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1.
Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 μmol L−1 h−1, dissolved inorganic carbon (DIC) production of 9–12 μmol L−1 h−1, very low NH4+ and Mn2+ concentrations, and constant NO3− production rates (0.4 - 0.7 μmol L−1 h−1) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6 μmol L−1 h−1 and very fast NO3− consumption (5.6–6.3 μmol L−1 h−1) and NH4+ production (1.4–1.5 μmol L−1 h−1). The latter rates were positively correlated to NO3− concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO3− concentrations and DIC and NH4+ productions of less than 1.3 and 0.1 μmol L−1 h−1, respectively. Mn2+ and CH4 were produced at constant rates (respectively 0.3 and 0.015 μmol L−1 h−1) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36 h following anoxic to oxic switches) showed rapid NH4+ and Mn2+ decreases to zero (1.6 and 0.8–2 μmol L−1 h−1, respectively) and very fast NO3− production (3 μmol L−1 h−1). This NO3− production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.  相似文献   

2.
Nitrification and nitrate reduction were measured simultaneously by a 15N-isotope dilution technique in the top 2 cm of sandy sediments in Great South Bay, Long Island, New York. Experiments were done at three times, under three different sets of environmental conditions. Nitrification rates remained between 0.010 and 0.015 μg-at N (g dry wt)−1 (24 h)−1 despite decreasing temperature. Nitrate reduction ranged from 0.02 to 0.11 μg-at N (g dry wt)−1 (24 h)−1. Nitrate reduction exceeded nitrification in two experiments. In the third, at low temperature and apparently high oxygen levels, rates of nitrification and nitrate reduction were comparable. We conclude that there is not a constant relationship between nitrification and nitrate reduction in this environment. Attempts to measure rates of nitrification by using the inhibitor chlorate were not successful.  相似文献   

3.
The Little Ice Age (∼1600–1900 AD) and 20th century sediment accumulation rates in Billefjorden, a subpolar fjord on Svalbard, were reconstructed by applying 210Pb, 137Cs and AMS 14C datings. The modern sediment accumulation rate decreases from more than 0.39 cm y−1 at the fjord head to 0.08 cm y−1 close to the fjord mouth. However, during the Little Ice Age the sediments accumulated at a much lower rate of 0.02 cm y−1 in the central fjord basin. This difference is most likely related to the rapid retreat of glaciers during the 20th century, when most of them withdrew up to 2 km. The post-Little Ice Age increases in temperature and a negative glacier mass balance resulted in a larger meltwater discharge transferring substantial amounts of sediments released from the glaciers, as well as those eroded from previously stored unconsolidated glacial sediments. A comparison of data from the subpolar fjords of Svalbard suggests that the increase in the sediment accumulation rate is a common trend, and further increases might be expected if climate warming continues. The properties of the fjord sediments (grain size, IRD, coarse-fraction composition, clay mineralogy) from the Little Ice Age and the 20th century showed no distinct differences. The change in the accumulation rate may be the most evident sedimentary record of this climatic change.  相似文献   

4.
Lignin phenols were measured in the sediments of Sepitiba Bay, Rio de Janeiro, Brazil and in bedload sediments and suspended sediments of the four major fluvial inputs to the bay; São Francisco and Guandu Channels and the Guarda and Cação Rivers. Fluvial suspended lignin yields (Σ8 3.5–14.6 mgC 10 g dw−1) vary little between the wet and dry seasons and are poorly correlated with fluvial chlorophyll concentrations (0.8–50.2 μgC L−1). Despite current land use practices that favor grassland agriculture or industrial uses, fluvial lignin compositions are dominated by a degraded leaf-sourced material. The exception is the Guarda River, which has a slight influence from grasses. The Lignin Phenol Vegetation Index, coupled with acid/aldehyde and 3.5 Db/V ratios, indicate that degraded leaf-derived phenols are also the primary preserved lignin component in the bay. The presence of fringe Typha sp. and Spartina sp. grass beds surrounding portions of the Bay are not reflected in the lignin signature. Instead, lignin entering the bay appears to reflect the erosion of soils containing a degraded signature from the former Atlantic rain forest that once dominated the watershed, instead of containing a significant signature derived from current agricultural uses. A three-component mixing model using the LPVI, atomic N:C ratios, and stable carbon isotopes (which range between –26.8 and –21.8‰) supports the hypothesis that fluvial inputs to the bay are dominated by planktonic matter (78% of the input), with lignin dominated by leaf (14% of the input) over grass (6%). Sediments are composed of a roughly 50–50 mixture of autochthonous material and terrigenous material, with lignin being primarily sourced from leaf.  相似文献   

