the opening and keeping the existing weir systems since 2023.

Surface and bottom water samples were collected from the Geum River (S1 and S2) and the Geum estuary (G1 to G12) and Yellow Sea (G13 to G15) in February and May 2018 and February and June 2021 (Figure 1c) using a Niskin water sampler (Model 1010, General Oceanics, Miami, FL, USA). The water samples in the G9, G10 and G15 sites for February 2021 were not collected because of technical problems, so the spatial distribution of nutrients and N* was not fully covered. The total water depth was described in the Supplementary Materials, and the bottom water, was collected at a distance of 0.3 to 0.5 m from the bottom sediments (Table S1). The in situ temperature and salinity were measured in the Geum Estuary water column using a CTD device (SBE 49 FastCAT, Seabird Scientific, Bellevue, WA, USA). The collected surface and bottom water samples were filtered using HDPE syringe filters (0.45 μm pore size; Advantec, Tokyo, Japan) and transferred to the laboratory on ice for nutrient and NO₃⁻ isotope analyses. One liter of seawater was filtered through #41 Whatman filter paper (GF/F) to determine the Chl-a concentrations and transferred to the laboratory under dark conditions.

2.2. Analyses of Nutrient and Chl-a Concentrations and NO₃⁻ Isotopic Values

NO₃⁻, PO₄⁻ and SiO₂ concentrations were measured using a nutrient auto-analyzer (Seal Analytical Inc., QuAAtro 39, Mequon, WI, USA). Herein, because [NO₂⁻] accounted for less than 0.1% of the dissolved inorganic nitrogen (DIN), we did not separate [NO₂⁻] and included it in the [NO₃⁻] value. As the specific analysis methods, the NO₃⁻ was reduced to NO₂⁻ by cadmium reduction and then the pink color of [NO₂⁻] was determined using UV at absorbance at 543 nm after the reaction with sulphanilamide and NEDA. The [PO₄⁻] reacted with acidified ammonium molybdate solution and then reduced to a blue complex. The absorbance for the blue color was measured at 690 nm using UV. For analyzing [SiO₂], the filtered seawater reacted with molybdate to formulate a silicomolybdate complex. Menthol and oxalic acid were used for the reduction of the silicomolybdate complex to make a blue color that was measured in UV at the 810 nm absorbance.

The Korean Standard Method for Marine Environments (KSM-ME) was followed for all nutrient analyses, and data reliability improved through simultaneous analyses of certified reference materials (KSM-ME, 2018). For Chl-a concentration measurements, the filter paper was placed in a 90% acetone solution for extraction. The absorbance of the extract solution was measured at 630, 647, 664, and 750 nm to calculate its concentration [23].

The riverine NO₃⁻ inputs from the Geum River to estuary were calculated by multiplying the Geum River’s discharge rate [NO₃⁻] at the S1 and S2 sites (Figure 1b). The monthly discharge data were obtained from the Korea Rural Community Corporation of South Korea.

The δ¹⁵N_NO3 and δ¹⁸O_NO3 isotopic values in NO₃⁻ for water samples were measured using a bacterial denitrification assay with a ThermoFinnigan GasBench PreCon trace gas concentration system (Thermo Scientifi, Bremen, Germany) interfaced to a Thermo Scientific Delta V Plus isotope-ratio mass spectrometer (Thermo Scientific, Bremen, Germany) at the UC Davis Stable Isotope Facility, USA [24]. After converting NO₃⁻ to N₂O using denitrifying bacteria, the gas sample was passed through a CO₂ scrubber, and N₂O was trapped and concentrated in liquid nitrogen cryo-traps. N₂O was carried by helium to the isotope ration mass spectrometry, and each isotopic value was measured. The N and O isotope values in NO₃⁻ were expressed as δ (‰) with respect to ^15/14N₂ gas in air and ^18/16O in Vienna Standard Mean Ocean Water. They were calibrated against the reference materials USGS-32, USGS-34, and USGS-35 supplied by NIST (National Institute for Standards and Technology, Gaithersburg, MD, USA) for δ¹⁵N_NO3 and ¹⁸O_NO3 [24]. The measurement errors were <0.4‰ for δ¹⁵N_NO3 and <0.5‰ for δ¹⁸O_NO3.

2.3. Analysis Using the SIAR Isotope Mixing Model

The SIAR isotope mixing model is an R package (Version R-4.3.2; R Core Team, Vienna, Austria) based on Bayesian statistical methods commonly employed in stable isotope studies. We used this model to determine the proportional contributions of NO₃⁻ pollution sources to a mixture, such as the estuary water column. Additionally, we used the δ¹⁵N_NO3 and δ¹⁸O_NO3 values measured from the Geum estuary water and the end-member values for each pollutant source referenced from previous literature [25,26,27].

