Nitrogen isotopic discrimination by water column nitrification in a shallow coastal environment

Temporal changes in nitrogen isotopic composition (δ¹⁵N) of the NO₃ ⁻ pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification on the change in the δ¹⁵N in the water column. The δ¹⁵N of NO₃ ⁻ in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion from NH₄ ⁺ to NO₃ ⁻. The significantly ¹⁵N-depleted NO₃ ⁻ in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water column nitrification (ɛ_nit = δ¹⁵N_substrate − δ¹⁵N_product) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO₃ ⁻ from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in the sediments was low (ɛ_denit = ∼1‰). δ¹⁵N_NO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ¹⁵N_NO3 is relatively small.     

Stable nitrogen isotope composition in sedimentary organic matter as a potential proxy of nitrogen sources for primary producers at a fringing coral reef

The stable nitrogen isotope ratio (δ ¹⁵N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable isotope ratios (δ ¹⁵N and δ ¹³C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef in Okinawa, Japan. Preliminary trials showed that δ ¹⁵N in SOM processed by the “wash-out method” for δ ¹³C analysis yielded similar δ ¹⁵N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition. The distribution of δ ¹⁵N and δ ¹³C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved inorganic nitrogen (DIN) sources on coral reef ecosystems.     

Diatoms grow faster using ammonium in rapidly flushed eutrophic Dokai Bay,Japan

The importance of the nitrogen source for phytoplankton growth in a highly eutrophic embayment, Dokai Bay, was investigated. The DIN concentration often exceeded 100 μM of which 40–70% was NH₄ ⁺. During two incubation experiments, the natural assemblage of mainly diatoms took up NH₄ ⁺ instead of NO₃ ⁻. The growth of two Skeletonema species isolated in Dokai Bay were significantly faster on NH₄ ⁺ (1.86 and 1.27 div. d⁻¹ respectively) than on NO₃ ⁻ (1.55 and 1.04 div. d⁻¹ respectively). Our results indicated that these diatoms could grow faster by using NH₄ ⁺ compared to NO₃ ⁻ in this eutrophic bay.     

Nitrate-nitrogen isotopic patterns in surface waters of the western and central equatorial pacific

To understand the processes transporting nitrate to the surface layer of the western and central equatorial Pacific, we measured the nitrogen isotopic ratio of nitrate (δ ¹⁵NO ₃ ⁻ ), which is a very useful tracer of the source of nitrate, above 200 m depth in this region in December 1999. δ ¹⁵NO ₃ ⁻ is higher (about 13.0‰) in the surface water than in the subsurface water (where it is about 6.5‰) due to isotopic fractionation during nitrate uptake by phytoplankton. The δ ¹⁵NO ₃ ⁻ value has a roughly linear relationship with the natural logarithm of nitrate concentration (ln[NO ₃ ⁻ ]). However, for values above 150 m depth, the intercept of this linear relationship varies with position from east to west. On the other hand, the data at 200 m depth at all observation stations are concentrated around a single point (ln[NO ₃ ⁻ ] = 2.5 and δ ¹⁵NO ₃ ⁻ = 6.5‰) and do not fit the linear relationships for the shallower values. To examine the meaning of the observed distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration we developed a box model including nitrogen and nitrogen isotopic cycles. By reproducing the observed relationship between δ ¹⁵NO ₃ ⁻ and nitrate concentration using this model we found that most nitrate is transported horizontally from the eastern equatorial Pacific. We also conducted case studies and investigated the effects of differences in pathways of nitrate transport on the distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration. From these studies we concluded that the observed linear relationships between δ ¹⁵NO ₃ ⁻ and ln[NO ₃ ⁻ ], having a common slope around 6‰ but different intercepts at each station, are evidence of the significant horizontal transport of nitrate to the surface water in this area.     

