Effects of tidal flat reclamation on sediment quality and hypoxia in Isahaya Bay

Abstract

1. Ariake Bay, which is located in western Japan, has a large tidal range (>6 m) and a vast tidal flat (200 km 2 ). In the early 1990s, the government-managed Isahaya Reclamation Project began in the western part of Ariake Bay. A 16-km 2 area of tidal flat in the inner part of Isahaya Bay was destroyed through reclamation and separated from the sea by a dyke, which created land and a freshwater reservoir.2. Since the initiation of the project, fishery yields around Isahaya Bay have dramatically decreased. The objective of this study was to clarify the relationship between the work associated with the Isahaya Reclamation Project and the recent environmental deterioration in Ariake Bay, with references to present sediment thickness and organic matter content, and hypoxic water distributions in Isahaya Bay.3. The organic matter load from the reservoir has increased since the initiation of the reclamation project and has been associated with a thick layer of fine sediment at the bottom of Isahaya Bay. The thickness of fine sediment and the total organic carbon content were higher in Isahaya Bay than in the freshwater reservoir.4. Based on measurements in August 2001, hypoxic water spread widely in and around Isahaya Bay; the lowest dissolved oxygen (DO) concentration (0.53 mg L À1) was observed just outside the dyke. An analysis based on a two-layered box model using data obtained in August 2001 showed that the DO consumption rate in the bottom layer was high (0.61 mg O 2 L À1 day À1 ), and that 22-41% of the total organic carbon load needed to induce the hypoxic water was derived from the reclamation area.5. Our findings strongly suggest that enclosed seas may suffer from eutrophic and hypoxic conditions because of their low seawater-exchange rate. A comprehensive conservation programme and environmental assessment including physical and material transport processes in the system is needed to manage the environment of the enclosed sea.

Seasonal and Intra-Annual Patterns of Sedimentary Evolution in Tidal Flats Impacted by Laver Cultivation along the Central Jiangsu Coast, China

“…Presently, studies relating to the impact of human activities on the sediment, hydrodynamics and ecological environment of tidal flats have concentrated on (1) artificial reclamation [16][17][18][19][20][21][22], (2) artificial introduction of plants [23][24][25][26], (3) seawall construction and repair [27][28][29] and (4) harbor and wind farm construction [30,31]. Despite significant interest, the effect of artificial seafood cultivation on the geomorphic evolution process has not been sufficiently studied and lacks thorough analysis due to the difficulty of data acquisition.…”

Multi‐decadal morpho‐sedimentary dynamics of the largest Changjiang estuarine marginal shoal: Causes and implications

“…In view of the co‐occurrence of intensive siltation promotion actions and the abrupt increase in shoal volume (Figures and ), the siltation promotion projects likely played a dominant role in recent siltation in the NHS under a regime of decreased riverine sediment input, stable tidal flow regime, and nearly unchanged sedimentary feature (Figures and ). The engineering‐promoted siltation on shoals, mostly through weakening hydrodynamics by construction of infrastructure, has been found to be highly effective, as shown in the Zuiderzee, Netherlands (Hoeksema et al, ), the Isahaya Bay, Japan (Hodoki & Murakami, ), and the Changjiang Estuary, China (Wei et al, ). Thus, the potential link between riverine sediment discharge and shoal evolution is further masked.…”

“…This pattern reveals a degradation of estuarine shoals under utilitarian reclamation, with the rate of reclamation substantially exceeding the progradation speed of the shoal. Examples can be found in the Saemangeum Reclamation Project, South Korea (Son & Wang, ), and the Isahaya Reclamation Project, Japan (Hodoki & Murakami, ), which resulted in decreases in the actual tidal flat area of 401 and 16 km 2 , respectively. Considering the possibly dim future of estuarine shoals in the face of rising sea level, intensified storms, and excessive human activities (Kirwan & Megonigal, ), we call for conservative reclamation.…”

“…In addition, the extensive construction of artificial structures within estuaries, including embankments, dikes, bridges, and groynes, tends to reduce tidal prisms, weaken regional tidal flow, and facilitate shoal accretion, as manifested in the Mersey Estuary, northern England (van der Wal, Pye, & Neal, ), the Changjiang Estuary, China (Wei et al, ), and the Seine Estuary, France (Cuvilliez, Deloffre, Lafite, & Bessineton, ). Reclamation projects generally lead to the degradation of supratidal and intertidal flats; examples include the Isahaya Reclamation Project in Japan (Hodoki & Murakami, ), the Saemangeum Reclamation Project in South Korea (Son & Wang, ), and large‐scale reclamation projects along the coast of The Netherlands (Hoeksema et al, ). Moreover, reclamation around bifurcation can induce flow diversion changes, which is a major cause of shoal accretion in the North Branch of the Changjiang Estuary (Dai, Fagherazzi, Mei, Chen, & Meng, ).…”

Impacts of human activities on tidal dynamics in a sexta-diurnal tidal resonant bay

“…For example, it changes tidal dynamics (e.g., Kang 1999;Park et al 2014), shrinks water volume (e.g., Gao et al 2014;Xiao et al 2019), alters sediment erosion and deposition (e.g., Flemming and Nyandwi 1994;Gao et al 2018), and may finally shorten the life of a bay. Large-scale land reclamation can also lead to damaged coastal habitats, rapid reductions in biological resources, and deterioration of marine ecosystems (e.g., Sato and Azuma 2002;Reise 2005;Hodoki and Murakami 2006;Yan et al 2013;Jickells et al 2016;Yuan et al 2016). Over the past three decades, land reclamation has prevailed along China's coastline due to rapid urbanization and massive economic growth (Liu and Diamond 2005;Pelling et al 2013;Song et al 2013;Zhang et al 2013Zhang et al , 2017Li et al 2018).…”