3. Results and Discussion

3.1. Riverine NO₃⁻ Inputs from the Geum River to the Geum Estuary between 2018 and 2021

In February during the dry season, the input of NO₃⁻ to the estuary from the river was 19 × 10³ and 6 × 10³ mol in 2018 and 2021, respectively, a 68% decrease. In May and June during the rainy and late-bloom seasons, the NO₃⁻ released from the river into the estuary decreased by 19% from 2018 (32 × 10³ mol) to 2021 (26 × 10³ mol) (Table 1). Similar to NO₃⁻ inputs, the [NO₃⁻] in February was 186 and 57 μmol L⁻¹ for 2018 and 2021, respectively, a 69% decrease. The [NO₃⁻] in May/June decreased by 35% from 2018 (115 μmol L⁻¹) to 2021 (75 μmol L⁻¹). The discharge between February 2018 and February 2021 was not considerably different; however, the discharge in June 2021 was slightly higher than that in May 2018. (Table 2). Thus, these results indicate that the weir opening could have contributed to a considerable reduction in the total amount of NO₃⁻ entering the estuary from the river.

3.2. Influence of the Weir Opening on Physicochemical Properties of the Geum Estuary

In both 2018 and 2021, the freshwater discharged from the river lowered the water column salinity near the estuary dam, ranging from 21 to 29 ‰ for both surface (Figure 2a) and bottom (Figure 2b) water near the estuary dam (G1 to G5). This freshwater supply causes weak stratification in the estuarine water column close to the estuary dam (Figure 2). In addition, freshwater spread was observed to be more extensive in 2021 than in 2018 (Figure 2). The low salinity did not extend further than 10 km from the estuary dam and at 10 km from the dam, the salinity was 30 to 32%, a typical offshore salinity (Table 2 and Figure 2). The water temperature was higher in 2021, likely related to the weather at the time of the survey (Table 2).

Because NO₃⁻ is the limiting factor for phytoplankton growth in marine ecosystems, monitoring [NO₃⁻] after opening of weirs is an important factor in assessing the change in water quality of estuarine ecosystems. Within 10 km from the estuary dam (G1 to G5), the surface water [NO₃⁻] in February of 2021 (55 ± 12 μmol L⁻¹; 18~81 μmol L⁻¹) was higher than that of 2018 (31 ± 4 μmol L⁻¹; 22~45 μmol L⁻¹) (Figure 3a,b; Table S4). Unlike the surface water, there was no difference in average [NO₃⁻] of the bottom water between February 2018 (25 ± 6 μmol L⁻¹; 20~37 μmol L⁻¹) and February 2021 (25 ± 7 μmol L⁻¹; 12~48 μmol L⁻¹) (Figure 4a,b; Table S3). When 10 km away from the estuary dam (G6 to G15), the [NO₃⁻] tended to be low, ranging from 1 to 8 μmol L⁻¹ in both the surface (Figure 3a,b) and bottom (Figure 4a,b) water in February of 2018 and 2021, with no difference in [NO₃⁻] during this time. Unlike February, a slight decrease in [NO₃⁻] in the surface water was observed from May 2018 (39 ± 7 μmol L⁻¹; 14~52 μmol L⁻¹) to June 2021 (29 ± 8 μmol L⁻¹; 6~5 μmol L⁻¹) near the estuary dam (Figure 3c,d). In addition, the influence of high NO₃⁻ inputs from rivers to the estuary was more extensive in May and June than in February, possibly attributed to the increase in discharge during the early rainy season (Figure 3). The possible explanation of reduction of [NO₃⁻] in the Geum River and estuary from 2018 to 2021 can be the decrease in the occurrence of harmful algae blooming. Following the monitoring of results conducted by the Ministry of Environment in South Korea, the current flow of river became faster, the biodiversity was resorted and O₂ concentration increased from 2018 to 2021 after weir opening. In addition, the abundance and composition of cyanobacteria, a main specie of HAB, was significantly decreased by 34% from 2018 to 2020 [28]. Thus, the improved water quality could reduce the occurrence of HAB, which, in turn, can decrease the amount of NO₃⁻ released from decomposition of HAB carcasses. The expansion of riparian areas such as sandbars, vegetated areas and wetlands after opening the weir may have increased the residence time of NO₃⁻ to contact with soils and aquatic plants, and thus enhancing the biological nitrogen removal processes including assimilation or denitrification.