Isotopic and elemental indicators of nutrient sources and status of coastal habitats in the Caribbean Sea,Yucatan Peninsula,Mexico

Nutrient inputs associated with coastal population growth threaten the integrity of coastal ecosystems around the globe. In order to assess the threat posed by rapid growth in tourism, we analyzed the nutrient concentrations as well as the δ¹⁵N of NO₃⁻ and macrophytes to detect wastewater nitrogen (N) at 6 locations along a groundwater-dominated coastal seagrass bed on the Caribbean coast of Mexico. We predicted that locations with greater coastal development would have higher concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (P), as well as δ¹⁵N of NO₃⁻, reflecting wastewater sources of N. However, concentrations of NO₃⁻ were not significantly different between developed (3.3 ± 5.3 μM NO₃⁻) and undeveloped (1.1 ± 0.7 μM) marine embayments. The most important control on DIN concentration appeared to be mixing of fresh and salt water, with DIN concentrations negatively correlated with salinity. The δ¹⁵N of NO₃⁻ was elevated at an inland pond (7.0 ± 0.42‰) and a hydrologically-connected tide pool (7.6 ± 0.57‰) approximately 1 km downstream of the pond. The elevated δ¹⁵N of NO₃⁻ at the pond was paralleled by high δ¹⁵N values of Cladophora sp., a ubiquitous green alga (10 ± 1‰). We hypothesize that inputs of nitrogen rich (NO₃⁻ > 30 μM) groundwater, characterized by ¹⁵N enriched signatures, flow through localized submarine groundwater discharges (SGD) and contribute to the elevated δ¹⁵N signatures observed in many benthic macrophytes. However, changes in nitrogen concentrations and isotope values over the salinity gradient suggest that other processes (e.g. denitrification) could also be contributing to the ¹⁵N enrichments observed in primary producers. More measurements are needed to determine the relative importance of nitrogen transformation processes as a source of ¹⁵N to groundwaters; however, it is clear that continued inputs of anthropogenic N via SGD have the potential to severely impact ecologically and economically valuable seagrass meadows and coral reefs along the Caribbean coast of Mexico.     

Temporal variability and characterization of physicochemical properties in the neritic area of Sagami Bay,Japan

Seasonal and interannual variations in physicochemical properties were investigated in the neritic area of Sagami Bay, Kanagawa, Japan, from December 2000 to December 2005. Physicochemical properties (i.e. temperature, salinity, density, dissolved oxygen and dissolved inorganic nutrient concentration) revealed clear seasonal variations, which were similar to each other during all 5 years. Temperature, salinity and dissolved inorganic nutrients showed rapid, drastic variations within a few days and/or weeks. These variations are related to sea levels, principally due to the shifting effects of the Kuroshio Current axis: they were strongly affected by the Kuroshio Water and other waters, when sea level difference was greater than ca. 35 cm and lower than ca. 15 cm, respectively. Temperature difference (DF _T ) increased with sea level difference, and the difference of salinity and dissolved inorganic nutrients (NH₄ ⁺-N, NO₃ ⁻+NO₂ ⁻-N, NH₄ ⁺+NO₃ ⁻+NO₂ ⁻-N, PO₄ ³⁻-P and SiO₂-Si) increased and decreased with DF _T , respectively. All these correlations are significant. Total dissolved inorganic nitrogen (N), phosphate (P) and silicate (Si) revealed seasonal variations in the ranges of 0.57–16.08, 0.0070–0.91 and 0.22–46.38 μM, respectively. From the regression equations between these elements allowed the following relation to be obtained; Si:N:P = 14.8:13.4:1. Dissolved inorganic nutrients were characterized by Si and/or P deficiency, especially in the upper layer (0–20 m depth) during summer. Single and/or combined elements are discussed on the basis of potential and stoichiometric nutrient limitations, which could restrict phytoplankton (diatom) growth as a limiting factor.     

Stable isotopes of carbon and nitrogen in suspended matter and sediments from the Godavari estuary

Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic matter. The mean isotopic ratios (δ¹³C_sed −25.1 ± 0.9, δ¹³C_sus −24.9 ± 1, δ¹⁵N_sed 8.0 ± 2 and δ¹⁵N_sus 6.5 ± 0.9‰) and elemental concentrations (C_sed 0.45 ± 0.2%, C_sus 0.9 ± 0.7%, N_sed 0.07 ± 0.05% and N_sus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of δ¹³C from the upper to lower estuary in both suspended (−27.5 and −24.3‰, respectively) and sedimentary (−26.2 and −24.9‰, respectively) phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive relationship exists between the δ¹³C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton. But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier.     

