Subproject A02 Material Cycling
Group A02-1 Elucidation of Key Processes of Carbon and Nitrogen Cycles in the Ocean
Evaluating ecological services of the ocean, e.g., carbon and nitrogen fixation and production of fisheries resources, requires understanding of the primary structure of biological production, which consists of new production and regenerated production. However, some recent studies have demonstrated that the conventional concept of biological production in the ocean should be greatly reconsidered. Our group focuses on processes in carbon and nitrogen cycles closely related to the marine biological production mechanism. Especially, the following three topics are examined: (1) the roles of nitrogen fixation and nitrification in primary production; (2) the behavior and metabolism of zooplankton; and (3) the microbiological interaction of dissolved organic matter. These cover the essential pathways of marine carbon and nitrogen cycles and include zooplankton, phytoplankton, bacteria, nutrients, and organic matter. Finally, we aim to provide novel information of the structure of marine biological production based on both the fluxes and the mechanisms of carbon and nitrogen cycles in different marine provinces. Subsequently, this information is expected to be combined with other information on the geographical distributions of ecological and biogeochemical parameters and to contribute to both the qualitative and quantitative evaluation of ecosystem services in different marine provinces.
Group A02-2 Studies on new mechanisms regulating marine biological production
A major objective of this study is to elucidate regulation mechanisms of marine phytoplankton productivity. The regulation mechanisms play an important role in marine biogeochemical cycles of several unique provinces in the Pacific Ocean. Our group focuses on interactions among inorganic and organic nutrients, trace metals, and light, as the fundamental regulation factors. A new regulation model of the phytoplankton productivity including co-limitation by the regulation factors will be developed from onboard multi-factorial nutrient manipulation experiments with consistent water property and rate measurements in representative provinces of the Pacific Ocean. It is essential to improve our understanding of the physicochemical speciation of trace metals in seawater as well as the variation in phytoplankton nutrient stoichiometry, because both exert a strong influence on the biological availability and ecological competition for the limiting elements among phytoplankton assemblages. The nutrient acquisition strategy of major phytoplankton groups will be analyzed from molecule and cell to community levels and by considering the time scales of day-night and seasonal changes in the physical environmental factors. The comparison of physiological activity and species composition between surface and sub-surface plankton communities in relation to the availability of light and macronutrients is another important subject that may give us new insight about environmental drivers determining biodiversity, water-column productivity, and response to climate change.
Group A02-3 Long-term change study on biogeochemical cycle and marine ecosystem
Through the retrospective analysis of decadal to multi-decadal observation data sets and model simulation, our group conducts studies on the long-term variation of nutrient cycles and the marine ecosystem on the basin to global scale. Our study goals are (1) to understand the regionally specific long-term variation of nutrients and ecosystem responses to climatic forcing or other global scale environmental change, and to discover the physical/chemical properties that drive the observed regional contrast, and (2) to evaluate how qualitative changes of the ecosystem (e.g., species composition, biodiversity, food web structure), which are caused by various environmental perturbations, could alter the rate of ocean uptake and vertical transport of carbon and consequently affect the global material cycle. Our initial approach is a regional comparison of spatio-temporal variations and trends by using the long-term data collected by ships of opportunity (SOOPs) and satellite observation, with a particular emphasis on the variation of plankton functional type (PFT). By analyzing ecosystem and nutrient data together with other biogeochemical observation data (e.g., dissolved inorganic carbon (DIC), sediment traps) and applying the newly developed 3D-PFT ecosystem model, we aim to understand the link between physical-chemical and biological processes from region to region.