Environmental Pollution 140 (2006) 453e462www.elsevier.com/locate/envpol

MEAD: An interdisciplinary study of the marine effectsof 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. Tjernstrom g, G. Svensson g, M. �Zagar g

a School of Environmental Sciences, University of East Anglia, East Anglia, UKb Department of Oceanography, Gøteborg University, Gøteborg, Sweden

c National Environmental Research Institute, Roskilde, Denmarkd Risø National Laboratory, Roskilde, Denmark

e ISAC-CNR, Rome, Italyf Atmospheric Science Programme, Department of Geography, Indiana University, Bloomington, IN 47405, USA

g Department of Meteorology, Stockholm University, Stockholm, Sweden

Received 10 September 2004; accepted 5 August 2005

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

Keywords: Nitrogen; Eutrophication; Atmospheric inputs; Kattegat

1. Introduction

The coastal seas are amongst the most valuable resourceson the planet but they are threatened by human activity. We

* Corresponding author. Tel.: C44 1603 593117; fax: C44 1603 591327.

E-mail address: t.jickells@uea.ac.uk (T. Jickells).! Deceased.

0269-7491/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2005.08.010

rely on the coastal area for mineral resources, waste disposal,fisheries and recreation and the effective management of theseconflicting uses requires collaborations between diverse groupsof users, scientists and coastal managers (e.g. v. Bodungen andTurner, 2001). In Europe, high population densities and highlevels of industrial activity mean that the pressures on coastalseas are particularly acute. One of the main problems con-cerning coastal seas is the rapid increase in the amounts ofnitrogen-based contaminants entering the water (henceforth

454 L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

referred to as nitrogen, recognising it does not include unreac-tive N2 gas). These nitrogen inputs, which come from manysources, particularly vehicles, industry and agriculture, canbe used by phytoplankton as nutrients promoting an increasein primary production leading to a range of deleterious effectsusually collectively described as eutrophication (Nixon, 1995;Jickells, 1998). Human activity has probably doubled the inputof nitrogen to the environment globally (Galloway et al.,1995). In Europe the increases in nitrogen have been greaterthan this, leading to real concern over the health of coastalwaters. Rivers have, until recently, been thought to be themost important source of nitrogen to the coastal seas, but wenow know that nitrogen inputs from the atmosphere are largeand can equal, or exceed, those from the rivers (e.g. Paerl,1995; Valigura et al., 2000), while atmospheric phosphorus in-puts are relatively small (Jickells, 1998). Atmospheric nitro-gen inputs are dominated by nitric acid/nitrate and ammonia/ammonium inputs by wet and dry deposition. These havevery different sources, reduced nitrogen primarily from animalfarming and nitrate from combustion sources emitted as NOand NO2. They also have different patterns of depositionwith ammonia rapidly deposited and oxidised nitrogen onlydeposited after oxidation to nitric acid, a process that cantake many hours or longer (see Spokes and Jickells, inpress).

The Kattegat (Fig. 1), the transitional area between theBaltic Sea and the North Sea, has received considerable atten-tion, because intense ammonia and nitrogen oxide emissionsin this region (see http://www.emep.int and http://www.ier.uni-stuttgart.de/public/de/organisation/abt/tfu/projekte/genemis)result in high rates of atmospheric deposition (Hertel et al.,2003). Symptoms of eutrophication are evident in this region(Møhlenberg, 1999; Meyer-Reil and Koster, 2000). A Chryso-chromulina sp. bloom in 1988, which first developed in thenorthern Kattegat, devastated fish farms along the western

3

21

Denmark

Sweden

Kattegat

Copenhagen

Fig. 1. Three stations for measurement of atmospheric chemical parameters

and meteorology were established during the MEAD project. Station One

was located in Lyngsa 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.

Scandinavian coast with significant economic losses(Moestrup, 1994). In the summer of 2002, levels of oxygenin the Great Belt region were the lowest ever measured forthat time of year (Ærtebjerg, 2002), and there appears to bea long term trend of decreasing deep water oxygen in thisarea (Ærtebjerg et al., 2003).