Interestingly, the [PO₄⁻] was higher in February 2021 (0.36 ± 0.02 and 0.39 ± 0.02 μmol L⁻¹ for surface and bottom water, respectively) than in February 2018 (0.27 ± 0.04 and 0.25 ± 0.04 μmol L⁻¹ for surface and bottom water, respectively) for both surface and bottom waters (Figure 3a,b and Figure 4a,b; Tables S2–S4). In May/June, the markedly increased trend for [PO₄⁻] was also observed from 2018 to 2021 for both surface (0.26 ± 0.08 μmol L⁻¹ for 2018 and 0.5 ± 0.12 μmol L⁻¹ for 2021) and bottom water (0.22 ± 0.05 μmol L⁻¹ for 2018 and 0.47 ± 0.12 μmol L⁻¹ for 2021). [PO₄⁻] (G1–G5) also tended to be higher in the nearby estuary dams than in the offshore dams (G6–G15) (Figure 3c,d and Figure 4c,d). Similar to the trend of [PO₄⁻], [SiO₂] was also higher in 2021 than in 2018 for both surface and bottom waters (Figure 3 and Figure 4). The average [SiO₂] of surface and bottom water in February was 1.05 ± 0.08 μmol L⁻¹ in 2018 and 5.0 ± 1.14 μmol L⁻¹ in 2021 (Figure 3a,b and Figure 4a,b; Table S4). In May/June, the [SiO₂] slightly increased from 2018 (7.43 ± 1.92 μmol L⁻¹) to 2021 (8.29 ± 1.20 μmol L⁻¹) in surface water (Figure 3c,d; Table S4). The bottom water trend was similar, and the concentrations tended to decrease from the estuary dam offshore. Notably, extremely high [PO₄⁻] in February 2018 was observed in G9, G14, and G15 (Figure 3a and Figure 4a), where seaweed aquaculture is located. In general, the growing season for seaweed is from October to February; therefore, the high [PO₄⁻] in February could be due to fertilization in seaweed aquaculture [29].

In summary, the total amount of NO₃⁻ inputs from the Geum River to the estuary decreased by approximately 68% in the dry season (February) and 19% in the early rainy season (May/June) of 2021. Additionally, atmospheric NO₃⁻ deposition is expected to have decreased due to the impact of COVID-19 [30]. In response, in the estuary, near the dam, [NO₃⁻] tended to increase during February 2021 (dry season), but decreased during May/June 2021 (early rainy season) when discharge increased. Unlike [NO₃⁻], [PO₄⁻] and [SiO₂] increased within 10 km of the estuary dam from 2018 to 2021. In particular, surface [SiO₂] increased sharply in February and June 2021 compared to 2018, and surface [PO₄⁻] also tended to increase sharply in June 2021 compared to 2018, when the discharge for May/June was high.

The trend of decreasing [NO₃⁻] in February and increasing [PO₄⁻] and [SiO₂] in May/June following the opening of the weir may be due to different nutrient removal mechanisms. For example, NO₃⁻ is primarily removed by biological processes, such as denitrification or uptake by plants [31,32], whereas PO₄⁻ and SiO₂ are removed by physical processes, such as deposition by binding to particles in the sediment [33,34]. A study by Maavara et al. [35] found that approximately 19% of nitrogen in reservoirs was removed through biological processes, but for phosphorus, about 44% was deposited and accumulated in the sediment inside the dam, because adsorption with particulate organic matter is the primary mechanism of phosphorus removal. Therefore, if the opened weirs could release more accumulated sediment, it is likely that large amounts of previously accumulated PO₄⁻ and SiO₂ would be discharged from the river into the estuary, thereby increasing their concentrations in the estuary water column. Legacy nutrients and contaminants that also accumulate and become trapped inside dams or reservoirs for long periods are often transported downstream at high flow rates following dam removal, resulting in degraded water quality and increased toxicant concentrations [36,37]. Therefore, it cannot be ruled out that the increased river runoff of suspended particles due to the opening of the weir will result in the release of phosphorus and silicate from the sediments and subsequent discharge into the estuary. Furthermore, it is possible that the faster flow rates and higher oxygen concentrations following weir opening may have alleviated the algal bloom [17], resulting in less decomposition of organic matter and reduced NO₃⁻ release from the river into the estuary during the early summer season (June 2021).

3.3. Change in Chl-a Concentrations after Weir Opening

To determine the limiting factor for phytoplankton growth in the Geum estuary, the value of N* (N* = [NO₃⁻] − 16 × [PO₄⁻]) was calculated using the ratio of NO₃⁻ to PO₄⁻ (16:1; Redfield Ratio). In general, a positive value of N* indicates that the water column has a sufficient external supply of NO₃⁻ such that PO₄⁻ is likely to be the limiting factor for primary production [38]. The results indicated that the water column near the estuary dam (G1 to G5) had a positive value for N* (Figure 3 and Figure 4) because of the large input of NO₃⁻ from the river, implying that PO₄⁻, rather than NO₃⁻, could regulate phytoplankton growth. However, the offshore had a negative value (Figure 3 and Figure 4), indicating that NO₃⁻ is a limiting factor for primary production, which is a general trend in the open ocean.

