The environmental impacts of an imaginary very large floating structure on marine environment, especially on carbon circulation around the structure, are examined using numerical simulation by means of coastal ecosystem model. After validation of coastal ecosystem model by comparing predicted results with observed ones, the impacts of the floating structure moored off Oppama or off Haneda are discussed. As a result of several computations, it is revealed that growth rate of sessile organisms adhering to the floating structure are smaller in the sea area off Haneda, where current velocity of sea water is slow, because particulate organic matter and oxygen are not supplied sufficiently. On the other hand, in the sea area off Oppama, seawater flows southward constantly at the speed of about 0.1m/s, and organic matter and oxygen are supplied appropriately, so sessile organisms can sustain their biomass. The biomass of sessile organisms per unit area of the floating structure moored off Oppama is much larger than that in the general tidal flat estuary. Further, the impacts of activities of sessile organisms on carbon circulation in Tokyo Bay are examined. Then it can be said that sessile organisms retain organic matters in the bay, which may otherwise flow out from the bay to the outer sea.