Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea

Environmental Pollution xxx (xxxx) xxx

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Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea* Dohyeong Kim a, Joohee Jo a, Bora Kim b, Jongseong Ryu b, Kyungsik Choi a, * a b

School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea Department of Marine Biotechnology, Anyang University, Ganghwa-gun, Incheon, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2018 Received in revised form 27 October 2019 Accepted 27 October 2019 Available online xxx

The effects of dike construction on the geomorphology and sedimentary processes of tidal flats were investigated using high-precision topographic profiling, short cores, and unmanned aviation vehicle (UAV)-assisted photogrammetry to understand their adverse consequences on the benthic ecosystem. Tidal flats at the south of Shinsi Island near one of the two sluice gates of the Saemangeum dike, display prominent morphologic features known as shelly sand ridges or cheniers (sensu Otvos, 2000) that have migrated landward about 5 m in a year. The tidal flats were dominated by erosion from winter to spring and by deposition during the remainder of the year except for the periods of heavy precipitation when tidal drainage channels became larger and deeper by headward erosion. With overall coarser-grained surface sediments, the presence of actively migrating wave-built cheniers are in stark contrast to muddy tidal flats with a monotonous morphology before the completion of the Saemangeum dike in 2006. Southeasterly waves reflected from the dike during winter to spring when north to northwesterly winds prevail account for the wave-induced onshore sediment transport and rapid morphologic changes in the tidal flats despite their location protected from offshore waves. The diversity and biomass of major macrofauna species tend to increase during rapid erosion and decrease during rapid deposition, highlighting the anthropogenic effect of dike-induced physical disturbance on the benthic ecosystem in the otherwise sheltered tidal flats. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Wave reflection Saemangeum dike Chenier Tidal flats Macrofauna

1. Introduction Coastal environments near the urban area have received huge development pressure to build coastal structures due to growing demand from both public and private sectors (Timmerman and White, 1997; Bulleri and Chapman, 2010; Wang et al., 2014). The coastal structures such as dikes, groins, and submerged breakwater alter coastal configuration significantly. Often such perturbation in physical setting resulted in hydrodynamic changes including wave reflection, strengthened longshore current and increased wave energy with coeval decrease of tidal prism (Isaacson, 1991; Ruggiero and McDougal, 2001; Ardhuin and Roland, 2012; Lee et al., 1999; Muller et al., 2006; Kang et al., 2009; Gao et al., 2014), and

* This paper has been recommended for acceptance by Dr. Yong Sik Ok. * Corresponding author. E-mail address: [email protected] (K. Choi).

associated sedimentologic and morphologic adaptations including beach erosion and reduced tidal-flat area, and the infilling of tidal channels (Plant and Griggs, 1992; Flemming and Nyandwi, 1994; Miles et al., 2001; Hood, 2004; Wang et al., 2012). These environmental changes are disastrous not only for the coastal structures but also for the benthic ecosystem. Previous studies on the adverse effect of the coastal structures focused on various issues including morphologic change, sediment transport, sedimentary process and benthic ecosystem immediate seaward of the structures (e.g., Lee et al., 1999; Borja et al., 2000; Jaramillo et al., 2002; Lee and Ryu, 2007, 2008; Otani et al., 2008; Park et al., 2009; Li et al., 2010; Naser, 2011; Nishijima et al., 2015; Yan et al., 2015; Duan et al., 2016; Shen et al., 2016; Yang et al., 2016; Liu et al., 2018). Surprisingly few studies have documented the impact of physical disturbance on the benthic ecosystem based on morphologic and sedimentologic observation. In particular, the impact and the role of waves reflected from the dike on the benthic ecosystem of tidal flats are less well understood.

