Search

Increasing inputs of reactive nitrogen have led to excessive phytoplankton growth in some coastal waters. Until recently, rivers were thought to be the most important nitrogen source but we now know that atmospheric inputs are large and can equal, or exceed, those from the rivers. These atmospheric nitrogen compounds have both agricultural sources (ammonia emitted from animal wastes) and combustion sources (nitrate derived from NOx emitted by vehicles and power stations). Our hypothesis is that atmospheric nitrogen deposition in summer to nutrient depleted, well lit, surface waters in coastal seas stimulates phytoplankton blooms. This paper summarises and compares studies conducted in the North Sea, the North East Atlantic Ocean and the Kattegat Sea. Budgeting approaches imply that the atmosphere can, under certain meteorological conditions and over short time periods, provide enough nitrogen to support a large increase in phytoplankton growth. This is not true in all areas and at all times and this emphasises the highly episodic nature of atmospheric deposition. However, productivity-based approaches suggest that atmospheric nitrogen inputs have little effect on phytoplankton growth. This may be because productivity in the North Sea and the Kattegat is controlled by internal recycling of nitrogen, even in the summer when inorganic nitrogen levels are very low. Over longer time scales, atmospheric inputs do increase the overall nitrogen stock in the water column. Reducing the input of nitrogen from the atmosphere will, therefore, reduce total nitrogen loads to coastal seas and hence may decrease eutrophication problems.

The study of aerosols and rainwater presented here demonstrates that episodic atmospheric deposition events associated with southeasterly flow are quantitatively significant for large areas of the North Atlantic Ocean. This paper considers aluminium and manganese, with predominantly crustal sources, and lead and zinc, which are mobilised into the atmosphere primarily through anthropogenic activity. High levels of all trace metals are associated with southeasterly flow from Europe as the air passes over heavily populated and industrialised regions before reaching the northeast Atlantic Ocean. Fluxes calculated using the 1% HNO3 acid soluble metal concentration show that, although the climatological norm for this area is westerly flow, short-lived southeasterly transport events dominate the input of trace metals to this ocean region. This material may be toxic to phytoplankton or may be represent a new source of nutrients to the biological community. A significant decrease in atmospheric lead levels in polluted air is seen between June 1996 and May 1997, reflecting the decrease in use of leaded fuels in Europe. Comparing atmospheric flux values to sediment trap metal fluxes shows that the atmosphere represents the dominant source of zinc to the deep ocean, whereas an additional, non-atmospheric, manganese source this required, perhaps from mobilisation of sedimentary material from the continental shelf or long range advection of manganese rich Saharan material.

Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ 15 N isotopic analysis. The isotopic enrichment factor, e , was calculated for both the species by subtracting the respective δ 15 N values of the coarse-mode fraction (>1 μm diameter) from those of the fine-mode fraction ( 4 NO 3 followed by scavenging of the HNO 3 produced appears to be significantly more important than in samples of onshore, marine dominated air. In contrast, ammonium appears to be transferred to the coarse mode during offshore flow largely via the dissolution and coagulation of aerosol ammonium, nitrate and sulphate-containing species. During onshore flow, the uptake of gaseous NH 3 , arising from the continued dissociation of NH 4 NO 3 , seems to become increasingly important.

