A solvent extraction method for the colorimetric determination of nanomolar concentrations of silicic acid in seawater

A solvent extraction method for measuring nanomolar concentrations of silicic acid in seawater is described. The procedure is based on the formation of beta silicomolybdic acid by reaction of silicic and molybdic acids at low pH, extraction of the combined acid into n-butanol and reduction with a mixture of p-methylaminophenol sulfate and sulfite. The concentration of the resulting blue silicomolybdous acid in the extract is determined colorimetrically. The method has 30 times the sensitivity and 14 times the precision of standard aqueous analyses. Molar absorbance is 2.29 × 10⁵ in seawater with a precision of ± 2.5 nM Si for concentrations <- 50 nanomolar. Sensitivity in seawater is 70% of that in deionized distilled water owing to a significant salt effect. Natural concentrations of arsenate, arsenite and germanic acid cause negligible interference; however, phosphate interference is equivalent to 11 ± 1 nM Si over a broad range of phosphate concentrations, resulting in an error of ± 1 nM in the corrected silicic acid concentration measurement.     

Evaluation of the indophenol method to measure NH4 in extracts from coastal marine sediments

It has been found that the measurement of NH₄⁺ in extracts of marine sediments was influenced by factors which inhibit the indophenol colour development, and through gel chromatography, diffusion, dilution and internal standard assays, it was possible to determine the extent of this interference. The measurement of NH₄⁺ in KCl extracts and porewater, using internal standards, indicated that colour development was inhibited 13% and 51%, respectively. Dilution of samples alleviated this inhibition, but more than five-fold dilution was necessary for some extracts. Diffusion of NH₃, collection and measurement generally gave concentrations comparable with those derived from the use of internal standards. These data suggest that the indophenol reagents should be added to sufficient diluted sediment extracts or to diffusates from extracts, to prevent interference to the colour development of NH₄⁺.     

Recovery of ammonium nitrogen by solvent extraction for the determination of relative 15N abundance in regeneration experiments

A new sample preparation method for ¹⁵N tracer measurement of ammonium regeneration in seawater is described. Ammonium nitrogen is incorporated into indophenol, extracted into dichloromethane and concentrated by evaporation. The isotopic ratio of the indophenol nitrogen is determined by emission spectroscopy. A small amount (0.2 μmol) of unlabeled nitrogen is introduced by reagents used during sample preparation. For samples of equal nitrogen content, the coefficient of variation is constant and the precision of the isotopic ratio determination is inversely related to the relative ¹⁵N abundance. Differences of 0.19 atom% ¹⁵N can be measured in 5.0 μM NH₄⁺ samples of 5.0 atom% ¹⁵N (SD = 0.07, n = 3, p = 0.05). Interface from forms of dissolved organic nitrogen prevalent in marine environments are negligible.     

光谱法测定海水中溶解态无机氮的研究进展

溶解态无机氮（dissolved inorganic nitrogen, DIN）主要由亚硝酸盐-氮（NO^-₂-N）、硝酸盐-氮（NO^-₃-N）和铵氮（NH⁺₄-N）组成,它们在海洋的生物地球化学循环过程中起重要作用。但人类活动向海洋输入了大量无机氮,导致一系列环境问题。为了更好地开展海洋氮循环研究和环境污染管理,需对海水中的DIN进行测定。在众多分析方法中,光谱法因其通用性好、适用范围广、所需设备简单,成为测定海水DIN的首选。本文总结了近10年来基于光谱法测定海水DIN的研究进展,包括紫外分光光度法测定NO^-₃-N、萘乙二胺分光光度法测定NO^-₂-N和NO^-₃-N、次溴酸盐氧化-分光光度法测定NH⁺₄-N、靛酚蓝分光光度法测定NH⁺₄-N、酸碱指示剂-分光光度法测定NH⁺₄-N、荧光法和化学发光法测定DIN等,比较了各分析方法的特点,并展望了光谱法测定海水DIN的发展趋势。总的来说,在分析方法上,新试剂的使用以及一些新合成材料的出现,丰富了DIN的分析手段;在分析仪器上,以流动分析技术为基础的分析仪器在DIN的实验室及现场分析中得到了广泛应用。DIN的分析方法均朝着简单便捷、全自动化、分析速度快、精确度高、可适用范围广的方向发展。     

