Blue mussels on a floating structure filter a lot of suspended particles in the ambient seawater. They also consume oxygen and release nutrients in their respiration and excretion. These activities of blue mussels may cause water quality to change, so the impacts of them on the surrounding environment should be assessed before construction of the floating structure. To predict quantitatively the change in the pelagic environment, numerical simulation by means of a model of blue mussels is one of the powerful tools. The model contains several parameters such as the relative filtration and respiration rates, whose numerical values should be determined appropriately. To obtain the numerical values of the parameters, decreasing rates of chlorophyll- a and dissolved oxygen are measured in the real sea, instead of determining them from the experimental results in the laboratory. As a result, decreasing rate of chlorophyll- a does not correspond to the concentration of chlorophyll-a around the blue mussels because the filtration of them is limited by low levels of salinity and dissolved oxygen. On the other hand, decreasing rate of dissolved oxygen generally agrees with the amount of dissolved oxygen in the seawater. Then the model of blue mussels is improved using the experimental results. Consequently, it is revealed that the numerical values of the relative filtration and respiration rates are getting larger and larger with increase in water temperature, and they change drastically at the salinity of 12-16 psu. Further, changes in these rates due to water temperature and salinity can be described well using the exponential and switch functions, respectively.