Direct atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters and can be enhanced by heterogeneous reactions between gaseous atmospheric nitrogen species and aerosol sea salt, which increase deposition rates. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland. Major-ion aerosol concentrations were determined and temporal patterns were interpreted with the use of air-mass back trajectories. Low levels of terrestrially derived material were seen during periods of clean, onshore flow, with respective concentration ranges for nitrate and ammonium of 0.47–220 and below detection limit to 340 nmol m −3 . Corresponding levels of marine derived material during these periods were high, with sodium concentrations ranging from 39 to 1400 nmol m −3 . Highest levels of terrestrially derived material were seen during polluted, offshore flow, where the air had passed recently over strong source regions of the UK and northern Europe, with concentration ranges of nitrate and ammonium of 5.6–790 and 9.7–1000 nmol m −3 , respectively. During polluted flow ∼40–60% of the nitrate was found in the coarse mode (>1 μm diameter) and under clean marine conditions almost 100% conversion was seen. In addition, our data suggests strong evidence for dissolution/coagulation processes that also shift nitrate to the coarse mode. Furthermore, such processes are thought also to give rise to the size-shifting of aerosol ammonium, since significant coarse-mode fractions of this species (∼19–45%) were seen at both sites. A comparison of the relative importance of nitrate and ammonium in the overall dry deposition of inorganic fixed-nitrogen at each site indicates that at Weybourne the mass-weighted dry deposition velocity of the latter is around double that seen at Mace Head with its resultant contribution to the overall inorganic nitrogen dry flux exceeding that of nitrate.

Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ15N isotopic analysis. For both species a significant difference in mean isotopic composition was seen between samples from Weybourne ( +6±6‰ for ammonium and +7±6‰ for nitrate) and Mace Head and RRS Challenger campaigns ( −9±8‰ for ammonium and −1±3‰ for nitrate). At each site a strong dependence of isotopic composition on the geographical origin of the sampled air mass was also observed. For aerosol ammonium, marine and terrestrially dominated samples were found to be isotopically distinct, perhaps reflecting the presence of oceanic sources of ammonia in addition to anthropogenic or natural terrestrial sources. Further distinctions were made within terrestrially dominated samples, possibly indicative of different types of animal husbandry regimes or other forms of anthropogenic activity. For aerosol nitrate, there was found to be generally less variation between samples at each site, although at Weybourne a significant difference was observed between the mean isotopic composition of samples originating from the northern UK ( +15±3‰ ) and that of those originating from the southern UK (+10±3‰) , suggesting that aerosol δ15N data might possibly facilitate source apportionment between NOx emissions from power stations and those from vehicle exhausts. The nitrate data also appeared to show seasonality with higher concentrations and lower δ15N values seen in the summer.

Converting measured concentrations into fluxes and using estimates of biological productivity in the coastal waters of the eastern Atlantic Ocean enables us to determine the role of the atmosphere as a source of biologically essential species, including nitrate and ammonium, to the marine biota. To understand the effects of the atmosphere as a source of nitrogen capable of promoting new production, we need to know both the seasonality of the input as well as the effects of extreme high deposition events which, while small in overall annual budget terms, maybe able to extend, or even promote, phytoplankton growth under nutrient depleted summer conditions. Aerosols and rainwater were collected at both Mace Head and at sea aboard RRS Challenger. Temporal patterns have been interpreted using airmass back trajectories which give the predicted air path prior to arrival at the sampling site. Low levels of both nitrate and ammonium are seen associated with marine westerly flow across the Atlantic and northerly air originating in the Arctic region. As expected, marine derived sodium, chloride, magnesium and seasalt sulphate are high during these periods. High concentration nitrate and ammonium events are seen associated with south-easterly flow where the airmass passes over the UK and northern Europe prior to arrival on the west coast of Ireland. In the polluted atmosphere, nitrate exists as nitric acid and as fine mode (1 μm diameter) sodium nitrate: HNO3 (g) + NaCl(s) → NaNO3(s) + HCl(g). This seasalt displacement reaction not only enhances dry nitrate deposition through more efficient gravitational settling of large particles, but also increases the efficiency of precipitational scavenging via inertial impaction. By looking at the size distribution of nitrate, we can see evidence for the seasalt displacement reaction. Under the polluted south-easterly flow, ∼40−60% of the nitrate occurs in the coarse mode fraction. Under clean marine conditions, the seasalt displacement reaction results in almost complete conversion of nitrate from the fine to the coarse aerosol mode. By converting measured wet and dry nitrate, ammonium and organic nitrogen concentrations into fluxes and comparing the data with estimates of biological productivity in the surface waters, our data suggest that ∼30% of new production in eastern Atlantic surface waters off Ireland can be supported by atmospheric inputs in May 1997 and that most of the input occurs during short lived, high-concentration, south-easterly transport events.DOI: 10.1034/j.1600-0889.2000.00062.x

