MEAD: An interdisciplinary study of the marine effects of atmospheric deposition in the Kattegat

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1 Environmental Pollution 14 (26) 453e462 MEAD: An interdisciplinary study of the marine effects of atmospheric deposition in the Kattegat L. Spokes a, T. Jickells a, *, K. Weston a, B.G. Gustafsson b, M. Johnsson b, B. Liljebladh b, D. Conley c, C. Ambelas-Skjødth c, J. Brandt c, J. Carstensen c, T. Christiansen c, L. Frohn c, G. Geernaert c, O. Hertel c, B. Jensen c, C. Lundsgaard c, S. Markager c, W. Martinsen, B. Møller c, B. Pedersen!, K. Sauerberg c, L.L. Sørensen d, C.C. Hasager d, A.M. Sempreviva e, S.C. Pryor f, S.W. Lund d, S. Larsen d, M. Tjernström g, G. Svensson g,m.zagar g a School of Environmental Sciences, University of East Anglia, East Anglia, UK b Department of Oceanography, Gøteborg University, Gøteborg, Sweden c National Environmental Research Institute, Roskilde, Denmark d Risø National Laboratory, Roskilde, Denmark e ISAC-CNR, Rome, Italy f Atmospheric Science Programme, Department of Geography, Indiana University, Bloomington, IN 4745, USA g Department of Meteorology, Stockholm University, Stockholm, Sweden Received 1 September 24; accepted 5 August 25 Atmospheric nitrogen deposition is an important factor in eutrophication processes in the Kattegat. Abstract 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. Ó 25 Elsevier Ltd. All rights reserved. Keywords: Nitrogen; Eutrophication; Atmospheric inputs; Kattegat 1. Introduction The coastal seas are amongst the most valuable resources on the planet but they are threatened by human activity. We * Corresponding author. Tel.: C ; fax: C address: t.jickells@uea.ac.uk (T. Jickells).! Deceased. rely on the coastal area for mineral resources, waste disposal, fisheries and recreation and the effective management of these conflicting uses requires collaborations between diverse groups of users, scientists and coastal managers (e.g. v. Bodungen and Turner, 21). In Europe, high population densities and high levels of industrial activity mean that the pressures on coastal seas are particularly acute. One of the main problems concerning coastal seas is the rapid increase in the amounts of nitrogen-based contaminants entering the water (henceforth /$ - see front matter Ó 25 Elsevier Ltd. All rights reserved. doi:1.116/j.envpol

2 454 L. Spokes et al. / Environmental Pollution 14 (26) 453e462 referred to as nitrogen, recognising it does not include unreactive N 2 gas). These nitrogen inputs, which come from many sources, particularly vehicles, industry and agriculture, can be used by phytoplankton as nutrients promoting an increase in primary production leading to a range of deleterious effects usually collectively described as eutrophication (Nixon, 1995; Jickells, 1998). Human activity has probably doubled the input of nitrogen to the environment globally (Galloway et al., 1995). In Europe the increases in nitrogen have been greater than this, leading to real concern over the health of coastal waters. Rivers have, until recently, been thought to be the most important source of nitrogen to the coastal seas, but we now know that nitrogen inputs from the atmosphere are large and can equal, or exceed, those from the rivers (e.g. Paerl, 1995; Valigura et al., 2), while atmospheric phosphorus inputs are relatively small (Jickells, 1998). Atmospheric nitrogen inputs are dominated by nitric acid/nitrate and ammonia/ ammonium inputs by wet and dry deposition. These have very different sources, reduced nitrogen primarily from animal farming and nitrate from combustion sources emitted as NO and NO 2. They also have different patterns of deposition with ammonia rapidly deposited and