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The effects of diatom abundance and biogenic silica (BSi) deposition on the population growth of tintinnid ciliates were investigated using incubations of sub-surface waters (10 m depth) of Suruga Bay, Japan in April and November 2018. Seawater for incubation was pre-filtered using a 200-µm mesh to remove mesozooplankton grazers. Initial concentrations of dissolved inorganic nitrogen were similar in both seasons, but silicic acid and phosphate were higher in April than in November. This allowed us to observe the growth of planktonic communities under varied silicic acid availability and explore the effects of BSi accumulation and food availability on the growth of tintinnids. Phytoplankton blooms dominated by diatoms formed in both, April and November incubations, but their biomass was higher in April than in November, demonstrating the effects of nutrients, particularly silicic acid, on diatom growth. The BSi concentration increased in both incubations but was higher in April than in November, confirming the effects of diatom abundance and size on BSi deposition. Tintinnid biomass also increased in both incubations but was higher in April than in November, showing that the biomass of tintinnid was affected by the BSi concentration. Species composition of diatoms affected the species composition of tintinnids by providing selected material for the construction of their lorica. Moreover, the results showed that the food supply mainly provided by pico-nanoflagellates via microbial loop and detritus may limit tintinnids’ population growth in natural planktonic ecosystems and this demonstrates tintinnids’ role as top predators in the microbial loop

Ocean surface acidification due to increasing atmospheric CO2 concentration is currently attracting much attention. Coccolithophores distribute widely across the world's oceans and represent a carbon sink containing about 100 million tonnes of carbon. For this reason, there is concern about dissolution of their shells, which are made of calcium carbonate, due to decreasing pH. In this study, intracellular calcification, photosynthesis, and mass transport through biomembranes of Emiliania huxleyi were modelled numerically for understanding biological response in calcifying organisms. Unknown parameters were optimised by a generic algorithm to match existing experimental results. The model showed that the production of calcium carbonate rather than its dissolution is promoted under an acidified environment. Calcite remains at saturation levels in a coccolith even when it is below saturation levels in the external seawater. Furthermore, a coccolith can dissolve even in water where calcite saturation exceeds 1, because the saturation may be below the threshold level locally around the cell membrane. The present model also showed that the different calcification rates of E. huxleyi with respect to rising CO2 concentrations reported in the literature are due to differences in experimental conditions; in particular, how the CO2 concentration is matched. Lastly, the model was able to reproduce differences in calcification rates among coccolithophore species. The above biochemical-kinetic model was then incorporated into an ecosystem model, and the behaviour of coccolithophores in the ecosystem and the influence of increases in CO2 concentration on water quality were simulated and validated by comparison with existing experimental results. The model also suggests that increased CO2 concentration could lead to an increase in the biomass ratio of coccolithophores to diatoms at high CO2 concentrations, particularly in oligotrophic environments, and to a consequent decrease in pH due to calcium dissolution.

Chlorophyll a fluorescence is increasingly being used as a rapid, non-invasive, sensitive and convenient indicator of photosynthetic performance in marine autotrophs. This review presents the methodology, applications and limitations of chlorophyll fluorescence in marine studies. The various chlorophyll fluorescence tools such as Pulse-Amplitude-Modulated (PAM) and Fast Repetition Rate (FRR) fluorometry used in marine scientific studies are discussed. Various commonly employed chlorophyll fluorescence parameters are elaborated. The application of chlorophyll fluorescence in measuring natural variations, stress, stress tolerance and acclimation/adaptation to changing environment in primary producers such as microalgae, macroalgae, seagrasses and mangroves, and marine symbiotic invertebrates, namely symbiotic sponges, hard corals and sea anemones, kleptoplastic sea slugs and giant clams is critically assessed. Stressors include environmental, biological, physical and chemical ones. The strengths, limitations and future perspectives of the use of chlorophyll fluorescence technique as an assessment tool in symbiotic marine organisms and seaplants are discussed.