Effects of Benthic Flux on Short Term Variations of Nutrients in Aburatsubo Bay

In order to investigate effects of benthic flux on the short-term variations in the distribution of nutrients in coastal waters, the concentrations of nutrients (PO₄ ^3-, NH₄ ⁺ NO₃ ^-, NO₂ ^- and H₄SiO₄) and other oceanographic parameters were measured every three hours over a 24-hour period at four fixed stations in the water column of Aburatsubo Bay, a shallow semi-enclosed inlet. Sediment cores were also taken from a fixed station once in each season over one year to quantitatively determine their benthic flux. Consistent linear negative correlations were found between their concentrations and salinity in the surface layers. This result suggests that fresh water was the main source of these nutrients and a physical mixing was the major process controlling their distribution. Monthly variations of PO₄ ^3- and NH₄ ⁺ monitored for 18 months in the bay also indicate that the high surf concentration of these nutrients was associated with the appearance of low salinity waters. On the other hand, in the bottom layers, a linear correlation between the concentration of the nutrients and salinity became weak, especially for NH₄ ⁺ and PO₄ ^3-. Their concentrations were higher than the predicted value from the conservative mixing between the fresh water and seawater, indicating the possibility of another source in the bottom layers. Benthic flux is suggested as a possible source. Pore water profiles of NH₄ ⁺ and PO₄ ^3- indicate their flux towards the overlying seawater, which is quantitatively consistent with their water column distributions.     

Improved chromatographic analysis of N:N ratios in ammonium or nitrate for isotope addition experiments

Estimating nitrogen transformation rates in aquatic ecosystems by isotope dilution techniques is simplified by directly measuring nitrogen isotopic ratios for NH₄⁺ in the water using high performance cation exchange liquid chromatography (HPLC). Modifications of HPLC conditions and implementation of a median-area method for retention time determination improved and linearized a previously reported sigmoid relationship between the retention time shift (RT_shift) of the NH₄⁺ peak and the ratio of [¹⁵NH₄⁺]: [Total NH₄⁺] in seawater fortified with ¹⁵NH₄⁺. Increasing the temperature of the HPLC column from 47 to 85 °C increased mobile phase buffer flow rate relative to column back pressure, decreased the retention time for NH₄⁺, and allowed the buffer pH to be optimized relative to the pK of NH₄⁺. The use of median-area rather than maximum-height to define the retention time of NH₄⁺ further improved the linearity (r > 0.995) of the relationship between the ratio [¹⁵NH₄⁺]: [Total NH₄⁺] and RT_shift over the range of isotope ratios. Reduction of NO₃⁻ to NH₄⁺ by adding zinc dust to acidified (pH 2) seawater or lakewater samples, followed by pH neutralization, and subsequent analysis of NH₄⁺ isotope ratios by HPLC, extended application of the method to isotope dilution experiments with NO₃⁻. Advantages of this direct-injection method over mass-measurement approaches traditionally used for isotope dilution experiments include small sample size and minimal sample preparation.     

Determination of ammonium regeneration rates in the oligotrophic ocean by gas chromatography/mass spectrometry

A method has been developed for the determination of ammonium concentration and isotopic enrichment in seawater samples at the low nanomolar range (10–100 nmol/kg). It is based on the reaction of phenol/hypochlorite with ammonium to form indophenol, with subsequent solid phase extraction, derivatisation and analysis by Gas Chromatography Mass Spectrometry. The precision of the method was maximised by incorporating a deuterated indophenol internal standard. A system was developed which generated seawater with extremely low ammonium concentrations thus matching sample and standard matrices for quantitative analysis. Data are presented from a study of ammonium regeneration rates at three stations in the oligotrophic North–East Atlantic where ambient ammonium concentrations were < 21 nmol/kg. Results suggested that ammonium availability for phytoplankton was limited by the rate of ammonium regeneration. Efficient ammonium assimilation contributed to the very low ambient ammonium concentrations measured at these stations. The study highlights the need for the accurate determination of ammonium regeneration rates in studies of new production, particularly in extreme oligotrophic conditions. If not corrected for isotope dilution, f-ratio estimates may be overestimated by 10.7–13.7%.     