oxidised nitrogen only deposited after oxidation to nitric acid, a process that can take many hours or longer (see Spokes and Jickells, in press). The Kattegat (Fig. 1), the transitional area between the Baltic Sea and the North Sea, has received considerable attention, because intense ammonia and nitrogen oxide emissions in this region (see and uni-stuttgart.de/public/de/organisation/abt/tfu/projekte/genemis) result in high rates of atmospheric deposition (Hertel et al., 23). Symptoms of eutrophication are evident in this region (Møhlenberg, 1999; Meyer-Reil and Köster, 2). A Chrysochromulina sp. bloom in 1988, which first developed in the northern Kattegat, devastated fish farms along the western Scandinavian coast with significant economic losses (Moestrup, 1994). In the summer of 22, levels of oxygen in the Great Belt region were the lowest ever measured for that time of year (Ærtebjerg, 22), and there appears to be a long term trend of decreasing deep water oxygen in this area (Ærtebjerg et al., 23). The MEAD Project (Marine Effects of Atmospheric Deposition) investigated how inputs of nitrogen from the atmosphere affect the chemistry and biology of the Kattegat and how this information can be used to help manage this coastal area. MEAD represents a unique collaboration between atmospheric and marine modellers and field scientists including ecologists, biogeochemists and physical oceanographers. The particular challenge in considering the effects of atmospheric inputs to marine systems is the wide range of space and time scales involved and the differences between these in the atmosphere and marine systems (Fig. 2). For example, atmospheric deposition events characteristically last a period of hours to a day or so, while the development of phytoplankton blooms can take many days to weeks. Such differences in time and space scales must be considered in developing programmes to investigate the effects of atmospheric deposition. In the MEAD approach described here we assess the likelihood of bloom development assuming that the biggest N inputs are most likely to trigger blooms. This is generally true but a relatively small N input under suitable physical conditions can trigger a bloom (Kononen, 21). Nutrient starved algae can rapidly assimilate nutrient inputs, although any blooms will take time to develop. MEAD field work involved atmospheric and water column chemical and physical measurements using ships, automated buoys and coastal stations. Field work was concentrated in the summer when phytoplankton growth conditions are optimal, but nutrient availability is restricted, although other Denmark Kattegat Copenhagen Sweden horizontal scale 1 km 1 km 1 km 1 m turbulent mixing in the atmosphere NOx oxidation turbulent mixing in the water atmospheric mesoscale processes algal blooms water exchange seasonal overturn climatic variations 1 cm new particles atmospheric processes water column processes Fig. 1. Three stations for measurement of atmospheric chemical parameters and meteorology were established during the MEAD project. Station One was located in Lyngså on the east coast of Denmark, Station Two was at Vesterøhavn on the Danish Island of Læsø and Station Three was near the town of Bua in Sweden. Cruises and mooring deployments were conducted between the Island of Læsø and the Swedish coast. 1 sec 1 min 1 hour 1 day 1 month 1 year 1 decade Fig. 2. A diagramatic representation of the different space and time scales of key processes involved in the atmospheric input of nitrogen to marine systems and the response of those systems to this deposition. Note that marine processes tend to operate on longer time scales and smaller spatial scales than similar processes in the atmosphere.

