Michio J. Kishi

An Ecological-Physical Coupled Model Applied to Station Papa

A vertical one-dimensional ecosystem model was constructed and applied to Station Papa. The model has seven compartments (phytoplankton, nitrate, ammonium, zooplankton, particulate organic matters, dissolved organic matters, dissolved oxygen) and was coupled with a mixed layer model for calculating diffusion coefficient which appears in the governing equations. The mixed layer model was driven by SST, SSS data observed at Station Papa in 1980 and ECMWF wind data for 1980, and the ecosystem model was driven by fixing nitrate concentration in deep layer to an observational value. The phytoplankton maximum in March was reproduced by the model although the maximum in fall-winter could not be reproduced. The model also suggests the importance of studying nitrification. As a whole, the model could reproduce characteristic features at Station Papa such as the summer ammonium maximum at 50 m depth, the summer dissolved oxygen maximum at 70 m depth and the absence of remarkable phytoplankton bloom.

Effects of interaction between two warm-core rings on phytoplankton distribution

Abstract A modeling study was conducted to investigate the effects of interaction between two warm-core rings on the phytoplankton distribution. Using a quasi-geostrophic reduced-gravity model, the interaction between two vortices was calculated to reproduce the partial vortex merger between two warm-core rings, 86B and OR. On the calculated flow field, the three-dimensional model of plankton dynamics generated high phytoplankton biomass at the edge of the merged vortex. In the model, phytoplankton grew due to nutrient transport into the euphotic zone by upward motion of water caused by meanders of the current followed by the partial vortex merger. Further, the model reproduced the horizontal distribution of phytoplankton observed in 86B.

Modeling of Spring Bloom in the Western Subarctic Pacific (off Japan) with Observed Vertical Density Structure

Effects of vertical stability on spring blooms of phytoplankton were investigated for the western subarctic Pacific ocean using a one-dimensional (depth) ecosystem model. In the model, vertical stability was expressed by diffusion constants calculated from observed density distribution. Dynamics of phytoplankton in blooms was calculated by the model using the vertical diffusion. Then, the calculated results were compared with the Coastal Zone Color Scanner (CZCS) data. The comparison shows that the shallow surface mixed layer causes early start days of spring blooms at inshore (northern) stations. In addition, spring blooms continue long at inshore (northern) stations since a water column has weak stability. This is because weak stability of a water column causes large nutrient supply from a deep layer and large diffusive transport of phytoplankton biomass from the subsurface maximum.

Numerical simulation model for quantitative management of aquaculture

Abstract A numerical model is developed for aquaculture management, which consists of four parts: (1) a current simulation model, which calculates tidal and wind-induced currents, (2) a chemical oxygen demand (COD) diffusion model, which calculates spatial distribution of COD using the simulated current, (3) a dissolved oxygen (DO) diffusion model, which calculates the spatial distribution of DO, and (4) an accumulation model, which calculates the distribution of deposits from fish aquaculture. Our model is capable of calculating the detailed spatial distribution of COD and DO by dividing the bay into many grid points. It also takes into consideration the effects of feed and fish in each raft, and the loading of COD from rivers. Using this model, we can assess the influence of the location or the area of aquaculture rafts on the ecological and/or environmental system. It is also of practical use in order to obtain a better distribution of rafts in bay areas, or to calculate the basic data for the renewal of licenses of an aquaculture business. The model is applied to Mikame Bay in Ehime Pref., west Japan, as a case study.

Sensitivity analysis of a coastal marine ecosystem

An ecosystem with four compartments, i.e. PO4-P, phytoplankton, zooplankton and detritus, in a coastal region was considered. Sensitivity analysis of a phosphorus flow model in well-mixed water without currents concludes that (1) the maximal photosynthetic rate,Vm, in Michaelis-Menten relation plays an important role in the distribution of biomass among compartments but the half saturation constant,Ks, is not so important, (2) the natural death rate of phytoplankton is important for the ecosystem, (3) the natural death rate of zooplankton is also effective on the ecosystem.A numerical experiment was also performed on the ecosystem with four compartments are also studied using a dynamical barotropic model of tidal currents of Mikawa Bay (Japan). Diffusion coefficient in diffusion equation plays the role of a linear smoothing parameter in the horizontal distribution of compartment. On the other hand, perturbations of biological parameters cause nonlinear variations in the horizontal distribution of compartment.