5.
In this study we examined the hypothesis that, under conditions of replete macronutrients and iron in the Southern Ocean, phytoplankton abundance and specific N uptake rates are influenced strongly by the processes of grazing and NH4 regeneration. NH4 and NO3 uptake rates by marine phytoplankton were measured to the northeast and northwest of the island of South Georgia during January-February 1998. Mean specific uptake rate for NO3 (vNO3) was 0.0026 h−1 (range 0.0013-0.0065 h−1) and for NH4 (vNH4) was 0.0097 h−1 (0.0014-0.0376 h−1). vNH4 was related positively with NH4 availability, which ranged from 0.1 to 1.5 mmol m−3 within the upper mixed layer. Ambient NH4 concentrations and vNH4 were both positively related to local krill biomass values, computed from mean values along acoustic transect segments within 2 km of the uptake measurement stations. These biomass values ranged from ∼1 g krill fresh mass m−2 in the northwest to >4 kg krill wet mass m−2 in the northeast. In contrast to the variability found with NH4 concentrations and uptake rates, vNO3 was more uniform across the sampling sites. Under these conditions, increasing NH4 concentration appeared to represent an additional N resource. However, high vNH4 tended to be found for stations with lower phytoplankton standing stocks, across a total range of 0.24-20 mg chlorophyll a m−3. These patterns suggest a coupling between phytoplankton biomass, vNH4 and krill in this region of variable but high krill biomass. Locally high concentrations of krill in parts of the study area appeared to have two opposing effects. On the one hand they could graze down phytoplankton stocks, but on the other hand, their NH4 excretion supported enhanced uptake rates by the remaining, ungrazed cells.  相似文献   

6.
The annual total and organic mercury bioaccumulation pattern of Scrobicularia plana and Hediste diversicolor was assessed to evaluate the potential mercury transfer from contaminated sediments to estuarine food webs. S. plana was found to accumulate more total and organic mercury than H. diversicolor, up to 0.79 mg kg−1 and 0.15 mg kg−1 (wet weight) respectively, with a maximum annual uptake of 0.21 mg kg−1 y−1, while for methylmercury the annual accumulation was similar between species and never exceeded 0.045 mg kg−1 y−1. The higher organic mercury fraction in H. diversicolor is related to the omnivorous diet of this species. Both species increase methylmercury exposure by burrowing activities and uptake in anoxic, methylmercury rich sediment layers. Integration with the annual biological production of each species revealed mercury incorporation rates that reached 28 μg m−2 y−1, and to extract as much as 11.5 g Hg y−1 (of which 95% associated with S. plana) in the 0.4 km2 of the most contaminated area, that can be transferred to higher trophic levels. S. plana is therefore an essential vector in the mercury biomagnification processes, through uptake from contaminated sediments and, by predation, to transfer it to economically important and exploited estuarine species.  相似文献   