https://doi.org/10.1016/j.envpol.2019.113507 0269-7491/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Over the past 40 years, Korean west coast has been straightened and deteriorated due to a series of reclamation for coastal infrastructure, culminated by the construction of the Saemangeum dike. With a world record dike-length of 33 km, the Saemangeum dike isolated an estuarine river mouth area of 409 km2 from offshore and sparked nationwide controversy over its adverse impact on the coastal environment. Despite predictions based on field observation and numerical modeling (e.g., Lee et al., 2008), the dike resulted in unexpected, substantial changes such as pronounced beach erosion (Lee and Ryu, 2007), intensified sediment transport (Lee and Ryu, 2008), and rapid deterioration of benthic ecosystem (Ryu et al., 2011, 2014). Although previous studies have documented various adverse effects of the Saemanguem dike, none of them linked dike-induced physical disturbance to the tidal-flat benthic ecosystem, especially in the seaward side of the dike. Tidal flats along the west coast of Korea are dominated by mixed energy conditions due to large tidal range and seasonally intensified wave activity (Choi, 2014). Depending on the exposure to waves, the tidal flats display contrasting sedimentologic and morphologic response. Open-coast tidal flats oscillate between mud-dominated flats during summer but become sand-dominated shoreface environment during winter when waves are generally larger (Yang et al., 2005). In contrast, tidal flats located in a semienclosed setting are tide-dominated and do not exhibit seasonality in surface sediment distribution and morphology due to meager wave influence (Kim et al., 1999; Yang et al., 2007). Despite the location protected against offshore waves, however, tidal flats at the southern embayment of Shinsi Island near the Saemangeum dike have wave-built shelly sand ridges or cheniers in the intertidal zone (sensu Otvos, 2000), which is typical for open-coast tidal flats. The present study aims to characterize morphodynamics and sedimentology of the tidal flats of Shinsi Island to understand dikeinduced perturbation on hydrodynamics and subsequent sedimentologic and morphologic response. In parallel, macrofaunal analysis of the surface sediments was conducted to evaluate the impact of dike-induced physical disturbance on the benthic ecosystem.

2. Study area Two studied tidal flats, South tidal flat and West tidal flat, are located at the south of Shinsi Island, which is the easternmost part of the Gogunsan archipelago that is now connected with the mainland by the Saemangeum dike (Fig. 1A). South tidal flat is a small, embayment-type tidal flat surrounded by basement rock with its entrance lying to the south (Fig. 1B, D, F). West tidal flat is an open-coast tidal flat with steep-gradient beaches developed near the high-tide level and bordered by an embankment to the north (Fig. 1B, C, E). Tides are semidiurnal with spring tidal range exceeding 6 m (Fig. 2A). Peak flood and ebb current speeds in the offshore area are up to 1 m/s and 0.8 m/s during spring tides, and 0.5 m/s and 0.6 m/s during neap tides (NCDSS, 2017). Tidal currents flow at 0.2 m/s to 0.3 m/s near the dike during spring tides. Waves are generally larger from winter to spring, with significant wave heights exceeding 3 m at the offshore mooring station P1 (MOF, 2017, Fig. 2B). Significant wave heights decrease to 0.6 m near the dike during winter. Waves propagate south to southeastward in the offshore area, whereas they veer to the north near the dike (MOF, 2017, Fig. 2D). Wind directions are seasonal, with northerly winds prevailing from winter to spring and southerly winds in summer (NCDSS, 2017, Fig. 2D). During the study period, heavy rainfall occurred in summer with a maximum rate of 275 mm per day and 105 mm per hour, which account for more than two-thirds of annual precipitation of 990 mm in 2017 (NCDSS, 2017, Fig. 2C).

