Advancing knowledge and cooperation for a healthy ocean and prosperous society
Considering the food chain/web in the ocean, the highest-level biological resources are supported by much larger amounts of lower-level biomass. To find the level of the ocean’s sustainable productivity, we need to know how much biomass could be generated through primary production. Primary production is based on the nutrient supply in the ocean’s euphotic zone. Quantitative evaluation of the transport of nutrients into the euphotic zone in a certain marginal sea, which we call the ‘nutrient footprint’ here, provides useful information on the variation in upper-level biological resources. Determining the origin of the nutrients for primary production is one of the most important issues in evaluating the changes in biological production caused by global warming in the marginal seas. Marginal seas are surrounded by multiple countries, and we need to integrate the information obtained by each country, which will require close international cooperation. To achieve common understanding of the nutrient footprint for primary production in the East China Sea (ECS), we coordinate field observations to evaluate the vertical transport of nutrients in the cooperative monitoring areas.
Another target is hypoxic water (or oxygen-minimum zones) formed in the bottom layer in the marginal seas, including coastal waters. Hypoxic waters are found in various regions, such as the ECS, the upper Gulf of Thailand, and the Sea of Okhotsk. However, the process of hypoxic water formation should be universal, while principal factors could depend on the region. In this project, we share the information and tools on the hypoxic waters of the various regions and try to find a way to sustain a healthy marine environment. The behavior of nutrients, dissolved oxygen, and organic matter in the bottom layer will be determined in the relevant areas. Vertical mixing also affects the formation of the bottom hypoxic waters.
The sea surface temperature (SST) in East Asian waters, including the East China and Yellow Seas as well as the Marginal Sea surrounded by Japan, Korea and Russia (MSJKR), is exhibiting a considerable increase at a rate of +1.14°C per century, which is a significantly higher rate than that of the global and North Pacific means (+0.55 and +0.53°C per century, respectively). It is feared that the SST increase may induce the frequent occurrence of meteorological hazards such as abnormally strong typhoons and low-pressure “bomb” events accompanied by strong winds and heavy rain through the increase in water vapor supply to the atmosphere from the oceans. In addition, tropical and subtropical species of not only fishes but also benthic life and shellfish have been frequently reported in the coastal areas of the Pacific Asian marginal seas. In particular, the regime shift in species in the southern MSJKR since the late 1990s is considered to be caused by the northward extension of tropical and subtropical ecosystems, which may influence future fisheries in the region via change in the ecosystem.
The long-term variations due to global warming and climate change bring structural changes in the oceans. It is known that the deep water in the MSJKR has its own thermohaline circulation system that is becoming weaker year by year via the increase in water temperature and increase in stratification, which inhibits vertical mixing. In addition, a decadal-scale variation in temperature has been detected in the deep water of the MSJKR, which is associated with the Arctic Oscillation, one of the well-known climate patterns. Such long-term variations could change the distribution of materials and the biogeochemical cycles through a change in the background flow condition. Indeed, oceanic acidification associated with the CO2 concentration in the sea water has been suggested in the abyssal MSJKR. Clear bi-decadal variations in dissolved oxygen and nutrient concentrations synchronized with the deep-water temperature variation have also been reported. These long-term changes in nutrient distribution may significantly affect the ecosystems in and around the seas.