7.
N2O Production, Nitrification and Denitrification in an Estuarine Sediment   总被引：1，自引：0，他引：1  
The mechanisms regulating N2O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N2O production with those in nitrification and denitrification using an experimental continuous-flow sediment–water system with15N tracer (15N-NO−3 addition). From Feburary to May, both nitrification and denitrification in the sediment increased (246 to 716 μmol N m−2 h−1and 214 to 1260 μmol N m−2 h−1, respectively), while benthic N2O evolution decreased slightly (1560 to 1250 nmol N m−2 h−1). Apparent diffusion coefficients of inorganic nitrogen compounds and O2at the sediment–water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0·68–2·0 times) in February. However, they increased to 8·8–52 times in May except for that of NO−2, suggesting that the enhanced NO−3 and O2supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 °C) presumably accelerated N2O production through nitrification, the observed decrease in sedimentary N2O production seems to be attributed to the decrease in N2O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N2O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO−3+NO−2 can be an important factor regulating N2O production in nearshore sediments.  相似文献   

8.
A cross-system analysis of bulk sediment composition, total organic carbon (TOC), atomic C/N ratio, and carbon isotope composition (δ13C) in 82 surface sediment samples from natural and planted mangrove forests, bank and bottom of tidal creeks, tidal flat, and the subtidal habitat was conducted to examine the roles of mangroves in sedimentation and organic carbon (OC) accumulation processes, and to characterize sources of sedimentary OC of the mangrove ecosystem of Xuan Thuy National Park, Vietnam. Sediment grain sizes varied widely from 5.4 to 170.2 μm (mean 71.5 μm), with the fine sediment grain size fraction (< 63 μm) ranging from 11 to 99.3% (mean 72.5%). Bulk sediment composition suggested that mangroves play an important role in trapping fine sediments from river outflows and tidal water by the mechanisms of tidal current attenuation by vegetation and the ability of fine roots to bind sediments. The TOC content ranged from 0.08 to 2.18% (mean 0.78%), and was higher within mangrove forests compared to those of banks and bottoms of tidal creeks, tidal flat, and subtidal sediments. The sedimentary δ13C ranged from − 27.7 to − 20.4‰ (mean − 24.1‰), and mirrored the trend observed in TOC variation. The TOC and δ13C relationship showed that the factors of microbial remineralization and OC sources controlled the TOC pool of mangrove sediments. The comparison of δ13C and C/N ratio of sedimentary OC with those of mangrove and marine phytoplankton sources indicated that the sedimentary OC within mangrove forests and the subtidal habitat was mainly composed of mangrove and marine phytoplankton sources, respectively. The application of a simple mixing model showed that the mangrove contribution to sedimentary OC decreased as follows: natural mangrove forest > planted mangrove forest > tidal flat > creek bank > creek bottom > subtidal habitat.  相似文献   

9.
The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234ThXS, the sediment-mixing coefficient (Db) was 4.3 ± 1.8 cm2 y−1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates (Sa), estimated using 137Cs and 210PbXS, were 0.16 ± 0.02 g cm−2 y−1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m−2 d−1, of which ∼10% or 2.9 ± 0.44 mmol C m−2 d−1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O2 were 26 ± 3.8 mmol m−2 d−1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m−2 d−1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m−2 d−1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m−2 d−1. The flux of DOC out of the sediments (DOCefflux) was 5.6 ± 1.3 mmol C m−2 d−1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.  相似文献   

10.
《Marine Chemistry》2007,103(1-2):84-96
An isotope dilution method has been developed to determine by gas chromatography/mass spectrometry (GC/MS) the rates of ammonium and nitrite oxidation in severely oligotrophic marine waters. The method is based on the formation of sudan-1 from nitrite, or from nitrate following reduction to nitrite. Samples were collected by solid phase extraction and purified by high performance liquid chromatography (HPLC). A deuterated sudan-1 internal standard was synthesized, purified by HPLC and used for quantitative analysis. Concentrations of NO2− and NO3− were generally < 2 nmol/kg and < 5 nmol/kg respectively, typical of oligotrophic surface waters, and turnover times for the inorganic N pools ranged from < 1 day to > 10 days. Significant rates of nitrification were measured in the surface oligotrophic ocean, with rates of ammonium and nitrite oxidation generally within the range of 10–500 pmol/kg/h. Consequently, a significant proportion of daily NO3− assimilation by marine phytoplankton is regenerated, and not new. In a case study of the oligotrophic gyre of the North Atlantic, the influence of NH4+ regeneration and nitrification on f-ratio values suggests that in the oligotrophic ocean, f-ratio values may be significantly, and sometimes grossly, overestimated.  相似文献   

