Number of Species (Table 2. Fig.3)

The number of species of marine algae was 7-9 in each area, and the relation between the areas was C<B<A. The number of species of red algae increased in winter. The number of species of sessile animals was 17-22 in each area, and the relation between the areas was C<B<A. The number of species of sessile animals in this study did not show any seasonal changes.

　

Dominant Species (Table 2)

The dominant species of marine algae were red algae (Chondrus sp., Gelidium elegans, Grateloupia lanceolata, etc.) in area A, red algae (Gracilaria textorii, etc.) and green algae (Enteromorpha spp., Ulva sp., etc.) in area B, green algae (Enteromorpha spp., Ulva sp., etc.) and brown algae (Ectocarpus sp.) in area C. The dominant species of sessile animals were arthropods (Balanus spp., etc.) in area A, mollusks (Omphalius rusticus, etc.) and tunicates (Ciona sp., etc.) in area B, mollusks (Crassostrea gigas, etc.) and arthropods (Balanus spp., etc.) in area C.

　

Biomass (Table 2. Fig.3)

The annual mean biomass of marine algae was 2,585-2,868 gW/m² and 99-138 gC/m², and the relation between the areas was B<C<A in wet weight, and A<C<B in carbon weight. This change is the reason that C/W ratio of green algae is much higher than it of other algae. The annual mean biomass of sessile animals was 1,152-1,496 gW/m² and 106-123 gC/m², and the relation between the areas was B<C<A. There was a high biomass of the sessile organisms in areas A and C in summer, and in area B in winter.

　

Production (Table 2)

The annual production of marine algae was 217-302 gC/m²/y, and the relation between the areas was A<C<B. The annual production of sessile animals was 299-346 gC/m²/y, and the relation between the areas was B<C<A.

　

　

　

Biomass of Trophic Level (Fig.4)

Biomass of the first trophic level was the highest of all the trophic levels at each area. There was a high biomass of suspension feeders at the second trophic level in areas A and C, and a high biomass of herbivores at the same level in area B.

　

Trophic Structure (Table 3)

The annual production of herbivores in the second trophic level in area A showed that the measured value was near the expected value. The annual production of herbivores in the second trophic level was markedly more than the expected value in area B, and markedly less than in area C. The annual production of the third trophic level in areas A and B showed that these measured values were near the expected values. The annual production of the third trophic level was less than the expected value in area C.

　

　

　

　

　

Suspended matter content of the seawater through the permeable rubble-mound breakwater in an artificial lagoon was less than of the outer seawater (Akai, 1984). It was considered that the water quality of Rinku Park Uchiumi, which has the investigation site, was almost homogeneous (Otsuka et al., 1998). In this study, the sessile organism community was examined in the response to flow and suspended matter content of seawater as physical environment.

　

The distribution of marine algae in area A, where red algae was dominant, was similar to the distribution in area B in winter because the seawater of the outer sea side flowed into the inner sea side. On the other hand, there was much thread-shaped brown algae, Ectocarpus sp., in area C in summer. Thin leaf-shaped, shell-shaped and thread-shaped algae were dominant in an area where the seawater flow was very low (Neushul, 1972). These show that the distribution of marine algae in area C is formed on the basis of the calm environment.

　

The distribution of sessile animals in area A, where cirripedes in the suspension feeders were dominant, was not similar to the distribution in area B, where Omphalius rusticus in herbivores was dominant. This suggests that suspended matter content of the seawater in the artificial lagoon is less than it of the outer seawater. These results show that the sessile organism community is formed according to the physical environment of each area in the artificial lagoon.

　

　

The annual production of the third trophic level in areas A and B was relatively similar to the expected value, but it in area C was less than the expected value. This indicates that a smooth material cycle was not formed in the calm area C because of few carnivores of the sessile organisms community in it.