Urea is an unstable and intermediate organic nitrogenous compound present in coastal environments and is derived from the excretion of some aquatic organisms and wastewater discharges. Urea plays an important role in the nitrogen cycle, where it is utilized by algae, including diatoms. However, there are very limited relevant data on the production, consumption, and degradation of urea because of the lack of appropriate measurement techniques. The conventional method is based on the formation of a colored product when urea reacts with diacetyl monoxime in a sulfuric acid solution. We examined the optimal conditions for the formation of the colored product; specifically, we evaluated different temperatures (22–80 °C), reaction times, mixing ratios of color reagents, and sample storage times. Application of the single mixed color-developing reagent (COLDER) at 70 °C resulted in the optimal formation of the colored product within a short reaction time of 60 min. This method was then used to measure dissolved urea in different coastal environments. The concentrations detected were as follows: 0.65–0.72, 0.49–0.58, and 1.09–2.28 µM urea-N at coral reef, seagrass, and mangrove sites, respectively. Our results showed high precision (SD = 0.02, CV = 1.2%), a low detection limit (0.03 µM urea-N), and a high recovery rate (94–99%). In summary, this high-temperature procedure for urea measurements should be valuable for obtaining high-precision data that can further the understanding of urea dynamics and its role in coastal ecosystems.

Diversion of water for irrigation from the Yellow River has impacted groundwater quality in the North China Plain (NCP). In this study, by using geochemical and isotope methods, groundwater origin, the spatial distribution of trace metals, pollution sources , and health risks were evaluated. Groundwater is recharged from surface water. The primary pollution components in surface water were B, Al, Se, Zn, Mn, and Ni-with concentrations exceeding standards by 100%, 84.4%, 64.4%, 31.1%, 20%, and 2.2%, respectively. In groundwater, exceeding standard rates for those elements were 100%, 100%, 61%, 25.7%, 39%, and 4.9%, respectively. The spatial distribution of polluted metals in surface water and groundwater was correlated, indicating an irrigation-influenced spatial distribution of trace metals in the groundwater. The trace metals were introduced via anthropogenic and geogenic activities. Zinc poses the most serious non-carcinogenic hazard for local residents. The ingestion pathway is much more likely to lead to zinc toxicity than the dermal absorption pathway. The carcinogenic Cd and Pb could result in an increased cancer risk for individuals exposed to the water. Non-carcinogenic hazard and carcinogenic risk attributable to groundwater is serious in the regions traversed by the rivers in the study area.

Irrigation projects have diverted water from the lower reaches of the Yellow River in China for more than 50 years and are unique in the world. This study investigated the effect of irrigation practices on the transfer and regional migration mechanisms of nitrate (NO3(-)) in surface water and groundwater in a Yellow River alluvial fan. Hydrochemical indices (EC, pH, Na(+), K(+), Mg(2+), Ca(2+), Cl(-), SO4(2-), and HCO(3-)) and stable isotopic composition (δ18O and δD) were determined for samples. Correlation analysis and principal component analysis (PCA) were performed to identify the sources of water constituents. Kriging was employed to simulate the spatial diffusion of NO3(-) and stable isotopes. Our results demonstrated that the groundwater exhibited more complex saline conditions than the surface water, likely resulting from alkaline conditions and lixiviation. NO3(-) was detected in all samples, 87.0% of which were influenced by anthropogenic activity. The NO3(-) pollution in groundwater was more serious than the common groundwater irrigation areas in the North China Plain (NCP), and was also slightly higher than that in surface water in the study area, but this was not statistically significant (p > 0.05). In addition, the groundwater sites with higher NO3(-) concentrations did not overlap with the spatial distribution of fertilizer consumption, especially in the central and western parts of the study area. NO3(-) distributions along the hydrogeological cross-sections were related to the groundwater flow system. Hydrochemical and environmental isotopic evidences indicate that surface water-groundwater interactions influence the spatial distribution of NO3(-) in the Piedmont of South Taihang Mountains.

We examined the production and partitioning of organic carbon under different inorganic nitrogen concentrations and phytoplankton composition by incubating unaltered seawater collected from 10, 100, 400 and 700 m depths off Suruga Bay, Japan. Particulate organic carbon (POC) accounted for 60–112% of net accumulated total organic carbon (TOC) during exponential growth and blooms of phytoplankton in all incubations regardless of the concentrations of nutrients and the forms of dissolved inorganic nitrogen (DIN) as well as the composition and size of phytoplankton. After the blooms of phytoplankton, microzooplankton (mainly ciliates) (>20 μm) increased rapidly in majority of incubations. This suggests that a large fraction of new production may also be transferred to higher trophic levels rather than immediately exported. After the depletion of initial DIN, accumulation of both POC and dissolved organic carbon (DOC) increased. DOC accounted for major fraction of TOC in most of the incubations. Continued accumulation of high C/N organic matter implies that recycling through the food web is not necessarily the dominant fate of organic carbon produced under low DIN condition but rather that a major fraction of the organic carbon is exported to deep water through sinking.