3 L. Spokes et al. / Environmental Pollution 14 (26) 453e factors beside nutrient supply, such as water column stability and light availability, are of course important and incorporated in our models. Under summer conditions, atmospheric deposition events are most likely to have a significant impact. The results obtained in MEAD were incorporated in computer models that allowed us to determine how atmospheric pollutants are transported in the atmosphere, deposited to the ocean and how this affects the growth of phytoplankton. These models have then been used to predict whether changing the amounts and types of nitrogenous contaminants entering the atmosphere could affect phytoplankton growth in coastal waters. We have also used existing monitoring data on phytoplankton abundance in the Kattegat in a retrospective analysis to identify bloom events and test for any relation between these and atmospheric deposition. Here we summarise the results of this programme and consider management options for nitrogen inputs into this region. While the work of MEAD is clearly site specific, the approach used and some of the conclusions drawn are of broader applicability to coastal seas. 2. Materials and methods Most of the methods for the individual components of the MEAD work have been published elsewhere. In these cases we summarise the methods briefly and provide appropriate references. Methods for the statistical treatment of long term data sets are described within Section 3.1. Here we focus on MEAD experimental and modelling methods. MEAD undertook water column studies of physical, chemical and biological parameters to describe the cycling of nutrients and the biogeochemical controls on phytoplankton activity (Fig. 1 for locations). In addition, MEAD participants developed a biogeochemical model of the phytoplankton ecosystem and embedded this within a basin scale hydrodynamic model for the Kattegat, using the field data to help calibrate the model. Mesocosm experiments were run on integrated surface water samples and incubated in clear 24 L polycarbamate bottles for 4 days in deck incubations at 5% of natural solar radiation. Triplicate mesocosms were used for each manipulation. Samples were taken one to two times each day for analysis of size fractionated production, chlorophyll a, particulate organic carbon and nitrogen (POC/N), dissolved inorganic nutrients and pigment/phytoplankton composition. Zooplankton samples were taken at the start and at the end of each experiment. Grazing experiments were run using mesocosm water in order to estimate grazing rates using the method of Landry and Hassett (1982). These field data provide important information about the regulation of phytoplankton activity in the Kattegat but cannot on their own identify the factors regulating the incidence of algal blooms. We addressed this problem using a modelling approach. Full details of the MEAD physical and chemical models are published elsewhere (Gustafsson, 2; Carstensen et al., 25), and we only present the pertinent results in this paper. The coupled biological-physical model was driven by the hydrodynamic model of Gustafsson (2) with a pelagic food-web developed by Vallino (2) calibrated with data from the Danish Aquatic Monitoring and Assessment Program and with data collected during the MEAD cruises. The model allows us to calculate the nitrogen inputs to surface waters from vertical mixing of deep waters. These values can then be compared to estimates of nitrogen inputs from rivers and from the atmosphere. MEAD conducted year round monitoring and intensive atmospheric process study campaigns at sites in and around the Kattegat (Fig. 1) and used these with models (Zagar et al., 23, in press) to estimate atmospheric nitrogen deposition fluxes. Wet deposition and particulate phase nitrogen deposition were estimated using size resolved deposition velocities (Spokes et al., 2) Gas phase deposition of ammonia and nitric acid were measured directly using relaxed eddy accumulation and detailed near surface profiles (Pryor and Sørenson, 22). 3. Results and discussion 3.1. Long term monitoring evidence for phytoplankton blooms in the Kattegat and the mechanisms which regulate them MEAD initially investigated the available data for bloom occurrence based on historic monitoring in Swedish and Danish coastal waters (see MEAD/). Using this database, statistical analyses were carried out to investigate the potential causeeeffect relationships between atmospheric deposition and phytoplankton abundance based on water column chlorophyll a measurements. The overall aim was to describe potential underlying mechanisms generating blooms. An operational statistical definition of blooms as observations exceeding the 97.5 percentile, based on the station specific chlorophyll a distributions, was formulated (Carstensen et al., 24), and chlorophyll a data were segregated into bloom and non-bloom situations. Trends and spatial patterns of the observed blooms were determined and differences in physical and chemical variables for blooms and non-blooms at all stations were investigated using a two-way analysis of variance (ANOVA) method. Highest average chlorophyll a concentrations (Fig. 3a) were seen in those parts of the Kattegat influenced by major freshwater tributaries as well as in the frontal area between the Kattegat and Skagerrak which has previously been noted to be relatively more productive (Richardson, 1985). Lowest concentrations were seen in the central south-eastern part of the Kattegat, a region dominated by Baltic Sea water. The number of blooms divided by the total number of observations (average of 8.7% over 36 stations, 1989e1999), however, revealed a different spatial pattern with blooms occurring more frequently in the western part of the Kattegat (Fig. 3b). Moreover, the spatial extent of a bloom was often limited to a localised region of the Kattegat. In general incidences of blooms are relatively low (Fig. 3b). The frequency of blooms in the Kattegat has remained relatively constant over the 199s, but interannual variations have been linked to external inputs of nitrogen (Carstensen et al., 24). This suggests that, on longer time scales, the frequency of blooms in the Kattegat has increased over time as inputs to the Baltic have risen throughout the industrial era. The ANOVA analysis revealed that bloom situations were associated with increases in salinity, nutrients, and decreases in temperature as well as increases in wind speed 3e5 days prior to the observed bloom. This suggests that blooms are most likely caused by entrainment of nutrient rich, higher salinity deep water into surface layers during wind forced mixing (Carstensen et al., 24). Moreover, summer blooms were dominated by large species indicating the possibility that size-dependent grazing could affect the development and probably also the fate of phytoplankton blooms.