3. Materials and methods 3.1. Ground survey and sedimentary analysis High-precision positioning of sampling locations and ground control points (GCPs) for unmanned aerial vehicle (UAV)-assisted photogrammetry was conducted using a real-time kinematic (RTK) GPS with 10 mm horizontal accuracy and 20 mm accuracy. Two shore-normal transects AA0 and BB0 were extracted from digital elevation models (DEMs) produced by UAV-assisted photogrammetry and profiled four times from February to November 2017 to detect seasonal variation in morphologic changes of West and South tidal flats (Fig. 3). Transects AA0 and BB0 are 350 m and 250 m, respectively. Elevations are corrected to the Incheon Datum level. Fourteen undisturbed cores were collected using stainless steel can corer (60 cm  10 cm  2 cm) in February and August 2017 at seven locations for the analysis of sedimentary facies of the tidal flats. The elevations of each core tops were measured for the estimation of sedimentation rate using an RTK GPS. Relief peels were made from the cores using epoxy and cheesecloth to enhance and preserve sedimentary structures for detailed sedimentological analysis. Surface sediments were collected at the seven locations four times from February to November 2017 to detect any temporal variability of sediment distribution. After the removal of organic matter using diluted hydrogen peroxide (H2O2), sediments were analyzed for grain size using a MasterSizer 3000 (Malvern Inc.). 3.2. Unmanned aviation vehicle (UAV)-assisted photogrammetry A total of 850 aerial photographs were taken at 60 m above the tidal-flat surface using DJI Phantom 4 Pro equipped with a 20megapixel sensor camera during four times of field survey. The aerial images were taken with an 80% overlap and were processed by Pix4D Mapper Pro (version 4.0.25) to create orthophoto mosaics. In each survey, 42 ground control points (GCPs) were acquired using the RTK-GPS and incorporated into the software to increase the accuracy of the mosaics (Smith and Vericat, 2015; Eltner et al., 2016). Spatial coverage was 400,000 m2. ArcGIS (version 10.1) was used to construct digital elevation models (DEMs) to visualize the detailed morphology of the tidal flats. Difference of DEM (DoD) was also calculated to characterize the spatial trend of deposition and erosion on the tidal flats between survey periods (Wheaton et al., 2009; Letortu et al., 2015). The resolution of orthophoto mosaics was 0.02 m, with RMS error being X: 0.04 m, Y: 0.02 m, Z: 0.05 m. A series of aerial photographs taken from 1985 to 2016 by National Geographic Information Institute of Korea (NGII, 2017) were analyzed to detect morphologic changes of the tidal flats. An analog image of 1985 had 1200 dpi resolution, whereas digital images from 2010 to 2016 had 0.25 m spatial resolution. 3.3. Macrofaunal analysis Macrofaunal analysis was conducted to understand the impact of sedimentation on the benthic ecosystem at ten stations in February, May, August, and November 2017. Seven of them are identical to those for sedimentological study. At each station, surface sediment samples were taken in triplicate for the top 30 cm of sediment using a cylindrical acryl corer with 9 cm diameter. Macrofauna were sampled with a 1 mm pore-size mesh. The filtered animals were then fixed in 4% buffered formalin solution and preserved in 70% ethanol for species identification, counting, and weteweight measurement. Taxa were identified to the species level, using a dissecting microscope and an optical microscope where necessary. Changes in tidal-flat elevation (i.e., deposition/ erosion) and mud content were correlated with faunal parameters

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 1. Map showing the location of Shinsi island tidal flats (A, B), transect profiles (AA0 , BB0 ) for surface morphology (C, D), and sampling stations for surface sediments, cores, and benthic organisms (S1~S7) (C, D). Field photographs of tidal beach in West tidal flat (E), chenier, and drainage channel in South tidal flat (F). AWS: automated weather station, GLB: Galmaeyeo Light Beacon.

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

Fig. 2. A) Predicted tidal curve of the study area from January to December 2017 showing macrotidal range. B) Significant wave height measured at P1 offshore Shinsi Island (MOF, 2017). C) Monthly precipitation data measured at AWS at Shinsi Island (NCDSS, 2017). Pronounced rainfall occurred during the summertime rainy season from June to September. Wind speed and significant wave height at P1 and P2 for February and November 2017 (MOF, 2017). Unlike the offshore area (P1), where wind directions lie at the same quadrant with wave directions, wind directions in the nearshore area near the present study (P2) are almost opposite to wave directions (MOF, 2017).

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 3. Repeated profiles along transects AA0 and BB0 , demonstrating active morphodynamics of tidal flats during the study period. Landward migration of the chenier is characteristic in South tidal flat (A, B). West tidal flat is relatively monotonous except steep tidal beach that retreated significantly from February to May 2017.

(number of species, total density, and biomass) by measuring the difference between the two adjacent values (e.g., May minus February, August minus May).