11.
Dissolved Fe, Mn and Al concentrations (dFe, dMn and dAl hereafter) in surface waters and the water column of the Northeast Atlantic and the European continental shelf are reported. Following an episode of enhanced Saharan dust inputs over the Northeast Atlantic Ocean prior and during the cruise in March 1998, surface concentrations were enhanced up to 4 nmol L− 1 dFe, 3 nmol L− 1 dMn and 40 nmol L− 1 dAl and returned to 0.6 nmol L− 1 dFe, 0.5 nmol L− 1 dMn and 10 nmol L− 1 dAl towards the end of the cruise three weeks later. A simple steady state model (MADCOW, [Measures, C.I., Brown, E.T., 1996. Estimating dust input to the Atlantic Ocean using surface water aluminium concentrations. In: Guerzoni. S. and Chester. R. (Eds.), The impact of desert dust across the Mediterranean, Kluwer Academic Publishers, The Netherlands, pp. 301–311.]) was used which relies on surface ocean dAl as a proxy for atmospheric deposition of mineral dust. We estimated dust input at 1.8 g m− 2 yr− 1 (range 1.0–2.9 g m− 2 yr− 1) and fluxes of dFe, dMn and dAl were inferred. Mixed layer steady state residence times for dissolved metals were estimated at 1.3 yr for dFe (range 0.3–2.9 yr) and 1.9 yr for dMn (range 1.0–3.8 yr). The dFe residence time may have been overestimated and it is shown that 0.2–0.4 yr is probably more realistic. Using vertical dFe versus Apparent Oxygen Utilization (AOU) relationships as well as a biogeochemical two end member mixing model, regenerative Fe:C ratios were estimated respectively to be 20 ± 6 and 22 ± 5 μmol Fe:mol C. Combining the atmospheric flux of dFe to the upper water column with the latter Fe:C ratio, a ‘new iron’ supported primary productivity of only 15% (range 7%–56%) was deduced. This would imply that 85% (range 44–93%) of primary productivity could be supported by regenerated dFe. The open ocean surface data suggest that the continental shelf is probably not a major source of dissolved metals to the surface of the adjacent open ocean. Continental shelf concentrations of dMn, dFe, and to a lesser extent dAl, were well correlated with salinity and express mixing of a fresher continental end member with Atlantic Ocean water flowing onto the shelf. This means probably that diffusive benthic fluxes did not play a major role at the time of the cruise.  相似文献   

12.
Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates, and growth efficiencies were studied in the Northern region of the Cananéia–Iguape estuarine system, which has recently experienced an intense eutrophication due to anthropogenic causes. Two surveys were carried out during spring and neap tide periods of the dry season of 2005 and the rainy season of 2006. This region receives large freshwater inputs with organic seston and phosphate concentrations that reach as high as 1.0 mg l−1 and 20.0 μM, respectively. Strong decreasing gradients of seston and dissolved inorganic nutrients were observed from the river/estuary boundary to the estuary/coastal interface. Gradients were also observed in phytoplankton and bacterial production rates. The production rates of phytoplankton were 5.6-fold higher (mean 8.5 μg C l−1 h−1) during the dry season. Primary production rates (PP) positively correlated with salinity and euphotic depth, indicating that phytoplankton productivity was light-limited. On the other hand, bacterial biomass (BB) and production rates (BP) were 1.9- and 3.7-fold higher, respectively, during the rainy season, with mean values of up to 40.4 μg C l−1 and 7.9 μg C l−1 h−1, respectively. Despite such a high BP, bacterial abundance remained <2 × 106 cells ml−1, indicating that bacterial production and removal were coupled. Mean specific growth rates ranged between 0.9 and 5.5 d−1. BP was inversely correlated with salinity and positively correlated with temperature, organic matter, exopolymer particles, and particulate-attached bacteria; this last accounted for as much as 89.6% of the total abundance. During the rainy season, BP was generally much higher than PP, and values of BP/PP > 20 were registered during high freshwater input, suggesting that under these conditions, bacterial activity was predominantly supported by allochthonous inputs of organic carbon. In addition, BB probably represented the main pathway for the synthesis of high-quality (low C:N) biomass that may have been available to the heterotrophic components of the plankton food web, particularly nanoheterotrophs.  相似文献   