Chl-a is a key indicator of algal biomass in estuarine environments and one of the most important parameters for assessing eutrophication in estuaries because it is closely related to various environmental factors [39]. Unfortunately, [Chl-a] was not measured in February 2018 during the dry season; therefore, it was not possible to monitor its changes during the late winter season between 2018 and 2021. However, for the comparison between May 2018 and June 2021, a decrease in surface water [Chl-a] was observed from June 2018 (11.1 ± 2.4 mg L⁻¹; 7.4~20.3 mg L⁻¹) to May 2021 (7.4 ± 1.4 mg L⁻¹; 5.1~11.5 mg L⁻¹) near the estuary dam (G1 to G5), even though the difference was not statistically significant. Thus, in order to verify the effect of the weir opening on the water quality of the Geum estuary, the long-term monitoring for phytoplankton compositions and [Chl-a] are needed. For the offshore (G6 to G15), with salinity above 30%, the surface [Chl-a] ranged similarly between 2018 (0.8~12.3 mg L⁻¹) and 2021 (1.5~9.6 mg L⁻¹), except for an outliner value in G13 (Table 2; Table S4).

Despite the decrease in NO₃⁻ input from the river to the estuary, the N* values near the estuary dam were positive in both 2018 (28) and 2021 (59), indicating that the inner estuary (G1 to G5) was already well supplied with NO₃⁻ and that the limiting factor for enhancing primary production could be [PO₄⁻] rather than [NO₃⁻]. As a result, a higher supply of PO₄⁻ after the opening of the weir could enhance phytoplankton growth in late bloom seasons, such as May/June. If more PO₄⁻accumulated within weirs is released from rivers and delivered to estuaries, such as with the legacy effect, primary productivity can be regulated by PO₄⁻ inputs. However, [Chl-a] can be affected by temperature, light availability, phytoplankton composition, and zooplankton predation pressure; therefore, long-term monitoring of a variety of factors, including nutrients and physical properties, is needed to assess the effect of weir opening on estuarine primary production.

From an offshore perspective, the [NO₃⁻] discharged from rivers to estuaries seems to be rapidly diluted offshore with increasing salinity, suggesting that river-sourced NO₃⁻ is unlikely to extend into or affect distant offshore areas. Thus, in the offshore far from estuaries, the NO₃⁻ deposited from the atmosphere could have a greater impact on primary production than NO₃⁻ or PO₄⁻ discharged from rivers.

3.4. No Change in NO₃⁻ Pollutant Sources after the Geum Estuary Weir Opening

The Bayesian isotope mixing model (SIAR in the R statistical package) was used to identify changes in the pollutant sources of NO₃⁻ between 2018 and 2021 [26,27]. The pollutant sources were classified into four categories—livestock waste, synthetic fertilizers, domestic sewage, and atmospheric deposition—and the isotopic end-member values for each source and biological fractionation were obtained from previous studies [25]. Because the isotopic values of the samples were not significantly different between the surface and bottom waters, both values were applied to the model. Most of the NO₃⁻ isotopic values ranged from 8 to 15‰ for δ¹⁵N_NO3 and >20‰ for δ¹⁸O_NO3 (Figure 5).

The result of the SIAR model indicated that chemical fertilizers (37 to 47%) and domestic sewage (36 to 40%) were the main contributors of most NO₃⁻ pollutants in the Geum estuary, followed by animal waste (12 to 23%) (Table 3 and Figure 5). The high proportion of chemical fertilizers and domestic sewage in the Geum estuary is because the land use around the estuary is dominated by forests (60%), agriculture (27%), and urban areas (6%) [40]. Despite the low percentage of urban areas, the high proportion of domestic sewage can be explained by the relatively low nitrogen removal rates in the advanced wastewater treatment facilities in South Korea (total nitrogen removal rate: 77% and total phosphorus removal rate: 95%) [41]. In addition, the relative contribution of atmospheric deposition was insignificant, possibly attributed to the river-dominated characteristics of the estuary.

The contribution of domestic sewage did not change considerably in 2021 compared to that in 2018, while that of livestock manure decreased, and chemical fertilizers increased (May–June comparison between 2018 and 2021) (Table 3 and Figure 5). It is difficult to identify pollutant source changes for livestock manure and chemical fertilizers because the land use that affects the source is unlikely to change notably over a three-year period. Thus, further research is required to determine whether the differences in contributions are due to a reduction in sources and changes in treatment processes.