4. Results 4.1. Tidal-flat morphology and morphodynamics 4.1.1. South tidal flat South tidal flat displays a rugged morphology with shelly sand ridges or cheniers in the lower intertidal zone below mean sea level (Fig. 1B, F, 3A, B). The cheniers are 180 m long, 30 m wide, and 0.6 m high, oriented NNE-SSW. The chenier has an asymmetric profile with a steep landward side (up to 3.3 ) and a gentle seaward side (0.6 ). With the NNE end attached to adjacent basement rock, the chenier is curved landward. Drainage tidal channels are developed at the seaward end (SSW) of the chenier (Figs. 1F and 4). The channels have become larger and deeper with noticeable headward erosion between May and August (Fig. 4B, C, F) when the channels enlarged up to 10 m wide and 0.5 m deep at the bankful stage. With continued headward erosion of about 15 m, the channels were infilled with sediments from August to November (Fig. 4D, G). Tidal flats on the landward side of the cheniers have a featureless morphology with a low slope gradient of 0.1.

Over the study period, the chenier has migrated landward about 5 m (Fig. 3A and B). The chenier has migrated about 2 m from February to May with a sedimentation rate of nearly 0.1 m on the tidal flats immediate landward of the chenier. From May to August, the chenier has migrated about 1 m further landward. Sedimentation continued on both sides of the chenier with up to 0.2 m on the seaward limb and less than 0.1 m on the landward limb. The most notable morphologic change occurred from August to November when the chenier continued to migrate further landward about 2 m, and its crest elevation was lowered by 0.2 m. Unlike previous time intervals, tidal flats on the landward side of the chenier were eroded by about 0.1 m. 4.1.2. West tidal flat West tidal flat has a concave-up profile (Fig. 3C). The tidal flat has no prominent morphologic feature except steep-sloped (~0.86 ) tidal beach near the high-tide level (Fig. 1E). Abrupt changes in the slope gradient are present at e 0.5 m elevation. Very low-relief swash bars are developed in the lower intertidal zone, which is not detectable on the transect profile. Remarkable erosion occurred on the tidal beach between February and May (Fig. 3C), when the beach has retreated about 9 m and was lowered by 0.8 m. In contrast, the tidal flat seaward of the beach experienced modest deposition of about 0.15 m. The transect profile did not change

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 4. A time-series of DEM and DoD of South tidal flat, illustrating landward migration of chenier and marked headward erosion of drainage channels between May and August 2017. Refer to Fig. 1B, D for the location of the South tidal flat.

noticeably during the remainder of the study period. 4.2. Surface sediments and sedimentary facies Surface sediments overall fine landward in South tidal flat. Sediments at stations seaward of the chenier (S1, S2, S3) are mainly composed of fine to very fine sands with mean grain sizes between 3.8 and 4.0 phi (Fig. 5). Sand contents vary from 53 to 60%, whereas mud contents range between 39 and 46%. Sediments at a station landward of the chenier (S4) are slightly finer-grained with mean grain sizes varying from 4.2 to 4.3 phi (Fig. 5A, C). Sand and mud contents are nearly the same. Surface sediments at S1, S2, and S3 show a marked fluctuation with a coarsening-trend in winter and a fining-trend in summer. In contrast, surface sediments at S4 display no seasonal trend in mean grain size (Fig. 5A). Surface sediments are highly bioturbated by mainly polychaete and crustacean, leaving indistinct laminations (Fig. 6A, B, C, D). Wave ripple cross laminations are locally preserved at S2 near the chenier (Fig. 6B). Shell fragments tend to be abundant near the chenier and the drainage channel. A mixture of sands and gravels with shell fragments constitute the base of the drainage channel as lag deposits (Fig. 6C). Surface sediments at S4 are dominantly muds with sparse shell fragments and indistinct laminations (Fig. 6D). Surface sediments in West tidal flat are coarser-grained than those in South tidal flat and show a landward fining trend (Fig. 5B, D). Sediments are composed mostly of fine to medium sands. Sand content varies between 55 and 83%, whereas mud content ranges between 15 and 35%. Sand content increases from S5 near the tidal beach to S7 most seaward location by 10e23%. Mean grain sizes vary from 3.2 to 4.4 phi. Except for S7, surface sediments show a marked seasonality in mean grain size and sorting value with sediments in May being the most fine-grained and poorly sorted. Like those in South tidal flat, sediments are highly bioturbated

(Fig. 6F). Landward dipping cross laminations are locally developed, which are interpreted as overwash deposits formed during the storm period (Fig. 6E).