The MEAD Project (Marine Effects of Atmospheric Depo-sition) investigated how inputs of nitrogen from the atmo-sphere affect the chemistry and biology of the Kattegat andhow this information can be used to help manage this coastalarea. MEAD represents a unique collaboration between atmo-spheric and marine modellers and field scientists includingecologists, biogeochemists and physical oceanographers. Theparticular challenge in considering the effects of atmosphericinputs to marine systems is the wide range of space andtime scales involved and the differences between these inthe atmosphere and marine systems (Fig. 2). For example,atmospheric deposition events characteristically last a periodof hours to a day or so, while the development of phytoplank-ton blooms can take many days to weeks. Such differencesin time and space scales must be considered in developingprogrammes to investigate the effects of atmosphericdeposition.

In the MEAD approach described here we assess the likeli-hood of bloom development assuming that the biggest Ninputs are most likely to trigger blooms. This is generally truebut a relatively small N input under suitable physical condi-tions can trigger a bloom (Kononen, 2001). Nutrient starvedalgae can rapidly assimilate nutrient inputs, although anyblooms will take time to develop.

MEAD field work involved atmospheric and water columnchemical and physical measurements using ships, automatedbuoys and coastal stations. Field work was concentrated inthe summer when phytoplankton growth conditions are opti-mal, but nutrient availability is restricted, although other

atmosphericmesoscaleprocesses

water column processes

1 sec 1 min 1 hour 1 day 1 month 1 year 1 decade

1 cm

1 m

1 km

10 km

100 km

hori

zont

al s

cale

climaticvariations

atmospheric processes

algalblooms

NOx oxidation

turbulentmixing in theatmosphere

seasonaloverturnwater

exchange

newparticles

turbulentmixing inthe water

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.

455L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

factors beside nutrient supply, such as water column stabilityand light availability, are of course important and incorporatedin our models. Under summer conditions, atmospheric de-position events are most likely to have a significant impact.The results obtained in MEAD were incorporated in computermodels that allowed us to determine how atmospheric pol-lutants are transported in the atmosphere, deposited to theocean and how this affects the growth of phytoplankton.These models have then been used to predict whether chang-ing the amounts and types of nitrogenous contaminants enter-ing the atmosphere could affect phytoplankton growth incoastal waters. We have also used existing monitoring data onphytoplankton abundance in the Kattegat in a retrospectiveanalysis to identify bloom events and test for any relation be-tween these and atmospheric deposition. Here we summarisethe results of this programme and consider management op-tions for nitrogen inputs into this region. While the work ofMEAD is clearly site specific, the approach used and someof the conclusions drawn are of broader applicability tocoastal 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 treat-

ment 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 embed-

ded 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 50% 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 phy-

toplankton 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, 2000; Carstensen et al., 2005), and we

only present the pertinent results in this paper. The coupled biological-physical

model was driven by the hydrodynamic model of Gustafsson (2000) with a

pelagic food-web developed by Vallino (2000) 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 pro-

cess study campaigns at sites in and around the Kattegat (Fig. 1) and used

these with models (�Zagar et al., 2003, in press) to estimate atmospheric nitro-

gen deposition fluxes. Wet deposition and particulate phase nitrogen deposi-

tion were estimated using size resolved deposition velocities (Spokes et al.,

2000) Gas phase deposition of ammonia and nitric acid were measured directly

using relaxed eddy accumulation and detailed near surface profiles (Pryor and

Sørenson, 2002).

3. Results and discussion

3.1. Long term monitoring evidence for phytoplanktonblooms in the Kattegat and the mechanisms whichregulate them

MEAD initially investigated the available data for bloomoccurrence based on historic monitoring in Swedish and Danishcoastal waters (see http://www.dmu.dk/AtmosphericEnvironment/MEAD/). Using this database, statistical analyses were carriedout to investigate the potential causeeeffect relationshipsbetween atmospheric deposition and phytoplankton abun-dance based on water column chlorophyll a measurements.The overall aim was to describe potential underlying mecha-nisms generating blooms. An operational statistical definitionof blooms as observations exceeding the 97.5 percentile, basedon the station specific chlorophyll a distributions, was formulat-ed (Carstensen et al., 2004), and chlorophyll a data were segre-gated into bloom and non-bloom situations. Trends and spatialpatterns of the observed blooms were determined and differen-ces in physical and chemical variables for blooms andnon-blooms at all stations were investigated using a two-wayanalysis of variance (ANOVA) method.