Incubation of unaltered deep-sea water and grazing experiment of nano- and micro- protozooplankton during incubation of deep-sea water were carried out to quantitatively characterize the planktonic structures of lower-trophic organisms and clarify the trophic pathways and controlling mechanisms involved. Phytoplankton biomass increased to 637 mg as carbon weight in a 500-l tank on Day 7 and was dominant in the planktonic structure of lower-trophic organisms. Nitrates in the incubation water was depleted after Day 7 and phytoplankton biomass decreased rapidly. On the other hand, bacteria, heterotrophic nano-flagellates and ciliates increased toward the end of incubation and were dominant in the later days of incubation. In grazing experiments on microbial organisms, bacterivory is more important for the carbon pathway in microbial food webs than herbivory when phytoplankton biomass is less than that of bacteria (low P/B conditions), while herbivory is more important than bacterivory when phytoplankton biomass is more than that of bacteria (high P/B conditions). Deep-sea water exhibited high phytoplankton productivity due to inherent high nutrients values. After depletion of nutrients, phytoplankton decreased (due also to enhanced nano- and micro-zooplankton grazing) and microbial organisms dominated. Thus, nutrients in the incubation water control the planktonic structure of lower-trophic organisms.

We examined the flows of nitrogen in two batch incubations of plankton assemblages under controlled conditions, using mesopelagic seawater from different depths. Observations included concentrations of nutrients, organic matter (particulate and dissolved) and plankton (biomass by species). Because nitrogen flows were not observed directly (as is generally the case), we examined them using a multi-element ecosystem model developed to simulate these experiments. Dynamic changes in the observed bulk POC/PON ratio (C/N ratio of particulate organic matter, POM) were consistent with a quota model for phytoplankton. To relate these changes to the regeneration of nitrogen, we included such a quota model and all major planktonic groups observed. By assimilating the data (all observations of carbon and nitrogen concentrations, including living carbon biomasses) into the model using the Monte Carlo Markov Chain, we accurately simulated the changes in bulk POC/PON ratio and its difference between incubations. Changes in phytoplankton composition dominated the changes in bulk POC/PON ratio. Bacteria dominated the regeneration of nitrogen in both incubations, and changes in bacterial regeneration strongly affected the bulk POC/PON ratio. After nitrate depletion, gross nitrogen regeneration varied inversely with observed POC/PON ratio and was constrained by this ratio. Thus, we have demonstrated a powerful new approach to constraining the flows of nutrients through ecosystems. Our results suggest that simultaneous observations of POC and PON could place an additional constraint on the supply of inorganic nitrogen in models of the oligotrophic ocean.

Vertical and seasonal characteristics of biogenic silica (BSi) dissolution in seawater were investigated by multiple dissolution experiments using seawater collected from surface and mesopelagic layers in Suruga Bay during the period 2002–2004. The dissolution rate coefficients calculated based on temporal changes of BSi concentration varied with the season of sample collection. They ranged from 0.023–0.057 day − 1 for surface samples and 0.0018–0.0025 day − 1 for mesopelagic samples for temperatures approaching in situ conditions. Experiments at various temperatures confirmed that BSi dissolution depends on temperature in natural seawater. Dissolution rate coefficient (day − 1 ) of BSi correlated significantly with temperature (°C), and Q 10 was 2.6. Addition of bioavailable organic matter to low-bioactivity seawater enhanced the protease activity and abundance of bacteria, and increased BSi dissolution rate by a factor of 1.4–2.0. There is clear evidence that BSi dissolution is accelerated by bacterial activity and potentially limited by bioavailable organic matter in natural seawater. Dissolution rates and total decreases of BSi concentration were lower during experiments using mesopelagic samples than in those using surface samples. This suggests that dissolution of BSi varies with depth and that BSi in the mesopelagic water is more resistant to the dissolution than that in the surface water. This lower dissolution rate was caused by lower temperature and lower bacterial activity due to less bioavailable organic matter in mesopelagic water. Our results provide a mechanistic understanding of variations in silica cycling within the seasonally and vertically differing marine environment.