4 456 L. Spokes et al. / Environmental Pollution 14 (26) 453e462 a) b) Fig. 3. (a) The spatial distribution of average surface chlorophyll a concentrations in mg L ÿ1 (May to August 1989e1999) in the Kattegat and (b) the spatial distribution of bloom frequencies as probabilities of observing a summer bloom. Dots show the monitoring stations used for the analysis and the spatial interpolation was done by ordinary Kriging. Results from Carstensen et al. (24) Marine nitrogen cycling in the Kattegat Mesocosm measurements confirm that the surface water phytoplankton of the Kattegat respond to nitrogen addition (Fig. 4) but generally not to phosphorus, since error bars for controls and P addition treatments overlap, while the response to N addition is significantly different. This justifies the MEAD focus on nitrogen inputs. However, measured grazing rates in the Kattegat are also very high, with 2e3% of the phytoplankton being grazed each day, providing the potential for nitrogen inputs to be rapidly assimilated into the food chain, rather than necessarily triggering blooms. chlorophyll µg L control t = control.5 N 1 N 5 N.5 P 1 N +.5 P Fig. 4. An example of the response of phytoplankton biomass (as measured in chlorophyll a concentrations) to nitrogen additions during a mesocosm experiment in August 2. Error bars represent standard deviations from replicate treatments. The addition of.5 mmol L ÿ1 NH 4 C produces a small but significant response and we have converted this to an equivalent flux to the surface layer for comparison to water column mixing and atmospheric deposition fluxes in Fig. 6. The physical and biogeochemical model estimates of inputs to the Kattegat indicate that there is considerable interannual variability in nitrogen inputs and a strong seasonality. Maximum inputs occur in the winter, dominated by riverine inputs and by wind induced vertical mixing, the latter being highly episodic. In summer, with low wind speeds and low river flows, atmospheric inputs dominate (Figs. 5 and 6). Variations in modelled phytoplankton biomass agree well with the measured data (Fig. 7) and show a pronounced biomass peak during the spring bloom and more constant concentrations during the summer, autumn and winter. The onset of nitrogen load (kt N y -1 ) nitrogen load (kt month -1 ) External sources of nitrogen land load upwelled flux Land load Upwelling Atmospheric Deposition atmospheric deposition J F M A M J J A S O N D Fig. 5. Estimated external inputs of nitrogen to the Kattegat showing (a) interannual variations and (b) seasonal variations.

