A preliminary study on an ecohydrodynamic model for the environmental assessment of a Very Large Floating Structure (VLFS) or Mega-Float in a bay is presented. The model consists of two parts, a hydrodynamic model and a marine ecosystem. Tidal currents and steady currents by the density anomaly are calculated in a bay with/without Mega-Float in the hydrodynamic model. A simplified marine ecosystem composed of nutrient, marine plankton and organic matter floating freely in the ocean is newly developed and combined with the hydrodynamic model. Numerical calculations with/without a Mega-Float in Tokyo Bay are demonstrated and the results are discussed by comparison with the existing computations and measurements.