Natural Tidal Flats Research Articles

英虞湾における干潟の歴史的変遷とその底質，底生生物への影響

The total areas of the existing natural and reclaimed tidal flat in Ago Bay were estimated by multispectrum aerial picture analysis. Approximately 70% of the tidal flat and shallow areas have already been lost because of land reclamation and dyke construction. The sediment quality and characteristics of macrobenthos in the natural tidal flats (estuary, in front of and behind the dykes) were investigated in this study. The macrobenthos in the natural tidal flat in front of and behind the dyke was poor in organic matter, because these sediments were relatively oligotrophic in front of the dyke and hypertrophic behind the dyke. Thus it is considered that the reclamation by constructing the dykes was one of the major causes of the deterioration of the benthic ecosystem and decrease in the biological productivity of Ago Bay.

The Planning and evaluation method of the artificial tidal flat by focusing on the basic natural condition of existing natural tidal flats

The Planning and evaluation method of the artificial tidal flat by focusing on the basic natural condition of existing natural tidal flats

Differences in invertebrate infaunal assemblages of constructed and natural tidal flats in New South Wales, Australia

The infauna at four natural and two constructed tidal flats were sampled to determine differences in assemblages on the southeastern coast of New South Wales, Australia. Spatial variation within location (tidal heights and subsites separated by 50–100 m) and between locations was incorporated into the design to investigate differences in species assemblages between constructed and natural locations. Overall, infauna at constructed locations were intermediate in abundance and species richness, although differences in abundance and species richness across tidal heights were observed at some locations. Multivariate analysis of species composition revealed distinct differences in species assemblages between constructed and natural locations, amongst tidal heights, and considerable spatial variation within any location. Inappropriate design of waterflow at the oldest constructed site is believed to be responsible for the low variability in species assemblages within this location which are characterised by early successional species that occur in disturbed tidal flats. The newer constructed tidal flat appeared to have adequate waterflow, high site variability in species composition and an infaunal assemblage typical of an early successional stage. The importance of measuring spatial variability in monitoring is stressed in order to make useful comparisons between constructed and natural tidal flats. This study showed that the successful construction of a mature and healthy tidal flat requires the maintenance of a significant waterflow area to ensure adequate tidal flow for the establishment of fauna.

Creation and adaptive management of a wild bird habitat on reclaimed land in Osaka Port

We examined the efficacy of habitat creation for shorebirds on reclaimed land in Osaka Port. The Wild Bird Park was laid out on a reclaimed land in Osaka Port in 1983. A modern history of the area as a habitat for shorebirds can be divided into four stages. First, many shorebirds visited the tidal flat of 30 ha that emerged on the south part of the Sakishima, which had been reclaimed from the sea in 1941, but was abandoned between 1950 and 1958 [Notes on Osaka Bay Waders. Private Publishing (in Japanese with English abstract)]. Second, the tidal flat disappeared because of the start of the reclamation of the north part of Sakishima in 1958, and a salt marsh of 85 ha emerged in the north part between 1974 and 1982. Third, the salt marsh disappeared due to the construction and the Wild Bird Park was opened on a part of the salt marsh in 1983. The park had a planted area of 6.5 ha and a sandy area of 12.8 ha, which included two ponds and a lagoon. Fourth, one of the ponds was restored to a tidal flat in 1995 after consultation among the manager, NPO, and scientist as a result of the monitoring. The area of tidal flat increased from 0.2 to 2.6 ha, and the number of shorebirds increased from 205 (the average of 1991–1995) to 1042 (1996). The abundance and species richness of shorebirds is equivalent to those at the first stage (1950s). The species composition of benthic animals had also changed; the dominant group was Chironomid larvae during the second and third stages and Polychaetes during the fourth stage, and the species richness was greatest at the fourth stage. However, the tidal flat of the park had fewer brachyurans and mollusks than natural tidal flats in Japan. This might result in a lower abundance of large sandpipers at the fourth stage compared with that at the first and second stages.

潜堤の設置が干潟生態系に及ぼす影響

The purpose of this study is to clarify the effects of submerged breakwater on an artificial tidal flat ecosystem. The studied tidal flat ecosystem is composed of three zones, that is, low-, high-and no-submerged-breakwater zones.The frequencies of shear stress being higher than the critical value (0.1N m-2) for silt transport in the low- and high-submerged-breakwater zones were lower than that in the no-submerged-breakwater zone. The silt and organic carbon contents in the low-and high-submerged-breakwater zones were higher than those in the no-submerged-breakwater zone. The slopes, reduction zone and permeability of sediment in the submerged breakwater zones were almost the same as those in natural tidal flats at Hiroshima Bay. However, the no-submerged-breakwater zone showed a sharper slope. The total population density of macrobenthos in the low-and high-submerged-breakwater zones were about twice as high as that in the no-submerged-breakwater zone. The dominant macrobenthic species in the submerged breakwater zones were Ceratonereis erthraeensis and Mediomastus sp., which are common in natural tidal flats at Hiroshima Bay. These results suggest that the construction of submerged breakwaters could change both the physiochemical and biological properties of artificial tidal flats similarly to those of natural tidal flats at Hiroshima Bay.

広島県尾道糸崎港における干潟再生事業

Although many artificial tidal flats had been constructed in Japan, there was not enough monitoring data that can prove the restoration of the natural environment after the construction of the tidal flats. Hiroshima Port and Airport Office at Chugoku Regional Development Bureau constructed three tidal flat as environment restoration projects in Onomichi-Itozaki Port area, Hiroshima, Japan. Total area of three tidal flats is c. a. 60 ha. After the completion of the tidal flats, geo-morphological change, sediment quality, benthos abundance, and sea-grass (Zostera marina) habitats have been monitored since 1984 to 2002. Especially, during FY 2000 to 2002, high biological diversity as natural tidal flats, abundance of many endangered spices, and 8 ha of sea-grass (Zostera marina) habitat restoration had been observed.