5 L. Spokes et al. / Environmental Pollution 14 (26) 453e a DIN flux (mg m -2 day -1 ) Southern Kattegat Central Kattegat DIN flux (mg m -2 day -1 ) Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 DIN flux (mg m -2 day -1 ) Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 b Kattegat area, daily values ATM. DEP [mg N/m 2 ] WET DEP [mg N/m 2 ] DRY DEP [mg N/m 2 ] PREC. [mm/day] APR MAY JUN JUL AUG SEP 1999 Fig. 6. (a) Modelled entrainment fluxes of nitrogen (in mg N m ÿ2 d ÿ1 ) for the period 1976e1999. Close-ups for the summers of 1989 and 1991 show that there is substantial variability in the flux between years. (b) Modelled atmospheric nitrogen deposition to the Kattegat using the ACDEP trajectory model, showing that wet deposition dominates the input of nitrogen to the Kattegat. In both figures, the line at 7 mg N m ÿ2 d ÿ1 (equivalent to a nitrogen concentration of.5 mmol L ÿ1 mixed into a 1 m water column, Fig. 4) is the statistically determined 1 year return of extreme atmospheric deposition (Carstensen et al., 24). It also represents a threshold for response based on mesocosm results see Fig. 4. This value has been subsequently used in the biological studies to represent an extreme atmospheric deposition event. the spring bloom occurs when incoming solar radiation is sufficiently high to promote phytoplankton growth. Nitrogen is stored in the water column during the winter months and is rapidly consumed during the spring bloom which begins in February and usually peaks in March. Following the spring bloom, phytoplankton growth is nitrogen limited for the remainder of the growth season. Output from the model suggests that the spring bloom ultimately fuels the growth of the grazer community during the summer. The mesocosm experiments and bioassays show that phytoplankton are limited by nitrogen

6 458 L. Spokes et al. / Environmental Pollution 14 (26) 453e462 Fig. 7. Temporal variation in measured and modelled (a) biomass, (b) inorganic nitrogen concentration and (c) grazer biomass. The x-axis is time in days between January 1, 1989 and December 31, Measured biomass is determined by multiplying measured chlorophyll a concentrations with a particulate organic carbon (POC) to chlorophyll a ratio equal to 4.2 mmol L ÿ1 C:1 mg L ÿ1 chl a. availability. However, the lack of biomass accumulation in the mesocosm experiments, coupled with the high measured grazing rates show that phytoplankton biomass is controlled both by limited nitrogen availability and by grazing. The good agreement between measured and modelled values through the period 1989 to 1998 has allowed us to extrapolate the model to the 1999 to 21 period and to determine the likely response of the biological community to changing inputs of nitrogen from the atmosphere (see Section 3.4) Atmospheric inputs of nitrogen to the Kattegat Gases and aerosols are removed from the atmosphere to the coastal seas by wet and dry deposition. Gases are also removed from the atmosphere onto aerosols through heterogeneous chemical reactions, particularly those involving seasalt. Reactions with seasalt are particularly important in coastal areas where the input of seasalt aerosols to polluted air masses occurs as marine and continental air mix (Jickells, 1998; Russell et al., 23). MEAD results demonstrate that seasalt aerosols are found well inland and this means that the role of seasalt interactions with atmospheric nitrogen species needs to be considered over land near the coast as well as over the sea. Gas phase HNO 3 reacts rapidly with aerosols such as seasalt, shifting nitrate onto the particulate phase. This process can enhance nitrate deposition by moving the nitrate onto larger aerosols. This effect has to be balanced against reduced deposition rates for HNO 3 gas and the overall balance of these processes is still uncertain (Jickells, 1998; Pryor and Barthelmie, 2; Russell et al., 23). Since ammonia has a very high deposition velocity, it is rapidly deposited, primarily as the gas, and therefore its deposition is fundamentally controlled by the local turbulence intensity in the region. Atmospheric turbulence is created by a combination of mechanical and thermal interactions between the atmosphere and the land or sea surface. Atmospheric measurements and high resolution modelling conducted during MEAD have shown that turbulence intensity in the Kattegat region is much more spatially and temporally variable than previously thought. Thus the entire Kattegat Sea region is influenced by the presence