Highest average chlorophyll a concentrations (Fig. 3a)were seen in those parts of the Kattegat influenced by majorfreshwater tributaries as well as in the frontal area betweenthe Kattegat and Skagerrak which has previously been notedto be relatively more productive (Richardson, 1985). Lowestconcentrations were seen in the central south-eastern part ofthe Kattegat, a region dominated by Baltic Sea water. Thenumber 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 occurringmore frequently in the western part of the Kattegat (Fig. 3b).Moreover, the spatial extent of a bloom was often limited toa localised region of the Kattegat. In general incidences ofblooms are relatively low (Fig. 3b). The frequency of bloomsin the Kattegat has remained relatively constant over the1990s, but interannual variations have been linked to externalinputs of nitrogen (Carstensen et al., 2004). This suggests that,on longer time scales, the frequency of blooms in the Kattegathas increased over time as inputs to the Baltic have risenthroughout the industrial era.

The ANOVA analysis revealed that bloom situations wereassociated with increases in salinity, nutrients, and decreasesin temperature as well as increases in wind speed 3e5 daysprior to the observed bloom. This suggests that blooms aremost likely caused by entrainment of nutrient rich, highersalinity deep water into surface layers during wind forcedmixing (Carstensen et al., 2004). Moreover, summer bloomswere dominated by large species indicating the possibilitythat size-dependent grazing could affect the developmentand probably also the fate of phytoplankton blooms.

456 L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

a) b)

Fig. 3. (a) The spatial distribution of average surface chlorophyll a concentrations in mg L�1 (May to August 1989e1999) in the Kattegat and (b) the spatial dis-

tribution 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. (2004).

3.2. Marine nitrogen cycling in the Kattegat

Mesocosm measurements confirm that the surface waterphytoplankton of the Kattegat respond to nitrogen addition(Fig. 4) but generally not to phosphorus, since error bars forcontrols and P addition treatments overlap, while the responseto N addition is significantly different. This justifies theMEAD focus on nitrogen inputs. However, measured grazingrates in the Kattegat are also very high, with 20e30% of thephytoplankton being grazed each day, providing the potentialfor nitrogen inputs to be rapidly assimilated into the foodchain, rather than necessarily triggering blooms.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

control 0.5 N 1 N 5 N 0.5 P 1 N +0.5 P

controlt = 0

chlo

roph

yll µ

g L-1

Fig. 4. An example of the response of phytoplankton biomass (as measured in

chlorophyll a concentrations) to nitrogen additions during a mesocosm exper-

iment in August 2000. Error bars represent standard deviations from replicate

treatments. The addition of 0.5 mmol L�1 NH4C produces a small but signifi-

cant 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 inputsto the Kattegat indicate that there is considerable interannualvariability in nitrogen inputs and a strong seasonality. Maxi-mum inputs occur in the winter, dominated by riverine inputsand by wind induced vertical mixing, the latter being highlyepisodic. In summer, with low wind speeds and low riverflows, atmospheric inputs dominate (Figs. 5 and 6).

Variations in modelled phytoplankton biomass agree wellwith the measured data (Fig. 7) and show a pronounced bio-mass peak during the spring bloom and more constant concen-trations during the summer, autumn and winter. The onset of

External sources of nitrogen

0

50

100

150

200

250

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

nitr

ogen

load

(kt

N y

-1)

nitr

ogen

load

(kt

mon

th-1

)

0

5

10

15

20

25

J F M A M J J A S O N D

land load

upwelled flux

atmospheric deposition

Land load

Upwelling

AtmosphericDeposition

Fig. 5. Estimated external inputs of nitrogen to the Kattegat showing (a) inter-

annual variations and (b) seasonal variations.

