Toward conducting ecological risk assessments (particularly exposure assessments) that consider the unique characteristics of microplastic particles (MPs) , there are methods for estimating the concentration of MPs, including the unmeasured size range, as well as concentration conversion methods. We aimed to apply these methods to the data measured at the Tokyo Bay area. First, we described the rescaling method for estimating the size distribution of MPs, including the unmeasured size range, by fitting the measured data of MPs collected from the surface water of the Tsurumi River estuary in Tokyo Bay to a continuous power law distribution. Then, we applied the rescaling method to the data measured at Tokyo Bay and estimated the number concentrations of MPs, incorporating the unmeasured size range. In addition, on the basis of the data measured at the Tsurumi River estuary, we showed the conversion of number concentrations to weight, volume, and surface area concentrations, which are often used in hazard assessments, suggesting that the order of concentration levels between sites may change depending on the concentration unit used. On the basis of these results, we discussed the issues on these rescaling and unit conversion methods.

Nutrients that enter Tokyo Bay become a source of primary production, absorbing CO₂ from the atmosphere and releasing O₂. These processes were investigated. Data from continuous measurements of water quality, flow velocity, and meteorology were used. With changing weather, the fluxes of O₂ and CO₂ frequently switched between absorption and release in opposite phases to each other. The O₂:CO₂ ratio due to biological metabolism was about 1:1, whereas the O₂:CO₂ ratio of sea surface fluxes was about 10:1, with annual averages of 65.4 for O₂ emission and 7.4 for CO₂ absorption in mmol m^-2 d^-1. The equilibration time for O₂ was short, and O₂ generated by primary production was quickly released to the atmosphere. On the other hand, the equilibration time for CO₂ was much longer than the residence time of the bay water, and the amount of CO₂ absorbed within the bay appeared to be about 1/10 of the CO₂ absorption capacity produced by nutrient influx. O₂ flux measurements are important in bays with short residence times. Vertical circulation flow in the bay was also measured and a conceptual diagram of primary production and O₂ and CO₂ transport for the entire bay was presented.