Takafumi Hirata

Takafumi Hirata
Hokkaido University | Hokudai · Faculty of Environmental Earth Science

PhD

About

61
Publications
20,484
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2,870
Citations
Citations since 2017
23 Research Items
1873 Citations
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2017201820192020202120222023050100150200250300
2017201820192020202120222023050100150200250300

Publications

Publications (61)
Article
Full-text available
The carbon to nitrogen (CN) ratio of phytoplankton connects the carbon and nitrogen cycles in the ocean. Any variation in this ratio under climate change will alter the amount of carbon fixed by photosynthesis, and ultimately the amount sequestered in the ocean. However, a consistent mechanistic explanation remains lacking for observed species‐spec...
Chapter
Differences in morphology, size, and pigmentation among various phytoplankton taxonomic groups impact their light absorption and scattering properties (e.g., Morel and Bricaud, 1981; Stramski and Kiefer, 1991; IOCCG, 2014), which modifies the color of the ocean. Optical satellite remote sensing enables the detection of backscattered sunlight emanat...
Conference Paper
Full-text available
Introduction Chum salmon (Oncorhynchus keta) is one of the important fishery resources in the North Pacific. Recently, under a changing climate, the return of chum salmon in Hokkaido has begun to decrease. More precise juvenile release operations are required for adapting to climatic changes, including warming sea temperature, and this can be accom...
Article
Full-text available
We present the performance of JAXA’s SGLI standard algorithms for estimating chlorophyll a (chl a) concentration and the light absorption coefficients of colored dissolved organic matter (CDOM) using recently compiled high-quality data obtained from oceanic to coastal waters. Prior to the evaluation of the algorithms, we first compare two forward m...
Article
Full-text available
The Kuroshio Large Meander (LM) is known to be highly aperiodic and can last from 1 to 10 years. Since a stationary cold core formed between the Kuroshio and the southern coast of Japan off Enshu-Nada and approaching warm saltier water on the eastern side of the LM changes the local environment drastically, many commercially valuable fish species d...
Article
Full-text available
Subsurface chlorophyll maxima are widely observed in the ocean, and they often occur at greater depths than maximum phytoplankton biomass. However, a consistent mechanistic explanation for their distribution in the global ocean remains lacking. One possible mechanism is photoacclimation, whereby phytoplankton adjust their cellular chlorophyll conte...
Article
The subarctic shelf of the Eastern Bering Sea (EBS) is one of the world's most productive marine environments, exposed to drastic climate changes characterized by extreme fluctuations in temperature, sea ice concentration, timing, and duration. These climatic changes elicit profound responses in species distribution, abundance, and community compos...
Article
Full-text available
Radiometric calibration utilizing the Moon as a reference source is termed as lunar calibration. It is a useful method for evaluating the performance of optical sensors onboard satellites orbiting the Earth. Lunar calibration provides sufficient radiometric calibration opportunities without requiring any special equipment, and is suitable for nano/...
Article
Climate change is triggering a global reorganization of marine life. Biogeographical transition zones, diversity-rich regions straddling biogeographical units where many species live at, or close to, their physiological tolerance limits (i.e., range distribution edges), are redistribution hotspots that offer a unique opportunity to understand the m...
Article
Full-text available
G. E. Hutchinson raised the paradox of how a number of phytoplankton species competing for the same resources are able to coexist in a relatively isotropic environment. As a key for solving the paradox, we focused on the limiting similarity which prohibits the coexistence of similar species. We expected that the limiting similarity will be mitigate...
Article
Full-text available
Spectrally resolved water-leaving radiances (ocean colour) and inferred chlorophyll concentration are key to studying phytoplankton dynamics at seasonal and inter-annual scales, for a better understanding of the role of phytoplankton in marine biogeochemistry; the global carbon cycle; and the response of marine ecosystems to climate variability, ch...
Article
Aim: To investigate the species-specific exposure and distributional responses of marine fish and invertebrate taxa to rapidly shifting climate in the Pacific Arctic, characterized by warming and cooling episodes, over the last 24 years. Location: Pacific Arctic region, eastern Bering Sea and Chukchi Sea. Methods: We examined the variations in th...
Article
Full-text available
Ecosystem models are used to understand ecosystem dynamics and ocean biogeochemical cycles and require optimum physiological parameters to best represent biological behaviours. These physiological parameters are often tuned up empirically, while ecosystem models have evolved to increase the number of physiological parameters. We developed a three-d...
Article
Full-text available
Ecosystem models are used to understand ecosystem dynamics and ocean biogeochemical cycles and require optimum physiological parameters to best represent biological behaviours. These physiological parameters are often tuned up empirically, while ecosystem models have evolved to increase the number of physiological parameters. We developed a three-d...
Preprint
Full-text available
To evaluate the group-specific primary production of diatoms, haptophytes, and cyanobacteria in the Kuroshio region, a novel satellite observation methodology using the SeaStar/SeaWiFS satellite instrument was developed. The method used bio-optical relationships between the group-specific production and bio-optical properties such as the photosynth...
Article
Full-text available
