1,721,007 research outputs found
Nutrients in estuaries — An overview and the potential impacts of climate change
No full textThe fate and cycling of macronutrients introduced into estuaries depend upon a range of interlinked processes. Hydrodynamics and morphology in combination with freshwater inflow control the freshwater flushing time, and the timescale for biogeochemical processes to operate that include microbial activity, particle-dissolved phase interactions, and benthic exchanges. In some systems atmospheric inputs and exchanges with coastal waters can also be important. Climate change will affect nutrient inputs and behaviour through modifications to temperature, wind patterns, the hydrological cycle, and sea level rise. Resulting impacts include: 1) inundation of freshwater systems 2) changes in stratification, flushing times and phytoplankton productivity 3) increased coastal storm activity 4) changes in species and ecosystem function. A combination of continuing high inputs of nutrients through human activity and climate change is anticipated to lead to enhanced eutrophication in the future. The most obvious impacts of increasing global temperature will be in sub-arctic systems where permafrost zones will be reduced in combination with enhanced inputs from glacial systems.Improved process understanding in several key areas including cycling of organic N and P, benthic exchanges, resuspension, impact of bio-irrigation, particle interactions, submarine groundwater discharges, and rates and magnitude of bacterially-driven recycling processes, is needed. Development of high frequency in situ nutrient analysis systems will provide data to improve predictive models that need to incorporate a wider variety of key factors, although the complexity of estuarine systems makes such modelling a challenge. However, overall a more holistic approach is needed to effectively understand, predict and manage the impact of macronutrients on estuaries
Analytical and environmental studies on some dissolved trace metals in sea water
No full textSIGLELD:D48469/84 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
The measurement of organically complexed Fe(II) in natural waters using competitive ligand reverse titration
No full textWhilst there is increasing evidence for the presence of stabilized FeII associated with organic matter in aquatic environments, the absence of a reliable method for determining FeII speciation in solution has inhibited the study of this aspect of Fe biogeochemistry. A technique is described here for the determination of FeII organic complexation in natural waters that is based on competitive ligand reverse titration and a model fit to experimental results, from which ligand concentration and a conditional stability constant can be obtained. Spectrophotometry was used to detect the Ferrozine (FZ) complex with reactive FeII, which in combination with a liquid waveguide capillary cell (LWCC) enabled high sensitivity and precision measurements of FeII to be made. A series of samples was collected in the Itchen River in Southampton, UK to test the method at a wide range of salinities including river water. Levels of FeII and total dissolved Fe were within previously reported values for this system. FeII was found to occur organically complexed with values for log?K?FeIIL (conditional stability constant for FeII-natural ligand complexes) of ?8 at salinities between 0 and 21, whilst no measurable complexation was detected at a salinity of 31. This work demonstrates that spectrophotometry can be used in combination with ligand competition to investigate metal speciation in natural waters
Dissolved Fe(II) in a river-estuary system rich in dissolved organic matter
No full textReduced iron, Fe(II), accounts for a significant fraction of dissolved Fe across many natural surface waters despite its rapid oxidation under oxic conditions. Here we investigate the temporal and spatial variation in dissolved Fe redox state in a high dissolved organic matter (DOM) estuarine system, the River Beaulieu. We couple manual sample collection with the deployment of an autonomous in situ analyser, designed to simultaneously measure dissolved Fe(II) and total dissolved Fe, in order to investigate processes operating on the diurnal timescale and to evaluate the performance of the analyser in a high DOM environment.Concentrations of dissolved Fe available to the ligand ferrozine are elevated throughout the estuary (up to 21 ?M in fresh water) and notably higher than those previously reported likely due to seasonal variation. Fe(II) is observed to account for a large, varying fraction of the dissolved Fe available to ferrozine (25.5 ± 12.5%) and this fraction decreases with increasing salinity. We demonstrate that the very high DOM concentration in this environment and association of this DOM with dissolved Fe, prevents the accurate measurement of dissolved Fe concentrations in situ using a sensor reliant on rapid competitive ligand exchange