　

The annual production of herbivores in the second trophic level in area A was relatively similar to the expected value, but it in area B was much more than the expected value. This means that marine algae used as the food of herbivores are insufficient, if the energy flow from marine algae to herbivores was appropriately formed, in area B. This is the reason that the annual production in the first trophic level in the sessile organism community does not contain it of microalgae. Therefore, it is thought that the production of microalgae is higher in area B and contributes to the water purification in the area with seawater flowing in the artificial lagoon.

　

On the other hand, the annual production of herbivores in the second trophic level in area C was much less than the expected value. It was shown that a smooth material cycle was not formed in the calm area C because of marine algae and few herbivores in the sessile organism community in it. The dominant species of this area were short-lived algae, Enteromorpha spp., Ulva sp. and Ectocarpus sp. Since these algae wither and die, and partly decompose on the seabed, these algae accumulate on the bottom of the artificial lagoon. Finally, it may be caused that the water quality and the quality of the bottom sediment become bad in the calm area of the artificial lagoon. These results indicate that some controls of the biomass of marine algae, e.g. harvesting, are required to keep and improve the water purification functions of the artificial lagoon.

　

　

The distribution and trophic structure of the sessile organisms in the artificial lagoon were different from each area corresponding to the physical environment. A smooth material cycle was formed in the area with seawater flowing, and contributed to the water purification in the artificial lagoon. However, the energy efficiency among the trophic levels of the sessile organism community was low in the calm area because of much green algae, and a smooth material cycle was not formed in it. It seems that 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. It was suggested that environmental deterioration was likely to be caused by much green algae in the calm area of the artificial lagoon. These results indicate that some controls of biomass of green algae, e.g. harvesting, are required to keep and improve the water purification functions of the artificial lagoon.

　

　

This study is a part of a research program of Uchiumi ecosystem modeling, which is cooperated by Mr. M. Sawada of Japan Port Consultants Co. Ltd., Dr. T. Nakanishi of SOH-GOH KAGAKU Inc. We would like to thank the students in Osaka Prefecture University and the staff of SOH-GOH KAGAKU Inc. for their cooperation with the field investigations. We also thank the reviewers for their constructive comments on the manuscript.

　

Akai, K. 1984. A Water Purification System in Enclosed Sea Area. In Proceedings of the 11th Architectural Technology Symposium, 76-79. (In Japanese)

Fuji, A. and K. Kawamura. 1970. Studies on the biology of the sea urchin-VII. bio-economics of the population of Strongylocentrotus intermedius on a rocky shore of southern Hokkaido. Bulletin of the Japanese Society of Scientific Fisheries. 36(8):763-775.

　

Neushul, M. 1972. Functional Interpretation of Benthic Marine Algal Morphology. In Contributions to the Systematics of Benthic Marine Algae of the North Pacific, edited by I. A. Abbott and M. Kurogi, 47-73. Japan: Japanese Society of Phychology, Kobe.

　

Otsuka, K., N. Nakatani, M. Miyachi, T. Nakanishi, N. Yoshimura and M. Sawada. 1998. Water purification mechanism of artificial lagoon in Rinku Park, 1st Report. Field investigations. J Kansai Society of Naval Architects, Japan. 229:211-219. (In Japanese)

Otsuka, K. and N. Nakatani. 2001. Estimation of Carbon and Nutrient Fixation Effects of an Artificial Lagoon in Osaka Bay. In Proceedings of OMAE '01, 20th International Conference on Offshore Mechanics and Arctic Engineering, OMAE-01-5036, 1-8.

Ryther, J.H. 1969. Photosynthesis and fish production in the sea. Science. 166:72-76.

Yamochi, S., H. Ariyama, T. Kusakabe, M. Sano, Y. Nabeshima, K. Mutsutani and T. Karasawa. 1995. Effects of a predominant sedentary organism of the coastal artificial structure on the eutrophication of the coastal area of Osaka Bay, 1. Growth and elimination of Mytilus edulis galloprovinciallis on the vertical wall. UMI-NO KENKYU. 4(1):9-18. (In Japanese)