To test the hypothesis that different phytoplankton cellular components undergo distinct decomposition processes, we examined bacterial decomposition of extracted dissolved organic matter (EX-DOM) and cell debris from the diatom (Skeletonema costatum) using long-term incubation experiments. The decay rates indicated that EX-DOM is extremely bioreactive (1.2-1.7 d-1), and that particulate organic carbon (POC) from cell debris is resistant to rapid microbial degradation (0.047-0.055 d-1). In the experiment with grazers, the percentage of accumulated dissolved organic carbon (DOC) in the EX-DOM and cell debris experiments was 5% and 64%, respectively, of the initial DOC concentrations derived from phytoplankton cells. The initial C/N ratio of DOM in the cell debris experiments was 19.2-19.7, higher than those in the EX-DOM experiments (5.5-6.0). The percentage of accumulated DOC in the cell debris experiment with grazers was 35% at day 13. Thereafter DOC accumulated until it reached 64%. Our data indicate that these two distinct phytoplankton-derived organic materials (EX-DOM and cell debris) decay and accumulate differently. These results suggest that structural components of phytoplankton cells may persist as semi-labile POC, and that DOC produced by solubilization of structural components of cell and phytoplankton-derived DOM with high C/N ratios may accumulate as semi-labile DOC in seawater.

Rate processes involved in the changes in the ratio of particulate organic carbon and particulate organic nitrogen (POC/PON ratio) in the decomposition of phytoplankton were clarified. First, we did two decomposition experiments using substrates whose high C/N ratio was more than two times higher than the Redfield ratio: (a) decomposition of Porphyridium purpureum (POC/PON = 17) and (b) decomposition of Skeletonema costatum (POC/PON = 6.5) in the presence of three times as much organic matter of glucose, which consequently increased the initial C/N ratio by more than a factor of four. In both experiments, the same mechanism was observed: POC decreased linearly while PON remained constant when the C/N ratio was high (>6). Then, we did decomposition experiments of extracted DOM of Skeletonema costatum in the presence and absence of a predator (heterotrophic nanoflagellates (HNF) and zooplankton). Finally, we developed a simple kinetic model of the phytoplankton decomposition process (PDP model). This model with fixed decomposition rate constants fits well with the results of the decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by a factor of more than three. Our simple PDP model can explain well both the formation processes of semi-refractory POM (SR-POM) and the processes involved in the changes in the POC/PON ratio.

The purpose of this study was to clear the fate of organic carbon and nitrogen in seawater and sediment, especially the difference between organic carbon and nitrogen in dissolved and particulate forms and to determine carbon sequestration by seagrass ecosystem. We had carried out four types experiments: the diel variation for 24 hour, and seasonal changes of DOC (Dissolved Organic Carbon)/DON (Dissolved Organic Nitrogen) and POC (Particulate Organic Carbon)/PON (Particulate Organic Nitrogen) in outside and inside of seagrass communities, and the enclosed chamber experiment for evaluation of release of organic matters from seagrass exuadation and the sediment, decomposition experiments in laboratory and in situ. POM (Particulate Organic Matter) of seagrass leaf litter is resistance to the biological degradation under 25°C. (The rate constant of decomposition is 0.0083 d -1 , namely remineralization time is about 120 days.) While POM in water column shows POC is more rapidly consumed to PON, and POC/PON ratio decresed with time of decomposition. DOM in water column shows DON is more rapidly consumed to DOC and DOC/DON ratio increased with time of decomposition. This suggests that rate of solubilization of DON from PON may dominate a rate of remineralization of nutrients in seagrass communities. We developed a kinetic model of the seagrass decomposition process (SDP) changes in POC/PON ratio with time as well as phytoplankton decomposition model (Fujii et al., 2002). The SDP model was adapted to the results of three kinds of decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by more than a factor of three. The apparent zeroth-order decrease in POC and the constant PON in the first labile decomposition process are well expressed by the SD model, and consequently, the calculated POC/PON ratio shows good agreement with the experimental result. A realistic model for CO 2 sequestration was also developted using net organic production (net organic production=gross organic production-respiration), sedimentation inside seagrass bed and sedimentation at the open ocean (=export flux). We calculated net sequestration of carbon into deep water after 100 years under two different sinking velocities which are for 6 m/d and 24 m/d. These results showed that 120 t-C for 6 m/d and 150 t-C for 24 m/d during 100 years were accumulated into deepwater, suggesting that 1.2 to 1.5 t-C/year for 31 t-C of gross production of seagrass/km 2 are accumulating into deep water. Seagrass ecosystem is very effective as one of the CO 2 sequestration.