4. Conclusions

Estuaries are transitional zones connecting rivers and offshore areas and function in toxicity removal and flooding control, and as a fisheries resource. However, estuaries have recently experienced eutrophication and deteriorating water quality owing to the large amount of nutrient input from rivers. Thus, improving the water quality of rivers is very important for the restoration of estuarine ecosystems because of the connectivity between estuaries and rivers. This study investigated the effect of opening a weir in a river, one of the implementations for restoring water quality in rivers on the physicochemical properties of estuaries. We measured the salinity, temperature, nutrients, and Chl-a concentrations in the Geum River, Geum estuary, and offshore area immediately after the weir opened in 2018 and stabilized in 2021. The results indicate that the NO₃⁻ inputs from rivers to estuaries significantly decreased from 2018 to 2021 by 19~68% for the late-bloom season (May/June); however, the trends of [NO₃⁻] in estuaries near the estuary dam fluctuated between 2018 and 2021. Unlike NO₃⁻, [PO₄⁻] and [SiO₂] were significantly higher in 2021 than in 2018 in February and May/June. A slight decrease in [Chl-a] was also observed during the late bloom season (May/June) from 2018 to 2021. Thus, large amounts of previously accumulated PO₄⁻ and SiO₂ within the weirs might have been released from the river into the estuary after opening the weirs, increasing their concentrations in the estuary water column. This supply of PO₄⁻ could enhance phytoplankton growth in estuaries, where PO₄⁻ is a limiting factor for primary production owing to the excessive inputs of NO₃⁻ from rivers. Therefore, in order to minimize the legacy effect of opening weirs on estuary ecosystems, the dredging accumulated in the sediments in the weirs can be considered. If the legacy sediment is critically toxic to aquatic or marine organisms, the complete removal of all sediments is the best way to protect the downstream or estuary ecosystems. However, if the legacy sediment is basically clean and its main problems is turbidity or occurrence of weak bloom, the gradual lowering of the height of the weir or gradually opening the weir can transport the sediment downstream or to the estuary in smaller portions, preventing a serious release of large volumes of sediments [42]. The next measure in reducing the legacy effect on downstream and the estuary is to conduct long-term monitoring of biogeochemical properties, including erosion, biodiversity, current flow, nutrients, primary productivity and toxic chemicals before and after opening the weirs. This information will be useful in the future to make scientific judgement about where the removal of weirs is ecologically meaningful.

Author Contributions

Formal analysis, B.R.; Investigation, M.H.L., H.S.K., D.Y. and Y.-S.J.; Resources, Y.-I.K., C.-U.P., J.M.P. and J.D.D.; Writing—original draft, M.H.L.; Writing—review & editing, H.K. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available in supplementary information.

Acknowledgments

We would like to thank the graduate student Jinhee Heo at Ewha Womans University; we will never forget her efforts and will remember her forever.

Conflicts of Interest

The authors declare that they have no known competing financial interests related to personal relationships that could have influenced the work reported in this study.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Location maps of the study site in Korean Peninsula (a), three weirs (Baekjebo, Gongjubo and Sejongbo) in the Geum River (b), and Geum Estuary and the sampling sites of seawater and river (c). The blue (G1 to G15) and yellow circles (S1 and S2), and the brown rectangle indicate Geum Estuary, Geum River and estuary dam, respectively.

Figure 2. Vertical properties of temperature and salinity in February of 2018 and 2021 (a), and May/June of 2018 and 2021 (b). The vertical sections include the sites of G1, G2, G3, G5, G6, G8, G11 and G14.

Figure 3. Nutrient concentrations (NO3−, PO4−, SiO2) and N* values in the surface water of the Geum Estuary in February 2018 (a) and 2021 (b) and May 2018 (c) and June 2021 (d). N* is calculated by following equation: N* = [NO3−] − 16 × [PO4−]. The 3 sites (G9, G10 and G15) in February 2021 were not sampled due to technical problems, so spatial distribution was no fully covered in February 2021.

Figure 4. Nutrient concentrations (NO3−, PO4−, Si) and N* values in the bottom waters of the Geum Estuary in February 2018 (a) and 2021 (b) and May 2018 (c) and June 2021 (d). N* is calculated by following equation: N* = [NO3−] − 16 × [PO4−]. The 3 sites (G9, G10 and G15) in February 2021 were not sampled due to technical problems, so spatial distribution was no fully covered in February 2021.

Figure 5. The δ15NNO3 and δ 18ONO3 isotopic values in the Geum Estuary water in February 2018 and 2021 (a) and May 2018 and June 2021 (b), and the isotopic end-members for pollutant sources. Open circles indicate the isotopic values in the Geun Estuary water and colored squares represent the ranges of isotopic values for pollutant end-members.

Table 1

The concentration and input amount of NO₃⁻ and discharge from the Geum River to the Geum Estuary in February and May 2018, and February and June 2021.

Properties	February 2018	February 2021	May 2018	June 2021
River discharge (L)	1.0 × 108	1.1 × 108	2.8 × 108	3.4 × 108
Riverine [NO3−] (μmol L−1)	186	57	115	75
Riverine input (mol)	19 × 103	6 × 103	32 × 103	26 × 103

Table 2

The average (±standard error) and range of physical properties and Chl-a concentrations in the Geum Estuary water in February 2018 and 2021, May 2018, and June 2021.

