Fumio Horiguchi

Model study of Lakes Shinji and Nakaumi : a coupled coastal lagoon system

Abstract To understand the physical and biological processes that occur in Lakes Shinji and Nakaumi, a coastal lagoon system, a hydrodynamic–ecological coupled model was applied. The model was run to hindcast these processes from March 1, 1995 to the end of September 1995. The simulation of physical processes suggests that the meteorologically induced sea level variation and a large episodic amount of freshwater discharge effectively caused the water exchange between the open sea and this lagoon system. The simulated results of the biological processes using the ecological model show good agreement with field data except for the phosphate concentration in the bottom water. It was also found that the microbiological processes in the water column, as well as in the sediment, contribute largely to the formation of oxygen-depleted water in the bottom layer. It was also clarified from the simulation that the two species of benthic bivalve, which have dense populations in this lagoon system, play important roles on removing organic particulate matter and nutrients from the system.

A Numerical Simulation of an Oil Spill in Tokyo Bay

Abstract An oil spill accident happened in Tokyo Bay on 2 July 1997. About 1500 m 3 of crude oil was released on the sea surface from the Japanese tanker Diamond Grace . An oil spill model is applied to simulate the fate of spilled oil. The Lagrangian discrete-parcel method is used in the model. The model considers current advection, horizontal diffusion, mechanical spreading, evaporation, dissolution and entrainment in simulating the oil slick transformation. It can calculate the time evolution of the partition of spilled oil on the water surface, in the water column and the sedimentation on the bottom. A continuous source at constant rate is set up as a tanker off the coast of Yokohama. The grid size is 1 km in the calculation domain. The residual flow simulated by a 3-D hydraulic model and observed wind data are used for advection. The simulated distribution of oil spreading agrees well with observations from satellite remote-sensing.

A Numerical Simulation of the Seasonal Cycle of Temperature, Salinity and Velocity Fields in Tokyo Bay

To understand when oxygen-depleted waters occur, how they develop and when they dissipate in inner Tokyo Bay, realistic simulations were attempted with fine spatial and temporal resolution by applying realistic time dependent external forcing. A 3D hydrodynamic model was driven by time-dependent external forcing factors/parameters such as solar radiation, air temperature, relative humidity, wind velocity, and fluvial discharge, under the open boundary conditions of 1995. A simulated time series of salinity and temperature agreed fairly well with observed data, except in summer. The model failed to reproduce the development of the surface mixed layer in summer. Several sensitivity analyses on the external forcing parameters such as wind velocity and vertical diffusivity were conducted to reproduce the mixed layer. However, changing these parameter values did not improve the model results.

Ecological risk assessment of TBT in Ise Bay.

An ecological risk assessment of tributyltin (TBT) in Ise Bay was conducted using the margin of exposure (MOE) method. The assessment endpoint was defined to protect the survival, growth and reproduction of marine organisms. Sources of TBT in this study were assumed to be commercial vessels in harbors and navigation routes. Concentrations of TBT in Ise Bay were estimated using a three-dimensional hydrodynamic model, an ecosystem model and a chemical fate model. Estimated MOEs for marine organisms for 1990 and 2008 were approximately 0.1-2.0 and over 100 respectively, indicating a declining temporal trend in the probability of adverse effects. The chemical fate model predicts a much longer persistence of TBT in sediments than in the water column. Therefore, it is necessary to monitor the harmful effects of TBT on benthic organisms.

Model study of environmental concentrations of TBT in Tokyo Bay - development of a Windows® version prototype

This study develops a model that predicts environmental concentrations and ecosystem exposure to tributyltin (TBT), which is an organotin compound used as an antifouling agent on ship hulls. In addition, using existing data, an average calculation for Tokyo Bay over four seasons was carried out using a three-dimensional flow model and ecosystem model. The calculation results for each season is stored in a database, and then combined with the chemical substance fate prediction model for marine areas. This model can be adapted for operation in Windows^(R) so that it is easy to use by other operators. Using the simple input of parameters, this prototype modeling system enabled detailed prediction of environmental concentrations and ecosystem exposure in a marine area. In the future, it will be possible to conduct a risk assessment of human exposure to TBT by improving the accuracy of parameters used in this modeling system.

Evaluation of ecotoxicity and fate of methylated butyltins in sediments and seawater from Tokyo Bay, Japan

We analyzed the fate of organotins in seawater and sediments from Tokyo Bay, Japan, by gas chromatography/inductively coupled plasma mass spectrometry. We also measured the toxicity of methylated butyltins by in vitro bioassays, the retinoid X receptor (RXR) activation method, and the marine luminescent bacterium Vibrio fischeri. Concentrations of tributyltin (TBT) and tributylmonomethyltin (TBMMT) in seawater were 0.0636 to 0.419 and 0.0050 to 0.108 ng Sn/L and in sediment were 7.51 to 17.8 and 3.67 to 6.87 ng Sn/wet weight g, respectively. Methylated butyltins did not activate RXR and were not toxic to bacteria. Tributylmonomethyltin in seawater would elute from sediment since TBMMT-to-TBT ratios showed a positive correlation (r(2) = 0.858) between sediment and deep seawater. Both methylation and debutylation of TBT seem to be major routes of decomposition of TBT in sediment. Methylation of TBT would not only cause subsequent volatilization but also decrease the toxicity of TBT species in the marine environment.

Intercomparison experiment of current meters in the upper ocean

Abstract Current meter intercomparison experiments were conducted near Onahama port in Fukushima Prefecture. Six current meters were deployed with a surface buoy mooring system. They were EM620, VMCM, ENDECO, IRCM-4, MTCM-4V, and U-EM. Significant correlations were obtained in the measurements of EM620, VMCM, ENDECO, and IRCM-4. The first two instruments are of the vector measuring type with internal vector averaging, and the rest are of the impeller sensor type without internal vector averaging. The difference between instruments with and without internal vector averaging could not be shown clearly.