Rates of C,N, P and Si recycling and denitrification at the US Mid-Atlantic continental slope depocenter

Benthic fluxes of O₂, titration alkalinity (TA), total inorganic carbon (TIC), Ca²⁺, NO₃⁻, NH₄⁺, PO₄³⁻, and Si(OH)₄ were measured by in situ benthic flux chamber incubations at 13 locations on the North Carolina continental slope. The majority of measurements were made at water depths of approximately 700–850 m, in the previously identified upper slope depocenter. This region is characterized by extremely high organic matter deposition rates and near saturation bottom water oxygen concentrations. Measured benthic fluxes of TA are reasonably correlated with O₂ benthic fluxes. Because bottom waters are supersaturated with respect to calcite and aragonite at these shallow water depths, these results demonstrate the importance of metabolically driven dissolution in this region. Subtraction of the calcium carbonate dissolution contributions from the TIC benthic fluxes suggests rates of organic matter remineralization ranging from 0.97 to 3.9 mol C m⁻² yr⁻¹ at the depocenter sites, a factor of 3–10 greater than estimated for the adjacent continental rise and upper slope areas. Because biological primary production in the overlying waters does not follow this pattern, these extremely high values are most likely supported by lateral inputs of highly reactive organic matter. Mass balance calculations indicate that despite the oxygenated bottom water conditions, 68% of the organic nitrogen released during organic matter remineralization processes is ultimately denitrified. The release of PO₄³⁻ from the depocenter sediments is equivalent to or larger than that predicted from the remineralization of Redfield organic matter. This implies either that PO₄³⁻ is preferentially released in this setting and that the accumulating sediments must be depleted in PO₄³⁻ relative to organic carbon or that another, non-organic, phase is contributing PO₄³⁻ to the system. The molar ratio of the Si benthic flux and organic carbon remineralization rate ranges from 0.30 to 0.86. This is significantly greater than the ratio reported for most pelagic diatoms. Possible reasons for this high ratio include the deposition of benthic diatoms that may have a larger Si : C ratio than pelagic diatoms, the near-bottom lateral input of partially reworked organic matter that may have an elevated Si : C ratio relative to fresh diatoms, preferential loss of carbon in sinking particulates or the release of Si from non-opaline materials.     

Annual Variation of Benthic Nutrient Fluxes in Shallow Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Abstract. Benthic fluxes of dissolved N. Si and P nutrients, alkalinity, dissolved inorganic C (DIC), and O₂ from sediments in the Gulf of Trieste (northern Adriatic, Italy) were measured monthly for 16 months, using laboratory incubated flux chambers at in siru temperatures in the dark. The annual average fluxes were: 02 = -19.3 ± 8.2, DIC = 13.7 ± 9.6, NO3 = -0.04 ± 0.16, NH4 = 0.3 ± 0.4. PO₄= 4.001 ± 0.01, Si = 0.9 ± 0.1 mmol m-² d^-1, with strong temporal fluctuations. The highest effluxes of all nutrients and DIC were observed in the summer. Small effluxes of DIC and NH4 and influxes of Si and PO4 were observed in late winter. Only NH₄ (ca. 50%) and Si (ca. 70%) fluxes were significantly correlated with temperature. This correlation suggests that the rate of downward input and the quality of sedimented organic matter (autochthonous and allochthonous) were superimposed on the temperature fluctuations. High DIC, NH₄ and Si effluxes observed in May 1993 during low temperature were due to the degradation of sedimentary organic matter produced by an early spring bloom of benthic microalgae which occurred about 6 weeks earlies while the autumn phytoplankton bloom was simultaneously reflected in enhanced benthic fluxes due to higher temperature. The role of benthic biological advection in this transport across the sediment-water interface, evaluated by comparison between measured benthic and calculated diffusive fluxes from nutrient pore water concentrations, was of minor importance. This is probably due to low infaunal activity throughout the year it was localized mostly in the narrow surficial layer. The annual average diffusive fluxes of NH4 and PO4 were higher than those measured, probably due to the presence of nitrificationdenitrifi-cation processes and redox-dependent chemical reactions at the oxic sediment-water interface, respectively. Only during bottom-water hypoxia in September 1993 did strong PO₄ effluxes prevail. Calculations based on the Redfield stoichiometry of oxic decomposition of organic N to NH4 and NO3, and differences between diffusive and measured NH₄ fluxes showed that denitrifkation averaged 0.8 mmol m^-2 d^-1. Significant correlations between NH₄ and PO₄ DIC and Si, and NH4 and Si fluxes suggested their parallel regeneration and utilization at the sediment-water interface. The nutrient fluxes observed were not significantly linked to O₂ consumption, suggesting also that anaerobic oxidation processes were important at the sediment-water interface in the gulf. The N, P and Si nutriqnts released from sediment pore waters are probably utilized in benthic microalgal and bottorn-hater primary production. This indicates that pelagic and benthic communities in the central part of the Gulf of Trieste function relatively independently of each other.     

Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)₄] in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)₄ and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138°E transect (between 30 and 34°N) and the 155°E transect (between 10 and 35°N) during the summers of 2007 and 2008. Surface Si(OH)₄ concentrations ranged from the detection limit (11 nmol L⁻¹) to 2462 nmol L⁻¹. High Si(OH)₄ concentrations (>1000 nmol L⁻¹) and dynamic fluctuations were generally observed north of 23°N, while consistently stable low concentrations of 415–751 nmol L⁻¹ were observed south of 23°N. Surface nitrate+nitrite (N+N) and phosphate (PO₄³⁻) were typically depleted to <20 nmol L⁻¹, except for PO₄³⁻ in the area south of 16°N. The majority of the study area was characterized by high-Si(OH)₄ and low-N+N and PO₄³⁻. However, submesoscale/mesoscale depressions of Si(OH)₄ were locally observed in the cyclonic eddy fields north of 23°N. Among a total of six Si(OH)₄ depressions within the eddies, a complete Si(OH)₄ depletion (<11 nmol L⁻¹) was observed on the cyclonic side near the Kuroshio axis (33.1°N, 138°E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)₄ was exhausted by diatoms. All of the Si(OH)₄ depressions were selective and not accompanied by local depressions of N+N and PO₄³⁻. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N₂ fixation.     

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO₂ uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO₃) and silicate (Si(OH)₄) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m⁻² d⁻¹, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km². We estimate that 0.15 Tmol C y⁻¹ is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on C_org export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.     

桑沟湾养殖海域营养盐和沉积物-水界面扩散通量研究

利用2006年4,7,11月和2007年1月4个航次对桑沟湾养殖海域的观测资料,分析了该海域营养盐分布、结构特征、主要控制过程以及沉积物-水界面扩散通量,结果表明,该海域的营养盐分布具有明显的季节变化,海水中NO3-,NO2-,PO43-,DOP,TDP和SiO32-浓度皆是秋季最高,而NH4+,DON,TDN浓度则为夏季最高;各种营养盐的最低值除DON外都出现在春季。春季湾内外海水交换不畅,再加上大型藻类海带等生长旺盛期的消耗,使营养盐浓度处于较低水平,在夏秋两季丰水期沿岸河流注入对该海域营养盐的影响较大,冬季无机营养盐浓度分布主要受沿岸流的影响。磷的结构变化较大,其中DOP百分含量在夏季最高,达到81%。从春季到秋季海水中TDN的结构变化从以DON为主转变成以DIN为主。硅和氮的原子比值全年变化不大,硅和氮和氮和磷原子比值春夏两季的高于秋冬季的。分析营养盐化学计量限制标准和浮游植物生长的最低阈值结果表明,磷是春夏两季桑沟湾浮游植物生长的限制性因素;春季硅浓度低于浮游植物生长的最低阀值,也是一个潜在的限制因素。计算结果显示桑沟湾沉积物释放的NH4+,SiO32-和PO43-对初级生产力的贡献较小,与其他浅海环境相比,桑沟湾沉积物-水界面的营养盐通量处于较低或中等水平。     

Early diagenetic redistribution and burial of phosphorus in the sediments of the southwestern East Sea (Japan Sea)