457L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

1976 1980 1984 1988 1992 1996 20000

100

200

300

400

500

600

700

DIN

flux

(mg

m-2

day

-1)

Southern KattegatCentral Kattegat

1991

Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 10

100

200

300

400

DIN

flux

(mg

m-2

day

-1)

Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 10

100

200

300

400

DIN

flux

(mg

m-2

day

-1)

1989

0.0

2.5

5.0

DR

Y D

EP [m

g N

/m2 ]

0

50

100

WET

DEP

[m

g N

/m2 ]

0

50

100

ATM

. DEP

[m

g N

/m2 ]

1 15 29 13 27 10 24 8 22 5 19 2 16APR MAY JUN JUL AUG SEP

1999

0

10

PREC

. [m

m/d

ay]

Kattegat area, daily valuesb

a

Fig. 6. (a) Modelled entrainment fluxes of nitrogen (in mg N m�2 d�1) 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 70 mg N m�2 d�1 (equivalent to a nitrogen concentration of 0.5 mmol L�1

mixed into a 10 m water column, Fig. 4) is the statistically determined 1 year return of extreme atmospheric deposition (Carstensen et al., 2004). 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 suf-ficiently high to promote phytoplankton growth. Nitrogen isstored in the water column during the winter months and israpidly consumed during the spring bloom which begins inFebruary and usually peaks in March. Following the spring

bloom, phytoplankton growth is nitrogen limited for the re-mainder of the growth season. Output from the model suggeststhat the spring bloom ultimately fuels the growth of the grazercommunity during the summer. The mesocosm experimentsand bioassays show that phytoplankton are limited by nitrogen

458 L. Spokes et al. / Environmental Pollution 140 (2006) 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, 1998. 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�1 C:1 mg L�1 chl a.

availability. However, the lack of biomass accumulation inthe mesocosm experiments, coupled with the high measuredgrazing rates show that phytoplankton biomass is controlledboth by limited nitrogen availability and by grazing. Thegood agreement between measured and modelled valuesthrough the period 1989 to 1998 has allowed us to extrapolatethe model to the 1999 to 2001 period and to determine thelikely response of the biological community to changing in-puts of nitrogen from the atmosphere (see Section 3.4).

3.3. Atmospheric inputs of nitrogen to the Kattegat

Gases and aerosols are removed from the atmosphere tothe coastal seas by wet and dry deposition. Gases are also re-moved from the atmosphere onto aerosols through heteroge-neous chemical reactions, particularly those involving seasalt.Reactions with seasalt are particularly important in coastalareas where the input of seasalt aerosols to polluted air massesoccurs as marine and continental air mix (Jickells, 1998;Russell et al., 2003). MEAD results demonstrate that seasalt

aerosols are found well inland and this means that the role ofseasalt interactions with atmospheric nitrogen species needsto be considered over land near the coast as well as over the sea.

Gas phase HNO3 reacts rapidly with aerosols such as sea-salt, shifting nitrate onto the particulate phase. This processcan enhance nitrate deposition by moving the nitrate onto largeraerosols. This effect has to be balanced against reduceddeposition rates for HNO3 gas and the overall balance of theseprocesses is still uncertain (Jickells, 1998; Pryor and Barthelmie,2000; Russell et al., 2003). Since ammonia has a very highdeposition velocity, it is rapidly deposited, primarilyas the gas, and therefore its deposition is fundamentallycontrolled by the local turbulence intensity in the region.Atmospheric turbulence is created by a combination of mechan-ical and thermal interactions between the atmosphere and theland or sea surface. Atmospheric measurements and high res-olution modelling conducted during MEAD have shown thatturbulence intensity in the Kattegat region is much more spa-tially and temporally variable than previously thought. Thusthe entire Kattegat Sea region is influenced by the presence

