Modeling and evaluating the ecosystem of sea-grass beds, shallow waters without sea-grass, and an oxygen-depleted offshore area

Abstract To investigate the ecological mechanism of sea-grass beds (area 1), shallow waters/tidal flats without sea-grass near the mouth of a river (area 2), and an oxygen-depleted offshore area (area 3), we developed a numerical model, which could represent the biochemical and physical processes of the coastal marine ecosystem comprised of the above three areas. The model can represent the dynamics of coupled cycle of carbon, nitrogen, phosphorous, and oxygen, dividing organic matter into five compartments viz: fast-labile POM, slow-labile POM, refractory POM, labile DOM, and refractory DOM. In addition, this model formulated three bacterial mineralization processes: oxic mineralization, suboxic mineralization, and anoxic mineralizaition. This model was applied to the “Jinno area (Jinno ecosystem)” of Atsumi Bay, Japan, where three types of areas (areas 1, 2, and 3) are all represented. The model reproduced the dynamics of the present field condition of the three areas accurately. From the analysis of the model results, we could indicate the characteristic processes of nutrient cycling in each different areas of an ecosystem. Biological fluxes, driven by suspension feeders, epiphytes and epifauna, were the main fluxes in area 1. Biological fluxes were also the main fluxes in area 2, but were largely driven by suspension feeders. In addition, physical fluxes of re-suspension in area 2 stood out compared to the other areas. Denitrification in area 2 is much larger than in areas 1 and 3. This is because the ratio of suboxic mineralization per all mineralization in that area is higher than in other areas. We also estimated the turnover rate of biological processes and physical processes in each area of the Jinno ecosystem to quantify the characteristics of nutrient cycling in each area. The turnover rate of biological or physical processes was defined as the ratio of the total biological or physical fluxes of a compartment to the mass of a compartment. The turnover rate of areas 1 and 2 were higher than area 3. The high turnover rate in area 1 is mainly due to the biological processes, whereas biological and physical processes were both important in area 2. This result means that the important driving force of nutrient cycling in sea-grass beds are the biological processes. Understanding the characteristics of the processes that cycle matter (carbon, nitrogen, phosphorus, and oxygen) through an ecosystem becomes the first step in drafting an environmental management plan.

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