The overall theme of my research is working at the interface between land
and ocean to understand the various effects of human activities on marine
life in subtropical and tropical areas. Demographic and social trends suggest
that eutrophication of coastal waters is likely to occur in coming a few
decades in the developing countries of Asia, Africa and Latin-America, most
of which are located in tropical and subtropical areas. The extensive studies
on nitrogen cycles at these areas, therefore, will be crucial to evaluate
the dominated mechanisms leading to eutrophication and simultaneous succession
of the biotic communities. I'm pursuing the study of interdiscipline; marine
chemistry, phisiology of marine plants, ecology of marine life, hydrology
in land, human sociology, and so on. Current research interests are followings.
- Groundwater discharge from adjacent terrestrial
areas is a potentially important nutrient source to coral reefs, since
adjacent lands are often overlaid with permeable bedrock such as limestone.
However, the estimates of terrestrial nitrogen loading, especially through
groundwater, had large uncertainties due to their episodic flowing in
space and time. I've been coping with this topic, combining traditional
technique at land areas (per-capita & land-use method, water balance &
nutrients concentrations determination in groundwater within the watershed)
and geochemical technique at ocean (e.g., 222Rn).
- Terrestrial nitrogen supplied via groundwater
or rainfall often arrives in pulses and is spatially and temporally variable.
Therefore, the measured short-term distribution of nitrogen concentrations
in the water column does not necessarily represent the long-term influence
of terrestrially derived materials. Further, nutrient budget model, in
which the contribution of several nutrients sources into the ecosystem
are estimated, does not indicate real nutrient sources for plants communities
in the ecosystem. On the other hand, stable nitrogen isotope ratio (d15N
values) in benthic macroalgae can give us great information about what
kinds of nitrogen is actually used by plants in the ecosystem, when d15N
values in focused N sources have distinct values from other sources. By
the other environmental factors with such chemical signatures, furthermore,
we would have better understanding about the factors controlling nutrients
supplies for the marine plants.
- The d15N signatures in macroalgae are variable
depending on the algal species, even if they are growing at same environment.
Isotope discrimination generally occurs through chemical reactions, because
a lighter isotope reacts more readily than a heavier isotope. For macroalgae,
it results in the lighter d15N of the tissue less than that of their N
sources, especially when the stock of algal internal N pool exceeds their
N demands and then remained 15N-enriched N leaks out of the cells. I'm
exploring this balance of N demands and N supplies, and consequent shift
of d15N values for each alga under various environmental conditions using
laboratory incubation system.
- Mangrove swamps, coral communities and seagrass
beds provide many important ecological functions, such as the substrate
for epiphyte and benthic fauna, and nursery ground for juvenile fish.
Further, their large capacities to absorb nutrients and attenuate water
motion take a role in buffering nutrient (DIN/P and POM) conditions in
water column. I've been conducting time-series monitoring around these
communities to investigate the significant role of these communities controlling
nutrients cycles at coastal waters in tropical and subtropical areas.
- In deep waters such as atoll and barrier reef, primary production and
their decomposition in water column are important process to better understand
nitrogen cycles in the system. I compared photosynthetic production in
water column with those in surface sediments at barrier reef in Palau
island, using O2 evolution methods and 13C-enriched incubation technique.
- The quantity and characteristics of this suspended particulate matter
(SPM) in the estuarine waters temporally and spatially fluctuate by an
order of magnitude, and influence a number of biological processes, including:
(i) food availability for bivalves and (ii) light availability for seagrasses.
Therefore, it is important to understand the mechanisms controlling the
increase of SPM and the shift of its qualities in shallow estuaries, especially
at the same resolution at which differences in species composition and
morphological characteristics of benthic organisms can be distinguished.
In addition to river water input, I've demonstrated seafloor topography
dependent mechanisms cause resuspension and temporal shifts of suspended
particulate matter characteristics in shallow estuaries.
- Ongoing deteriorations of corals due to the global and local scales-disturbances
have greatly changed species compositions at coral reefs. In addition
to the nutrients dynamics in ecosystem level, I'm also interested in nutrients
dynamics in community level. Natural abundance of 15N and 13C, and fatty
acids compositions in organisms existing in the local communities are
effectively employed to follow the pathway of energies. 15N and 13C dilution
method are also used in laboratory incubations.
I was engaged in an integrated research project "Human impacts
on Urban Subsurface Environments" conducted at Research Institute for
Humanity and Nature, Kyoto, Japan. Most global environmental studies have
long been focused on the environmental issues above the ground surface
such as air pollution, global warming, seawater pollution, and decrease
in biodiversity. Although subsurface environments are also important for
human life in the present and future, the issues have been largely ignored
because of the invisibility of the phenomena and difficulty of the evaluations.