Encystment and excystment of <Emphasis Type="Italic">Gyrodinium instriatum</Emphasis> Freudenthal et Lee

In the present study, we have investigated the conditions influencing encystment and excystment in the dinoflagellate Gyrodinium instriatum under laboratory conditions. We incubated G. instriatum in modified whole SWM-3 culture medium and in versions of modified SWM-3 from which NO₃ ⁻, PO₄ ³⁻, NO₃ ⁻ + PO₄ ³⁻, or Si had been omitted and observed encystment. Percentage encystment was high in the media without N and without P, while the percentage encystment in the medium lacking both N and P was highest. Moreover, to investigate N or P concentration which induced the encystment, Gyrodinium instriatum was also incubated in media with different concentrations of inorganic N and P; the concentrations of NO₂ ⁻ + NO₃ ⁻ and PO₄ ³⁻ were measured over time. The precursors of cysts appeared within 2 or 3 days of a decrease in NO₂ ⁻ + NO₃ ⁻ or PO₄ ³⁻ concentration to values lower than 1 μM or 0.2 μM, respectively. When cysts produced in the laboratory were incubated, we observed excystment after 8–37 days, without a mandatory period of darkness or low temperature. We incubated cysts collected from nature at different temperatures or in the dark or light and observed excystments. Natural cysts excysted at temperatures from 10 to 30°C, in both light and dark, but excystment was delayed at low temperatures. These studies indicate that G. instriatum encysts in low N or P concentration and excysts over a wide temperature range, regardless of light conditions, after short dormancy periods.     

Contents and stable isotopic compositions of organic carbon and total nitrogen in the surface sediment of two coastal bays in Korea

Surface sediment from the coastal bays of Gwangyang and Masan in South Korea were analyzed for their contents and isotopic values of organic carbon and total nitrogen. The sources and diagenetic alteration of organic matter were also assessed. Total organic carbon varied from 0.22% to 3.48% (average = 1.40%, n = 75), and C/N ratios varied from 2.4 to 15.2 (average = 8.79, n = 75). δ¹³C_org ranged from −19.92‰ to −25.86‰ (average = −21.21‰, n = 75), and δ¹⁵N_TN ranged from 8.57‰ to 3.93‰ (average = 6.49‰, n = 75). Total organic carbon in both areas was associated with grain-size, with higher contents in finer grained sediment. The high carbon content observed in Masan Bay sediment correlated with its higher C/N ratio. δ¹³C_org and δ¹⁵N_TN varied widely, attributable to various influences such as the input of terrestrial organic matter and diagenetic alteration. The depleted δ¹³C_org and higher δ¹⁵N_TN observed in the sediment of Gwangyang Bay reflected terrestrial supply, implying that biogeochemical processes, i.e. bacterial degradation, were more active in Masan Bay sediment, which showed less depleted δ¹³C_org and higher δ¹⁵N_TN than Gwangyang Bay sediment. δ¹⁵N_TN was the more useful indicator of biogeochemical processes in the highly anoxic sediment. These results indicate that the δ¹³C_org and δ¹⁵N_TN of sedimentary organic matter in coastal bays can indicate the source and degree of diagenetic alteration of sedimentary organic matter.     