4.3. Sediment-macrofauna relationship A total of 101 species were identified at ten sampling stations, amounting to 1948 individual/m2 of density and 1008.2 g WW/m2 of biomass. The surface elevation of each station fluctuated about 1.3 cm on average between successive surveys. Rapid deposition of 0.32 m occurred at S9 from May to August, whereas erosion reached up to 0.26 m at S1 from February to May. The relationship between sedimentation, mud content, and macrofaunal response is illustrated in Fig. 7. The number of species per station correlates negatively with depositional trends (r ¼ e 0.4, p < 0.05 in Fig. 7A). Among 34 species, a polychaete species Heteromastus filiformis, a dominant subsurface deposit feeder, showed a significantly negative correlation between the density and depositional trends (r ¼ e 0.385, p < 0.05 in Fig. 7C). The density of Heteromastus filiformis decreased from spring (May) to summer (August) when deposition took place and increased from summer (August) to autumn (November) when tidal flats experienced marked erosion. The density of a muddy sand-favoring polychaete species Lumbrineris longifolia is proportional to the mud content (r ¼ 0.497, p < 0.05): a marked increase from winter (February) to spring (May) followed by a decrease from spring (May) to summer (August) (Fig. 7B). The density of a polychaete species Sigambra tentaculata also exhibited a significantly positive correlation with mud content (r ¼ 0.462, p < 0.05), but did not show any distinct seasonal pattern (Fig. 7D). Other faunal parameters did not show any meaningful correlation with the temporal changes in the surface elevation and mud content.

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 5. Histograms showing temporal and spatial variation of mean grain size and sorting of surface sediments for South tidal flat (A) and West tidal flat (B).

5. Discussion 5.1. Morphodynamics and sedimentary processes of tidal flats

Fig. 6. Representative core photographs showing temporal and spatial variation of sedimentary structures and grain size. (A) Core 3 at S3 (August), (B) Core 2 at S2 (February), (C) Core 2 at S2 (August), (D) Core 4 at S4 (August), (E) Core 5 at S5 (February), (F) Core 7 at S7 (August). Note the sharp transition from laminated mud to overlying bioturbated shelly sand in Core 2 (B), reflecting the onset of winter storms. Shelly deposits (CL) in Core 3 demarcate lag deposits of the drainage channel (C). BM: bioturbated mud, LM: laminated mud, MM: massive mud, CL: channel lag, CS: crosslaminated sand, BS: bioturbated sand.

Tidal flats along the west coast of Korea display a wide range of the spectrum regarding morphodynamics and sedimentary processes (Choi, 2014). Contrast arises mainly due to the morphologic setting of the tidal flats relating to offshore waves. Because of distinct seasonal variation in the intensity of offshore waves due to monsoon climate, open-coast tidal flats experience erosion and are dominated by sands during winter to spring when waves are persistently strong, whereas the flats are subject to accumulation of fine sediments during summer to fall when waves are generally subdued (Yang et al., 2005; Choi, 2014). Where the seasonal wave activity comes together with a supply of coarse-grained sediments, prominent wave-built morphology such as cheniers appeared on the open-coast tidal flats (Lee et al., 1994; Yang et al., 2006; Ryu et al., 2008). In contrast, tidal flats located within an embayment display less distinct seasonal oscillation in morphology and sedimentary processes, as the flats are effectively sheltered from offshore waves (Choi, 2014). This is particularly the case where the entrance of the embayment lies in the opposite direction to the prevailing winds (Choi, 2014). However, noticeable erosion and the coarsening of the flats may occur without any wave activity in the sheltered embayment (Ryu, 2003; Ryu et al., 2004), when heavy summertime rainfalls accentuate runoff discharge (Choi, 2011). Morphodynamics and sedimentary processes of the studied tidal flats are different from those in the other embayment-type flats along the west coast of Korea in that the former display a wave-built chenier despite their location protected against offshore waves. Although the chenier has migrated at slower rate than those from open-coast Baeksu tidal flat (75 m per year; Yang et al., 2005)

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 7. Plots showing a negative relationship between sedimentation and number of species (A), between sedimentation and density of Heteromastus filiformis (polychaete) (C), positive relationship between mud content and density of Lumbrineris longifolia (polychaete) (B) and Sigambra tentaculata (polychaete) (D).