Organic complexation has been suggested to be an important mechanism by which the biogeochemistry of transition metals is altered in natural waters. We have conducted complexing ligand titrations on rainwater, collected during the winter and spring from a semi-urban U.K. location, using cathodic stripping voltammetry with tropolone as the added ligand. The results show that organic ligands capable of binding copper are present in all the precipitation samples measured. As a consequence of the method used, only those copper-natural ligand complexes with conditional stability constants (1 : 1 stoichiometry-log K CuL ′) between 11 and 14 have been identified. Within this stability region, ligand concentrations vary from 10.2 to 34.5 nM, very similar to the total copper levels which range between 10.2 and 33.2 nM. While total copper has a concentration of the order of 10 −8 M, free Cu 2+ ions exist at levels of just 10 −11 −10 −12 M, showing that copper is largely complexed in the rainwater samples measured. The existence of such low free ion concentrations must be considered when assessing the catalytic role of rain and aerosol trace metals in atmospheric reactions and determining the impact of atmospheric inputs on surface water biogeochemistry. Conditional stability constants increase with the detection window used (from log K CuL ′) = 11.4 at a detection window centred at log α CuAL = 2.68 to log K CuL ′) = 12.57 at log α CuAL = 4.47). As the method is specific to a limited range of conditional stabilities, dependent on the strength and amount of competitive ligand used, this concentration dependence suggests the presence of a wider range of natural ligands than those identified here. The source of the organic complexant in precipitation is unclear. We postulate, based on stable isotope measurements made on rains collected at the same location (Cornell et al. , 1995), that the organic ligands are likely to be terrestrial in origin.

THE concentrations of bioavailable iron in the surface waters of some ocean regions may indirectly modulate climate by controlling phytoplankton productivity and thus the amounts of carbon dioxide1 and dimethyl sulphide (DMS) that are exchanged with the atmosphere. Oxidation of DMS is involved in the formation of atmospheric sulphate particles, which can exert a climate cooling effect2 directly (by scattering and absorbing solar radiation), and indirectly (by affecting cloudiness and hence global albedo). But direct evidence supporting the hypothesis that DMS production in the ocean is affected by iron availability is lacking. Here we report changes in the concentrations of DMS in response to in situ iron-enrichment during two ecosystem-scale experiments designed to investigate the biological and chemical effects of iron fertilization of under-productive surface ocean waters3,4. The first such experiment revealed a limited overall biological response3 and no significant changes in DMS concentrations, although the concentrations of its biochemical precursor doubled5. The second experiment, designed to better mimic the natural process of iron enrichment, elicited a much stronger biological response4, and DMS concentrations increased by a factor of 3.5. This result provides direct support for an important link in the iron–DMS–climate hypothesis.

Experiments have been conducted to determine whether the speciation of the nitrate-nitrite and iodate-iodide redox couples is influenced by light and organic matter in both deionised and sea waters. The results obtained indicate that photochemical production of nitrite occurs and that the rate of this reaction is dependent on the concentration of organic matter present in the system. The findings suggest that nitrate undergoes not only direct photochemical reaction, acting itself as a primary chromophore, but also a photochemically induced reduction through organic matter. This reaction is, however, unlikely to influence productivity to any great extent as most phytoplankton can use both species as nutrients. It may, however, be a source of the highly reactive hydroxyl radical and of NO which has the potential to degas to the atmosphere and there help to regulate ozone concentration. Experiments conducted on the iodine redox system similarly indicate that photochemically induced production of iodide occurs. The results show that the reaction is not biologically mediated and that the presence of organic matter is essential for iodide photoproduction. At the concentrations of added iodate used in the experiments no direct 1:1 relationship between iodate and iodide is seen. It is suggested that, under these conditions, organic matter is not only essential for the photochemical production of iodide but that it is also the limiting species for the reduction reaction.

