(3) Requirements on Modeling a Multiple Ecosystem

As mentioned above, hypoxia generation/annihilation and its ripple effects shown in Figure 1 are formulated by mutual interaction between benthic-pelagic ecosystems and between central bay-tidal flat ecosystems, i.e. a multiple ecosystem. Therefore, when evaluating the ecological response of the hypoxic estuary, the model is required to contain (a) the benthic and pelagic systems in both the central bay and tidal flat areas, and (b) mutual interactions between the benthic and pelagic systems, and between the central bay and tidal flat areas. In recent years, a number of benthic-pelagic coupling models have been developed (Baretta and Ruardij, 1988; Baretta et al., 1995; Baretta-Bekker and Baretta, 1997; Sohma et al., 2001; Sohma et al., 2004; Luff and Moll, 2004), with several models describing diagenetic (metabolic) processes in detail (Luff and Moll, 2004).

Furthermore, the requirements in items (1), (2) and (3) mentioned above are not independent from each other, because vertical micro scale mechanisms in the benthic system (items (1) and (2)), or the daytime scale dynamics in the tidal flat area (item (2)) is controlled by the ecosystem network in the multiple ecosystem (item (3)). Thus, the ecosystem model which treats all the requirements in items (1), (2) and (3) at the same time (i.e. describing both (a) the micro scale spatial resolution in benthic and (b) the ecosystem dynamics of daytime scale, plus (c) including the benthic-pelagic ecosystem both in the central bay and tidal flat simultaneously) is important/useful to evaluate the ecosystem dynamics of a hypoxic estuary. However, such models or model studies had not existed.

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On the basis of the philosophical and technological background mentioned above, a new ecosystem model, the Ecological Connectivity Hypoxia Model (ECOHYM), the first model to meet all the requirements in items (1), (2) and (3) simultaneously (i.e. describing both (a) the micro-scale spatial resolution in benthic and (b) the ecosystem dynamics of daytime scale, plus (c) including the benthic-pelagic ecosystem both in the central bay and tidal flat) was developed. With this, the challenges for the clarification of the mechanisms of hypoxia and for the prediction of the ecosystem response and tolerance to environmental measures, development, impact and disturbance were performed from two perspectives: (1) the whole estuary, composed of temporal-spatial mutual linkage of benthic-pelagic or central bay-tidal flat ecosystems (holism), and (2) each physical-biochemical process, contributing to oxygen production/consumption (reductionism).

For ECOHYM to complete these challenges, firstly, the selection of treating the physical and biochemical processes in the model is significant. It should be performed based on the confirmation of the known knowledge of the hypoxia and its related phenomena mechanically. Each selected physical-biochemical process should be formulated based on the latest scientific knowledge as much as possible. This approach is derived from reductionism. The additional requirement for ECOHYM is not the apprehension of each physical and biochemical processes fragmentary nor the description of the ecosystem by superposition (the stack) of them, but is the description of the mechanical linkage and interaction of the each process. Therefore, the numerical construction (Sohma, 2005b), which can describe the autonomous response/the feedback effect due to the entanglement of each process and can estimate the dynamics of the ecosystem as a whole, has to be applied to ECOHYM. Modeling the internal mechanisms of the benthic and pelagic ecosystems or the tidal flats and central bay ecosystems, and also linking each ecosystem by such a numerical construction enables us to regard the whole estuarine ecosystem as the ecosystem of temporal-spatial mutual linkage of the benthic and pelagic systems or the central bay and tidal flat areas. This approach is derived from holism.

The success of the development of such a model reveals where and how much each modeled physical and biochemical processes contribute relatively on the oxygen consumption and production, and leads to the clarification of hypoxic mechanisms. In addition, it enables us to predict the response of the whole ecosystem of the estuary to the environmental measures and perturbations quantitatively and qualitatively, while considering the ripple effects of the measures/perturbations through the entanglement of physical and biochemical processes. These results are also linked to establishing the foundation of the cost-performance evaluation on the environmental improvement technology, such as nutrient reduction, dredging, sand capping, and tidal flat restoration. Here, “quantitatively” means the direction/trend of the temporal and spatial dynamics.

On modeling, several assumptions have to be forced to the ambiguous/unknown/ uncertain processes. However, the fact that model outputs the result of mechanical interactions of many processes, suggests the possible breakthrough on some of the yet-to-be- defined processes reductively/reversely from the perspective of the whole ecosystem balance

(the holistic approach), although they have not been clarified yet through the research of each process piece by piece. Such a method is sometimes effective on advancing the frontiers of science.

Because all existing models are the condensed/simplified description of the real system, any model includes some approximations and assumptions. However, if the political measure is argued by reference to the model output/results, and if the persons engaging in the decisions of political measures may discuss about the output/results with sufficient understanding of the approximations and assumptions imposed in the model, the model can be expected to be used in the consensus building on making the action plan of the coastal management.

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5.1. Construction of the Model

ECOHYM is composed of two models: a hydrodynamics model and an ecological model for the benthic and pelagic systems. The ecological model is generalized to enable its application to both the central bay and tidal flat area. The whole construction of ECOHYM is illustrated in Figure 2. The hydrodynamics model is calculated independently from the ecological model, whereas the ecological model receives input of the flow-temperature field from the hydrodynamics model. Therefore, the physical field calculated in the hydrodynamics model such as advection, eddy diffusion, and temperature, is not affected by the model variables (plankton, detritus, nutrients and dissolved oxygen etc.) treated in the ecological model, but model variables of the ecological model is moved passively by the physical field. More specifically, physical field transports the model variables of the ecological model and changes the biochemical balance among the model variables at any computational grid. As such, hydrodynamics changes the biochemical processes which are formulated by model variables. The concept, “eco-hydrodynamics”, emphasizes the importance of this interaction among the physical field and biochemical processes. The interaction between the central bay and tidal flat areas is also performed through the physical field (flow field and eddy viscosity) calculated by the hydrodynamics model. The physical processes playing the transportation in the sediment or sediment-water interface such as molecular diffusion, irrigation, bioturbation and burial etc. are treated in the ecological model, while not considered in the hydrodynamics model.

Hereinafter, to clarify the definition of central bay and tidal flat areas in the model, the central bay area is defined as that area where the transparency/depth is so low/deep that it hinders benthic algae, sea-grass and sea-weed photosynthesis. The tidal flat area is defined as that area where the transparency/depth is so high/shallow that benthic algae, sea-grass and sea-weed photosynthesis occurs. Areas of submersion/emersion with tidal level shift are included in the tidal flat area. In the eutrophic estuary in Japan, the central bay area usually has a higher potential to become hypoxic than the tidal flat areas.

Ecological Connectivity Hypoxia Model