459L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

of the adjacent coastline and by the islands within the Kattegat.This means that nowhere in the atmosphere over the Kattegatare conditions such that continental influences can be excluded,neither in chemical constituents, nor in the small-scale mete-orological conditions. This influence is manifested as hori-zontal gradients in practically all parameters, as heterogeneousand complex meteorological conditions (jets, sea breeze cir-culations, etc.) or both (�Zagar et al., in press). Thus it is notpossible to describe atmospheric transport and deposition us-ing relatively simple terrestrial or oceanic parameterisations,but rather the individual atmospheric situation must be mod-elled. This conclusion is probably true for all coastal regionsand suggests that coastal shelf regions need to be treated asatmospheric transitional zones between marine and terrestrialsystems on scales of many tens of kilometres at least (�Zagaret al., 2003). Using improved estimates of atmospheric turbu-lence in coastal regions, MEAD results have shown that drydeposition of gas phase ammonia close to the coast is muchmore important than previously thought and can be an impor-tant source of nitrogen to the Kattegat (Pryor and Sørensen,2002), particularly in view of the large and daily variable(Ambelas Skøth et al., 2004) 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 thatlocal dry deposition can exceed the basin average by at leastan order of magnitude or more (�Zagar et al., 2003). Moredetailed modelling is required to provide a more completeand detailed picture.

During dry periods the dry deposition of nitrogen is veryimportant. However MEAD campaigns, long term monitoringand modelling all indicate that, on long time scales, atmo-spheric deposition to the central Kattegat is dominated bywet deposition (Fig. 6). This input is highly episodic and largenitrogen inputs can occur during short rain events. For exam-ple, a single rain event delivered over 40% of the total monthlyrain flux in May 2001. Such large rain events are not uncom-mon in this and many other regions (Spokes and Jickells, inpress; Valigura et al., 2000). This emphasises that long termdata sets are needed in order to accurately quantify the magni-tude and variability of atmospheric deposition.

3.4. External inputs of nitrogen to surface waters,a budget approach

The results of the MEAD assessment of the significance ofatmospheric inputs of nitrogen to the Kattegat are summarisedin Fig. 8. The average annual nitrogen budget for the KattegatSea shows that ~24% of the external nitrogen input to the re-gion is from the atmosphere while twice this amount entersthrough deep water entrainment. This suggest that, on anannual basis, the atmosphere is not the dominant source ofnitrogen. However, the atmosphere is more important in thesummer (Fig. 5). During this summer period, approximately40% of the external nitrogen load to the surface waters isfrom the atmosphere.

However, only extreme atmospheric deposition events arelikely to be sufficiently large to produce direct effects on

the phytoplankton community (Fig. 6). Results from ex-perimental and modelling studies and from the MEADretrospective analysis suggest that atmospheric inputs are un-likely alone to cause phytoplankton blooms in the Kattegat(Carstensen et al., 2005). Deep water entrainment of nu-trient rich bottom water during storm events is a muchmore likely cause since the model results indicate thissource is the largest source of short term N inputs. Atmo-spheric deposition and water column mixing are, however,correlated, mainly because both are, to some extent, relatedto wind velocity, and the nitrogen input from both processesfuels phytoplankton growth (Hasager et al., 2003). Consider-ing the aggregated supply of nitrogen to the surface waters,there are many summer days (Fig. 6a) where the externalsupply of nitrogen has the potential to result in an increasein chlorophyll a above 0.5 mg L�1 (a realistic daily increasechlorophyll a required to sustain a bloom). The dominantsource of nitrogen on such days is water column mixing(Carstensen et al., 2005). So although atmospheric deposi-tion is, on average, the dominant source of nitrogen to sur-face waters in the summer, the periods with excess nitrogensupply likely to result in bloom development are dominatedby fluxes from below the pycnocline.

Importantly, however, the atmosphere does represent a sig-nificant source of nitrogen to the Kattegat on an annual basisand, as a result, contributes to the total water column nutrientload in surface and deep waters and hence to eutrophicationpressures. Atmospheric inputs may also act to sustain bloomdevelopment initiated by N inputs from below as discussedearlier.