13.
The response of a sub-arctic, deep-sea macrofaunal community to a simulated food sedimentation event was studied by means of a stable isotope “pulse-chase” experiment. A food pulse was simulated by adding 500 mg C m−2 of 13C-labelled diatoms, Chaetoceros radicans, to sediment cores retrieved from 1080 m in the Faroe-Shetland Channel. Carbon uptake by specific macrofaunal groups was quantified after 3 and 6 days of incubation. The carbon uptake of the dominant taxon (Polychaeta) was quantified at the genus-, and where possible, species-level, representing a data resolution that is rare in deep-sea tracer studies. The macrofaunal community reacted rapidly to the diatom addition, with 47% and 70% of the animals illustrating 13C-enrichment after 3 and 6 days, respectively. Approximately 95% of C uptake was located in the upper 2 cm due to the particularly shallow vertical distribution of the macrofaunal community and the nonexistent tracer subduction by burrowing species. Polychaetes of the families Ampharetidae and Cirratulidae were among the most heavily labelled with above background enrichment reaching 1300‰. Approximately 0.8 and 2.0 mg C m−2 were processed by the macrofauna after 3 and 6 days, representing 0.2% and 0.4% of the added carbon, respectively. It was not possible to differentiate sub-surface deposit-feeding polychaetes from predator/scavenger- and omnivorous polychaetes using their natural ??15N signatures. However, the combination of natural abundance ??15N data and 13C-labelling experiments proved to be useful for elucidating trophic relations in deep-sea food webs. This study confirms that macrofauna play an active role in the short-term carbon cycling at bathyal depths even at sub-zero temperatures and highlights the need for detailed knowledge of the community structure in understanding carbon processing patterns and early diagenesis of organic matter in marine sediments.  相似文献   

14.
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  相似文献   

15.
Extracellular enzyme activities were compared among surface water, bottom water, and sediments of the Delaware Estuary using six fluorescently labeled, structurally distinct polysaccharides to determine the effects of suspended sediment transport on water column hydrolytic activities. Potential hydrolysis rates in surface waters were also measured for the nearby shelf. Samples were taken in December 2006, 6 months after a major flood event in the Delaware Basin that was followed by high freshwater run-off throughout the fall of 2006. All substrates were hydrolyzed in sediments and in the water column, including two (pullulan and fucoidan) that previously were not hydrolyzed in surface waters of the Delaware estuary. At the time of sampling, total particulate matter (TPM) in surface waters at the lower bay, bay mouth, and shelf ranged between 31 mg l−1 and 48 mg l−1 and were 2 to 20 times higher than previously reported. The presence of easily resuspended sediments at the lower bay and bay mouth indicated enhanced suspended sediment transport in the estuary prior to our sampling. Bottom water hydrolysis rates at the two sites affected by sediment resuspension were generally higher than those in surface waters from the same site. Most notably, fucoidan and pullulan hydrolysis rates in bay mouth bottom waters were 22.6 and 6.2 nM monomer h−1, respectively, and thus three and five times higher than surface water rates. Our data suggest that enhanced mixing processes between the sediment and the overlying water broadened the spectrum of water column hydrolases activity, improving the efficiency of enzymatic degradation of high molecular weight organic matter in the water with consequences for organic matter cycling in the Delaware estuary.  相似文献   

