¹Graduate School of Engineering, The University of Tokushima

Tokushima, Tokushima Pref., JAPAN

n-yoshi@sogokagaku.co.jp

　

²Graduate School of Engineering, Osaka Prefecture University

Sakai, Osaka Pref., JAPAN

　

　

Artificial lagoons have been constructed in enclosed sea areas in order to conduct restoration of water quality and to enhance floral and faunal settlement. Rinku Park Uchiumi, which formed an artificial lagoon, in Osaka Bay, Japan was constructed in 1996. Water exchange of the water body in Uchiumi periodically occurs through the permeable rubble-mound breakwater by the tide. The purpose of this study is to evaluate the environment of the artificial lagoon by examining the community structure of sessile organisms, and to optimize the management for the good condition, which keeps and improves the water purification functions, of the artificial lagoon.

　

Investigation of the sessile organisms was carried out in the permeable rubble-mound breakwater of the seaward (area A) and the shoreward side (area B), and on the rubble-mound inside the lagoon (area C) in summer and winter. The results showed that the sessile organism community depended on the area. The sessile organism community in area A was composed of diverse organisms in each trophic level, such as marine algae and some animals. There were also diverse organisms in area B with seawater flowing, but the number of species in area B was lower than in area A. The rubble-mound in area C were thickly overgrown with green algae, and the number of species in area C was low. This showed that green algae were dominant in such a calm environment as an artificial lagoon.

　

Generally speaking, the presence of green algae in the calm area is taken as a positive sign and as leading to an improvement of the water quality due to the assimilation of nutrients by algae. However, eutrophication occurred because of few herbivores and much green algae which decomposed on a seabed. As a result for better control of the environment of the artificial lagoon, the green algae must be regularly harvested.

　

　

The water quality in enclosed sea areas encircled by big industrial cities, such as northwest part of Osaka Bay, is generally chronically bad since the pollution load is much higher than the natural purification capacities. In order to purify the seawater, several coastal eco-technologies have been proposed.

　

Artificial lagoons, which are calm water areas surrounded by permeable rubble-mound breakwaters, were proposed as one of the coastal eco-technologies applicable to the enclosed sea (Akai, 1984). It has been pointed out that artificial lagoons may have some water purification functions, such as wave breaking re-aeration, biological contact oxidation in the permeable rubble-mound breakwater, increased transparency due to the sinking of suspended matter, carbon and nutrient fixation and improved oxygen supply from algae, creation of the biological material cycle.

　

Some investigations on the water purification functions of permeable rubble-mound breakwaters have been carried out. The biological water purification mechanism of the artificial lagoon in Rinku Park in Osaka Bay was investigated (Otsuka and Nakatani, 2001). Their approach was according to an ecosystem modeling technique, which consists of field investigations and numerical analysis using an ecosystem model. The results of estimations showed that the carbon and nutrient fixation effects by marine algae was large and the total fixation effects decreased year by year because of the accumulation of the sedimentary organic matter. However, environmental evaluation of an artificial lagoon based on the sessile organism community has not been discussed from the viewpoint of ecology in these investigations.

　

The purpose of this study is to clarify the community structure of sessile organisms, based on the biota and biomass in the artificial lagoon, to evaluate the environment of the artificial lagoon by the sessile organism community, and to optimize the management for the good condition, which keeps and improves the water purification functions, of the artificial lagoon.

　

　

　

Rinku Park, the investigation site in this study, is located near the Kansai International Airport in Osaka Bay (Fig.1). In this park, there is an artificial lagoon so-called Uchiumi which is surrounded by rubble-mound revetment including transmittable breakwater and sand beach (Fig.2). Rinku Park Uchiumi, which is about 120 m long, 50 m wide, and 1.5 m deep in average, was constructed in 1996. The water area and volume of Uchiumi are about 6,600 m² and 10,000 m³ in average, respectively, and over 40% of the seawater is exchanged between inner and outer lagoon areas at flood tide.

　

Uchiumi of the investigation site was divided into three areas (A, B and C) in this study. The wave dissipating concrete blocks on the permeable rubble-mound breakwater on the open sea side were designated as "area A", and the stones on the inner lagoon side as "area B" and the stones used in the rubble-mound surrounding the inner lagoon side as "area C" (Fig.2).

　

　

　

　

　

Field investigations were performed on August 6, 1998 in summer and February 17, 1999 in winter. The sampling points of each area in the artificial lagoon were 1 point in area A, and 2 points in areas B and C (Fig.2). The mean of 2 points data was used in the sampling data in areas B and C. The sampling water level was about 1 m under the mean water level in Osaka Bay.

　

Quadrat samples (30 cm * 30 cm) of the sessile organisms in the artificial lagoon were taken from the sloping surface of the blocks and stones using a scraper, followed by checking for the organisms present.

　

The samples of the sessile organisms were identified, and the wet weight was measured for each species. For sessile animals which had a shell, the wet weight was measured including the shell. Data on wet weight were converted into 1 m * 1 m. Moreover, a part of dominant species measured for dry weight was examined for amounts of carbon by using a Carbon and Nitrogen Analyzer.

　

　

Calculation of Biomass of the Sessile Organisms

The annual mean of biomass averaged wet weight of summer and winter was converted into the amount of carbon using the ratio of carbon to wet weight (= C/W ratio, see Table 1), and into the annual production using the ratio of annual production to annual mean biomass (= P/B ratio). The P/B ratio 2.19 of Ulva pertusa (Fuji and Kawamura, 1970) was used for marine algae because of much green algae. The P/B ratio 2.82 of Mytilus galloprovincialis of Osaka Bay (Yamochi et al., 1995) was used for sessile animals because of many suspension feeders which had a shell. In addition, the C/W ratio of three sessile animals, Omphalius rusticus, Crassostrea gigas and Balanus sp., was taking carbon weight including the shell into consideration.

　

　

Analysis of the Community Structure of Sessile Organisms

The classification of the trophic level of sessile organism community was as follows. (1) The first trophic level which was the producers was made of marine algae. (2) The second trophic level which was the primary consumers was made of herbivores (Polyplacophora, Gastropoda, Echinoidea, etc.) and suspension feeders (Porifera, Bivalvia, Balanus spp., Bryozoa, Tunicata, etc.). (3) The third trophic level which was the higher consumers was made of carnivores (Actiniaria, Gastropoda, Crustacea, Asteroidea, etc,).

　

In order to evaluate the trophic structure, the annual production of each trophic level on the basis of this investigation was compared to the expected value. The relation between the first and second trophic levels (only herbivore) and between the second and third trophic levels were examined in this study. The expected value was calculated from 15 % of the ecological efficiency (Ryther, 1969) of the coastal sea shore. It was considered that a smooth material cycle was formed if the measured value of annual production was close to the expected value. Since a smooth material cycle was the good condition of ecosystem in the artificial lagoon, it was regarded as good environment of the artificial lagoon.