Previous work has shown that the type and pH history of an aerosol governs trace metal solubility in rainwater. This study concentrates on the crustal elements Al, Fe and Mn and identifies additional processes which affect dissolution not only in the atmosphere but also on mixing into seawater. Aerosol dissolution experiments (at aerosol concentrations of about 30 mg 1−1) show manganese exhibiting high solubility at the low pH values typical of clouds (54±2.5% at pH 2, with results expressed in mole percent units) with 85% of this increase occurring within 6 hours of acidification. The percentage dissolution decreases to 50% at pH values representative of rainwater (pH 5.5) and to 26±4% at pH 8, typical of seawater. No such dramatic solution phase removal occurs at pH 8 in the presence of inorganic anions (to a final solubility of 44±2%). Thus the extent of manganese dissolution depends strongly on whether aerosols are cycled through acidic environments and on subsequent inorganic complexation once rainwater mixes into sea. Aluminium shows highest dissolution (7.1±0.6%) at low pH with 78% of this increase occurring within 6 hours of acidification. Rapid solution phase removal occurs on increasing the pH to that representative of rainwater (to 0.9±0.4% with 87% of this decrease occurring within 15 min). As a consequence of acid cycling and aluminium's amphoteric nature, solubility is enhanced at seawater pH (2.3±0.3%) over that in rain. Iron shows a strong pH-solubility relationship with highest solubility at low pH (4.7±0.2%), 70% of this value being reached within 6 hours of acidification, and decreasing rapidly to 0.17% as pH is raised to 8. Addition of inorganic anions at pH 8 to simulate mixing into seawater causes a further decrease in solubility, perhaps due to anion induced colloid destabilisation. Photochemical reduction also effects solubility under low pH conditions with Fe(II) comprising 1% of the total iron in the Saharan Aerosol used and 8.4% in an Urban material at a pH of ≈ 2. This element shows rapid solution phase removal with increasing particulate load which is tentatively rationalised in terms of a simple Kd approach.

During an intercomparison field experiment, organized at the Atlantic coast station of Mace Head, Ireland, in April 1991, aerosol samples were collected by four research groups. A variety of samplers was used, combining both high- and low-volume devices, with different types of collection substrates: Hi-Vol Whatman 41 filter holders, single Nuclepore filters and stacked filter units, as well as PIXE cascade impactors. The samples were analyzed by each participating group, using in-house analytical techniques and procedures. The intercomparison of the daily concentrations for 15 elements, measured by two or more participants, revealed a good agreement for the low-volume samplers for the majority of the elements, but also indicated some specific analytical problems, owing to the very low concentrations of the non-sea-salt elements at the sampling site. With the Hi-Vol Whatman 41 filter sampler, on the other hand, much higher results were obtained in particular for the sea-salt and crustal elements. The discrepancy was dependent upon the wind speed and was attributed to a higher collection efficiency of the Hi-Vol sampler for the very coarse particles, as compared to the low-volume devices under high wind speed conditions. The elemental mass size distribution, as derived from parallel cascade impactor samplings by two groups, showed discrepancies in the submicrometer aerosol fraction, which were tentatively attributed to differences in stage cut-off diameters and/or to bounce-off or splintering effects on the quartz impactor slides used by one of the groups. However, the atmospheric concentrations (sums over all stages) were rather similar in the parallel impactor samples and were only slightly lower than those derived from stacked filter unit samples taken in parallel.