The primary goal of this project is to evaluate the relationships between
the stages of development of cities and various subsurface environmental
problems, including extreme subsidence, groundwater contamination, and
subsurface thermal anomalies. Therefore, we are focusing on 7 Asian
mega-cities, Tokyo, Osaka, Seoul, Taipei, Manila, Bangkok, Jakarta, which
are located in different developing stages.
- Nitrate (NO3-) contamination in groundwater is severe problem in the
subsurface environments at developing mega cities, as well as other chemical
and metal contamination. Spatial and temporal shift of the source of NO3-
pollutions and the potential of NO3- reduction in the subsurface environments
were investigated using 15N and 18O signatures in NO3- at targeted Asian
cities. Combined use of multiple tracers of both radioactive isotopes
and stable isotopes, and other geochemical components like CFCs are also
introduced to understand hydrological processes. I hope that I can trace
the historical change of pollutants and their sources by collecting the
groundwater from different depth layer.
- Fresh groundwater is important pathway to bring land-derived nutrients
to coastal waters. However, uncontrolled groundwater pumping during these
decades at the developing cities has caused severe subsidence throughout
the city areas, and has dramatically changed hydraulic potential in the
aquifers. It is likely that contribution of fresh-groundwater-derived
nutrients to the demand of coastal ecosystem is different depnding on
the management of groundwater at land areas. (cf, at shallow coastal areas,
the term ''groundwater'' include both fresh water with land-derived nutrients,
and recirculated seawater with marine-born nutrients including mineralized
ones in sediments.). To investigate whether fresh groundwater fluxes exist
along the coastal line, and to quantify the flux of groundwater and associated
chemical components into the water, combined approaches (seepage meter,
resistivity monitoring, isotope tracers in water and nutrients) are introduced
in my study.
- Geographical Information System (GIS) is used to integrate huge volume
of data set from socioeconomic data to material data in water. Visualized
information facilitates us to understand the interaction among environmental
factors. I plan to release the results of my survey on the browser basis.
- Land-derived materials and marine-born phytoplankton sink to lower layer and accumulate in a sequence on the sediment. Therefore, vertical chemical profiles in the sediment cores are used as a proxy to show environmental shift in the past. We reconstruct past environments in Asian coastal areas using C, N and P contents and their stable isotopes.
The supply of nutrients (N, P and Si) and trace metals into the surface ocean is one of important factors to control phytoplankton biomass and species, and subsequently consumers in upper trophic level. We are studying about material cycles at East China Sea (ECS) and Japanese coastal areas (especially Kyusyu area, western Japan), which are one of the most damaged ecosystems in the world, combining stable isotopes techniques and molecular structure analysis. Based on the scientific data and discussions, we hope to propose appropriate management for the sustainable fishery resources and conservation of marine ecosystems.
Nagasaki area faces to East China Sea and is suffered from yellow sand (fine sand from the desert in China and Mongolia) and anthropogenic dust. Theses atmospheric depositions has negative impact on human health, while nutrients and trace metals contained in the dust has potential to enhance phytoplankton growth at the oligotrophic waters. We are surveying the source identification of atmospheric depositions and calculating their fluxes to the ocean using back trajectory analysis and chemical analyses (isotopes of H, O, C, N, Nd and Sr) in the particles collected in aerosol sampler.
Atmospheric depositions is important nutrients source for the phytoplankton growth in addition to the land-derived one, diffusion fluxes from sediment and upwelling water.From the viewpoint of nutrient supply, we check the main factor controlling the primary production in the coastal waters, using chemical analyses of sinking particles collected in sediment trap.
Nutrient fluxes from Changjiang River are important sources to support primary production in East China Sea. However construction of Three Gorge Dam (the biggest dam in the world) and South-North Water Transfer Project (canal from Changjiang River to Yellow River) would largely shift (decrease) river water and associated nutrient fluxes to East China Sea. Especially, silicate and phosphorus input have been decreasing compared with nitrogen fluxes. We are studying about the impact of alternative nutrient supplies (other new nutrient sources and the use of DOM through microbial loop) combining stable isotopes in N and O isotopes in nitrate, O isotope in phosphate, DON/P analyses. Moreover, specific mechanisms for phytoplankton to adapt nutrient depleted conditions are another important topic in near future.
At inner bay areas, phytoplankton bloom tend to occur at lower euphotic zone where nutrient diffused from the sediment reach, especially when nutrients in the surface layer was depleted due to lesser precipitation in dry season. We are evaluating each nutrient flux using temperature, oxygen and the amount of organic matter in surface sediment. Both in situ experiments and laboratory incubation experiment are conducted to evaluate these fluxes.