This study was carried out in order to understand the early diagenetic redistribution of phosphorus and relevant mass balance in the sediments of the East Sea. In two cruises during May 1993 and October 1995, 11 box cores were collected in the southwestern part of the East Sea. Dissolved phosphorus and iron were analyzed in the porewater from the cores. Sediment samples were analyzed for solid-phase P species and solid-phase Fe oxyhydroxide by sequential extraction.Phosphorus speciation results show that organic P is the major chemical form of phosphorus in young sediments within the upper 50 cm of sediment. However, the authigenic fraction of total P increases with depth, indicating the precipitation of carbonate fluorapatite (CFA) in the sediments. The authigenic CFA (Ca₅(PO₄)_2.6(CO₃)_0.4F) was formed and buried at rates of 11–110 μmol cm⁻² kyr⁻¹. The main source of dissolved phosphorus for the precipitation of CFA is organic P. Dissolved phosphorus, released from the decomposition of organic P, diffuses upward to return to bottom water, or is sorbed to iron oxides in the oxidized sediments. As sedimentation proceeds, the iron oxide-bound P is released in the reduced layer and enters the dissolved phase, which contributes P to the formation of CFA in addition to that contributed by the organic P.The burial flux of reactive P (iron oxide-bound P+authigenic P+organic P) is 0.09–0.53 g P m⁻² yr⁻¹ that accounts for 18–58% of the reactive P arriving at the sediment/water interface. The burial flux of reactive P is high in the upper and lower continental margin sediment. The burial flux of reactive P in the Ulleung Basin sediment is less than those in the continental margin sites by a factor of 6, indicating that the reactive P burial flux is mainly dependent on sedimentation rate.     

Utility of an automatic water sampler to observe seasonal variability in nutrients and DIC in the Northwestern North Pacific

Concentrations of nutrients (NO₃, NO₂, Si(OH)₄, PO₄ and dissolved inorganic carbon (DIC) were measured in a series of seawater samples collected over approximately 15 months in 2005 and 2006 by an automatic water sampler (Remote Access Sampler, RAS) in the Northwestern North Pacific. Seasonal variability and concentrations of NO₃ + NO₂ (NO_x and Si(OH)₄ were comparable to previous shipboard observations, although there were small errors associated with measurements of PO₄ and DIC. Concentrations of these nutrients began to decrease in late April. After the end of June, NO_x and Si(OH)₄ decreased rapidly, with large fluctuations. After October, these nutrients increased again until late spring 2006. The ratio of the decrease of Si(OH)₄ to that of NO_x suggests that numbers of biogenic opal-producing creatures, such as diatoms, increased after the end of June. This conclusion was supported by a rapid increase in biogenic opal flux recorded in a sediment trap at 150 m. The relationship between NO_x concentrations at the RAS depth of 35 m and NO_x integrated over the upper 100 m was determined using previous shipboard hydrocast data. This relationship was used to estimate integrated mixed layer NO_x concentration from RAS data. Estimated new production based on seasonal drawdown of integrated NO_x averaged approximately 156 mg-C m⁻²day⁻¹ annually, which agrees with previous estimates. Thus, an automatic seawater sampler that documents annual maximum and minimum nutrient concentrations and episodic events such as storms and spring blooms, which might be missed by an ordinary research vessel, will contribute to time-series observations of nutrients and, by extension, biological pump activity.     

Composition and bacterial utilization of free amino acids in tropical mangrove sediments

The composition and bacterial utilization of dissolved free amino acids (DFAA) in tropical mangrove sediments was examined. Amino acid concentrations (300–900 ng total DFAA ml⁻¹) and composition were similar to that of other organic-rich, anaerobic sediments with lowest and highest concentrations in the low and mid intertidal zones, respectively. The non-protein amino acid, β-glutamic acid, rarely reported in previous studies, was found as a major component of the interstitial DFAA pool. Intracellular amino acids from some cultured strains of sulphate-reducing bacteria (e.g. Desulfobacter app) showed the presence of β-glutamic acid as a major cellular constituent suggesting that these bacteria may be a source of this amino acid in mangrove pore waters. In high intertidal sediments, bacterial growth rates (μ) correlated significantly with total DFAA concentrations with depth.Amino acid concentrations and composition differed significantly between sediments and overlying tidal waters. Flux chamber experiments showed negligible amino acid flux out of the sediments in untreated chambers, but rates of amino acid flux ranged from 27 to 69 mgN m⁻² day⁻¹ (= 81–207 mg C.m⁻² day⁻¹) in chambers where poisons were applied to the sediment surface. Such fluxes could account for between 9–38% and 5–19% of the nitrogen and carbon required to support the levels of bacterial productivity measured in surface (0–1 cm) sediments. These experiments suggest that bacterial populations in surface sediments are capable of utilizing all of the amino acid flux to the sediment-water interface in tropical mangroves.     