We conducted experiments at Fukido mangrove, northeastern Ishigaki Island, Okinawa, Japan in March 1999, to quantify the nitrogen cycle for simulations of nitrogen cycling and export. We measured concentrations of TON (total organic nitrogen), nitrate (NO3– + NO2–), ammonia (NH4+) in the creek water, and as physical parameter, measured flow rate and water level. Concentrations of TON and NO3– + NO2– were higher at high tide and lower at low tide, whereas NH4+ concentration had an opposite trend. Export flux of NO3– + NO2– was positive (62 mol·d–1) but export of TN (total nitrogen) was estimated –21 mol·d–1, so that Fukido mangrove is a source of nitrogen to its coastal community. And we constructed a model of nitrogen cycle and simulated the export fluxes. Simulated TON export was only 6.6%. In the simulations, the vegetation was a net sink for nitrogen, meaning that it is string nitrogen as biomass. There is no mechanism to input into the ecosystem in this model. Nitrogen fixation may be a significant source of nitrogen to the mangrove ecosystem, and we will need to conduct more studies to clarify here its role in the nitrogen cycle.

Decomposition experiments were conducted on cultured phytoplankton (Skeletonema costatum) in seawater containing decomposer and consumer of size less than 500 µm. We determined the decomposition rates of bulk particulate organic matter (POM), the ratio of labile to semi-refractory fractions in the POM, and the POM carbon/ nitrogen (C/N) ratio during decomposition. To identify the kinetic mechanisms involved in the reactions of different order (e.g., first- and second-order), we studied the sensitivity of reaction rates to the initial concentration of POM, ranging from 2.4 to 71 mg-C L−1. The results showed that decomposition consists of two first-order reactions: decomposition of labile and of semi-refractory particulate organic carbon (POC). The decomposition rate constants found for labile (0.13 day−1 at 20°C), and semi-refractory POC (0.008 day−1 at 20°C), and the carbon weight ratio of semi-refractory POC (13% at 20°C), were insensitive to the initial organic matter concentration. The time-dependence of the C/N ratio was also independent of this initial concentration. The decomposition rate constants and the content of semi-refractory POC did not change, regardless of the absence or presence of 25–500 µm organisms in natural seawater.

This work constitutes the first report on the morphology and chemical identification of biological micro-particles in rain water at two sites (Shizuoka and Tsukuba) from Japan. Micro-particles in rain water were deposited directly onto electron microscopic grids using ultracentrifugation. Particles with diameters more than 0.2 µm were examined using an electron microscope equipped with an energy-dispersive X-ray analyzer. Biological particles in rain water were found to be bacteria and leaf debris, and most of those particles were associated with minerals. Peaks of phosphorus and potasium in the X-ray spectra, which are characterized as chemical components of bacteria or leaves, were observed in all the biological particles. These particles in rain water were present in number percentages of 23 to 34% of the total insoluble particles, with concentrations of 1.6×104 to 2.4×104 cm−3 of rain water. This finding suggests that these biological particles are captured effectively in cloud water and/or precipitation, and that they may play a role in cloud condensation and/or ice nucleation.

Only about half of all the CO2 that has been produced by the burning of fossil fuels now remains in the atmosphere. The CO2 missing from the atmosphere is the subject of an important debate. One of the great possibility of the missing CO2 is to know the pool and turn-over time of oceanic organic carbon, especially dissolved organic carbon in the carbon cycle. Measurement of dissolved organic carbon in seawater has been much argued for a long time. An essential discussion is described in relation to the understanding “Nature”.