Inputs of glacially derived dissolved and colloidal iron to the coastal ocean and implications for primary productivity
No full textGlacial meltwaters draining shield bedrock under the Greenland Ice Sheet (GIS) contain <0.4 ?m “total dissolved” Fe (TDFe) with an average flow weighted concentration of ?53 nM. The concentrations of <0.03 and 0.03–0.4 ?m Fe vary over the ablation period, with weighted means for each of these fractions being respectively 22.4 nM and 30.8 nM. These concentrations are lower estimates as an adjacent larger glacier (a more representative source of glacial meltwater) had higher dissolved Fe concentrations, and reactions of meltwaters with proglacial sediments could also enhance dissolved Fe concentrations. This source of TDFe is additional to the reactive (oxyhydr)oxide phases identified by Raiswell et al. (2006) that are also introduced to adjacent polar seas from glaciers. The Fe concentrations in the shield bedrock underlying the GIS are lower than those of other crustal rocks (4.0% cf. 6.2%), but we argue that these Fe concentrations are not limiting on the total dissolved Fe concentrations we measure. The biogeochemical weathering processes operating on the subglacial debris and suspended sediment in our catchment are likely to be similar to those in other glaciated catchments. Therefore the meltwater Fe concentrations reported here can be used to give a first estimate of global fluxes of meltwater dissolved Fe to coastal polar waters. A lower estimate of the global flux of TDFe from glacial meltwaters is ?75 × 106 moles Fe/a. This glacial meltwater input of Fe to adjacent polar waters will be greatest around Greenland where there are highest annual meltwater discharges. However, the greatest impact of this source of glacial meltwater Fe is anticipated to be in Antarctic high nutrient low chlorophyll (HNLC) waters where phytoplankton productivity is typically limited by availability of Fe. For Antarctic waters the estimated meltwater Fe (TDFe) input is about 10% of that suggested to come from sea ice melting, but glacial inputs continue throughout the austral summer ablation period after sea ice melt is complete. <br/
Trace metals in seawater
No full textThe major advances in knowledge of the concentrations and distributions of trace metals in the seas and oceans have occurred since the mid 1970s. This progress reflects major breakthroughs in procedures for essentially contamination-free (clean) sampling, the adoption of clean methodologies for handling and analysis of samples, and improvements in the analytical methods themselves. The extent of progress by the early 1980s was demonstrated by the major collection of papers edited by Wong et al. 1 Overviews of the subject have been given by Burton and Statham 2 and Bruland. 3 With the improved knowledge of the true variations in the concentrations of metals within the ocean, many systematic features in their distributions have become apparent. It has been possible to classify their marine geochemical behaviors and to interpret these in terms of the interactions of the dissolved chemical species with particulate material, which reflect the fundamental bonding properties of the elements.4
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Glacial flours as a potential source of Fe(II) and Fe(III) to polar waters
No full textSupply of Fe to high nitrate low chlorophyll (HNLC) waters, such as the Southern Ocean, is the principle limiting factor for primary production and the associated carbon uptake. Glacial meltwaters have recently been shown to provide a globally substantial input of Fe to both Antarctic and Arctic polar waters. The particulate Fe flux in glacial runoff is several orders of magnitude greater than the dissolved Fe flux and yet little is known, to date, about the potential bioavailability of this particulate Fe. A fraction of particulate Fe may be bioavailable to organisms and thus have a significant impact on biota in HNLC zones. Here, we describe in detail the composition and speciation of Fe released from four different glacial flours. We show that the fraction of labile Fe varies markedly between different flours and is not proportional to total Fe. Furthermore a small fraction of the particulate Fe is available to the Fe(II) selective ligand ferrozine, which indicates active redox cycling on the surface of the flour
Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge
No full textFe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is ?Fe(II)–FeS = +0.60 ± 0.12‰.The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be ?0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are ?0.89 ± 0.11‰ and ?0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature
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