7 L. Spokes et al. / Environmental Pollution 14 (26) 453e of the adjacent coastline and by the islands within the Kattegat. This means that nowhere in the atmosphere over the Kattegat are conditions such that continental influences can be excluded, neither in chemical constituents, nor in the small-scale meteorological conditions. This influence is manifested as horizontal gradients in practically all parameters, as heterogeneous and complex meteorological conditions (jets, sea breeze circulations, etc.) or both (Zagar et al., in press). Thus it is not possible to describe atmospheric transport and deposition using relatively simple terrestrial or oceanic parameterisations, but rather the individual atmospheric situation must be modelled. This conclusion is probably true for all coastal regions and suggests that coastal shelf regions need to be treated as atmospheric transitional zones between marine and terrestrial systems on scales of many tens of kilometres at least (Zagar et al., 23). Using improved estimates of atmospheric turbulence in coastal regions, MEAD results have shown that dry deposition of gas phase ammonia close to the coast is much more important than previously thought and can be an important source of nitrogen to the Kattegat (Pryor and Sørensen, 22), particularly in view of the large and daily variable (Ambelas Skøth et al., 24) Danish ammonia emissions. Most operational modelling is carried out at a coarse resolutions (O15 km) but MEAD studies using a 5 km grid size show that local dry deposition can exceed the basin average by at least an order of magnitude or more (Zagar et al., 23). More detailed modelling is required to provide a more complete and detailed picture. During dry periods the dry deposition of nitrogen is very important. However MEAD campaigns, long term monitoring and modelling all indicate that, on long time scales, atmospheric deposition to the central Kattegat is dominated by wet deposition (Fig. 6). This input is highly episodic and large nitrogen inputs can occur during short rain events. For example, a single rain event delivered over 4% of the total monthly rain flux in May 21. Such large rain events are not uncommon in this and many other regions (Spokes and Jickells, in press; Valigura et al., 2). This emphasises that long term data sets are needed in order to accurately quantify the magnitude and variability of atmospheric deposition External inputs of nitrogen to surface waters, a budget approach The results of the MEAD assessment of the significance of atmospheric inputs of nitrogen to the Kattegat are summarised in Fig. 8. The average annual nitrogen budget for the Kattegat Sea shows that ~24% of the external nitrogen input to the region is from the atmosphere while twice this amount enters through deep water entrainment. This suggest that, on an annual basis, the atmosphere is not the dominant source of nitrogen. However, the atmosphere is more important in the summer (Fig. 5). During this summer period, approximately 4% of the external nitrogen load to the surface waters is from the atmosphere. However, only extreme atmospheric deposition events are likely to be sufficiently large to produce direct effects on Skagerrak From Land Deep water 47 3 Mixed layer 12 Atm. deposition Denitrification the phytoplankton community (Fig. 6). Results from experimental and modelling studies and from the MEAD retrospective analysis suggest that atmospheric inputs are unlikely alone to cause phytoplankton blooms in the Kattegat (Carstensen et al., 25). Deep water entrainment of nutrient rich bottom water during storm events is a much more likely cause since the model results indicate this source is the largest source of short term N inputs. Atmospheric deposition and water column mixing are, however, correlated, mainly because both are, to some extent, related to wind velocity, and the nitrogen input from both processes fuels phytoplankton growth (Hasager et al., 23). Considering the aggregated supply of nitrogen to the surface waters, there are many summer days (Fig. 6a) where the external supply of nitrogen has the potential to result in an increase in chlorophyll a above.5 mg L ÿ1 (a realistic daily increase chlorophyll a required to sustain a bloom). The dominant source of nitrogen on such days is water column mixing (Carstensen et al., 25). So although atmospheric deposition is, on average, the dominant source of nitrogen to surface waters in the summer, the periods with excess nitrogen supply likely to result in bloom development are dominated by fluxes from below the pycnocline. Importantly, however, the atmosphere does represent a significant source of nitrogen to the Kattegat on an annual basis and, as a result, contributes to the total water column nutrient load in surface and deep waters and hence to eutrophication pressures. Atmospheric inputs may also act to sustain bloom development initiated by N inputs from below as discussed earlier Management options to reduce atmospheric N inputs Baltic Reducing nitrogen loads to the Kattegat will reduce eutrophication pressures in the region. Here we consider various emission reduction scenarios and estimate their impact on phytoplankton activity in the Kattegat. As noted earlier, ammonia is rapidly deposited close to the emission source while nitrate undergoes long range transport before deposition. We estimate, based on results from the Lagrangian air Permanent burial Denitrification Fig. 8. Budget of biologically active nitrogen for the Kattegat (units kt yr ÿ1 ) differentiated into fluxes into the mixed layer and deep water (grey).

