As surface mixing begins in autumn, the subsurface DO maximum rapidly goes deeper level, in particular, it is found below the euphotic zone in the Tsushima Warm Current (TWC) region (below 100 m). The weakened photosynthetic production and intrusion of the low-oxygen Tsushima Warm Water (TWW) into depths between 50 and 100 m moves the peak DO concentration to below the euphotic zone in the TWC region.
The low-oxygen properties of the TWW formed passing through the East China Sea (ECS) are not realistically simulated without consideration of the biogeochemical activities.
Below the euphotic zone, the decomposition of detritus (DET) consumes the DO concentration all the year round. The prominent biological reduction in DO concentration was found in the TWC region at the depths below 150 m because the water is poorly ventilated by the relatively shallow MLD. The negative biological effect on the DO concentration outcrops to the surface by the winter deep convection in the SPG region.
The improved performance of the DO variations identified in Chapter 2 allowed to realistically represent the long-term simulation of the physical and biogeochemical environments in the lower layers. The possible causes leading the long-term decrease in DO concentration of the lower layers were quantitatively evaluated in Chapter 3: 1.
Cessation of the new water supply (“P×B”); 2. Enhancement in biological decomposition by the increase in falling organic materials (“B”); 3. Enhancement in vertical advection of
It is clarified that the biological production at the upper layer of the EJS has been enhanced during the last few decades. The enhancement in the biological activities was caused by the intensification of the TWC. The strengthening of the TWC induced the deepening of the mixed layer in the path of the Offshore Branch and the Liman Cold Current attributed to the increase in temperature and salinity. The deepening of the MLD increased the DIN concentration available in the euphotic zone and led to the enhancement of photosynthetic production.
The rise in biological production in the upper layer has increased the sedimentation of organic matters toward the lower layer and it finally accelerated the decline in DO concentration by encouraging the decomposition process. This “B” was identified to decrease the DO concentration by 2.0×10−2 ml l−1 over the deep and bottom waters from 1990 to 2012. It accounts for 7% relative to the total decline in DO concentration. In addition to changes in the biogeochemical environment, the variation in physical conditions with the long-term perspective also influences on the DO trend. The physical processes without biological decomposition result in the increase in about 35% of the total reduction in DO concentration over the lower layer (“P”).
The decrease in DO concentration of 128% relative to the total decrement is attributed to the first possible cause, that is the fundamental biological consumption with the passage of time under the supply of new water almost ceased (“P×B”). Therefore, the cessation of the deep convection is the crucial cause of the long-term decrease in the DO concentration in the deep layer. However, we found the changes in physical and biogeochemical environments can decelerate or accelerate the decline in DO concentration from a few to tens of percent in the deep layer.
It may be instructive to discuss the future change in oxygen environment as a natural extension of the present coupled model. The bottom water is expected to become anoxic in about 400 years (5.1 ml l−1 / 0.013 ml l−1 year−1) from 1990, if the DO concentration is continuously declined by the present decreasing trend. This prediction is almost as the same scale as that of Gamo (1999), who forecasted the bottom water will be anoxia within 300 years. We need to monitor consistently the variations in the ocean environment to prepare the depletion of the DO concentration.
The expansion of hypoxia and oxygen minimum zones due to sustained decreases in DO concentration has come up as a major problem in coastal water and open oceans around the world. Hypoxia is typically defined by a range from 1.4 to 2.0 ml l−1 of the DO concentration (IPCC, 2013; Hofmann et al., 2011). The deep waters of the EJS are expected to be hypoxic in a few hundred years if the current decline rate of the DO concentration continues. The hydrologic and biogeochemical changes will delay or advance hypoxia by the several years to decades.
Hypoxia leads to reduction in marine diversity and harmful effects on the organisms such as decrease in growth and reproduction, forced migration, decline of suitable habitat, increased vulnerability to predation, and disruption of life cycles (Vaquer-Sunyer and Duarte, 2008). The realistic prediction of the DO trend by the coupled model can support the preparation of countermeasures for hypoxic waters.
an important role to accumulate carbon in the ocean. Thus, the numerical model, providing the realistic distribution and trend of the DO concentration, can be further used to estimate the long-term fluctuations in export flux of carbon and thereby predict the ocean's ability to control carbon dioxide.
Feely et al. (2004) estimated the export flux of carbon using Oxygen Utilization Rate (OUR) and Redfield ratio in the Pacific Ocean. They calculated OUR by the ratio of apparent oxygen utilization (AOU) representing the biological consumption of oxygen to water mass age estimated by using pCFC-12 method. The multiplication of OUR and Redfield ratio (C:O = 117:170, that is, Rc:o = 0.688) provides Organic Carbon Remineralization Rate (OCRR). The export flux of carbon is obtained by the vertical integration of OCRR. The application of this method to our results would enable to assess the carbon storage capacity of the EJS, and it remains one of the future studies.
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