Behavior of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in coral reef waters in relation with biological processes was studied with incubation experiments and field observations in May 2008 and 2009 at the fringing reef of Sesoko Island, Okinawa, Japan. Reef sea water (RSW) and coral mucus added RSW collected from Acropora digitifera (AcrRSW) and Montipora digitata (MonRSW) were incubated for one day in situ and then for 77 days in the laboratory under dark condition . The results indicated that the behavior of DON was different compared to that of DOC in RSW and mucus added (AcrRSW and MonRSW) during dark incubation. Concentration of DON increased from 8.3 µM to 11.8 µM for AcrRSW and 4.0 µM to 15.4 µM for MonRSW during dark incubation period. The increasing rates for DON in AcrSRW and MonRSW were 0.05 µM day -1 and 0.1 µM day -1 respectively. On the other hand DOC concentration decreased from 129.0 µM to 75.0 µM for AcrRSW and 75.1 µM to 64.7 µM for MonRSW, with decreasing rates of 0.7 µM day -1 and 0.1 µM day -1 respectively. We assume that the increase of DON may be determined by difference between rates of inputs of organic matter mainly from mucus and rate of degradation of dissolved organic matter in the water column. These results suggest that recycling of DON is slow than that of DOC in coral reef ecosystem.

Fluorescence and dissolved organic carbon (DOC) measurements of porewaters from the Santa Barbara Basin, the Guaymas Basin, and the upper sections of the Nankai Trough suggest that ultraviolet fluorescence ([lambda][sub ex] = 325 nm, [lambda][sub em] = 450 nm) may be used as a first order estimate of DOC in anoxic marine porewaters. The majority of porewater organic carbon appears to be fluorescent, while a constant [approximately]1 mM DOC, probably the low molecular weight compounds, is not fluorescent. These data are consistent with a model in which low molecular weight compounds dissolved in porewater act as the common intermediate between labile sedimentary organic matter and remineralization or polymerization products. Fluorescence may also be used to sensitively estimate benthic DOC fluxes to the overlying water column. Results from the Santa Barbara Basin, if representative of global anoxic oceanic regions, indicate that DOC release from anoxic sediments is not a major source of oceanic DOC when compared to internal recycling rates, but may be comparable to external input or permanent removal processes. 36 refs., 3 figs.

A direct comparison is presented here between dissolved organic carbon (DOC) analyses done with the historical routine wet chemical oxidation method (persulfate oxidation), the high temperature catalytic oxidation method of Sugimura and Suzuki (MRI instrument), and a commercial high temperature combustion analyzer (Shimadzu instrument). The sample set used for the comparison included stations in a tidal urban river, the salinity gradient of an estuary, and continental shelf and slope waters, including those to a depth of 2000 m. Both high temperature combustion methods measured considerably more DOC (1.5 to 3.5 times) in the coastal waters than did the conventional persulfate method. In the fresh water and estuary, the commercial instrument measured only slightly higher values than did the conventional method, while the MRI instrument measured almost twice the concentration. Throughout all the environments, the MRI instrument measured about twice the concentration measured by the commercial instrument. An inverse linear correlation can be demonstrated between DOC and apparent oxygen utilization with data from each of the methods, but it is argued that this does not necessarily imply ‘oceanographic consistency’.

The high-temperature catalytic oxidation (HTCO) method for the determination of dissolved organic carbon in seawater, reported by Sugimura and Suzuki (1988, Marine Chemistry, 24, 105–131) has been improved to provide more valid data on the levels of dissolved organic carbon (DOC). The improvements are as follows. (1) An open-close valve, made from an alloy of iron, molybdenum, nickel and chromium and without a silicone septum, is used as the injection port. (2) Copper oxide and sulfix (mixture of AgO and CoO) are packed with a 3% platinum (Pt) catalyst into the combustion tube for the complete elimination of halogenated and sulfur compounds. (3) The top of the catalyst is covered with Pt gauze to prevent the Pt catalyst from flying upwards. (4) A glass water-trap, containing phosphoric acid and silver nitrate in solution, is connected just below the outlet of the combustion tube. (5) A high-sensitivity, infra-red gas analyser (Beckman model 880) is used. The blank value for the system is checked using water prepared by passage through the improved HTCO system. The total blank value (system blank plus water blank) ranges from 15 to 30 μM C. Of the total blank volume, the blank value for the system which was examined using water collected from the HTCO system, was less than 3 μM C. The greatest contributions to the blank value in this machine are made by the organic matter originally present in deionized water. The oxidation efficiency of the catalyst with different levels of Pt was examined for measurements of DOC. Although there is little difference in detected concentrations of DOC from the surface to deep waters with levels of 1.5, 3 and 5% Pt in the catalyst, the alumina impregnated with higher levels of Pt is much better for the complete oxidation of the DOC in seawater. The size of the combustion tube, the quality of the catalyst and the uniform distribution of the furnace temperature are key elements in determining the oxidative capacity of the system. It is also important to prevent backflushing just after the injection of the sample and to prevent the catalyst from moving in the combustion tube for the accurate measurement of DOC in seawater.