8 46 L. Spokes et al. / Environmental Pollution 14 (26) 453e462 pollution model ACDEP (Hertel et al., 23), that almost half of the ammonia input to the Kattegat is from Danish sources but less than 1% of the oxidised nitrogen is. Hence different emission management strategies are required for the two dominant forms of inorganic nitrogen. Several emission reduction strategy scenarios have been run with the ACDEP model in order to provide atmospheric nitrogen input data to the MEAD marine model. The impact of changing agricultural practices was investigated by reducing ammonia emissions and the effect of decreasing emissions from combustion sources such as road traffic and industry was determined through reduction in nitrogen oxide (NO x ) levels. Scenarios considered modifications to emissions throughout Europe and at a more local level focussing on Denmark as the land area immediately upwind under prevailing westerly winds. As with all scenarios the options considered were meant to be illustrative rather than reflect specific policy options Scenario modelling The emission reduction scenarios tested with the ACDEP model were: 5% reduction in all NH 3 emissions 5% reduction in all NO x emissions 5% reduction in all NH 3 and NO x emissions no Danish emissions. The results (Table 1) show that Danish emissions contribute about 2% of the atmospheric nitrogen deposition to the Kattegat. A 5% reduction in all ammonia emissions and a 5% reduction in all nitrogen oxide emissions across Europe each lead to an approximately 2% reduction in atmospheric nitrogen deposition. This 5% reduction in both ammonia and nitrogen oxide emissions is predicted to reduce atmospheric nitrogen depositions by only approximately 4% because of long range transport into the area and because some of the emitted material is transported beyond the Kattegat. Reducing sulphur dioxide levels appears to have little effect on nitrogen deposition to the Kattegat even though formation of ammonium sulphate aerosols is a very important process in the atmosphere. Using these revised estimates of atmospheric nitrogen inputs in the marine coupled hydrodynamic-biochemical model has allowed us to estimate how changing nitrogen emissions Table 1 Percentage reduction in nitrogen deposition to the Kattegat following decreases in nitrogen emissions to the atmosphere Scenario Percentage change in nitrogen deposition to the Kattegat No Danish emissions ÿ2 ÿ17 ÿ24 ÿ5% NH 3 emissions ÿ21 ÿ22 ÿ19 ÿ5% NO x emissions ÿ24 ÿ25 ÿ21 ÿ5% NH 3 and NO x emissions ÿ43 ÿ41 ÿ4 Table 2 Changes in summer primary production in the Kattegat Sea for different emission reductions scenarios Scenario Percentage change in primary production No Danish emissions ÿ4. ÿ3.2 ÿ6. ÿ5% NH 3 emissions ÿ4. ÿ4.2 ÿ4.2 ÿ5% NO x emissions ÿ5. ÿ4.2 ÿ5. ÿ5% NH 3 and NO x emissions ÿ8.6 ÿ7.6 ÿ8.8 will impact primary production in the Kattegat. The effect of reduction scenarios was quantified in terms of summer (May to August) primary production, biomass and bloom frequency. The summer primary production from the various scenarios are shown in Table 2. The predicted change in summer production approximately follows the change in load and the results suggest that for every 1% reduction in atmospheric load, a 2% reduction in primary production is seen. The effect is not directly proportional because recycling of nitrogen from water column organic and particulate N reservoirs sustains a component of the primary productivity in coastal seas (Weston et al., 24). The results in Table 2 shows that the model predicts that large reductions in atmospheric deposition will result in modest reductions in biological production. The effect of nitrogen deposition on bloom frequency during the summer has also been calculated and the model also predicts slightly reduced biomass and bloom magnitude (results not shown). 