Nitrogen dynamics within a wind-driven eddy

Wind-driven cyclonic eddies are hypothesized to relieve nutrient stress and enhance primary production by the upward displacement of nutrient-rich deep waters into the euphotic zone. In this study, we measured nitrate (NO₃⁻), particulate carbon (PC), particulate nitrogen (PN), their stable isotope compositions (δ¹⁵N-NO₃⁻, δ¹³C-PC and δ¹⁵N-PN, respectively), and dissolved organic nitrogen (DON) within Cyclone Opal, a mature wind-driven eddy generated in the lee of the Hawaiian Islands. Sampling occurred in March 2005 as part of the multi-disciplinary E-Flux study, approximately 4–6 weeks after eddy formation. Integrated NO₃⁻ concentrations above 110 m were 4.8 times greater inside the eddy (85.8±6.4 mmol N m⁻²) compared to the surrounding water column (17.8±7.8 mmol N m⁻²). Using N-isotope derived estimates of NO₃⁻ assimilation, we estimated that 213±59 mmol m⁻² of NO₃⁻ was initially injected into the upper 110 m Cyclone Opal formation, implying that NO₃⁻ was assimilated at a rate of 3.75±0.5 mmol N m⁻² d⁻¹. This injected NO₃⁻ supported 68±19% and 66±9% of the phytoplankton N demand and export production, respectively. N isotope data suggest that 32±6% of the initial NO₃⁻ remained unassimilated. Self-shading, inefficiency in the transfer of N from dissolved to particulate export, or depletion of a specific nutrient other than N may have led to a lack of complete NO₃⁻ assimilation. Using a salt budget approach, we estimate that dissolved organic nitrogen (DON) concentrations increased from eddy formation (3.8±0.4 mmol N m⁻²) to the time of sampling (4.0±0.09 mmol N m⁻²), implying that DON accumulated at rate of 0.83±1.3 mmol N m⁻² d⁻¹, and accounted for 22±15% of the injected NO₃⁻. Interestingly, no significant increase in suspended PN and PC, or export production was observed inside Cyclone Opal relative to the surrounding water column. A simple N budget shows that if 22±15% of the injected NO₃⁻ was shunted into the DON pool, and 32±6% is unassimilated, then 46±16% of the injected NO₃⁻ remains undocumented. Alternative loss processes within the eddy include lateral exchange of injected NO₃⁻ along isopycnal surfaces, remineralization of PN at depth, as well as microzooplankton grazing. A 9-day time series within Cyclone Opal revealed a temporal depletion in δ¹⁵N-PN, implying a rapid change in the N source. A change in NO₃⁻ assimilation, or a shift from NO₃⁻ fueled growth to assimilation of a ¹⁵N-deplete N source, may be responsible for such observations.     

N2 fixation variability in the oligotrophic Sulu Sea, western equatorial Pacific region over the past 83 kyr

N₂ fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic nitrate inventory. However, it is not known how, when, and where N₂ fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition (δ¹⁵N) over the last 83 kyr from a sediment core (KH02-4 SUP8) taken in the Sulu Sea in the western equatorial Pacific region; data allow the N₂ fixation variability in the sea to be reconstructed. Sediments, sinking, and suspended particulate organic matter (POM) all have lighter isotopic values compared to the δ¹⁵N values of substrate nitrate (av. 5.8‰) in North Pacific Intermediate Water. These lighter δ¹⁵N values are regarded as reflecting N₂ fixation in the Sulu Sea surface water. A δ¹⁵N mass balance model shows that N₂ fixation rates were significantly enhanced during 54–34 kyr in MIS-3 and MIS-2. It has been speculated that higher interglacial denitrification rates in the Arabian Sea and the eastern tropical Pacific would have markedly decreased the global oceanic N inventory and contributed to the increase in N₂ fixation in oligotrophic regions, but such a model was not revealed by our study. It is possible that changes in N₂ fixation rates in the Sulu Sea were regional response, and accumulation of phosphate in the surface waters due to enhanced monsoon-driven mixing is thought to have stimulated enhancements of N₂ fixation during MIS-3 and MIS-2.     

An Ecosystem Model Including Nitrogen Isotopes: Perspectives on a Study of the Marine Nitrogen Cycle

We have developed an ecosystem model including two nitrogen isotopes (¹⁴N and ¹⁵N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ¹⁵N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of δ¹⁵N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the δ¹⁵N of nitrate measured in seawater and the seasonal variations in δ¹⁵N of sinking particles obtained from sediment trap experiments. Simulated δ¹⁵N of phytoplankton are determined by δ ¹⁵N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated δ¹⁵N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of δ ¹⁵N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated δ¹⁵N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of δ¹⁵N of ammonium in winter and a sensitive study varying the parameter of nitrification rate.     

Benthic metabolism and the fate of dissolved inorganic nitrogen in intertidal sediments