Population dynamics of ‘red tide’ organisms in eutrophicated coastal waters — Numerical experiment of phytoplankton bloom in the East Seto Inland Sea, Japan

Abstract A system of simulation models on the outbreak of ‘red tide’, a large-scale phytoplankton bloom, is developed by identifying a model structure based on the marine ecological and hydrological conditions in the East Seto Inland Sea, Japan. The models consists of two parts: one is to determine hydraulic conditions of water movements in the sea, and the other is to calculate biological interactions among red tide organisms and nutrients under the diffusive and advective transportation associated with the calculated water movement. The comparison between the results simulated by our models and the observed biological or physical data on ‘red tide’ has shown: (a) the amount of nutrients and oceanographic characteristics are basic factors influencing the rapid increase in phytoplankton population; (b) the alternate migration of both phyto- and zooplanktons in the vertical direction according to a light/dark cycle causes a dense phytoplankton population in the upper layer; and (c) the tidal current plus water movement induced by the wind has a great impact on phytoplankton dynamics.

Causes and consequences of spring phytoplankton blooms in Otsuchi Bay, Japan

Abstract A time-dependent, three-dimensional model simulates some important biological and physical processes in Otsuchi Bay in the northern part of Japan during the spring bloom in 1990. The spatial distribution of chlorophyll and primary productivity in the model is determined primarily by light condition and secondarily by the strength of vertical advection which is caused by a compensated current of strong westerly wind induced by the surface current. Nutrients are not the limiting factors during the spring bloom and zooplankton does not have an important influence on chlorophyll levels.

Prediction of phytoplankton growth in a Warm-Core Ring using three dimensional ecosystem model

In order to simulate the response of the primary production dynamics to the decay of the Warm-Core Ring (WCR) 86-B off the east coast of Japan, we made a numerical model which consists of a three dimensional physical model (modified GCM) and we used the same biological model that Frankset al. (1986) did. According to the three dimensional model, the well known sub-surface chlorophyll maximum was reproduced but horizontal distributions of Chl.a and NO3−-N showed different patterns corresponding to different initial conditions of nutrient. This is because the weak vertical velocity in the WCR does not play an important role on the ecosystem but only the light intensity and the balance between uptake and vertical diffusion of dissolved nutrient is important. This result differs from that of Frankset al. (1986). The two WCRs interaction model suggests that a weak upwelling could exist between two WCRs accompanied by baroclinic instability.

Phytoplankton growth response to wind induced regional upwelling occurring around the Izu Islands off Japan

From frequent field observations performed in coastal waters around the Izu Islands, Japan, a clear regional upwelling associated with the wind was detected baside Niijima Island. Nutrient supply by the upwelling into the euphotic zone was confirmed, and the subsequent phytoplankton growth supported by the upwelled nutrients was evaluated. The upwelling and the nutrient supply occurred within a day over an area ofca. 400 km2, and phytoplankton growth response occurred only a few days after the upwelling. Such regional upwelling is considered to be one of the major mechanisms supporting the high productivity of coastal waters.

Numerical simulation model for quantitative management of mariculture

Abstract A numerical model is developed for mariculture management, which consists of four parts: (1)current simulaton model which calculates tidal and wind induced currents, (2)COD diffusion model which calculates spatial distribution of COD using simulated current, (3)DO diffusion model which calculates spatial distribution of dissolved oxigen, and (4)accumulation model which calculates distribution of deposits from mariculture of fish. Our model is capable of (1) calculating the detailed spatial distribution of COD and DO by dividing the bay area in to many grid points, (2) taking into consideration the effects of feed and fish in each raft, and the loading of COD from rivers. Using this model, we can assess the influence of the location or the area of the mariculture rafts on the ecological and/or environmental system. It is also of practical use in order to obtain a better distribution of rafts in bay areas, or to calculate the basic data for the renewal of licenses. Application is made to Mikame Bay in Ehime Pref., west part of Japan, as a case study.