Jota Kanda

Tracking the Fukushima Radionuclides

Ongoing radionuclide monitoring and tracking efforts are required following the nuclear accident at the Fukushima Daiichi Nuclear Power Plant. On 11 March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) lost cooling capability during the magnitude-9.0 Tohoku earthquake and the subsequent tsunami (1). The incident led to severe damage of the plant and the release of large amounts of radionuclides to the environment. Local contamination still prevents over 100,000 residents from returning to their homes. Detailed maps are beginning to provide a picture of the contamination patterns (see the first figure), but as radionuclides migrate and diffuse through the environment (see the second figure), continual monitoring is required to guide remediation and ensure human safety.

Seasonal phytodetritus deposition and responses of bathyal benthic foraminiferal populations in Sagami Bay, Japan: preliminary results from “Project Sagami 1996–1999”

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.

New production and photosynthetic rates within and outside a cyclonic mesoscale eddy in the North Pacific subtropical gyre

Abstract Photosynthetic rates, autotrophic nitrogen assimilation, and the uptake and assimilation of nitrate were measured at three stations in the center of, near the edge of, and outside of a cyclonic mesoscale eddy near the island of Hawaii. Areal photosynthetic rates near the edge and at the center of the eddy were 31% and 66% higher, respectively, than outside the eddy. Nitrate uptake was equated to the drawdown of nitrate in incubated samples. Nitrate assimilation was calculated from the incorporation into particulate matter of 15 N derived from K 15 NO 3 . Ambient nitrate concentrations were about 8 nM, and uptake of nitrate was undetectable above the nitracline. Addition of 55 nM K 15 NO 3 spikes to samples taken above the nitracline stimulated an uptake and assimilation of nitrate which averaged 20–24 nM day −1 . The stimulated nitrate uptake occurred at comparable rates during the day and night, but almost all stimulated assimilation occurred at night. Much of the stimulated nitrate uptake which occurred during the day was apparently excreted in a reduced form, most but not all of which was assimilated at night. In deeper samples that contained greater than 350 nM nitrate, almost all assimilation of nitrate occurred during the day. Nitrate uptake outside the eddy and the upward diffusion of nitrate outside the eddy were estimated to be 10–15 mg N m −2 day −1 . The f -ratio was estimated to be 0.2 outside the eddy and 0.8 at the center of the eddy. Relative growth rates were about 70% outside the eddy and either nutrient-saturated or nearly so within the eddy. Areal chlorophyll a (chl a ) concentrations were about 32% higher at the center of the eddy than outside. The differences in photosynthetic rates between the three stations could be largely explained by the differences in relative growth rates and chl a biomass. In samples taken from below the top of the nitracline, dark assimilation of 14 C increased calculated photosynthetic rates by 25–35%, the percentage increase being positively correlated with nitrate concentration. No comparable dark assimilation was observed in samples taken from above the nitracline. This dark assimilation may reflect the release and subsequent uptake of carbon assimilated by phytoplankton exposed to recently upwelled water.

Determination of ammonium in seawater based on the indophenol reaction with o-phenylphenol (OPP)

Abstract A method for the determination of ammonium in seawater is described. The method is based on the indophenol (or the Berthelot) reaction with o-phenylphenol [(1,1′-biphenyl)-2-ol, abbreviated as OPP] as an alternative of phenol. Unlike phenol, OPP is available as stable flaky crystal with no caustic odor. This nature of OPP can eliminate much of the inconvenience associated with the use of phenol. The sensitivity of detection is comparable to that of the common phenol-based methods. The indophenolic compound obtained from the reaction with OPP is more hydrophobic than that from the reaction with phenol. Thus the indophenolic compound can be extracted directly from the alkaline reaction solution of the present method either with organic solvent or with reversed-phase column. The extracted indophenol can be used to recover ammonium nitrogen for the determination of 15-N isotope ratio. The experimental procedure of the extraction of indophenolic compound is relatively simple compared to the other existing methods.

Seasonal and annual variation in the primary production regime in the central part of Sagami Bay

Abstract Seasonal variations in the primary production regime in the upper water column were assessed by shipboard observations using hydrocasts and natural fluorescence profiling at a fixed station in the central part of Sagami Bay, Japan. The observations were conducted as a part of ‘Project Sagami’ dedicated to the interdisciplinary study of seasonality in bathyal benthic populations and its coupling with water column processes. Based on the time-series observations at intervals of about 1 to 2 months, primary productivity in terms of chlorophyll abundance appeared to be elevated during the spring of 1997, but the observed peaks of biomass were much less significant in the spring of 1998. Meanwhile, the organic matter flux, as indicated by sediment trap data and benthic observations, had a significant peak in the spring of 1998 as well, and its magnitude was comparable to that in 1997. Satellite images of ocean color obtained during the spring of 1997 indicate the importance of events with time scales much shorter than a month, and suggest qualitative differences in the phytoplankton community in the euphotic zone for each bloom event during this period. The possible mechanisms that could yield the spring maximum of material input to the benthic community are discussed.