OCEANIC dissolved organic carbon (DOC) is one of the Earth's largest carbon reservoirs, but until recently its role in the carbon cycle has been neglected. New methodology1, however, has led to larger estimates of DOC concentrations and also to renewed interest in the biochemical lability of DOC2. Previous work found that the mean age of DOC in the surface ocean was > 1,000 years3. To examine the lability of DOC in greater detail, we have conducted experiments to estimate DOC turnover rates in the upper ocean. We directly observed rapid DOC turnover by bacterioplank-ton during the spring phytoplankton bloom in the North Atlantic ocean. Potential turnover rates, measured in 0.8-um filtered samples, ranged from 0.025 to 0.363 per day, and were consistent with bacterial biomass production and uptake of dissolved nitrogen (NH+4, NO-3 and urea). Our results indirectly suggest that cycling of dissolved organic nitrogen (DON) differs from that of DOC. The high estimates of DOC concentrations and turnover rates repeated here, if found to be general, would seem to demand changes in models4 of carbon cycling and of the ocean's role in buffering increases in atmospheric CO2.

Dissolved organic matter (DOM) has an important role in ocean chemistry, in the remineralization of inorganic nutriens, feed web by heterotrophic organisms or as ligand of trace metals. Recent measurements of dissolved organic carbon (DOC) concentration in seawater using a high temperature catalytic oxidation (HTCO) method influenced strongly the biogeochemical ocean community. The HTCO method produced substantially higher values with more possibilities to be related to the consumption of oxygen. The first diel observations of DOC concentrations were done during the bloom study of JGOFS in 1989. DOC concentrations changed from 20 μM C to 50 μM C. This change is primarily due to the physical mixing of water and to biological processes. It was observed that there is a very weak negative correlation between the DOC inventory (0–50 m depth) and primary production. It seems that the rate of decomposition of DOC or DOM affects the rate of primary production. Using a two box model the DOM downward flux was calculated. It is likely that the DOM downward flux of about 5 GtC yr-1 plays an important role in the oceanic carbon cycle. The turnover time of DOC is calculated to be 2.9 years in the upper ocean layers and 170 years in the deep waters.

[1] An evaluation of DOC export with the formation of North Pacific Intermediate Water is presented. Data from sites representing the relevant North Pacific water masses (subtropical, subpolar, and subtropical transitional) are used to demonstrate the process. We suggest that dissolved organic carbon (DOC)-replete Oyashio (subpolar) water mixes with DOC-depleted Kuroshio (subtropical) water in the mixed water region east of Japan. The new intermediate water, formed at a rate of 2–5 Sv, exports DOC at 13 ± 6 Tg DOC yr � 1 . On the basis of these findings we present an alternative model to explain the 14 C-DOC age gradient recently reported for intermediate depths of the North Pacific. INDEX TERMS: 4806 Oceanography: Biological and Chemical: Carbon cycling; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4283 Oceanography: General: Water masses; KEYWORDS: dissolved organic carbon; DOC; North Pacific Intermediate Water; carbon export; carbon

A method is described for the rapid, precise determination of non-volatile dissolved organic carbon in seawater in concentrations between 0 and 2000 μM. The oxidation is carried out on a platinum catalyst at 680 °C under an oxygen atmosphere after the sample has been freed of inorganic carbon, and the concentration of the CO 2 generated is measured with a non-dispersive IR gas analyzer. The determination can be carried out on board ship with a precision of ±2% using a sample volume of 200 μl. The results obtained using this method are of a much higher value than those obtained using persulfate oxidation methods. The molecular weight dependency of the results clearly indiates that the above discrepancy is caused by the low oxidation efficiency of the persulfate oxidation method against high-polymer organic matter dissolved in seawater. The results reveal that the concentration of dissolved organic carbon in surface water is about 300 μM and decreases with depth. An examination of molecular weight distribution indicates that the concentration of high-polymer organic carbon decreases rapidly from surface to deeper layers, with molecular weight ranging from 1.8 x 10 3 to 6 x 10 4 Dalton. It is noted that there is an inverse correlation between the concentration of dissolved organic carbon and apparent oxygen utilization (AOU). Frozen or acidified preservation of unfiltered samples does not give reliable results. An essential requirement for dissolved carbon analysis is membrane filtration just after sampling and real-time analysis on board. Because of the well-defined principle of the oxidation process, its reliability, the ease of sample handling and of the analytical procedure on board or in the land laboratory, and the consistency of the oceanographic parameters, this method is much more suitable for the analysis of marine dissolved organic carbon than the methods used previously.