16.
The relationship between sedimentary Fe inputs and net seagrass population growth across a range of Posidonia oceanica meadows growing in carbonate Mediterranean sediments (Balearic Islands, Spain; SE Iberian Peninsula, Spain; Limassol, Cyprus; Sounion, Greece) was examined using comparative analysis. Sedimentary Fe inputs were measured using benthic sediment traps and the net population growth of P. oceanica meadows was assessed using direct census of tagged plants. The meadows examined ranged from meadows undergoing a severe decline to expanding meadows (specific net population growth, from −0.14 yr−1 to 0.05 yr−1). Similarly, Fe inputs to the meadows ranged almost an order of magnitude across meadows (8.6–69.1 mg Fe m−2 d−1). There was a significant, positive relationship between sedimentary iron inputs and seagrass net population growth, accounting for 36% of the variability in population growth across meadows. The relationship obtained suggested that seagrass meadows receiving Fe inputs below 43 mg Fe m−2 d−1 are vulnerable and in risk of decline, confirming the pivotal role of Fe in the control of growth and the stability of seagrass meadows in carbonate sediments.  相似文献   

17.
Nitrification, fuelled by ammonium is the pivotal oxidative pathway to nitrogen cycling. In spite of its ecological significance, the factors regulating nitrification rates in the benthic realm remain poorly understood. The present study therefore examines some of the factors like ammonium, nitrite, nitrate, organic carbon, iron and manganese on down-core variability in benthic nitrification rates in two different mangrove ecosystems, one under the influence of ferromanganese ore mining (experiment) and the other relatively undisturbed (control). We hypothesize that besides organic carbon, iron could also influence the rate of nitrification. The study also contrasts the distributive pattern of autotrophic and heterotrophic nitrifiers in the two regions. The concentration of iron at the control site ranged from 1.1% to 15.1% while at the experimental site it ranged from 2.9% to 46%. The levels of organic carbon at control and experimental sites ranged from 0.02% to 6.9% and 0.1% to 6.5%, respectively. The nitrification rates at the control and experimental sites are comparable and ranged from 3.2 ± 1.2 to 18.4 ± 1.9 ng at-N g(sediment)−1 h−1 and 2.7 ± 1.5 to 18.2 ± 0.6 ng at-N g(sediment)−1 h−1, respectively. While the abundance of heterotrophic nitrifiers at both the sites ranged from 102–3 cells g−1 sediment, the autotrophic nitrifiers at the experimental site was higher by an order at 103 cells g−1 sediment reflecting the relatively higher refractile nature of organic carbon at the experimental site (Straus and Lamberti, 2000). Though organic carbon and nitrification rates are similar in both the sites, the underlying mechanisms governing the processes could be different. Our studies suggest that at the control site, heterotrophic nitrifiers govern nitrification rates (r = 0.28, p < 0.05, n = 64) using organic carbon (r = 0.32, p < 0.01, n = 64). At the experimental site, nitrification was governed more by autotrophic nitrifiers (r = 0.43, p < 0.001, n = 64) at the expense of iron (r = 0.47, p < 0.001, n = 64). Therefore at the experimental site with higher load of iron, autotrophic nitrification could be more important. It is therefore inferred that both the quality of organic carbon and quantity of iron govern nitrification rates in these mangrove swamps.  相似文献   