We determined patterns of benthic metabolism and examined the relative importance of denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) as sinks for nitrate (NO₃⁻) in intertidal sediments in the presence and absence of benthic microalgal (BMA) activity. By influencing the activity of BMA, light regulated the metabolic status of the sediments, and, in turn, exerted strong control on sediment nitrogen dynamics and the fate of inorganic nitrogen. A pulsed addition of ¹⁵N-labeled NO₃⁻ tracked the effect and fate of dissolved inorganic nitrogen (DIN) in the system. Under illuminated conditions, BMA communities influenced benthic fluxes directly, via DIN uptake, and indirectly, by altering the oxygen penetration depth. Under dark hypoxic and anoxic conditions, the fate of water column NO₃⁻ was determined largely by three competing dissimilatory reductive processes; DNF, DNRA, and, on one occasion, anaerobic ammonium oxidation (anammox). Mass balance of the added ¹⁵N tracer illustrated that DNF accounted for a maximum of 48.2% of the ¹⁵NO₃⁻ reduced while DNRA (a minimum of 11.4%) and anammox (a minimum of 2.2%) accounted for much less. A slurry experiment was employed to further examine the partitioning between DNF and DNRA. High sulfide concentrations negatively impacted rates of both processes, while high DOC:NO₃⁻ ratios favored DNRA over DNF.     

Determination of experimentally enriched15N in nitrate nitrogen based on an improved method of azo dye formation

A method has been developed for determination of¹⁵N isotope ratio in nitrate nitrogen, which is a major analytical step in tracer experiments for studies of nitrate metabolism in the marine environment. The method is based on diazotization of nitrite with sulfanilic acid following reduction of nitrate to nitrite by a cadmium-copper column. The diazonium compound is then subject to the azo coupling reaction with 2-naphthol, and the azo dye formed is extracted by a solid phase extraction column. The dye eluted from the column is collected, and total nitrogen and¹⁵N content of the dye are determined by mass spectrometry. Sulfanilic acid can also remove preexisting nitrite by heating the sample under acidic conditions before passing through the cadmium-copper reduction column. The average recovery of nitrate nitrogen was 86%. A procedure for reducing the background nitrogen that derives from the analytical operations has been developed; background nitrogen was limited to about 0.25 μg-atomN. The variation in the background nitrogen levels reflects the range of error in¹⁵N determination of nitrate nitrogen by this method. Application of the present method to a¹⁵NO₃ ⁻ isotope dilution experiment for determination of nitrification rate in sea water is demonstrated.     

基于双还原体系与膜进样质谱快速测定15N加富水样中15NO3-的方法

海洋中的氮循环是海洋生物地球化学研究的热点领域之一,而硝化过程是氮循环的关键一环,准确获取硝化速率对于丰富海洋氮循环的认识至关重要。¹⁵N标记同位素技术是目前国际上最为广泛使用的硝化速率测定方法,该方法的核心在于准确测定¹⁵N加富样品产生的¹⁵NO₂^-和¹⁵NO₃^-的含量,但目前的方法普遍存在测试时间较长、测试成本较高、所需样品体积较大或者检测限较高等问题。研究以低成本的膜进样质谱作为¹⁵N加富样品测试设备,建立了基于镉柱与氨基磺酸双还原体系测定¹⁵N加富样品中¹⁵NO₃^-含量的方法。经条件优化实验确定的具体方法:采用1 mol/L HCl配制15 mmol/L的氨基磺酸（SA）作为反应试剂除去样品原有的NO₂^-,然后利用镉柱将¹⁵NO     

Evaluation of spatial correlation between nutrient exchange rates and benthic biota in a reef-flat ecosystem by GIS-assisted flow-tracking

A Geographic Information System (GIS)-aided flow-tracking technique was adopted to investigate nutrient exchange rates between specific benthic communities and overlying seawater in a fringing reef of Ishigaki Island, subtropical Northwestern Pacific. Net exchange rates of NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, PO₄ ³⁻, Total-N and Total-P were estimated from concentration changes along the drogue trajectories, each of which was tracked by the Global Positioning System and plotted on a benthic map to determine the types of benthic habitat over which the drogue had passed. The observed nutrient exchange rates were compared between 5 typical benthic zones (branched-coral (B) and Heliopora communities (H), seaweed-reefrock zone (W), bare sand area (S), and seagrass meadow (G)). The dependence of nutrient exchange rates on nutrient concentrations, physical conditions and benthic characteristics was analyzed by multiple regression analysis with the aid of GIS. The spatial correlation between nutrient exchange rates and benthic characteristics was confirmed, especially for NO₃ ⁻ and PO₄ ³⁻, which were usually absorbed in hydrographically upstream zones B and W and regenerated in downstream zones H and G. NO₃ ⁻ uptake in zones B and W was concentration-dependent, and the uptake rate coefficient was estimated to be 0.58 and 0.67 m h⁻¹, respectively. Both nutrient uptake in zone W and regeneration in zone H were enhanced in summer. The net regeneration ratio of NO₃ ⁻/PO₄ ³⁻ in zone H in summer ranged 5.2 to 34 (mean, 17.4), which was somewhat higher than previously measured NO₃ ⁻/PO₄ ³⁻ for sediment pore waters around this zone (1.1–8.5). Nutrient exchanges in zone S were relatively small, indicating semi-closed nutrient cycling at the sediment-water interface of this zone. NH₄ ⁺ efflux from sediments was suggested in zone G. The data suggest that the spatial pattern of nutrient dynamics over the reef flat community was constrained by zonation of benthic biota, and that abiotic factors such as nutrient concentrations and flow rates, influenced nutrient exchange rates only in absorption-dominated communities such as zones B and W.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area,northern South China Sea