Properties	February 2018	February 2021	May 2018	June 2021
		Surface water
Temperature	3.6 ± 0.5	5.5 ± 0.8	14.5 ± 0.7	22.4 ± 1.0
(°C)	(2.9–4.6)	(4.5–6.6)	(13.4–15.6)	(20.8–24.2)
Salinity	29.5 ± 3.5	26.0 ± 4.7	26.9 ± 3.6	25.4 ± 5.2
(‰)	(20.9–32.1)	(20.1–31.5)	(21.3–30.9)	(13.8–31.1)
Chl-a	–	4.3 ± 1.3	8.0 ± 5.4	7.1 ± 4.6
(μg L−1)		(2.4–6.2)	(0.7–22.8)	(1.5–20.3)
		Bottom water
Temperature	3.1 ± 0.3	4.7 ± 0.6	13.6 ± 1.0	21.7 ± 1.2
(°C)	(2.7–3.6)	(4.0–6.2)	(11.4–15.0)	(19.4–23.6)
Salinity	30.4 ± 2.5	28.8 ± 4.2	29.8 ± 2.0	29.3 ± 2.2
(‰)	(25.6–32.1)	(19.0–31.6)	(24.0–31.3)	(24.4–31.1)
Chl-a	–	4.6 ± 1.2	5.2 ± 3.2	4.6 ± 3.2
(μg L−1)		(2.8–6.3)	(0.8–11.3)	(1.8–11.9)

Table 3

Relative percentage for NO₃⁻ pollutant sources in the Geum Estuary in February and May 2018 and February and June 2021.

Pollutant Sources (%)	February 2018	February 2021	May 2018	June 2021
Manure	23	15	17	12
Chemical fertilizer	37	39	39	47
Sewage	38	40	38	36
Atmospheric deposition	2	6	6	5

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jmse11122251/s1, Table S1: The water depth in the Geum Estuary; Table S2: Descriptive statistics of physiochemical properties in the surface water (G1 to G15) in February/May 2018 and February/June 2021; Table S3: Descriptive statistics of physiochemical properties in the bottom water (G1 to G15) collected in February/May 2018 and February/June 2021; Table S4: The p-value for The Mann-Whitney comparison test of chemical properties between 2018 and 2021 samples collected in the Geum Estuary surface and bottom waters (G1 to G15). The p-value < 0.05 (bold values) indicated that the values between 2018 and 2021 were statistically significantly different.

References

1. Barbier, E.B.; Hacker, S.D.; Kennedy, C.; Koch, E.W.; Stier, A.C.; Silliman, B.R. The value of estuarine and coastal ecosystem services. Ecol. Monogr.; 2011; 81, pp. 169-193. [DOI: https://dx.doi.org/10.1890/10-1510.1]

2. Lee, Y.G.; An, K.G.; Ha, P.T.; Lee, K.Y.; Kang, J.H.; Cha, S.M.; Cho, K.H.; Lee, Y.S.; Chang, I.S.; Kim, K.W. et al. Decadal and seasonal scale changes of an artificial lake environment after blocking tidal flows in the Yeongsan Estuary region, Korea. Sci. Total Environ.; 2009; 407, pp. 6063-6072. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2009.08.031] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/19765805]

3. Jeong, Y.H.; Yang, J.S.; Park, K. Changes in water quality after the construction of an estuary dam in the Geum river estuary dam system, Korea. J. Coast. Res.; 2014; 30, pp. 1278-1286. [DOI: https://dx.doi.org/10.2112/JCOASTRES-D-13-00081.1]

4. Ralston, D.K.; Brosnahan, M.L.; Fox, S.E.; Lee, K.D.; Anderson, D.M. Temperature and residence time controls on an estuarine harmful algal bloom: Modeling hydrodynamics and Alexandrium fundyense in Nauset estuary. Estuaries Coasts; 2015; 38, pp. 2240-2258. [DOI: https://dx.doi.org/10.1007/s12237-015-9949-z] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26692827]

5. Pess, G.R.; McHenry, M.L.; Beechie, T.J.; Davies, J. Biological Impacts of the Elwha River Dams and Potential Salmonid Responses to Dam Removal. Northwest Sci.; 2008; 82, pp. 72-90. [DOI: https://dx.doi.org/10.3955/0029-344X-82.S.I.72]

6. Bednarek, A. Undamming Rivers: A Review of the Ecological Impacts of Dam Removal. Environ. Manag.; 2001; 27, pp. 803-814. [DOI: https://dx.doi.org/10.1007/s002670010189] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/11393315]

7. Holtcamp, W. Fish return to undammed Elwha River. Nature; 2012; 27, pp. 803-814. [DOI: https://dx.doi.org/10.1038/nature.2012.10948]

8. Magilligan, F.J.; Graber, B.E.; Nislow, K.H.; Chipman, J.W.; Sneddon, C.S.; Fox, C.A. River restoration by dam removal: Enhancing connectivity at watershed scales. Elem. Sci. Anth.; 2016; 4, 000108. [DOI: https://dx.doi.org/10.12952/journal.elementa.000108]

9. Ministry of Land (MOL), Transport and Maritime Affairs (South Korea). Masterplan for the Four Major Rivers Project; Ministry of Land (MOL), Transport and Maritime Affairs (South Korea): Sejong, Republic of Korea, 2009.

10. Ministry of Land (MOL), Transport and Maritime Affairs (South Korea). Plan for Constructing Senery along the Geum River; Ministry of Land (MOL), Transport and Maritime Affairs (South Korea): Sejong, Republic of Korea, 2010.

