Akio Sohma

A New Coastal Marine Ecosystem Model Study Coupled with Hydrodynamics and Tidal Flat Ecosystem Effect

A new coastal marine ecosystem model was developed, which was composed of pelagic and benthic ecosystems, and was applied to Mikawa Bay, Japan. This model deals with variations of biochemical and physical interactions among dissolved oxygen and C-N-P species (composition formed out of carbon, nitrogen and phosphorus elements) so that it resolves the flux dynamics of carbon, nitrogen, phosphorus and oxygen elements. The physical and biochemical mechanism figured in this model is constructed for the purpose of simulating the estuarine lower trophic ecosystem, in areas where the sea was too deep for light to reach the sea-bottom. As a result of coupling the benthic with pelagic system, the effect of process of sedimentation and nutrient diffusion back to the pelagic system could be indicated. In addition, by implementing the tidal flat ecosystem models calculation result, the integrated model can include the effect of water purification in tidal flats where the light can reach the sea-bottom, and where seaweed, sea grass and benthic algae exist. In this study, the model indicates that oxygen-depleted water exists at the sea-bottom especially in summer mainly caused by an increase of oxygen consumption in the benthic system and a decrease of the vertical mixing water process. Furthermore, by comparing the case--with the tidal flat ecosystem model and the case without it, the effect of water purification of tidal flat estuaries was indicated. From the viewpoint of a short time scale, the tidal flat has the potential to restrict red tide (rapid increase of phytoplankton), and from the viewpoint of a long time scale, it restricts the sedimentation of detritus. Restricting the sedimentation prevents oxygen-depleted water occurring in the coastal marine system of Mikawa Bay.

Blue carbon in human-dominated estuarine and shallow coastal systems

AbstractEstuarine and shallow coastal systems (ESCS) are recognized as not only significant organic carbon reservoirs but also emitters of CO2 to the atmosphere through air–sea CO2 gas exchange, thus posing a dilemma on ESCS’s role in climate change mitigation measures. However, some studies have shown that coastal waters take up atmospheric CO2 (Catm), although the magnitude and determinants remain unclear. We argue that the phenomenon of net uptake of Catm by ESCS is not unusual under a given set of terrestrial inputs and geophysical conditions. We assessed the key properties of systems that show the net Catm uptake and found that they are often characteristic of human-dominated systems: (1) input of high terrestrial nutrients, (2) input of treated wastewater in which labile carbon is highly removed, and (3) presence of hypoxia. We propose that human-dominated ESCS are worthy of investigation as a contributor to climate change mitigation.

New Numerical Model Study on a Tidal Flat System – Seasonal, Daily and Tidal Variations

Abstract We developed a new numerical model to investigate the dynamics of tidal flat ecosystems and their role in water quality in terms of the carbon cycle. This model was applied to Isshiki, a natural tidal flat area, which is the largest in Mikawa Bay, Japan. This model dealt with variations of biochemical or physical interaction among dissolved oxygen and C–N–P species (comprised of carbon, nitrogen, and phosphorus elements) both on a short-time scale ( 4 –N, NO x –N, and PO 4 –P are more sensitive to daily environmental variation than to seasonal environmental variation. This means that the rotation speed of these materials in the tidal flat area is fast. Here, we defined the rotation speed as the ratio of total fluxes of substance to the mass of the substance. Phytoplankton with a high rotation speed in the tidal flat area means that the tidal flat has the potential to recover from rapidly increasing phytoplankton: red tide. The model also indicated that the peculiar feature of the tidal flat is the mineralization of organic material. The effect on a long term base, is that it prevents the accumulation of sediment, which results in controlling the increase of oxygen consumption in benthic system, which is the cause of oxygen depleted water.

CO2 Uptake in the Shallow Coastal Ecosystems Affected by Anthropogenic Impacts

Shallow coastal ecosystems (SCEs) are generally recognized as not only significant organic carbon reservoirs but also as sources for CO2 emission to the atmosphere, thus posing a dilemma regarding their role in climate change mitigation measures. However, we argue that SCEs can act as sinks for atmospheric CO2 under a given set of biogeochemical and socioeconomic conditions. The key properties of SCEs that show net uptake of atmospheric CO2 are often characteristic of human-dominated systems, that is, high nutrient inputs from terrestrial systems, input of treated wastewater in which labile carbon has been mostly removed, and the presence of hypoxic waters. We propose a new perspective on the potential of human-dominated SCEs to contribute to climate change mitigation, both serving as carbon reservoirs and providing direct net uptake of atmospheric CO2, in light of human systems–ecosystem interactions. Namely, if we view the land and a SCE as an integrated system, with appropriate management of both wastewater treatment and SCE, we will be able to not only suppress CO2 release but also capture and store carbon.