Experiments have been conducted to determine whether the speciation of the manganese and iron redox couples can be influenced by light and organic matter in seawater. The results indicate that photochemically induced reduction of Mn(IV) to Mn(II) occurs through the mediation of humic acids. The rate of the reaction is dependent on many variables, some of which are identified here. Stirring plays an important role in the reductive dissolution process, the effect of which is dependent on the organic loading. It is suggested that this results from an adsorptive competition between organic matter and photoproduced reductants such as H2O2. Photochemically induced reduction reaction, at least in part, controls the speciation and thus biological availability of manganese in surface seawater. The results obtained from studies of the iron redox system show generally poor reproducibility and no conclusive evidence for photochemically induced reduction of Fe(III) under seawater conditions. These results are perhaps understandable given the fast reoxidation reactions by hydrogen peroxide and oxygen at seawater pH and the use of membrane filtration to separate soluble and insoluble iron. Reproducible results are obtained in stirred seawater and show a gradual decrease in levels of 0.2 μm filterable iron over time. The light replicates show a slower rate of iron loss from solution than in the dark. The possible implications of these results for iron bioavailability in seawater are discussed.

The atmosphere has now been recognized as a major source of both trace metals and nutrients to the oceans, with wet deposition being a major contributor to the flux. Solution pH has been suggested to be the major control on metal solubility in rainwater, but for many trace metals this relationship is not a simple one. Aerosols are typically exposed to [approximately]10 condensation/evaporation cloud cycles before removal in rain and, as a result of H[sub 2]SO[sub 4] and HNO[sub 3] uptake and SO[sub 2] oxidation, cloudwater pH can be very low. Laboratory studies have been conducted, using Saharan and Urban aerosols, to assess the effect of pH on trace metal solubility. The results for the crustal elements Al and Fe in the Saharan aerosol show that metal solubility is a strong function of pH with highest solubilities seen under low pH conditions, comparable to those found in clouds. Increasing the pH to simulate neutralization of acidity by ammonia and crustal material results in almost complete removal of these elements from the solution phase, suggesting that a simple relationship between pH and solubility should exist in rainwater. For Al and Fe in the Urban aerosol there is evidence that some ofmore » the material solubilized at low pH is kept in solution at high pH, perhaps as the result of complexation by organic matter. Manganese shows high solubility after the initial acidification from both the Saharan and Urban materials with only slight removal from solution at increased pH. For this element it appears, therefore, that the pH-dependent dissolution process is not reversible. It is tentatively suggested that Fe in the Urban aerosol, under these experimental conditions, is under solubility control.« less

[1] Ammonium salts represent a major component of fine mode marine aerosol that play an important role in climate regulation, atmospheric pH control and nutrient supply to the oceans. We report here the results of analyses of aerosol samples collected in remote areas of the Atlantic Ocean. We show the isotopic abundance of aerosol ammonium varies with concentration, with isotopically light ammonium (−5 to −8‰) associated with the lowest ammonium concentrations compared to isotopically heavy ammonium (+10‰) associated with higher ammonium concentrations. We interpret the light ammonium signature as arising from marine emissions of ammonia. These results provide further evidence for the existence of a marine ammonia source and offers a method to define the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and thereby investigate the role of these emissions in climate regulation.

The flux of nitrogen species from the atmosphere into the ocean, with emphasis on coastal waters, was addressed during the ANICE project (Atmospheric Nitrogen Inputs into the Coastal Ecosystem). ANICE focused on quantifying the deposition of atmospheric inputs of inorganic nitrogen compounds (HNO3, NO3-, NH3 and NH4+) into the North Sea and the processes governing this deposition. The Southern North Sea was studied as a prototype. Because the physical and chemical processes are described, as opposed to empirical relations, the results can potentially be transferred to other regional seas like the Mediterranean, the North Atlantic continental shelf area and the Baltic. Two intensive field experiments were undertaken, centred around the offshore tower Meetpost Noordwijk and the Weybourne Atmospheric Observatory in East Anglia (UK). Long-term measurements were made on a ferry sailing between Hamburg and Harwich/Newcastle. These measurements provided data for sensitivity studies of a variety of problems associated with the coastal region that are not easily evaluated with larger scale models, to constrain models and to test model results. Concentrations of nitrogen compounds over the North Sea and the resulting deposition presented in this paper were obtained with the Lagrangian transport-chemistry model ACDEP. The average annual deposition in 1999 was 906kg Nkm-2. The results are compared with experimental data from the ferry. Effects of temporal and spatial variations are evaluated based on experimental results and small-scale model studies. In particular, effects of the aerosol size distribution on the nitrogen deposition are discussed. © 2003 Elsevier Ltd. All rights reserved.