Distributions and biogeochemistries of methylamines and ammonium in the Arabian Sea

The distributions of monomethylamine (MMA), dimethylamine (DMA), trimethylamine (TMA) and ammonium (NH⁺₄) were investigated in the Arabian Sea. The data set presented is the first to describe the distribution of MAs on an oceanic scale. Throughout the region concentrations of NH⁺₄ were up to two orders of magnitude greater than those of the MAs. MMA (0–66 nM) was generally the most abundant MA, whilst TMA was only found at concentrations <4 nM. Low concentrations of MAs in open-ocean meso- and oligotrophic regions contrasted with the elevated levels recorded in the highly productive coastal upwelling waters of the NW Arabian Sea. In total the MAs contributed <1% dissolved organic nitrogen (DON). Depth maxima of MMA and DMA were generally associated with those of Chla, and in offshore regions, also with those of NH⁺₄ (above the thermo-, oxy- and nitrataclines). Maxima of TMA were recorded at the base of the thermo- and oxyclines, resolved from the other analytes. Through correlation studies, a degree of diatom specific MMA production was inferred (R=0.65, p<0.001) and microzooplankton grazing found to influence significantly all aqueous MA concentrations. Enhanced correlation of MMA concentrations with mesozooplankton abundance was attributed to their ability to graze diatoms. These observations are analogous to those made of equivalent oceanographic regimes in the Mediterranean Sea (Gibb et al., 1994) and support the idea that MA concentrations in seawater are primarily regulated by the productive aspects of their biological dynamics. We postulate that the nitrogen taken up in nutrient-rich, diatom-dominated regions of the Arabian Sea will be used both biosynthetically and anabolically. This may be accompanied by introduction of MMA and DMA into the aqueous phase through enzymatic precursor degradation, nitrogen detoxification, senescence or lysis and accelerated through grazing pressures, particularly that of mesozooplankton on diatoms. In contrast, under the more oligotrophic conditions recorded in the remote Arabian Sea, those species of phytoplankton with a lower nitrogen demand are favoured, e.g., prymnesiophytes and dinoflagellates. Correspondingly lower MA concentrations are recorded in these regions.     

浒苔对NH+4-N与NO-3-N吸收的相互作用

在国内首次研究了大型海洋绿潮藻浒苔(Ulva prolifera)对NH4+-N与NO 3--N两种氮源的选择吸收作用。结果表明:当两种氮源等浓度比例存在时,随着NH4+-N与NO3--N浓度升高,藻体对NH4+-N的吸收速率逐渐升高,而对NO3--N吸收受到抑制;当NO3--N和NH 4+-N高浓度比存在时,藻体对NH4-N的吸收速率随着NO3--N/NH4+-N比例的升高和NH4-N浓度的下降而降低;当NO3--N和NH4+-N低浓度比存在时,藻体对NH+4-N保持较高的吸收速率,而对NO3--N的吸收效率随着NO3--N浓度的降低而降低;浒苔具有同时利用水体中较高浓度的NH+4-N和NO3--N的能力,只有当NH4+-N或NO3--N浓度较低时,才以吸收相对应的氮源为主。这说明浒苔能够快速、大量地吸收水体中氮源,为爆发性增殖贮备物质条件。同时,即便两种氮源同时存在,浒苔对NH+4-N的吸收速率也远高于对NO3--N的吸收速率,因此,控制NH4+-N的大量输入仍是预防浒苔绿潮爆发的关键。     