4. Conclusions Our statistical analysis shows that summer phytoplankton blooms within the Kattegat are rare. Results from the MEAD project suggest that the main factor regulating nitrogen supply and bloom occurrence in the Kattegat is entrainment of nitrogen from deep waters. Other than those results available from the continuous plankton recorder surveys (e.g. Reid et al., 1998), the MEAD retrospective analysis is one of the few systematic studies of bloom frequencies carried out (Hasager et al., 23). These results were supported by modelled fluxes from the ACDEP atmospheric deposition model (Hertel et al., 23) and a hydrodynamical model of the Baltic Region (Gustafsson, 2). Model results showed that nitrogen input through entrainment is highly episodic and carries more nitrogen to the surface layer during short-term pulses (!1 week) than atmospheric deposition events do (Carstensen et al., 24). This analysis of the modelled fluxes combined with the observed blooms also showed that a substantial portion of the blooms were generated in the frontal area of the northern Kattegat through entrainment mainly and thereafter transported to other parts of the Kattegat and adjacent seas. Atmospheric deposition fluxes were correlated with the entrainment fluxes, although with a minor nitrogen contribution (1e2%). Thus, atmospheric deposition plays a minor direct role in combination with entrainment mechanisms for the formation of blooms in the Kattegat, although the possibility of large local deposition needs to be further investigated using

9 L. Spokes et al. / Environmental Pollution 14 (26) 453e high-resolution modelling. Atmospheric deposition may contribute to the sustaining of algal blooms once initiated. Atmospheric inputs make an important contribution to the overall nitrogen economy of the Kattegat both directly and via their contribution to fluvial inputs and lateral transport from the Baltic Sea and hence to the total eutrophication pressure. Although bloom events are rare, they can be very damaging and it is clear that reductions in the overall nitrogen stock are likely to reduce bloom intensity. Reductions of atmospheric nitrogen inputs need to consider both nitrate and ammonia and possibly organic nitrogen (Cornell et al., 23). The atmospheric chemistry of nitrate and ammonia are very different despite their close correlation in the atmosphere (Spokes and Jickells, in press). In particular, ammonia is much more rapidly deposited than NO x because this component must first be oxidised to nitric acid and nitrate aerosols before significant deposition can occur. Existing monitoring and modelling efforts probably underestimate the role of ammonia deposition, particularly in coastal waters close to emission sources and high resolution physical modelling is required to correctly describe the atmospheric turbulence in coastal areas which governs ammonia deposition. The differing sources and atmospheric chemistry of oxidised and reduced nitrogen means that regulation of ammonia inputs requires management of local, primarily agricultural, emissions, while regulation of nitrate requires European wide regulation of combustion sources. Since both inputs are of similar magnitude, substantial reductions of atmospheric nitrogen inputs to the Kattegat, and other coastal seas require, in practice, strategies to reduce both inputs. MEAD has demonstrated that marine and atmospheric scientists working together with a combined modelling and measurement campaign can tackle complex problems that cross traditional scientific boundaries and deliver results with direct policy implications. The specific results of MEAD are of course directly relevant to the Kattegat, but the approach adopted and the general principles developed can be applied to any coastal sea. Acknowledgements MEAD was funded by the EU under contract number EVK3-CT We thank all who helped with fieldwork, particularly the crew of the Skagerrak. References Ambelas Skøth, C., Hertel, O., Gyldenjerne, S., Ellerman, T., 24. Implementing a dynamical ammonia emission parameterisation in the large-scale air pollution model ACDEP. Journal of Geophysical Research 19, D6 (Art. No. D636 MAR 23 24). Ærtebjerg, G., 22. Monitoring Cruise Report. NERI, Denmark. Ærtebjerg, G., Andersen, J.H., Hansen, O.H., 23. Nutrients and eutrophication in Danish marine waters. 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