To improve our understanding of the role of phytoplankton for marine ecosystems and global biogeochemical cycles, information on the global distribution of major phytoplankton groups is essential. Although algorithms have been developed to assess phytoplankton diversity from space for over two decades, so far the application of these data sets has...
Article
Full-text available
Phytoplankton are composed of diverse taxonomical groups, which are manifested as distinct morphology, size and pigment composition. These characteristics, modulated by their physiological state, impact their light absorption and scattering, allowing them to be detected with ocean color satellite radiometry. There is a growing volume of literature...
Article
Full-text available
To advance our understanding of competition and coexistence in phytoplankton species within a functional group, such as a diatom group, we developed a numerical model composed of 240 within-tropic-level virtual species that can actually or potentially compete. We then explored how the phytoplankton assembly is structured by deterministic or stochas...
Article
Anthropogenically-induced global warming is expected to decrease primary productivity in the subtropical oceans by strengthening stratification of the water column and reducing the flux of nutrients from deep-waters to the sunlit surface layers. Identification of such changes is hindered by a paucity of long-term, spatially-resolved, biological tim...
Article
Full-text available
Past model studies have projected a global decrease in marine net primary production (NPP) over the 21st century, but these studies focused on the multi-model mean and mostly ignored the large inter-model differences. Here, we analyze model simulated changes of NPP for the 21st century under IPCC's high emission scenario RCP8.5 using a suite of nin...
Article
Full-text available
Past model studies have projected a global decrease in marine net primary production (NPP) over the 21st century, but these studies focused on the multi-model mean and mostly ignored the large inter-model differences. Here, we analyze model simulated changes of NPP for the 21st century under IPCC's high emission scenario RCP8.5 using a suite of nin...
Article
Full-text available
Diatoms are the major marine primary producers on the global scale and, recently, several methods have been developed to retrieve their abundance or dominance from satellite remote sensing data. In this work, we highlight the importance of the Southern Ocean (SO) in developing a global algorithm for diatom using an Abundance Based Approach (ABA). A...
Article
The Sea of Okhotsk is one of the most productive ocean regions in the world. However, the in situ bio-optical properties, which are crucial for satellite ocean-color of the productivity, remain uncertain in this region because little data have been available. We conducted an in situ observation and evaluated the bio-optical properties in terms of c...
Article
Full-text available
Modeling and monitoring plankton functional types (PFTs) is challenged by the insufficient amount of field measurements of ground truths in both plankton models and bio-optical algorithms. In this study, we combine remote sensing data and a dynamic plankton model to simulate an ecologically sound spatial and temporal distribution of phyto-PFTs. We...
Article
Full-text available
We investigated the mechanisms of phytoplankton competition during the spring bloom, one of the most dramatic seasonal events in lower-trophic level ecosystems, in four state-of-the-art Plankton Functional Type (PFTs) models: PISCES, NEMURO, PlankTOM5 and CCSM-BEC. In particular, we investigated the relative importance of different ecophysiological...
Article
Full-text available
We compare the spatial and temporal representation of phytoplankton functional types (pPFTs) in four different Dynamic Green Ocean Models (DGOMs; CCSM-BEC, NEMURO, PISCES and PlankTOM5) to derived phytoplankton distributions from two independent satellite estimates, with a particular focus on diatom distributions. Global annual mean surface biomass...
Article
Full-text available
Geo-engineering proposals to mitigate global warming have focused either on methods of carbon dioxide removal, particularly nutrient fertilization of plant growth, or on cooling the Earth's surface by reducing incoming solar radiation (shading). Marine phytoplankton contribute half the Earth's biological carbon fixation and carbon export in the oce...
Article
Full-text available
Modeling and monitoring plankton functional types (PFTs) is challenged by insufficient amount of field measurements to ground-truth both plankton models and bio-optical algorithms. In this study, we combine remote sensing data and a dynamic plankton model to simulate an ecologically-sound spatial and temporal distribution of phyto-PFTs. We apply an...
Article
Full-text available
Ocean color measured from satellites provides daily, global estimates of marine inherent optical properties (IOPs). Semi-analytical algorithms (SAAs) provide one mechanism for inverting the color of the water observed by the satellite into IOPs. While numerous SAAs exist, most are similarly constructed and few are appropriately parameterized for al...
Article
Phytoplankton functional types (PFTs) derived from a global 3D marine ecosystem model, MEM-OU, are compared with those observed by satellite. The climatological field of the large phytoplankton (diatoms) and small phytoplankton (nano- and picoplankton) showed the model and satellite observation generally agrees in their spatial distribution but sho...
Article
Full-text available
We compared the functional response of a biogeochemical data assimilation model versus an empirical satellite-derived algorithm in describing the variation of four phytoplankton (diatoms, cyanobacteria, coccolithophores and chlorophytes) groups globally and in 12 major oceanographic basins. Global mean differences of all groups were within ~ 15% of...
Article
We investigated the mechanisms of phytoplankton competition during the spring bloom, one of the most dramatic seasonal events in lower-trophic level ecosystems, in four state-of-the-art Plankton Functional Type (PFTs) models: PISCES, NEMURO, PlankTOM5 and CCSM-BEC. In particular, we investigated the relative importance of different ecophysiological...
Article