沙海蜇(Nemopilema nomurai)代谢及分解过程对水环境理化因子及浮游植物影响的研究

从上行控制角度,通过野外采样和围隔培养实验,研究了水母的代谢及分解过程对水体环境中pH、溶解氧、营养盐组成的影响,以及该过程中浮游植物的变化。实验结果表明,沙海蜇在代谢过程中短时间内会大量消耗水体中的溶解氧（dissolved oxygen,DO）,使水体出现低氧和轻度酸化。代谢过程释放出大量营养盐,使水体中的溶解无机氮（dissolved inorganic nitrogen,DIN）浓度在24h内增加为原来的12倍,溶解无机磷（dissolved inorganic phosphorus,DIP）浓度增加了40多倍,进而引起水体中叶绿素a（chlorophyll a,chl a）浓度的增加。沙海蜇的分解过程使水体表现出明显的低氧（缺氧）和酸化现象。沙海蜇生物量越大,分解时间越长,对水体的改变程度越明显,此外,还释放出大量的营养盐并改变原有的营养盐结构,可以刺激甲藻和绿藻的生长,甚至可能引发藻华。     

Nitrogen uptake kinetics in the Ross Sea, Antarctica

The first estimates of uptake kinetic parameters for NH₄⁺, NO₃⁻, and urea in the Ross Sea, Antarctica were measured on three cruises during austral late winter–early spring 1996 (pre-bloom), late spring 1997 (bloom development), and summer 1997 (bloom decline). Nitrogen (N) uptake experiments were conducted with water collected at the 50% light penetration depth using trace-metal clean protocols and ¹⁵N tracer techniques. At all sites, ambient NO₃⁻ concentrations ranged from 5.8 to 30.5 μg-at N l⁻¹ and silicic acid concentrations were greater than 62.0 μg-at Si l⁻¹. The following trends were observed. First, based on maximum uptake rates (V_max), apparent N utilization followed the order NO₃⁻>NH₄⁺>urea during the pre-bloom and bloom development cruises. During the summer cruise, as the bloom was declining, the apparent order of utilization was NH₄⁺>NO₃⁻>urea. Second, evidence for possible repression of NO₃⁻ uptake by elevated NH₄⁺ concentrations was only observed at one site. Third, the kinetic parameters of NH₄⁺ uptake rates corrected for isotope dilution were compared with the kinetic parameters determined from uncorrected rates. In this comparison, the measure of substrate affinity, α (α=V_max/K_s) increased by an average of 4.6-fold when rates were corrected for isotope dilution, but values of V_max remained unchanged. Fourth, using bacterial production data, the magnitude of bacterial N uptake was estimated. Assuming that all bacterial N demands were met with NH₄⁺, the estimated bacterial portion of NH₄⁺ uptake ranged from <1%, when the ratio of bacteria to autotrophic biomass was low, to 35%, when bacterial abundance and biomass were highest. Finally, dramatic changes in NH₄⁺ uptake capacity were observed at one station (Stn. O), where kinetic parameters were measured during all three cruises. We hypothesize that a mutualistic relationship exists between phytoplankton and heterotrophic bacteria, and that the creation of microzones of high NH₄⁺ concentrations contributed to the changes seen at this station.     