长江水体溶解态无机氮和磷现状及长期变化特点

于2006年2、5、8和11月对长江从攀枝花至河口和上游的两条支流雅砻江和嘉陵江的溶解态无机氮(NO-3-N、NO-2-N和NH+4-N)和磷酸盐(PO43--P)进行了取样调查,同时结合长江营养盐的历史数据,分析了长江水体中溶解态无机氮、磷的长期变化特点。结果表明,长江NO-3-N、NH+4-N、DIN(包括NO-3-N、NO-2-N和NH+4-N)和PO3-4-P浓度从上游到下游显示出增加趋势,但存在季节差异;NO2-N浓度总体较低,在长江中下游(武汉—南京)浓度较高。长江从上游到下游DIN通量的变化主要受径流量的影响,从上游到下游单位面积年产N量逐渐升高;PO3-4-P输送通量从上游往下游呈增加趋势,也主要受径流量控制,但从季节变化来讲,PO3-4-P的月输送通量受其浓度的控制更加明显。自20世纪60年代来,长江水体中NO3--N、NO2--N、DIN和PO3-4-P的浓度都处于缓慢上升趋势,但到80年代上升速度明显加快;不同阶段DIN和34PO-P的季节变化特点也不尽相同,反映了其来源的差异。目前,长江水体中溶解态无机氮、磷浓度与国内及国际河流相比处于中等水平。     

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.     

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.     

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.     

A benthic Si mass balance on the Congo margin: Origin of the 4000 m DSi anomaly and implications for the transfer of Si from land to ocean

To elucidate the origin of the silicic acid (DSi) anomaly observed along the 4000 isobath on the Congo margin, we have established a benthic Si mass balance and performed direct measurements of biogenic silica (bSiO₂) dissolution in the deep waters and in the sediments. Results strongly suggest that the anomaly originates from the sediments; the intensity of DSi recycling is consistent with the degradation of organic matter, as observed from Si:O₂ ratios in the benthic fluxes compared to that ratio observed in the anomalies. Strong imbalances, observed in both the Si and C mass balances, suggest that the biogenic matter that degrades and dissolves in these sediments near 4000 m does not come from pelagic sedimentation. It is probably not coming also from the deep channel, because observations were similar in the deep channel vicinity (site D) and further south, far from its influence (site C). The composition of the sediments, with an Si:C ratio close to that observed on continental shelves, suggests that this matter is coming from downslope transport. A first estimate of the magnitude of this flux at global scale, close to 12 T mol Si yr⁻¹, suggests that it may be an important path for transferring Si from land to ocean.     

Sediment-water exchange of Mn,Fe, Ni and Zn in Galveston Bay,Texas

In-situ benthic flux studies were conducted at three stations in Upper Galveston Bay twice during March 1996 to directly measure release rates of dissolved Mn, Fe, Ni and Zn from the sediments. Results showed reproducible increases with time in both replicate light and light–dark benthic chambers, resulting in average fluxes of −1200±780, −17±12, −1.6±0.6 and −2.4±0.79 μmol m⁻² day⁻¹ for Mn, Fe, Ni and Zn, respectively. Sediment cores collected during 1994–1996 showed that surficial pore water concentrations were elevated compared to overlying water column concentrations, suggesting diffusive release from the sediments. Diffusive flux estimates of Mn and Zn agreed in direction with chamber fluxes measured on the same date, but only accounted for 5–38% of the measured flux. Diffusive fluxes of Fe agreed with measured fluxes at the near Trinity River station but overestimated actual release in the mid and outer Trinity Bay regions, possibly due to inaccurate determination of the Fe pore water gradients or rapid oxidation processes in the overlying water at these stations.In general, measured fluxes of Mn and Ni were higher in the mid Trinity Bay region and suggested a mechanism for the elevated trace metal concentrations previously reported for this region of Galveston Bay. However, the fluxes of Fe were highest in close proximity to the Trinity River, supporting the elevated Fe concentrations measured in this region during this and other studies, and decreased towards middle and outer Trinity Bay. Trace metal turnover times were between 0.1 and 1.2 days for Mn, between 1.3 and 4.6 days for Fe, and between 27 and 100 days for Ni and 12–20 days Zn, and were considerably shorter than the average Trinity Bay water residence time (1.5 years) for this period. Comparing area averaged benthic inputs to Trinity River inputs shows the sediments to be a significant source of trace metals to Galveston Bay. However, while benthic inputs of trace metals were measured, water column concentrations remained low despite rapid turnover times for Mn and Fe, suggesting removal of these metals from the water column after release from the sediments.     