Dissolved and particulate forms of mercury in seawater were determined. To avoid errors due to adsorption, total mercury was collected on XAD-2 resin after converting inorganic mercury to oxinate immediately after sampling. The organic mercury was trapped directly on XAD-2 resin. Both total and organic mercury adsorbed on XAD-2 resin were finally reduced to metallic mercury, which was determined by the cold vapor AAS method. In the temperature zone in the western North Pacific, total mercury is 14 ng l−1 in surface waters and 6 ng l−1 between 500 and 5,000 m. Organic and particulate mercury are 7 ng l−1 in surface waters and 2 ng l−1 between 500 and 5,000 m. Particulate forms are 3 to 10% of the total. Organic mercury comprises one-third to a half of the total. The molecular weight of organic mercury compounds was determined to be about 9 × 103, indicating its proteinaceous nature.

In order to clarify the chemical forms of minor metallic elements occurring in the ocean, a new method of separation of organic compounds of metals in sea water by using XAD-2 resin was contrived. By applying the new method of determination, it is found that, (1) More than 80 % of V, Fe, Cd and Cu dissolved in sea water are present in organic forms regardless of the depth; (2) A most part of Co and Pb are present in organic forms near the surface, but they change into inorganic forms in the deep layer up to 50 to 60%; (3) Of the total amounts of dissolved Al, Ni, Zn, Ag, Mo, and U, less than 30 % are present in organic forms in sea water anywhere in the ocean; (4) Up to 45% of Se is in organic forms.

We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV-radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOCHTC) is about twice that of the UV-oxidizable DOC (DOCuv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOCuv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOCres). We report accelerator mass spectrometry (AMS) Δ14C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ14C values of the humic material are less than those of DOCuv from a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOCuv in the ocean.

Article synthese regroupant les progres techniques recents realises au Japon dans le domaine de l'analyse chimique d'especes minerales et organiques presentes dans l'eau de mer

A detailed examination of a high-temperature catalytic oxidation method for liquid samples in the analysis of total nitrogen dissolved in seawater is reported. The nitrogenous compounds in liquid samples are oxidized on a platinum catalyzer at 680°C under oxygen atmosphere and the generated NO 2 is absorbed into a chromogenic reagent, followed by a spectrophotometric determination. The results of this method are much higher than those of wet oxidation methods. Molecular size dependency of the results clearly indicates that the above discrepancy is caused by the low oxidation capacity of the wet oxidation method against high-polymer organic matter dissolved in seawater. The results revealed that the concentration of total nitrogen in seawater is nearly constant from surface to bottom, ranging from 30 to 40 μM 1 −1 , which organic nitrogen concentration higher in the surface layer, and a rapid decrease with depth. An examination of molecular size distribution indicates that the concentration of high-polymer organic nitrogen decreases rapidly from surface to deeper layers, with molecular sizes ranging from 5 × 10 3 to 3 × 10 4 . Because of the well-defined principle of the oxidation process, its reliability, ease of sample handling and of analytical procedure on board or in the land laboratory, the present method is much more suitable for the marine analytical chemistry of total and organic nitrogen than the other previous methods.

The content of tellurium in sea water is not yet known because of its low concentration and the lack of an appropriate means for separation and determination up to now. By using a new method of XAD-Bismuthiol-II system to separate and determine a minute amount of tellurium ion in sea water, the content Te(IV) in surface water in the open sea was determined as ranging from 0.3 to 0.9 ng 1-1.

The selenium content and its chemical form were studied in water collected from nine main rivers in Japan. It was confirmed that there are two different chemical forms of selenium in river water, i.e., the tetra- and hexa-valent inorganic ions, but no organic form is found. The total selenium content of river water ranged from 33 to 94 ng l-1 with a weighted average of 57 ng l-1. The fraction of selenium contained in suspended matters was below 10% of the total.