Using a decade of satellite ocean-colour observations and a model that links chlorophyll-a to the size of the phytoplankton cells, parameterised using pigment data from the Indian Ocean, we examine the implications of the Indian Ocean Dipole (IOD) for phytoplankton size structure. The inferred interannual anomalies in phytoplankton size structure a...
Article
Satellite Phytoplankton Functional Type Algorithm Intercomparison Workshop; Sapporo, Japan, 22-23 November 2011 Satellite observations of ocean color have become synonymous with derivations of chlorophyll a concentration as a proxy for phytoplankton biomass. In addition, a number of satellite algorithms for estimating the phytoplankton community st...
Article
Satellite remote sensing of ocean colour is the only method currently available for synoptically measuring wide-area properties of ocean ecosystems, such as phytoplankton chlorophyll biomass. Recently, a variety of bio-optical and ecological methods have been established that use satellite data to identify and differentiate between either phytoplan...
Article
Full-text available
Error-quantified, synoptic-scale relationships between chlorophyll-a (Chl-a) and phytoplankton pigment groups at the sea surface are presented. A total of ten pigment groups were considered to represent three Phytoplankton Size Classes (PSCs, micro-, nano- and picoplankton) and seven Phytoplankton Functional Types (PFTs, i.e. diatoms, dinoflagellat...
Article
The MarQUEST (Marine Biogeochemistry and Ecosystem Modelling Initiative in QUEST) project was established to develop improved descriptions of marine biogeochemistry, suited for the next generation of Earth system models. We review progress in these areas providing insight on the advances that have been made as well as identifying remaining key outs...
Article
A three-component model was developed which calculates the fractional contributions of three phytoplankton size classes (micro-, nano- and picoplankton) to the overall chlorophyll-a concentration in the Atlantic Ocean. The model is an extension of the Sathyendranath et al. (2001) approach, based on the assumption that small cells dominate at low ch...
Article
An important goal in ocean colour remote sensing is to accurately detect different phytoplankton groups with the potential uses including the validation of multi-phytoplankton carbon cycle models; synoptically monitoring the health of our oceans, and improving our understanding of the bio-geochemical interactions between phytoplankton and their env...
Article
We derived size-specific photosynthetic rates for eastern boundary upwelling regions using satellite ocean colour, by a simple approach that links photosynthetic rate to phytoplankton size classes with the optical absorption of phytoplankton. Comparisons with a conventional chlorophyll-based photosynthetic algorithm and with in situ measurements sh...
Article
The distribution function of the ocean nadir radiance, defined by the upward radiance-to-irradiance ratio, is investigated as functions of the absorption coefficient and the volume scattering function to understand their relationship rather than to develop a numerical algorithm. It is shown for oceanic waters that the distribution function is direc...
Article
Based on the view of ecological pattern being steady state, rather than an equilibrium phenomenon, we assert that, if real, ecological biomes and provinces in the ocean should be detectable in surface fields obtained from satellite data as coherent, co-varying spatial regions with a high degree of permanence. Likewise, hierarchy is an important pro...
Article
An irradiance inversion model to estimate the in situ absorption coefficient of seawater has been developed for the Ultraviolet-A (UVA) wavelength domain. Input parameters are sun angle and the up-and downward planar irradiances measured for at least two depths. The present method does not require seawater to be sampled, and is a discrete wavelengt...
Article
We have developed a model linking phytoplankton absorption to phytoplankton size classes (PSCs) that uses a single variable, the optical absorption by phytoplankton at 443 nm, aph(443), which can be derived from the inversion of ocean colour data. The model is based on the observation that the absolute value of aph(443) co-varies with the spectral...
Article
Full-text available
A new expression for relationship between the surface irradiance reflectance and Inherent Optical Properties (IOPs) of homogeneous seawater is presented. Physical parameters largely responsible for the relationship are identified. These are the average cosines of the backward volume scattering function and of the downward radiance. Lorenz-Mie compu...
Article
Full-text available
We review the concept of phytoplankton functional types (PFTs) in marine ecosystems as a means of advancing bio-mechanistic models that can be coupled to the global carbon cycle and the Earth's climate system. Conventional classification of phytoplankton by size may seem arbitrary, but there appears clear links between size and environmental charac...
Article
Effects of shape of particle scattering phase function on the ocean colour reflectance are examined by means of radiative transfer simulations. The simulations suggest that different shape of particle phase function may cause 19% of discrepancy in the reflectance for oceanic waters, even if the backscattering probability of suspended particles does...
Article
Full-text available
A semianalytical approach to the problem of determining inherent optical properties from satellite and in situ ocean color data is presented. The model uses empirically derived spectral slopes between neighboring wavebands in combination with radiative transfer modeling to determine the spectral absorption (alpha) and backscatter (b(b)); these valu...
Article
An attempt to retrieve the volume scattering function (VSF) of source-free and no-inelastic-scattering ocean water is made from the upwelling irradiance Eu and downwelling irradiance Ed. It will be shown, from the radiative transfer equation, that the VSF of seawater can be calculated by the planar irradiances when the scattering phase function of...

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