11. Lee, H.J.; Park, G.K.; Cheon, S.U. Effects of weir construction on phytoplankton assemblages and water quality in a large river system. Int. J. Environ. Res. Public Health; 2018; 15, 2348. [DOI: https://dx.doi.org/10.3390/ijerph15112348]

12. Lee, S.J.; An, K.G. Influence of Weir Construction on Chemical Water Quality, Physical Habitat, and Biological Integrity of Fish in the Geum River, South Korea. Pol. J. Environ. Stud.; 2019; 28, pp. 2175-2186. [DOI: https://dx.doi.org/10.15244/pjoes/91843]

13. Kim, H.G.; Lee, H.Y.; Joo, G.J.; Kim, H.G.; Lee, H.Y.; Joo, G.J. Trait-based algal community assembly associated with Pectinatella magnifica (Bryozoa, Phylactolaemata). Algae; 2019; 34, pp. 99-109. [DOI: https://dx.doi.org/10.4490/algae.2019.34.5.3]

14. Im, R.Y.; Kim, J.Y.; Choi, J.Y.; Do, Y.; Joo, G.J. Changes of river morphology in the mid-lower part of Nakdong River basin after the 4 Large River Project, South Korea. Korean J. Ecol. Environ.; 2015; 48, pp. 188-194. [DOI: https://dx.doi.org/10.11614/KSL.2015.48.3.188]

15. Im, R.Y.; Kim, J.Y.; Nishihiro, J.; Joo, G.J. Large weir construction causes the loss of seasonal habitat in riverine wetlands: A case study of the Four Large River Projects in South Korea. Ecol. Eng.; 2020; 152, 105839. [DOI: https://dx.doi.org/10.1016/j.ecoleng.2020.105839]

16. Ministry of Environment (MOE) (South Korea). Occurrence of Algae (Green Algae) and Corresponding Annual Report; Ministry of Environment: Sejong, Republic of Korea, 2016.

17. Ministry of Environment (MOE) (South Korea). Analysis Report on the Monitoring the Opening 16 Weirs of 4 Rivers (2017.06~2018.12); Ministry of Environment: Sejong, Republic of Korea, 2019.

18. Kim, S.K.; Joo, G.J.; Choi, J.Y. Effect of Gated Weir Opening on the Topography and Zooplankton Community of Geum River, South Korea. Land; 2022; 11, 529. [DOI: https://dx.doi.org/10.3390/land11040529]

19. Ministry of Environment (MOE) (South Korea). Press Release: Finalized Changes to the National Integrated Water Management Plan. Available online: https://www.water.go.kr/front/bbs/bbsView.do?bbsSn=35&nttSn=7525&menuCode=S024003 (accessed on 21 September 2023).

20. Lee, S.Y.; Choi, Y.S.; Joo, S.; Jeong, G.; You, Y.H. Foraging behavior and preys in relation to feeding site types of the Eurasian oystercatcher (Haematopus stralegus osculans) during the breeding season in Yubu Island, Korea. Korean J. Ecol. Environ.; 2018; 51, pp. 184-191. [DOI: https://dx.doi.org/10.11614/KSL.2018.51.2.184]

21. Kang, S.; Kim, J.H.; Joe, Y.J.; Nam, S.I.; Shin, K.H. Long-term environmental changes in the Geum Estuary (South Korea): Implications of river impoundments. Mar. Pollut. Bull.; 2021; 168, 112383. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2021.112383]

22. Cho, J.; Song, Y.; Kim, T.I. Numerical modeling of estuarine circulation in the Geum River estuary, Korea. Procedia Eng.; 2016; 154, pp. 982-989. [DOI: https://dx.doi.org/10.1016/j.proeng.2016.07.586]

23. Moon, C.R.; Kang, D.J.; Park, M.O.; Noh, J.H.; Yoo, S.J.; Moon, J.E.; Shin, K.H.; Kim, Y.S.; Choi, J.K.; Suh, Y.S. An inter-laboratory comparison study on chlorophyll a determination in seawater. Sea J. Korean Soc. Oceanogr.; 2014; 19, pp. 76-87. [DOI: https://dx.doi.org/10.7850/jkso.2014.19.1.76]

24. Sigman, D.M.; Casciotti, K.L.; Andreani, M.; Barford, C.; Galanter, M.; Böhlke, J.K. A Bacterial Method for the Nitrogen Isotopic Analysis of Nitrate in Seawater and Freshwater. Anal. Chem.; 2001; 73, pp. 4145-4153. [DOI: https://dx.doi.org/10.1021/ac010088e]

25. Kendall, C.; Elliott, E.M.; Wankel, S.D. Tracing anthropogenic inputs of nitrogen to ecosystems. Stable Isot. Ecol. Environ. Sci.; 2007; 2, pp. 375-449.