This component of EUROTRAC has concentrated on two themes; the magnitude of atmospheric fluxes to the North Sea and the bioavailablity of such atmospheric inputs of metals as evidenced by their solubility. Good progress has now been made in both areas. The solubility of metals in rain-water is now reasonably well understood, and the atmospheric fluxes of metals and nitrogen to the North Sea have been quantified and the main factors controlling them rationalised. Furthermore much of what has been learnt about inputs to the North Sea is applicable to other coastal areas, providing evidence of several important processes in such environments which need to be included in transport and deposition models.

Within the EUROTRAC programme, the main objectives of our group were devoted to a better understanding of the homogenous and heterogeneous processes which control the atmospheric aqueous phase chemistry and the evolution of paniculate aerosol (mineral and carbonaceous) in cloud droplets, before their deposition. These objectives were conducted mainly in marine environment to study atmospheric systems rather homogenous and characterised by long residence times.

The Atmospheric Nitrogen Inputs into the Costal Ecosystem (ANICE) project was initiated to improve the accuracy and performance of model tools used to estimate the atmospheric nitrogen deposition to the sea. The experimental work includes the use of a ferry for long-term measurements of gaseous species, aerosols and rain water during one year, and two intensive field campaigns. During the first ANICE intensity experiment, situations were encountered in which the air flow connected the different stations. Differences in nitrate and ammonium concentrations between the various stations were analyzed in terms of the processes determining the concentrations of these species and to quantify their variations with fetch.

Rain water sampling in marine areas was undertaken by four groups working independently at the same time at the same place. The analytical results of the samples collected exhibit large variations and the main cause of these variations is the “non-rain deposition” as sea salt during the rain itself. The collected water is a mixture of rain water and variable amounts of more concentrated sea salt transported by the wind. This non-rain deposition is influenced by the exposure geometry of the sampler to the wind flow. Because sample conductivity and pH were measured using exactly the same protocol each time, a quantitative model based on simple conductivity and pH measurements is proposed to compute the amount of dissolved sea salt within samples collected under marine conditions. The acid balance of the rain water appears to be not influenced by this non rain deposition. Conclusions given by this work should be taken into account to ensure the data quality of rain sampling and analysis, especially if joint comparative experiments are conducted at different sites.

Atmospheric circulation patterns over the North Sea occasionally result in episodes of high concentrations of various contaminants. Synergistic chemical effects can act to enhance the deposition of nitrogen during these events resulting in highly episodic nitrogen inputs to the North Sea. Two such episodes are described and their causes interpreted. The problems of devising sampling strategies to quantify these events and assess their effects on phytoplankton communities are discussed.

Dissection of the basilar artery caused sudden coma and death in a 40-year-old man. Atypical clinical features were explained at necropsy. A ventral dissection of the artery within its outer layers resulted in destruction of the pontine tegmentum with sparing of the basis pontis. An unsuspected defect in the internal elastic lamina in the left internal carotid artery was also found suggesting a more generalised disorder of arterial walls. Basilar artery dissection should be considered in the diagnosis of coma in young people.