Nitrogen dynamics during the Arabian Sea Northeast Monsoon

This investigation focused on the weaker and less well understood of the two Arabian Sea monsoonal wind phases, the NE Monsoon, which persists for 3–4 months in the October to February period. Historically, this period has been characterized as a time of very low nutrient availability and low biological production. As part of the US JGOFS Arabian Sea Process Study, 17 stations were sampled on a cruise in January 1995 (late NE Monsoon) and, 15 stations were sampled on a cruise in November 1995 (early NE Monsoon). Only the southern most stations (10° and 12°N) and one shallow coastal station were as nutrient-depleted as had been expected from the few relevant prior studies in this region. Experiments were conducted to ascertain the relative importance of different nitrogenous nutrients and the sufficiency of local regeneration processes in supplying nitrogenous nutrients utilized in primary production. Except for the southern oligotrophic stations, the euphotic zone concentrations of NO₃⁻ were typically 5–10-fold greater than those of NO₂⁻ and NH₄⁺. There was considerable variation (20–40-fold) in nutrient concentration both within and between the two sections on each cruise. All nitrogenous nutrients were more abundant (2–4-fold) later in the NE Monsoon. Strong vertical gradients in euphotic zone NH₄⁺ concentration, with higher concentrations at depth, were common. This was in contrast to the nearly uniform euphotic zone concentrations for both NO₃⁻ and NO₂⁻. Half-saturation constants for uptake were higher for NO₃⁻ (1.7 μmol kg⁻¹ (s.d.=0.88, n=8)) than for NH₄⁺ (0.47 μmol kg⁻¹ (s.d.=0.33, n=5)). Evidence for the suppressing effect of NH₄⁺ on NO₃⁻ uptake was widespread, although not as severe as has been noted for some other regions. Both the degree of sensitivity of NO₃⁻ uptake to NH₄⁺ concentration and the half-saturation constant for NO₃⁻ uptake were correlated with ambient NO₃⁻ concentration. The combined effect of high affinity for low concentrations of NH₄⁺ and the effect of NH₄⁺ concentration on NO₃⁻ uptake resulted in similarly low f-ratios, 0.15 (s.d.=0.07, n=15) and 0.13 (s.d.=0.08, n=17), for early and late observations in the NE Monsoon, respectively. Stations with high f-ratios had the lowest euphotic zone NH₄⁺ concentrations, and these stations were either very near shore or far from shore in the most oligotrophic waters. At several stations, particularly early in the NE Monsoon, the utilization rates for NO₂⁻ were equal to or greater than 50% the utilization rates for NO₃⁻. When converted with a Redfield C : N value of 6.7, the total N uptake rates measured in this study were commensurate with measurements of C productivity. While nutrient concentrations at some stations approached levels low enough to limit phytoplankton growth, light was shown to be very important in regulating N uptake at all stations in this study. Diel periodicity was observed for uptake of all nitrogenous nutrients at all stations. The amplitude of this periodicity was positively correlated with nutrient concentration. The strongest of these relationships occurred with NO₃⁻. Ammonium concentration strongly influenced the vertical profiles for NO₃⁻ uptake as well as for NH₄⁺ uptake. Both NO₂⁻ and NH₄⁺ were regenerated within the euphotic zone at rates comparable to rates of uptake of these nutrients, and thus maintenance of mixed layer concentrations did not require diffusive or advective fluxes from other sources. Observed turnover times for NH₄⁺ were typically less than one day. Rapid turnover and the strong light regulation of NH₄⁺ uptake allowed the development and maintenance of vertical structure in NH₄⁺ concentration within the euphotic zone. In spite of the strong positive effect of light on NO₂⁻ uptake and its strong negative effect on NO₂⁻ production, the combined effects of much longer turnover times for this nutrient and mixed layer dynamics resulted in nearly uniform NO₂⁻ concentrations within the euphotic zone. Responses of the NE Monsoon planktonic community to light and nutrients, in conjunction with mixed layer dynamics, allowed for efficient recycling of N within the mixed layer. As the NE Monsoon evolved and the mixed layer deepened convectively, NO₂⁻ and NO₃⁻ concentrations increased correspondingly with the entrainment of deeper water. Planktonic N productivity increased 2-fold, but without a significant change the new vs. recycled N proportionality. Consequently, NO₃⁻ turnover time increased from about 1 month to greater than 3 months. This reflected the overriding importance of recycling processes in supplying nitrogenous nutrients for primary production throughout the duration of the NE Monsoon. As a result, NO₃⁻ supplied to the euphotic zone during the NE Monsoon is, for the most part, conserved for utilization during the subsequent intermonsoon period.     