The Shenhu area is one of the most favorable places for the occurrence of gas hydrates in the northern continental slope of the South China Sea. Pore water samples were collected in two piston cores (SH-A and SH-B) from this area, and the concentrations of sulfate and dissolved inorganic carbon (DIC) and its carbon isotopic composition were measured. The data revealed large DIC variations and very negative δ ¹³C-DIC values. Two reaction zones, 0–3 mbsf and below 3 mbsf, are identified in the sediment system. At site SH-A, the upper zone (0–3 mbsf) shows relatively constant sulfate and DIC concentrations and δ ¹³C-DIC values, possibly due to bioturbation and fluid advection. The lower zone (below 3 mbsf) displays good linear gradients for sulfate and DIC concentrations, and δ ¹³C-DIC values. At site SH-B, both zones show linear gradients, but the decreasing gradients for δ ¹³C-DIC and SO₄ ²⁻ in the lower zone below 3 mbsf are greater than those from the upper zone, 0–3 mbsf. The calculated sulfate-methane interface (SMI) depths of the two cores are 10.0 m and 11.1 m, respectively. The depth profiles of both DIC and δ ¹³C-DIC showed similar characteristics as those in other gas hydrate locations in the world oceans, such as the Blake Ridge. Overall, our results indicate an anaerobic methane oxidation (AMO) process in the sediments with large methane flux from depth in the studied area, which might be linked to the formation of gas hydrates in this area.     

北海湾近10 年来溶解无机氮含量变化及其与环境因子的关系

根据北海湾1995～2004 年10a 间8 个航次的调查资料, 分析研究了该湾溶解无机氮的含量变化及其与环境因子的关系。结果表明: 该湾溶解无机氮(DIN)含量具有春夏季较高、秋冬季较低的变化特点, 突出了南流江迳流的重大贡献作用; 在DIN 中, 所有航次均具有NO₃^-含量较高、NH₄⁺含量次之、NO₂^- 含量最低的分布特征, 而且除2004 年秋季外, NO₃^- 所占DIN 的比值均在63.38%以上, 已成为该湾DIN 的主要存在形式; 通过不同时期的对比分析, 得出该湾DIN 具有春秋季显著上升、冬夏季显著下降的变化规律, 增养殖排废起主导影响作用; 相关分析显示, DIN 与环境因子之间的相关性, 以与盐度出现的机率最多、显著性也最高, 与COD 的相关性次之, 与其余环境因子的相关性依次排列为pH>DO>Chl.a, 与3 种形态氮之间则以NO₃^- 和NO₂^- 最为密切, 7 个航次达到了显著正相关水平, 而与NH₄⁺的显著正相关只出现在迳流影响较小的秋冬季节。     

A tidal front in winter influenced by river discharge

Water quality of Osaka Bay is greatly influenced by freshwater discharge from rivers, to the effect of salinity playing a major role in forming the stratification. The tidal front is expected to appear in such an area even in winter considering the theory of the formation of a tidal front. From the field observation, we recognized a tidal front in winter for the first time in Osaka Bay. The critical value of a parameter log(H/U ³) for generation of a tidal front is obtained as a function of river discharge and cooling effect through sea surface. Differences of nutrients (NH₄–N, PO₄–P) concentration across the front are not clear, probably because they are utilized by phytoplankton easily. But in (NO₂–N)+(NO₃–N), DIN, PP and T–P, there is a discontinuity structure across the front, even if the absolute value of concentration difference is small.