and Gomso Bay (23 m per year; Ryu et al., 2008), the presence of actively migrating chenier indicates that waves played a vital role in the sediment transport. Besides, a seasonal change in the surface sediment textural composition is similar to what observed in the open-coast tidal flats (Yang et al., 2005), where surface sediments are coarser-grained during winter months due to increased wave activity. Locally surface sediments became coarser-grained during summer in South tidal flat, which resulted from increased surface runoff by heavy rainfall. As evidenced from the headward erosion of the drainage channel that breached the chenier (Fig. 4), the heavy rainfall imposed direct shear stress on the tidal-flat surface and lowered the erosion threshold of muddy sediments with the input of freshwater (Tolhurst et al., 2006; Choi, 2011). Accentuated runoff discharge also facilitated seaward export of the eroded sediments by reinforcing ebb-tidal asymmetry, which explains summertime erosion on the tidal flats the presence of coarse-grained sediments in the semi-enclosed bay (Ryu, 2003; Ryu et al., 2004). Overall, the morphodynamics and sedimentary processes of the studied tidal flats attest to the characteristic features of open-coast tidal flat rather than those of embayment-type tidal flat. 5.2. Influence of the Saemangeum dike The construction of the Saemangeum dike resulted in notable morphologic and sedimentologic changes immediately seaward of the dike (Lee and Ryu, 2007, 2008). Vigorous erosion took place across the sluice gates of the dike, which are located in the tidal channels. The erosional scours have deepened and expanded since the construction of the dike (Lee and Ryu, 2008), presumably due to concentrated outflow during the opening of the gate at high tide. Sediments entrained from the scours appear to be advected from the dike and accumulated to form shallow shoals in the offshore

region where the channels lose their configuration. Lee and Ryu (2008) also noted that seafloor sediments became sandier after the dike construction. Nearshore sediments were transported towards the dike, and most of them were redistributed northwesterly along the dike (Lee and Ryu, 2007). The rest of them were transported southeasterly along the dike. Tidal flats and beaches at the southeastern end of the dike experienced marked erosion during spring tides that coincided with high wave activity (Lee and Ryu, 2007). The expansion of the sandy seafloor and increased longshore sediment transport along the dike are all suggestive of intensified wave activity promoted by the Samangeum dike. Vintage aerial photographs indicate that the chenier in South tidal flat started to develop since 2007 (Fig. 8). The chenier had migrated about 200 m between 2010 and 2016 at a rate of 19 m/ year with peak migration occurring between 2012 and 2014 when the chenier displaced landward about 100 m at a rate of 50 m/year, which is an order of magnitude faster than the rate observed in this study. The decreased migration rate of the chenier reflects reduced wave influence as the chenier approached higher elevation (Otvos, 2000). The presence of actively migrating chenier attests to the fact that South tidal flat has been exposed to significant wave activity after the dike construction (Fig. 9). Recently observed wave data at the two mooring stations (P1 and P2 in Fig. 1A) indicate those wave directions are significantly modified by the dike (MOF, 2017). Unlike waves at the offshore station (P1), those at the nearshore station (P2) immediate seaward of the dike have propagated in the opposite direction to the prevailing winds during winter to spring (Figs. 2D and 9). Such counterintuitive wave direction at P2 was not the case before the dike construction when the chenier was not developed in South tidal flat (Fig. 9). Regional wave model based on the observation before the dike construction indicates that waves around P2 have propagated mainly from W, WNW, and NW, which

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 8. A time series of aerial photographs exhibiting a marked increase of sandy sediments on South tidal flat since 2007 when Seamanguem dike was completed. Notable landward migration of cheniers occurred since 2010. Image courtesy of NGII (2017).

is similar to the prevailing wind direction during winter to spring (MAFF, 1989; 1991). The drastic changes of wave directions at P2 before and after the dike construction point to the fact that the dike reflected wind-induced waves (e.g., Ruggiero and McDougal, 2001; Wolf et al., 2011; Ardhuin and Roland, 2012, Fig. 9). Continued buildup and landward migration of the chenier imply that sustained and increased onshore supply of coarse-grained sediments occurred in South tidal flat. Wave shoaling and breaking at the seaward side of the dike and channelized outflow at the sluice gate contributed scouring of the seafloor and subsequent resuspension of coarse-grained sediments (Lee and Ryu, 2008). The entrained coarse-grained sediments were likely to be transported onshore by flood-dominated residual currents and waves (e.g.,