Effect of changes in water salinity on ammonium, calcium, dissolved inorganic carbon and influence on water/sediment dynamics

The effect of a sudden increase in salinity from 10 to 37 in porewater concentration and the benthic fluxes of ammonium, calcium and dissolved inorganic carbon were studied in sediments of a small coastal lagoon, the Albufera d'Es Grau (Minorca Island, Spain). The temporal effects of the changes in salinity were examined over 17 days using a single diffusion-reaction model and a mass-balance approach. After the salinity change, NH₄⁺-flux to the water and Ca-flux toward sediments increased (NH₄⁺-flux: 5000–3000 μmol m⁻² d⁻¹ in seawater and 600/250 μmol m⁻² d⁻¹ in brackish water; Ca-flux: −40/−76 meq m⁻² d⁻¹ at S=37 and −13/−10 meq m⁻² d⁻¹ at S=10); however, later NH₄⁺-flux decreased in seawater, reaching values lower than in brackish water. In contrast, Ca-flux presented similar values in both conditions. The fluxes of dissolved inorganic carbon, which were constant at S=10 (55/45 mmol m⁻² d⁻¹), increased during the experiment at S=37 (from 30 mmol m⁻² d⁻¹ immediately after salinity increase to 60 mmol m⁻² d⁻¹ after 17 days).In brackish conditions, NH₄⁺ and Ca²⁺ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order reaction for NH₄⁺ production and a first-order reaction for Ca²⁺ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH₄⁺.The mass balance for 17 days indicated a higher retention of NH₄⁺ in porewater in the littoral station in seawater conditions (9.5 mmol m⁻² at S=37 and 1.6 mmol m⁻² at S=10) and a significant reduction in the water consumption at both sites (5 mmol m⁻² at S=37; 35/23 mmol m⁻² at S=10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations (−10/−1 meq m⁻² at S=37; 50/90 meq m⁻² at S=10) and was linked to a higher efflux of CO₂ to the atmosphere, because of calcium carbonate precipitation in water (675/500 meq m⁻²). These results indicate that increased salinity in shallow coastal waters could play a major role in the global carbon cycle.     

Basin-scale distribution of NH<Subscript>4</Subscript><Superscript>+</Superscript> and NO<Subscript>2</Subscript><Superscript>−</Superscript> in the Pacific Ocean

We used more than 25,000 nutrient samples to elucidate for the first time basin-scale distributions and seasonal changes of surface ammonium (NH₄ ⁺) and nitrite (NO₂ ^?) concentrations in the Pacific Ocean. The highest NH₄ ⁺, NO₂ ^?, and nitrate (NO₃ ^?) concentrations were observed north of 40°N, in the coastal upwelling region off the coast of Mexico, and in the Tasman Sea. NH₄ ⁺ concentrations were elevated during May–October in the western subarctic North Pacific, May–December in the eastern subarctic North Pacific, and June–September in the subtropical South Pacific. NO₂ ^? concentrations were highest in winter in both hemispheres. The seasonal cycle of NH₄ ⁺ was synchronous with NO₂ ^?, NO₃ ^?, and satellite chlorophyll a concentrations in the western subtropical South Pacific, whereas it was synchronous with chlorophyll-a but out of phase with NO₂ ^? and NO₃ ^? in the subarctic regions.     

The Interactive Effects of Elevated CO<Subscript>2</Subscript> and Ammonium Enrichment on the Physiological Performances of <Emphasis Type="Italic">Saccharina japonica</Emphasis> (Laminariales,Phaeophyta)