Landform Conservation and Flood Control: the Issue of the Chitose Diversion Channel Project in Hokkaido, Japan

Landform conservation is the main task of geomorphology in the 21st Century. Since landforms provide the templates for the development of wildlife habitats, landform change is likely to cause habitat loss, inducing a reduction of biodiversity. From the viewpoint of ‘geomorphic species’, specific landforms such as natural rivers, tidal flats and coral reefs are endangered species in the Japanese islands. To avoid the further destruction of natural rivers, environmentally appropriate flood control plans are necessary. The analysis of the Chitose Diversion Channel project in Hokkaido, Japan, revealed that (1) the construction of this diversion channel has caused not only environmental problems but also serious social conflicts relating to habitats within and beyond the drainage basin; (2) excavation of a deep channel changed the groundwater supply which previously supported a natural river system, and (3) selection of a very high discharge value as the target of flood control was the main reason for planning this project. From a landform conservation perspective, the method adopted in Japan is unsuitable because it does not incorporate the scientific procedure necessary for selecting the ‘best’ peak discharge corresponding to the target rainfall with a certain recurrence period. An alternative flood control plan, combining the selection of a lower peak discharge, construction of retention pools and the restoration of meandering river channels is proposed to avoid the further destruction of natural rivers.

PROBLEMS OF ARTIFICIAL TIDAI FLAT

Biological survey was conducted for three artificial tidal flats in Japan. As compared with natural tidal flat, artificial tidal flats are unstable in features, poor in biodiversity, and feeds fewer shorebirds. It is necessary for a principle that active natural tidal flats are strictly conserved, inactive flats are recovered, lost flats are restored, and local people are involved in planing. An environmental impact assessment and mitigation method should be conducted logically and scientifically. Recovery or restoration should be undertaken by the principles and guidelines for wetland restoration. Consensus of the stakeholders are indispensable to the decision making of tidal flat recovery plan.

CHARACTIERISTICS OF THE TOPOGRAPHY AND SOIL IN FORESHORE TIDAL FLATS AND ITS TYPE CALSSIFICATION

A series of field surveys investigating the stability of natural/manmade tidal flats were conducted from June 2000 to April 2001, i. e., the profile of the seabed and the stiffness and grain-size distribution of sediment were measured. Five natural and seven artificial tidal flats were investigated including one natural and three artificial sand beaches. The terrace of the foreshore region in most natural tidal flats consists of a main flat part and a multiple bar-trough part (multi-bar). The former serves as an important habitat, while the latter is needed to ensure stability of the main flat part against wave actions which occupy the area from the ebb tide shoreline to 200-350m shoreside. Types of forcshorc tidal flats are classified in the condition of the incident wave height and the grain-size of bottom sediments.

Factors to determine the functions and structures in natural and constructed tidal flats

The purpose of this study was to evaluate the effects of design criteria and environmental conditions of the site of created tidal flats, on the structure and function of the man-made ecosystem. Seven constructed tidal flats and three natural tidal flats were studied taking into consideration the difference in the location (wave height and tidal current), inflow of river water, slope of tidal flat and age after the construction was completed. Parameters studied were physico–chemical and biological characteristics of soils and rate of respiration. The natural tidal flats had higher contents of silt, nitrogen and organic matter compared to the constructed ones. The natural ones had a reductive zone below 2 cm, whereas the constructed ones had an aerobic zone from the surface to below 20 cm. The bacterial population in the soil of the constructed tidal flats were one to two orders of magnitude lower than that in the natural ones. Biomass of macrobenthos and microbial respiration rate, however, were not significantly different between the natural and the constructed tidal flats. There was an exceptional constructed tidal flat with similar physico–chemical and biological characteristics as the natural ones. It is most probable that sufficient conditions to have similar tidal flats to natural ones are not only the slope, but also the location in enclosed bay or calm coastal area. Thus, to make man-made tidal flats with the same characteristics as those in natural one, man-made tidal flats should be designed and/or located to enhance the accumulation of silt on tidal flat. It is important to select a place having low water motion and to construct a flat with small slope.

自然および人工干潟の有機物浄化能の定量化と広島湾の浄化に果たす役割

The objective of this study is to estimate the purification capacity for organic matter (carbon) in natural and man-made tidal flats. In this study, we measured respiration rates and biomass for diatom+bacteria+meiobenthos and macrobenthos in the permeable layer of soil in tidal flats. Furthermore, the role of these tidal flats for the purification of organic matter in Hiroshima Bay was studied.The man-made tidal flats had deeper permeable layer for purification of sea water compared to the natural tidal flats. The carbon biomass of bacteria and macrobenthos were not significantly different between the natural and the man-made tidal flats. The respiration rates by diatom+bacteria+ meiobenthos and macrobenthos were not significantly different between the natural and the man-made tidal flats. However, the purification capacity by diatom+bacteria+meiobenthos and macrobenthos in man-made tidal flats was higher than that in natural ones. It is most probable that permeable layer of soil for purification capacity in man-made tidal flats was deeper compared to the natural ones. Purification capacity of natural and man-made tidal flats was compared with external and internal organic loading into Hiroshima Bay. The percent contribution of tidal flats was only about 0.5n%. Even if there were restoration of the area as in 1940s, the constribution will be 1.6%. The purification capacity of organic matter in tidal flats is not large as expected compared with pollution loading in Hiroshima Bay.