Estuaries and Coasts (2009) 32:1165–1175 DOI 10.1007/s12237-009-9217-1
Enhanced Benthic Nutrient Flux During Monsoon Periods in a Coastal Lake Formed by Tideland Reclamation Seok-Hwi Kim & Kangjoo Kim & Minhyung Lee & Hwa-Jin Jeong & Won-Jang Kim & Jong-Gyu Park & Jae-sam Yang
Received: 18 January 2009 / Revised: 12 August 2009 / Accepted: 14 August 2009 / Published online: 9 September 2009 # Coastal and Estuarine Research Federation 2009
Abstract Sediment and water quality were investigated in an artificial coastal lake (Saemangeum Lake, Korea) that was formed by constructing a 33-km long sea-dyke offshore from the mouths of two adjacent rivers, which discharge into the Yellow Sea. Sediment showed drastic increases in fine fraction (silt and clay) after the dyke construction. TN, TP, and OC contents in the sediment showed the similar spatial variation to that of fine fraction. A mixing model indicated benthic fluxes of nutrients such as PO4, NH4, and SiO2, which were considerably elevated during the summer monsoon season. It is revealed that this phenomenon was associated with the development of strong salinity stratification, elevated water temperature, and increased groundwater discharge. However, a change in the sedimentation environment due to dyke construction is suggested as the primary reason for the enhanced benthic fluxes. Keywords Tideland reclamation . Saemangeum . Benthic flux . Salinity stratification . Groundwater discharge . Organic-rich sediment
S.-H. Kim : K. Kim (*) : M. Lee : H.-J. Jeong Department of Environmental Engineering, Kunsan National University, Jeonbuk 573-701, Korea e-mail:
[email protected] W.-J. Kim Korea Rural Community & Agricultural Corporation, 576-804 Jeonbuk, Korea J.-G. Park : J.-s. Yang Department of Oceanography, Kunsan National University, Jeonbuk 573-701, Korea
Introduction The goal of the Saemangeum Tideland Reclamation Project (STRP) is to create 283 km2 of new land and 118 km2 of fresh water reservoir by constructing a 33 km-long dyke offshore of the mouths of the Mankyeong and Dongjin Rivers in Korea (Fig. 1). The STRP is one of the biggest ever tideland reclamation projects in the world. The offshore dyke was fully connected in April 2006 and ‘Saemangeum Lake’ has been formed inside the dyke after its construction. However, the water quality of the two rivers draining into the lake is poor and the sluice gates of the lake (Shinsi and Garyeok Gates; Fig. 1) are therefore generally left open to allow for dilution with seawater of better quality. During the last several hundred years, the reclamation of tidal flats has been a common practice to create usable land area in Korea. The scale of reclamation has expanded since the mid-twentieth century. However, the attempt to create a large-scale fresh coastal lake in Korea as a part of a tideland reclamation effort (i.e., Shihwa Lake with 60 km2 in reservoir surface area) was a failure due to severe water quality degradation after the construction of offshore dyke. When Shihwa Lake began to be filled with freshwater, strong anoxia and highly elevated NH4, PO4, and H2S levels were observed (Han and Park 1999). As a result, the plan to convert Shihwa Lake into a freshwater reservoir was abandoned, and today the lake water is instead diluted with seawater. On account of this experience, nationwide concern has since arisen over Saemangeum Lake, which is twice the size of Shihwa Lake. Benthic nutrient flux is widely accepted as one of the main causes of water quality degradation in eutrophic lakes (Jacoby et al. 1982; Istvánovics and Somlyódy 2001; Lai and Lam 2008). The decomposition of organic matter
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Mankyeong and Dongjin Rivers are 1,606 and 1,149 km2, respectively. Due to poor input water quality, the dilution of nutrient concentrations with seawater will continue until or unless water quality is substantially improved. The tidal range in the Saemangeum area was about 6 m before the dyke was constructed but is now less than 1 m even when the sluice gates are fully open (KORDI 2006). Weather data from the past 30 years show mean monthly air temperatures ranging from −0.4°C (January) to 25.7°C (August) in the study area. The mean annual precipitation is 1,204 mm, of which more than 65% generally occurs during summer (June to September). Fig. 1 Location of Saemangeum Lake area. Solid circles and numbers in parentheses represent sampling stations and the average water depths at each station, respectively
sometimes creates anoxic conditions in deep waters and/or in the shallow porewater of the bottom sediments and induces the release of nutrients such as PO4 and NH4 (Van Luijn et al. 1999; Burger et al. 2007; Beutel 2006; Spears et al. 2008). During the summer months, organic matter decomposition and benthic flux are enhanced due to high water temperatures and the development of thermal stratification (Pettersson et al. 2003; Markou et al. 2007; Jiang et al. 2008; Wilhelm and Adrian 2008). Groundwater is another important nutrient source in coastal environments (Taniguchi et al. 2002; Kim et al. 2005; Burnett et al. 2007). According to Kim et al. (2005), groundwater supplies approximately 57% of the silica associated with river discharge to the Yellow Sea. This study was initiated to investigate the factors that impact the water quality of Saemangeum Lake following the offshore dyke construction. In the course of this study, we observed benthic nutrient fluxes to be considerably enhanced during the summer rainy season. This paper discusses the factors causing this phenomenon and projects the future water quality of the lake based on our findings.
Study Area The watershed of Saemangeum Lake (Saemangeum watershed) encompasses 2,755 km2 of the western coast of the Korean Peninsula, and drains via the Mankyeong and Dongjin Rivers into the Yellow Sea (Fig. 1). The Saemangeum watershed is famous in Korea for its flat topography. Its land-use is predominantly agricultural with scattered urban and industrial areas. The annual discharge of the Mankyeong River (1,100×106 m3/year) is about 1.6 times greater than that of the Dongjin River (700×106 m3/year) due to its larger drainage area. The drainage areas of the
Sampling and Methods For this study, eight monitoring stations (MK1, MK3, MK4, and MK7 on the Mankyeong side; DJ2, DJ4, DJ6, and DJ7 on the Dongjin side) were selected in the lake (Fig. 1). Water depths at each station ranged between 1.3 and 8.9 m (Fig. 1). The qualities of both surface and bottom waters were monitored at each station on 21 occasions over a 1.5-year period that lasted from May 2006 to October 2007. The monitoring was performed approximately biweekly in 2006 and monthly in 2007. Bottom water samples were collected using a van Dorn sampler at 50 cm above the bottom. Surface water samples were collected directly. Water temperature, salinity, pH, and dissolved oxygen (DO) were measured in the field by placing a multi-probe sensor (YSI 660XML) directly into the lake. Water samples were immediately filtered through membranes (Hydrophilic PVDF 0.45 μm, Millipore MillexHV). Filtrates were stored in 200 mL polyethylene bottles without any headspaces. The samples were kept at 4°C until analyses. All chemical species except Si were analyzed within 3 days of sampling. Dissolved nutrients such as NH4, NO3, NO2, PO4, and SiO2 were colorimetrically measured using a UV-visible spectroscopy (UV1700, Shimadzu) according to Solorzano (1969) for NH4-N and Parsons et al. (1984) for others. Sediment samples were collected using a grab sampler, which was designed to minimize wash-out during retrieval. All surface sediments (