Environmental challenges such as ocean acidification and eutrophication influence the physiology of kelp species. We investigated their interactive effects on Saccharina japonica (Laminariales, Phaeophyta) under two pH conditions [Low, 7.50; High (control), 8.10] and three NH ₄ ⁺ concentrations (Low, 4; Medium, 60; High, 120 μM). The degree of variation of pH values in the culture medium and inhibition rate of photosynthetic oxygen evolution by acetazolamide were affected by pH treatments. Relative growth rates, carbon, nitrogen, and the C:N ratio in tissue samples were influenced by higher concentrations of NH ₄ ⁺ . Rates of photosynthetic oxygen evolution were enhanced under elevated CO₂ or NH ₄ ⁺ conditions, independently, but these two factors did not show an interactive effect. However, rates of NH ₄ ⁺ uptake were influenced by the interactive effect of increased CO₂ under elevated NH ₄ ⁺ treatment. Although ocean acidification and eutrophication states had an impact on physiological performance, chlorophyll fluorescence was not affected by those conditions. Our results indicated that the physiological reactions by this alga were influenced to some extent by a rise in the levels of CO₂ and NH ₄ ⁺ . Therefore, we expect that the biomass accumulation of S. japonica may well increase under future scenarios of ocean acidification and eutrophication.     

The interaction of Np(V)O2+ with common mineral surfaces in dilute aqueous solutions and seawater

The extent and kinetics of Np(V)O₂⁺ adsorption from dilute aqueous solutions and seawater onto a variety of synthetic and natural solids were determined at 25°C and 1 atm total pressure. Extensive and complex adsorption reactions were found, contrary to speculations in the literature that NpO₂⁺ should behave as a simple monovalent ion with a low affinity for surfaces. When normalized to adsorption per unit solid surface area, the ranking for the synthetic solids was aragonite ? calcite > goethite ? MnO₂ ≈ clays. Natural materials generally followed the same behavior patterns as their synthetic counterparts. The dissolved/adsorbed ratio was found to be constant over a wide range (10^?13–10^?7M) of NpO₂⁺ concentrations. At higher concentrations the extent of adsorption decreased until a solubility limit was reached at approximately 10^?5 M.Solution composition had the most significant influence for NpO₂⁺ adsorption on goethite, where much more extensive adsorption occurs in dilute solutions than in seawater. When seawater is added to a dilute solution, extensive desorption of NpO₂⁺ from goethite occurs. Tests conducted on NpO₂⁺ adsorbed on carbonates indicated that it remained in the V oxidation state.There is a growing consensus that Pu dissolved in natural waters also occurs dominantly in the V oxidation state as PuO₂⁺ ion. Consequently, these results for NpO₂⁺ may serve as a guide for Pu behavior when also in the V oxidation state. The fact that most adsorbed Pu is found in the III or IV oxidation states indicates that reduction of Pu may occur subsequent to adsorption in the V oxidation state.     

Influence of the environmental factors on the intensity of the oxygen,ammonium, and phosphate metabolism in the agar-containing seaweed <Emphasis Type="Italic">Ahnfeltia tobuchiensis</Emphasis> (Ahnfeltiales,Rhodophyta)

A complex study of the influence of various environmental factors on the rate of the oxygen (M_{O 2}), ammonium (M_{NH 4}), and phosphate (M_{PO 4}) metabolism in Ahnfeltia tobuchiensis has been carried out in situ in the Izmena Bay of Kunashir Island. The following environmental factors have been included into the investigation: the photosynthetically active radiation (PAR); the ammonium (NH₄); the phosphate (PO₄); and the tissue content of carbon (C), nitrogen (N), phosphorus (P), and chlorophyll a (Chl). The population of agar-containing seaweed A. tobuchiensis forms a layer with a thickness up to 0.5 m, which occupies about 23.3 km²; the population’s biomass is equal to 125000 tons. The quantitative assessment of the organic matter production and nutrient consumption during the oxygen metabolism (M_{O 2}) has been carried out for the whole population. It has been shown that the daily rate depends on the PAR intensity, the seawater concentrations of PO₄ and NH₄, and the tissue content of N and P (r ² = 0.78, p < 0.001). The daily NH₄ consumption averages 0.21 μmol/(g_DW h) and depends on the NH₄ and O₂ concentrations in the seawater and on the C and Chl a content in the algal tissues (r ² = 0.64, p < 0.001). The daily PO₄ consumption averages 0.01 μmol/(g_DW h) and depends on the NH₄ concentration in the seawater and on the P content in the algal tissues (r ² = 0.40, p < 0.001).     

