A comparative study of materials flows between seagrss-and seaweed-based ecosystemes in tropical lagoon.

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The Purpose of this study is to evaluate the cycling of carbon and nitrogen and their coupling with the metabolisms of biological communities in seagrass beds and in seaweeds in tropical lagoon. We made an extensive field observation in the tropical lagoon located at south of Suva, Fiji from 1994-1996. In the lagoon, there is a dense Syringodium seagress bed and also Halimeda population around the seagrass bed. Following are the some outcomes from the field survey.(1) Combing with the available data of aero-photographs and our field observation, the seagrass bed near the Dravuni Island expanded their size to about 6 times during the last 20 years. This increase in size was also confirmed for the latest 3 years field observation, indicating 22m expansion of seagrass bed to the shore side. The island has a population of some 100 peoples, which is rather stable during the last 20 years. But, the village equipped water supply system from the well about 5-6 years ago. Possible reason for th … More e seagrass increase is the change of nutrients load from the island due to the increase of water usage.(2) Together with the change of seagrass bed, we found the change of epibenthic community distribution such as Ascidians and Lagenophyra. Some of those animals harbor symbiotic alage, which are significant primary producers in the tropical lagoon. The symbiotic character of those animals seems to be important factor controlling the small scale distribution with in seagrass bed and Halimeda communities.(3) Stable isotope analyzes of 15-N and 13-C in various biological communities and suspended organic materials in the seagrass bed and its vicinity gave some interesting insight of food web structure in this ecosystem. Using the significant difference in both 13-C and 15-N rations in seagrass and their epiphytic algae, which are partially due to the nitrogen fixation by blue green algae, filamentous algae was suggested as a primary food source of epibenthic and other benthic animals. Active nitrogen fixation by filamentous blue green was also confirmed experimentally using C2H2 method. Less