18.
The temporal variability of estuarine sedimentation has been investigated in the northernmost part of the Wadden Sea (Denmark), using an estuarine sedimentary sequence at Ho Havn. The sedimentary sequence appears to have been deposited within the last ∼ 2000 yr based on detailed luminescence dating of the estuarine mud, whose ages range between 225 ± 40 and 2050 ± 300 yr. The age-depth profile reveals that the sedimentation rate has varied considerably in the past. Estuarine sedimentation was very rapid ∼ 1400 yr ago; the ages over almost 1 m of sediment are indistinguishable. After this accretion rate of ∼ 9 mm a− 1, the rate dropped abruptly to ∼ 0.3 mm a− 1 some time between 1340 and 970 yr ago. This slow rate of accretion continued until ∼ 350 yr ago, when it accelerated to ∼ 1.3 mm a− 1. These abrupt changes in the accretion rate are possibly related to local sea level fluctuations, thus the period with low accretion rate most probably reflects a situation with a stable or decreasing relative sea level. The rapid deposition of ∼ 0.9 m of sediment within about one century some 1400 yr ago shows that large amounts of fine-grained sediment were available for deposition in the region at that time, and an increasing relative sea level was most probably responsible for the creation of the accommodation space for sedimentation. Recent studies on mudflats and salt marshes in the region also tend to show high accretion rates, indicating that the coastal lagoons could be less vulnerable and threatened by a future sea level rise than generally believed.  相似文献   

19.
Bacterial productivity (BP) and respiration (BR) were examined in relation to primary productivity (PP) for the first time in a shallow tropical ecosystem (Cochin Estuary), India. The degree of dependence of BP (6.3–199.7 μg C L−1 d−1) and BR (6.6–430.4 μg C L−1 d−1) on PP (2.1–608.0 μg C L−1 d−1) was found to be extremely weak. The BP/PP (0.05–8.5) and PP/BR (0.02–7.9) ratios widely varied in the estuary depending on the season and location. There was a seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter input through rivers which enhanced the bacterial heterotrophic activity and very high pCO2 (106–6001 μatm) levels. The heterotrophic zones were characterized by low PP but high bacterial production and respiration leading to oxygen undersaturation and exceptionally high pCO2. We propose that the CO2 supersaturation caused by increased bacterial respiration (in excess of PP) was a result of bacterial degradation of allochthonous organic matter. This indicates that sources other than planktonic compartment need to be explored to understand the C-cycling in this estuary. These results are of particular relevance to tropical ecosystems in general, where the bulk of world's river discharges occur.  相似文献   

20.
In the seasonally stratified Gulf of Aqaba Red Sea, both release by phytoplankton and oxidation by nitrifying microbes contributed to the formation of a primary nitrite maximum (PNM) over different seasons and depths in the water column. In the winter and during the days immediately following spring stratification, formation was strongly correlated (R2 = 0.99) with decreasing irradiance and chlorophyll, suggesting that incomplete reduction by light limited phytoplankton was a major source of . However, as stratification progressed, continued to be generated below the euphotic depth by microbial oxidation, likely due to differential photoinhibition of and oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of the δ15N and δ18O in the combined and pool, suggesting that assimilation and nitrification were co-occurring in surface waters. As stratification progressed, the δ15N of particulate N below the euphotic depth increased from −5‰ to up to +20‰.N uptake rates were also influenced by light; based on 15N tracer experiments, assimilation of , , and urea was more rapid in the light (434 ± 24, 94 ± 17, and 1194 ± 48 nmol N L−1 day−1 respectively) than in the dark (58 ± 14, 29 ± 14, and 476 ± 31 nmol N L−1 day−1 respectively). Dark assimilation was 314 ± 31 nmol N L−1 day−1, while light assimilation was much faster, resulting in complete consumption of the 15N spike in less than 7 h from spike addition. The overall rate of coupled urea mineralization and oxidation (14.1 ± 7.6 nmol N L−1 day−1) was similar to that of oxidation alone (16.4 ± 8.1 nmol N L−1 day−1), suggesting that mineralization of labile dissolved organic N compounds like urea was not a rate limiting step for nitrification. Our results suggest that assimilation and nitrification compete for and that N transformation rates throughout the water column are influenced by light over diel and seasonal cycles, allowing phytoplankton and nitrifying microbes to contribute jointly to PNM formation. We identify important factors that influence the N cycle throughout the year, including light intensity, substrate availability, and microbial community structure. These processes could be relevant to other regions worldwide where seasonal variability in mixing depth and stratification influence the contributions of phytoplankton and non-photosynthetic microbes to the N cycle.  相似文献   

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