Long-term monitoring of the sedimentary processes in the central part of Sagami Bay, Japan: Rationale, logistics and overview of results

Abstract Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic–pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic–pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic–pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment–water interface were also documented.

Particle dynamics in the deep water column of Sagami Bay, Japan. I: origins of apparent flux of sinking particles

Abstract Temporal variations of sinking particle flux, together with their organic chemical properties, were monitored in the deep basin of Sagami Bay, Japan, using sediment traps with very high time resolutions from March 1997 to August 1998. At a height of 350 m above the bottom (about 1200 m water depth), the averaged total mass flux was more than 1000 mg/m2/day, which is about 10 times higher than those obtained for open ocean regions near Sagami Bay. While large amounts of phytodetritus, derived from phytoplankton blooms in the surface water, were transported downward in spring, the following extraordinary patterns in the temporal variability of sinking particle flux were also observed: (1) A sustained large flux of sinking particles during low productive periods from summer to winter in 1997. (2) An episodic increase of sinking particle flux in June 1998. (3) A difference in the temporal variability of sinking particles between the spring bloom periods of 1997 and 1998. The content of total organic carbon (TOC) and the stable carbon isotopic ratio (δ13C) of TOC demonstrated that the large fluxes observed in (1) and (2) could be attributed to the resuspension of phytodetritus deposited on the sea floor during the spring bloom period, and the abrupt erosion of surface sediment on the continental slope, respectively. The concentration of suspended particles in the deep water column affect the apparent flux of sinking particles. At the same time, sinking particles exported from surface waters during the spring bloom both decrease and increase suspended particle concentration through scavenging and rebound processes, respectively. Finally, the apparent difference in sinking particle flux between 1997 and 1998, (3), could be explained by differences in the extent of the scavenging process, which depend on the flux and quality of exported particles from the surface waters.

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay, Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by 15N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (KI), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high KI, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower KI). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, KIvalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.

Specific growth rates and grazing mortality of chroococcoid cyanobacteria Synechococcus spp. in pelagic surface waters in the sea

Abstract Specific growth rates and grazing mortality of phycoerythrin-dominant (PE-dominant) Synechococcus spp. populations have been evaluated in oceanic surface waters off Japan. Diel changes in cell numbers and frequency (patterns) of dividing cells (FDC) of the Synechococcus population indicated that cell divisions occurred throughout the day and that a maximum is reached at dusk. The specific growth rate of the Synechococcus population determined by diffusion chamber cultures in which the micrograzers were removed by 1-μm Nuclepore filters, and by the FDC method, was as high as 0.10·h −1 which can double the population size in 10 μm) and less abundant heterotrophs (mostly ciliates) determined by diffusion chamber cultures was 0.04 and 0.06 · h −1 , respectively. As a result, the growth rates and grazing mortality are expected to be balanced and the PE-dominant Synechococcus maintained its population size at a certain low level in the study areas.

Release of Pu Isotopes from the Fukushima Daiichi Nuclear Power Plant Accident to the Marine Environment Was Negligible

Atmospheric deposition of Pu isotopes from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has been observed in the terrestrial environment around the FDNPP site; however, their deposition in the marine environment has not been studied. The possible contamination of Pu in the marine environment has attracted great scientific and public concern. To fully understand this possible contamination of Pu isotopes from the FDNPP accident to the marine environment, we collected marine sediment core samples within the 30 km zone around the FDNPP site in the western North Pacific about two years after the accident. Pu isotopes ((239)Pu, (240)Pu, and (241)Pu) and radiocesium isotopes ((134)Cs and (137)Cs) in the samples were determined. The high activities of radiocesium and the (134)Cs/(137)Cs activity ratios with values around 1 (decay corrected to 15 March 2011) suggested that these samples were contaminated by the FDNPP accident-released radionuclides. However, the activities of (239+240)Pu and (241)Pu were low compared with the background level before the FDNPP accident. The Pu atom ratios ((240)Pu/(239)Pu and (241)Pu/(239)Pu) suggested that global fallout and the pacific proving ground (PPG) close-in fallout are the main sources for Pu contamination in the marine sediments. As Pu isotopes are particle-reactive and they can be easily incorporated with the marine sediments, we concluded that the release of Pu isotopes from the FDNPP accident to the marine environment was negligible.