26. Parnell, A.C.; Inger, R.; Bearhop, S.; Jackson, A.L. Source partitioning using stable isotopes: Coping with too much variation. PLoS ONE; 2010; 5, e9672. [DOI: https://dx.doi.org/10.1371/journal.pone.0009672] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/20300637]

27. Nam, T.H.; Ryu, H.S.; Kang, T.; Han, Y.; Kim, J.; Lee, K.; Hwang, S.; Kim, K. Quantifying nitrogen source contribution ratios using stable isotope method: Application of Bayesian mixing model. J. Korean Soc. Water Environ.; 2019; 38, pp. 510-519.

28. Ministry of Environment (MOE) (South Korea). Press Release: Observation Monitoring Comprehensive Information System for Weirs for 4 Major Rivers. 2021; Available online: https://www.me.go.kr/home/web/index.do?menuId=10525 (accessed on 15 September 2021).

29. Hwang, E.K.; Choi, H.G.; Kim, J.K. Seaweed resources of Korea. Bot. Mar.; 2020; 63, pp. 395-405. [DOI: https://dx.doi.org/10.1515/bot-2020-0007]

30. Yoon, J.E.; Son, S.; Kim, I.N. Capture of decline in spring phytoplankton biomass derived from COVID-19 lockdown effect in the Yellow Sea offshore waters. Mar. Pollut. Bull.; 2022; 174, 113175. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2021.113175] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34844148]

31. Forshay, K.J.; Stanley, E.H. Rapid nitrate loss and denitrification in a temperate river floodplain. Biogeochemistry; 2005; 75, pp. 43-64. [DOI: https://dx.doi.org/10.1007/s10533-004-6016-4]

32. Kreiling, R.M.; Richardson, W.B.; Cavanaugh, J.C.; Bartsch, L.A. Summer nitrate uptake and denitrification in an upper Mississippi River backwater lake: The role of rooted aquatic vegetation. Biogeochemistry; 2011; 104, pp. 309-324. [DOI: https://dx.doi.org/10.1007/s10533-010-9503-9]

33. Rao, D.N.; Ghosh, V.R.D.; Sam, P.; Yadav, K.; Sarma, V.V.S.S. Phosphate removal through adsorption on suspended matter in the Bay of Bengal: Possible implications to primary production. J. Earth Syst. Sci.; 2021; 130, 22. [DOI: https://dx.doi.org/10.1007/s12040-020-01524-8]

34. Pan, G.; Krom, M.D.; Zhang, M.; Zhang, X.; Wang, L.; Dai, L.; Sheng, Y.; Mortimer, R.J. Impact of suspended inorganic particles on phosphorus cycling in the Yellow River (China). Environ. Sci. Technol.; 2013; 47, pp. 9685-9692. [DOI: https://dx.doi.org/10.1021/es4005619]

35. Maavara, T.; Chen, Q.; Van Meter, K.; Brown, L.E.; Zhang, J.; Ni, J.; Zarfl, C. River dam impacts on biogeochemical cycling. Nat. Rev. Earth Environ.; 2020; 1, pp. 103-116. [DOI: https://dx.doi.org/10.1038/s43017-019-0019-0]

36. Van Meter, K.J.; Basu, N.B.; Van Cappellen, P. Modelled historical and predicted future nitrogen loads for the Mississippi River, indicating that legacy nitrogen loading from agriculture may prevent reduction of the Gulf of Mexico dead zone for decades. Science; 2018; 360, pp. 427-430. [DOI: https://dx.doi.org/10.1126/science.aar4462]

37. Stanley, E.H.; Doyle, M.W. Trading off: The ecological effects of dam removal. Front. Ecol. Environ.; 2003; 1, pp. 15-22. [DOI: https://dx.doi.org/10.1890/1540-9295(2003)001[0015:TOTEEO]2.0.CO;2]

38. Tett, P.; Droop, M.R.; Heaney, S.I. The Redfield ratio and phytoplankton growth rate. J. Mar. Biol. Assoc. UK; 1985; 65, pp. 487-504. [DOI: https://dx.doi.org/10.1017/S0025315400050566]

39. Guo, J.; Zhang, C.; Zheng, G.; Xue, J.; Zhang, L. The establishment of season-specific eutrophication assessment standards for a water-supply reservoir located in Northeast China based on chlorophyll-a levels. Ecol. Indic.; 2018; 85, pp. 11-20. [DOI: https://dx.doi.org/10.1016/j.ecolind.2017.09.056]

40. Ministry of Environment (MOE) (South Korea). The Classification of 1975 to 2000 Land Use Using a Satellite Imagery; Ministry of Environment: Sejong, Republic of Korea, 2006.

41. Ministry of Environment (MOE) (South Korea). Study on the Assessment of Aging Public Wastewater Treatment Facilities and Feasibility Investigation for Improvement; Ministry of Environment: Sejong, Republic of Korea, 2019.

42. EPAUnited States Environmental Protection Agency. Dam removal requirement, processes and techniques. National Management Measures to Control Nonpoint Source Pollution from Hydromodification; United States Environmental Protection Agency: Washington, DC, USA, 2007; Chapter 9