Biogenic fluxes in the Cariaco Basin: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO₃) and long-chain C₃₇ alkenones (ΣAlk₃₇) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r²=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO₃ and ΣAlk₃₇ were strongly correlated (0.6<r²<0.8) with those of OC. The major exception was the lower than expected ΣAlk₃₇ fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m⁻² yr⁻¹ at 275 m) and diminished ΣAlk₃₇ fluxes (e.g., 5 mg m⁻² yr⁻¹ at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m⁻² yr⁻¹) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m⁻² yr⁻¹) over the 1996–1999 study period. CaCO₃ burial fluxes (30–40 g m⁻² yr⁻¹), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m⁻² yr⁻¹) but comparable to those obtained from the shallowest trap (i.e. 38 g m⁻² yr⁻¹ at 275 m). In contrast, the burial rates of ΣAlk₃₇ (0.4–1 mg m⁻² yr⁻¹) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m⁻² yr⁻¹), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m⁻² yr⁻¹) and elevated ΣAlk₃₇ fluxes (∼2 mg m⁻² yr⁻¹) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.     

The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific

The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO^-₃, and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is influenced seasonally by coastal upwelling. Nevertheless, sediment trap material evidently records the isotopic composition of new nitrogen sources, since average δ¹⁵N is generally indistinguishable from δ¹⁵N values for subsurface NO^-₃. Surficial sediments are also very similar to the average δ¹⁵N value of the sediment traps, being within 1‰. This difference in δ¹⁵N between trap material and sediment is much less than the previously observed 4‰ difference for the deep sea. Better organic matter preservation at our margin sites is a likely explanation, which may be due to either low bottom O₂ concentrations or higher organic matter input to the sediments. All sites have δ¹⁵N for sub-euphotic zone NO^-₃ (8–10‰) substantially elevated from the oceanic average (4.5–5‰). This isotopic enrichment is a result of denitrification in suboxic subsurface waters (Gulf of California) or northward transport of denitrification influenced water (Monterey Bay and San Pedro Basin). Our results therefore suggest that downcore δ¹⁵N data, depending on site location, would record the intensity of denitrification and the transport of its isotopic signature along the California margin. Temporal variations in δ¹⁵N for the sediment traps do appear to respond to upwelling or convective injections of NO^-₃ to surface waters as a result of isotopic fractionation during phytoplankton uptake. Overall, though, the coupling between NO^-₃ injection, δ¹⁵N, and flux is looser than previously observed for the open-ocean, most likely the result of the smaller time/space scales of the events. In the Gulf of California, wintertime convective mixing/upwelling does produce distinct δ¹⁵N minima co-occurring with particle flux maxima. Interannual variations are apparent in this region when these winter-time δ¹⁵N minima fail to occur during El Niño conditions. There appears to be a positive relationship between the Southern Oscillation Index (SOI) anomaly and annual average δ¹⁵N. One explanation calls for hydrographic changes altering the δ¹⁵N of subeuphotic zone NO^-₃.     

长江口及邻近海区营养盐结构与限制

通过研究长江口及邻近海域溶解无机氮(DIN=NO₃-+NO₂-+NH₄+)、磷酸盐(PO₄3-)、硅酸盐(SiO₃2-)所表征的营养盐区域结构特征及影响因素,在分析营养盐绝对限制情况的基础上,划分了潜在相对营养限制区域。结果表明,123°E以西近岸表层区域DIN/P比值全年均高于16,而Si/DIN除秋季外基本小于1,显示出长江冲淡水影响下"过量氮"的特征。春夏季河口锋面区(31°~32.5°N,122.5°~124°E)硅藻的大量生长可使DIN/P异常升高和Si/DIN异常降低。秋季研究区域北部DIN/P西低东高且Si/DIN西高东低是由于在高DIN、低PO₄3-的长江冲淡水影响下,近岸受相对低DIN、高SiO₃2-的苏北沿岸流南下入侵影响而被分割而成。冬季长江口门东北部存在的高DIN/P和低Si/DIN区则主要由于寡营养盐的黑潮水深入陆架,向东北输送的部分长江冲淡水和增强的苏北沿岸流共同作用造成DIN升高所致。利用Redfield比值进行了不同站位表层潜在相对营养限制情况的区分。近岸123°E以西受高DIN、SiO₃2-长江冲淡水影响,四季多呈现PO₄3-潜在相对限制,而在春夏季由于浮游植物的大量吸收PO₄3-,造成局部PO₄3-绝对限制及潜在相对限制。春夏季氮限(DIN潜在相对限制)一般发生在外海部分站位,但较为零散。秋季除了东南外海大部分站位外,受苏北沿岸流影响在长江口北部近岸也存在氮限。随着低DIN/P的黑潮表层水(KSW)的入侵加强,冬季外海氮限站位增多。硅限(SiO₃2-潜在相对限制)在夏季发生在赤潮高发区,而冬季南部存在较多硅限站位表明KSW中SiO₃2-相对较为缺乏。     