The ANICE (Atmospheric Nitrogen Inputs into the Coastal Ecosystem) project addressed the atmospheric deposition of nitrogen to the North Sea, with emphasis on coastal effects. ANlCE focused on quantifying the deposition of inorganic nitrogen compounds to the North Sea and the governing processes. An overview of the results from modelling and experimental efforts is presented. They serve to identify the role of the atmosphere as a source of biologically essential chemical species to the marine biota. Data from the Weybourne Atmospheric Observatory (UK) are used to evaluate the effect of short episodes with very high atmospheric nitrogen concentrations. One such episode resulted in an average deposition of 0.8 mmol N m-2 day-1, which has the potential to promote primary productivity of 5.3 mmol C m-2 day-1. This value is compared to long-term effects determined from model results. The total calculated atmospheric deposition to the North Sea in 1999 is 948 kg N km-1, i.e. 0.19 mmol N m-2 day -1 which has the potential to promote primary productivity of 1.2 mmol C m-2 day-1. Detailed results for August 1999 show strong gradients across the North Sea due to adjacent areas where emissions of NOx and NH3 are among the highest in Europe. The average atmospheric deposition to the southern part of the North Sea in August 1999 could potentially promote primary production of 2.0 mmol C m-2 day-1, i.e. ∼ 5.5% of the total production at this time of the year in this area of the North Sea. For the entire study area the atmospheric contribution to the primary production per m2 is about two-third of this value. Most of the deposition occurs during short periods with high atmospheric concentrations. This atmospheric nitrogen is almost entirely anthropogenic in origin and thus represents a human-induced perturbation of the ecosystem. © 2003 Elsevier Ltd. All rights reserved.

This paper summarises the results of the EU funded MEAD project, an interdisciplinary study of the effects of atmospheric nitrogen deposition on the Kattegat Sea between Denmark and Sweden. The study considers emissions of reactive nitrogen gases, their transport, transformations, deposition and effects on algal growth together with management options to reduce these effects. We conclude that atmospheric deposition is an important source of fixed nitrogen to the region particularly in summer, when nitrogen is the limiting nutrient for phytoplankton growth, and contributes to the overall eutrophication pressures in this region. However, we also conclude that it is unlikely that atmospheric deposition can, on its own, induce algal blooms in this region. A reduction of atmospheric nitrogen loads to this region will require strategies to reduce emissions of ammonia from local agriculture and Europe wide reductions in nitrous oxide emissions. 2005 Elsevier Ltd. All rights reserved.

Jickells, T.D. ... et. al.-- 34 pages, 6 tables, 10 figures We report the results of an experiment in the Northeast Atlantic in which sulphur hexafluoride (SF6) was released within an eddy and the behaviour of trace gases, nutrients and productivity followed within a Lagrangian framework over a period of 24 days. Measurements were also made in the air above the eddy in order to estimate air–sea exchange rates for some components. The physical, biological and biogeochemical properties of the eddy resemble those of other eddies studied in this area, suggesting that the results we report may be applicable beyond the specific eddy studied. During a period of low wind speed at the start of the experiment, we are able to quantitatively describe and balance the nutrient and carbon budgets for the eddy. We also report concentrations of various trace gases in the region which are similar to those observed in other studies and we estimate exchange rates for several trace gases. We show that the importance of gas exchange over other loss terms varies with time and also varies for the different gases. We show that the various trace gases considered (CO2, dimethyl sulphide (DMS), N2O, CH4, non-methane-hydrocarbons, methyl bromide, methyl iodide and volatile selenium species) are all influenced by physical and biological processes, but the overall distribution and temporal variability of individual gases are different to one another. A storm disrupted the stratification in the eddy during the experiment, resulting in enhanced nutrient supply to surface waters, enhanced gas exchange rates and a change in plankton community, which we quantify, although overall productivity was little changed. Emphasis is placed on the regularity of storms in the temperate ocean and the importance of these stochastic processes in such systems This is contribution no. ACP175 of the NERC ACSOE Thematic Programme. NERC grants GST/02/1276 supported the work of Jickells and Spokes, GST/02/1278 the work of Liss and Nightingale together with core funding to the Plymouth Marine Laboratory (PML) while the ACSOE core programme supported Broadgate and the shiptime costs, NERC grant GR9/3467 to G.S. supported Moncoiffé and Gilpin. Steinke was supported by NERC grant GR3/10956. The University of Newcastle upon Tyne Research Committee provided funding to support T. Frost. The work of Simó and Pedrós-Alió was supported by the Spanish McyT grant MAR97-1885-E