MOF, 2017). During the period between May and August, when wave activity is generally low (MOF, 2017), muddy sediments were notably deposited in South tidal flat. The muddy sediments were originated from the subtidal region and transported by flooddominated residual currents. The Saemanguem dike resulted in a flood dominance and a notable decrease in tidal current speed (Lee et al., 2008), which is attributable to the decrease of tidal prism as a result of straightened coastlines and the increase of coastal gradient (e.g., Hood, 2004; Yang et al., 2006; Shi et al., 2011). The reduced tidal currents in front of the dike allow mud deposition in the subtidal channel during fair weather. The fine-grained sediments can be easily resuspended during the wavy conditions and transported onshore by flood-dominant residual currents to promote

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Fig. 9. Schematic illustration of directions of tidal currents and waves around the study area before and after the construction of the Saemangeum dike. The tidal flat was mudprone and dominated by tidal currents with insignificant waves due to their location sheltered from offshore waves before dike construction. The tidal flat became sandier due to pronounced wave activity as a result of wave reflection at the dike, accounting for rapid onshore migration and buildup of cheniers.

mud deposition in the sheltered tidal flat. 5.3. Impact of morphologic and sedimentologic changes on macrofaunal distribution Deposition and erosion on a highly dynamic tidal flat determine the physical stability of the bed, which is crucial for the recruitment potential of the animals (Sanders, 1968; Bouma et al., 2001a, b). Thus, the changes in morphology or sedimentation and mud content are important to determine benthic community structures in the intertidal areas (Zühlke and Reise, 1994; Herman et al., 2001). The Saemangeum dike induced rapid morphologic and sedimentologic changes in the tidal flats inside the dike by promoting sedimentation of mud due to the reduction of tidal currents (Lee and Ryu, 2008), resulting in a completely different benthic community with the increase of the opportunistic species in the flats (Ryu et al., 2011, 2014). Muddy sediments with high organic content are known to increase the numbers and biomass of mud-affinity species such as polychaete in the tidal flats (e.g., Cahoon et al., 1999; Vanaverbeke et al., 2011). An increase of mud content may lead to advantageous conditions for Lumbrineris longifolia, which favored sediment habitat with high mud content (unpublished data of authors). However, H. filiformis and P. linea which favored muddy environments and dominated in Korean tidal flats (Park et al., 2014;

Ryu et al., 2014) showed no significant correlation with changing mud content, warranting further studies to verify their response to mud deposition (Fig. 7). Unlike mud content, temporal changes in sedimentation or morphology tend to correlate with the diversity and density of major benthic macrofauna in the study area. Deposition between May and August is linked with low energy conditions and continued onshore sediment transport. Reduced diversity and the dominance of polychaetes imply that habitat quality has been deteriorated, presumably due to excessive supply of nutrients associated with the sediment supply (e.g., Hyland et al., 2005). High sedimentation rates could suffocate benthic animals, leading to an unstable and harsh environment for some benthic macrofauna (Blanchard and Feder, 2003). A similar observation was made from the Ganghwa tidal flat, where notable siltation takes place due to the large-scale reclamation (Choi et al., 2010). The diversity and density of benthic macrofauna increased between May and February and between November and August when the study area experienced notable erosion by intensified wave activity and increased runoff (Fig. 7A, C). The abundance of stress-tolerant species such as H. filiformis reflects a hostile condition for benthic macrofauna induced by the heightened energy level (e.g., Amaro et al., 2007). The increased density of benthic macrofauna may facilitate the erosion of tidal-flat sediments (Herman et al., 2001;

Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507

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Ysebaert et al., 2005). 6. Conclusions Tidal flats at the south of Shinsi Island have experienced pronounced erosion during winter to spring and overall deposition during summer and fall except for the periods of heavy rainfall when tidal drainage channels became enlarged with marked headward erosion. Although their location is protected from offshore waves, the tidal flats were strongly influenced by waves, which is exemplified by the landward migration of wave-built cheniers. This is in stark contrast to the tidal flats before the construction of the Saemangeum dike in 2006 when the flats were muddier with no apparent wave-induced morphology. The dike reflected waves generated by prevailing offshore winds and promoted prolonged wave activity onto the flats, resulting in an opencoast type morphodynamics in the otherwise protected tidal flat. Sustained onshore sediment flux with seasonal wave activity produced a drastic sedimentation pattern oscillating between rapid erosion and rapid deposition, leading to an unstable and harsh environment for major macrofauna species. Dike-induced physical disturbance promoted morphologic instability of the studied tidal flats, which control the diversity and biomass of major macrofauna species on the tidal flats with positive and negative feedback. Acknowledgments This study is supported by the projects entitled “Quantitative Estimation of Morphodynamics and Sediment Transport in the Macrotidal Intertidal Environment Based on UAV Measurement and Hydrodynamic Observation (NRF-2016R1A2B4009501)” funded by the Ministry of Science, ICT and Future Planning of Korea granted to KSC and “Integrated Management of Marine Environment and Ecosystems Around Saemangeum (2014-0257)” funded by the Ministry of Oceans and Fisheries of Korea granted to KSC and JR and “Marine Ecosystem-based Analysis and Decision-making Support System Development for Marine Spatial Planning (Grant No. 20170325), funded by Ministry of Oceans and Fisheries of Korea (MOF) granted to JR. Authors wish to thank anonymous reviewers for their insightful comments to clarify the focus of the manuscript. References Amaro, T.P.F., Duineveld, G.C.A., Bergman, M.J.N., Witbaard, R., Scheffer, M., 2007. The consequences of changes in abundance of Callianassa subterranea and Amphiura filiformis on sediment erosion at the Frisian Front (south-eastern North Sea). Hydrobiologia 589, 273e285. Ardhuin, F., Roland, A., 2012. Coastal wave reflection, directional spread, and seismoacoustic noise sources. J. Geophys. Res.: Oceans 117, C00J20. Blanchard, A.L., Feder, H.M., 2003. Adjustment of benthic fauna following sediment disposal at a Site with multiple stressors in Port Valdez, Alaska. Mar. Pollut. Bull. 46, 1590e1599. Borja, A., Franco, J., Perez, V., 2000. A marine biotic index to establish the ecological quality of soft-bottom benthos within European estuarine and coastal environments. Mar. Pollut. Bull. 40, 1100e1114. Bouma, H., Duiker, J.M.C., Vries, P.P.d., Herman, P.M.J., Wolff, W.J., 2001a. Spatial pattern of early recruitment of Macoma balthica (L.) and Cerastoderma edule (L.) in relation to sediment dynamics on a highly dynamic intertidal sandflat. J. Sea Res. 45, 79e93. Bouma, H., Vries, P.P.d., Duiker, J.M.C., Herman, P.M.J., Wolff, W.J., 2001b. Migration of the bivalve Macoma balthica on a highly dynamic tidal flat in the Westerschelde estuary, The Netherlands. Mar. Ecol. Prog. Ser. 224, 157e170. Bulleri, F., Chapman, M.G., 2010. The introduction of coastal infrastructure as a driver of change in marine environments. J. Appl. Ecol. 47, 26e35. Cahoon, L.B., Nearhoof, J.E., Tilton, C.L., 1999. Sediment grain size effect on benthic microalgal biomass in shallow aquatic ecosystems. Estuaries 22, 735e741. Choi, K.-H., Lee, S.-M., Lim, S.-M., Walton, M., Park, G.-S., 2010. Benthic habitat quality change as measured by macroinfauna community in a tidal flat on the west coast of Korea. J. Oceanogr. 66, 307e317. Choi, K.S., 2011. External controls on the architecture of inclined heterolithic stratification (IHS) of macrotidal Sukmo Channel: wave versus rainfall. Mar.

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Please cite this article as: Kim, D et al., Influence of dike-induced morphologic and sedimentologic changes on the benthic ecosystem in the sheltered tidal flats, Saemangeum area, west coast of Korea, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113507