The effect of pressure on the chemical potentials of seawater components,on ionic species in seawater,and on calcite saturation levels

The activities of most of the major seawater components at 1,001 bars have been estimated, and values for the ions deduced. Equations giving the effect of pressure on the activities of ionic species in seawater ($\text{S = 35‰}$) have been developed. The species covered are: NaSO₄^?, MgSO₄⁰, CaSO₄⁰, H⁺, the free base (NH₃), the HCO₃^?/CO₃² activity ratio and the ion activity product of calcium carbonate. Comparison of the latter with the “ideal” solubility of calcite (pure solid in equilibrium with a mixed electrolyte solution) indicates a degree of saturation compatible with the trends indicated by in situ measurements.     

Tidal time-scale variation in nutrient flux across the sediment–water interface of an estuarine tidal flat

We determined the range of the tidal variations in nutrient flux across the sediment–water interface and elucidated mechanisms of the flux variation in two estuarine intertidal flats (one sand, one mud) in northeastern Japan. Nutrient flux was measured using in situ light and dark chambers, which were incubated for 2 h, 2–6 times per day. Results showed that nutrient concentration in overlying water varied by tide and was also affected by sewage-treated water inflow. The nutrient fluxes responded quickly to the tidal variation in overlying water chemistry and the range of the variation in flux was as large as the seasonal-scale variation reported in previous studies. In the sand flat, salinity increase likely enhanced benthos respiration and led to increases in both O₂ consumption and PO₄³⁻ regeneration under low illumination, while benthic microalgae were likely to actively generate O₂, uptake PO₄³⁻ and suppress PO₄³⁻ release under high illumination (>900 μmol photons m⁻² s⁻¹). Also in the mud flat, PO₄³⁻ flux was related with O₂ flux, although the range of temporal variation in PO₄³⁻ flux was small. In both the flats, NH₄⁺ flux was always governed by NH₄⁺ concentration in the overlying water; either an increase in NH₄⁺ uptake or a decrease in NH₄⁺ release was observed as the NH₄⁺ concentration rose due to inflow of river water or input of sewage-treated water. Although NO₃⁻ tended to be released in both tidal flats when low NO₃⁻ concentration seawater dominated, their relationship was likely to be weakened under conditions of low oxygen consumption and suppressed denitrification. It is likely that tidal variation in nutrient flux is governed more by the nutrient concentration than other factors, such as benthic biological processes, particularly in the case where nutrient concentration in the overlying water is relatively high and with wide amplitude.     

Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary

Phytoplankton NH₄⁺ and NO₃⁻ uptake was examined along the longitudinal salinity gradient of the Delaware Estuary over several seasonal cycles using ¹⁵N-tracer techniques. Saturated nitrogen uptake rates increased directly with water temperature and reached a maximum of 380 nmol Nl⁻¹h⁻¹ during summer. This temperature dependence was related primarily to changes in the rate of maximum chlorophyll specific uptake, which varied exponentially between 2 and 70 nmol N [μg Chl h]⁻¹ over a temperature range of 2–28°C. Despite these high uptake rates, balanced growth (C:N7:1) could be maintained over the diel light cycle only by highly efficient nitrogen uptake at low light intensities and dark uptake below the photic zone and at night (dark UPTAKE=25% maximum uptake).Ammonium fulfilled 82% of the annual phytoplankton nitrogen demand in the estuary despite dominance of NO₃⁻ in the ambient dissolved inorganic nitrogen pool. The predominance of NH₄⁺ uptake occurred because of the general suppression of NO₃⁻ assimilation at NH₄⁺ concentrations in excess of 2 μ . This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO₃⁻ concentrations found in the estuary contribute to this pattern. Nitrate was a significant source of nitrogen only during periods of high phytoplankton production in summer, and when NH₄⁺ concentrations were low towards the end of the spring bloom.