Spatial and temporal variability of the ratios between the nitrate and phosphate concentrations in the Sea of Okhotsk

The relation between the nitrate and phosphate concentrations in the Sea of Okhotsk and the bordering waters of the Pacific Ocean were studied. The surveys were carried out in the autumn, spring, and summer of 2001–2002. For the deepwater part of the sea, the relation [NO^? ₃] = ((14.88 ± 0.07) × [PO^3? ₄] ? 5.46 ± 0.17) was found. The coefficients in the equation given are statistically different from those in the similar equation for the Pacific waters: [NO^? ₃] = (16.05 ± 0.15) × [PO^3? ₄]-(7.23 ± 0.36). In the northern part of the sea; on the shelf; in the slope area; and, especially, in the deep waters of the TINRO Depression, the linear dependence between the phosphate and nitrate concentrations was distorted. This feature was described in terms of nitrate deficiency. The maximum values of this deficiency were found in the near-bottom waters. The principal processes that might cause the nitrate deficiency were considered: the difference in the oxidation rates of the nitrogen and phosphorus organic compounds, the matter transfer between the continent and the sea, the different efficiency of the biogenic burial of nitrogen and phosphorus in the bottom sediments, and the denitrification in the upper layer of the bottom sediments. It was shown that the most probable cause of the nitrate deficiency was the denitrification. The loss of inorganic nitrogen owing to the supply of the waters of the Sea of Okhotsk to the Pacific Ocean was estimated as ～2.5 × 10¹¹ mol N/year.     

Benthic fluxes in a tropical Estuary and their role in the ecosystem

In-situ measurements of benthic fluxes of oxygen and nutrients were made in the subtidal region of the Mandovi estuary during premonsoon and monsoon seasons to understand the role of sediment–water exchange processes in the estuarine ecosystem. The Mandovi estuary is a shallow, highly dynamic, macrotidal estuary which experiences marine condition in the premonsoon season and nearly fresh water condition in the monsoon season. The benthic flux of nutrients exhibited strong seasonality, being higher in the premonsoon compared to the monsoon season which explains the higher ecosystem productivity in the dry season in spite of negligible riverine nutrient input. NH₄⁺ was the major form of released N comprising 70–100% of DIN flux. The benthic respiration rate varied from −98.91 to −35.13 mmol m⁻² d⁻¹, NH₄⁺ flux from 5.15 to 0.836 mmol m⁻² d⁻¹, NO₃⁻ + NO₂⁻ from 0.06 to −1.06 mmol m⁻² d⁻¹, DIP from 0.12 to 0.23 mmol m⁻² d⁻¹ and SiO₄⁴⁻ from 5.78 to 0.41 mmol m⁻² d⁻¹ between premonsoon to monsoon period. The estuarine sediment acted as a net source of DIN in the premonsoon season, but changed to a net sink in the monsoon season. Variation in salinity seemed to control NH₄⁺ flux considerably. Macrofaunal activities, especially bioturbation, enhanced the fluxes 2–25 times. The estuarine sediment was observed to be a huge reservoir of NH₄⁺, PO₄³⁻ and SiO₄⁴⁻ and acted as a net sink of combined N because of the high rate of benthic denitrification as it could remove 22% of riverine DIN influx thereby protecting the eco system from eutrophication and consequent degradation. The estuarine sediment was responsible for ∼30–50% of the total community respiration in the estuary. The benthic supply of DIN, PO₄³⁻ and SiO₄⁴⁻ can potentially meet 49%, 25% and 55% of algal N, P and Si demand, respectively, in the estuary. Based on these observations we hypothesize that it is mainly benthic NH₄⁺ efflux that sustains high estuarine productivity in the NO₃⁻ depleted dry season.