3.5. Management options to reduce atmospheric N inputs

Reducing nitrogen loads to the Kattegat will reduce eutro-phication pressures in the region. Here we consider variousemission reduction scenarios and estimate their impact onphytoplankton activity in the Kattegat. As noted earlier,ammonia is rapidly deposited close to the emission sourcewhile nitrate undergoes long range transport before deposition.We estimate, based on results from the Lagrangian air

Deep water

30

47

Skagerrak

Mixed layerBaltic

63

66

34

56

102

From LandAtm.

deposition

39

28

96 52Denitrification26

Denitrification Permanent burial

Fig. 8. Budget of biologically active nitrogen for the Kattegat (units kt yr�1)

differentiated into fluxes into the mixed layer and deep water (grey).

460 L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

pollution model ACDEP (Hertel et al., 2003), that almost halfof the ammonia input to the Kattegat is from Danish sourcesbut less than 10% of the oxidised nitrogen is. Hence differentemission management strategies are required for the twodominant forms of inorganic nitrogen. Several emission reduc-tion strategy scenarios have been run with the ACDEP modelin order to provide atmospheric nitrogen input data to theMEAD marine model. The impact of changing agriculturalpractices was investigated by reducing ammonia emissionsand the effect of decreasing emissions from combustion sour-ces such as road traffic and industry was determined throughreduction in nitrogen oxide (NOx) levels. Scenarios consideredmodifications to emissions throughout Europe and at a morelocal level focussing on Denmark as the land area immediatelyupwind under prevailing westerly winds. As with all scenariosthe options considered were meant to be illustrative rather thanreflect specific policy options.

3.6. Scenario modelling

The emission reduction scenarios tested with the ACDEPmodel were:

� 50% reduction in all NH3 emissions� 50% reduction in all NOx emissions� 50% reduction in all NH3 and NOx emissions� no Danish emissions.

The results (Table 1) show that Danish emissions contributeabout 20% of the atmospheric nitrogen deposition to theKattegat. A 50% reduction in all ammonia emissions anda 50% reduction in all nitrogen oxide emissions across Europeeach lead to an approximately 20% reduction in atmosphericnitrogen deposition. This 50% reduction in both ammoniaand nitrogen oxide emissions is predicted to reduce atmo-spheric nitrogen depositions by only approximately 40%because of long range transport into the area and becausesome of the emitted material is transported beyond theKattegat. Reducing sulphur dioxide levels appears to havelittle effect on nitrogen deposition to the Kattegat even thoughformation of ammonium sulphate aerosols is a very importantprocess in the atmosphere.

Using these revised estimates of atmospheric nitrogen in-puts in the marine coupled hydrodynamic-biochemical modelhas 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

1999 2000 2001

No Danish emissions �20 �17 �24

�50% NH3 emissions �21 �22 �19

�50% NOx emissions �24 �25 �21

�50% NH3 and NOx emissions �43 �41 �40

will impact primary production in the Kattegat. The effect ofreduction scenarios was quantified in terms of summer (Mayto August) primary production, biomass and bloom frequency.The summer primary production from the various scenarios areshown in Table 2. The predicted change in summer productionapproximately follows the change in load and the results sug-gest that for every 10% reduction in atmospheric load, a 2% re-duction in primary production is seen. The effect is not directlyproportional because recycling of nitrogen from water columnorganic and particulate N reservoirs sustains a component ofthe primary productivity in coastal seas (Weston et al., 2004).

The results in Table 2 shows that the model predicts thatlarge reductions in atmospheric deposition will result inmodest reductions in biological production. The effect of nitro-gen deposition on bloom frequency during the summer has alsobeen calculated and the model also predicts slightly reducedbiomass and bloom magnitude (results not shown).

4. Conclusions

Our statistical analysis shows that summer phytoplanktonblooms within the Kattegat are rare. Results from the MEADproject suggest that the main factor regulating nitrogen supplyand bloom occurrence in the Kattegat is entrainment of nitrogenfrom deep waters. Other than those results available from thecontinuous plankton recorder surveys (e.g. Reid et al., 1998),the MEAD retrospective analysis is one of the few systematicstudies of bloom frequencies carried out (Hasager et al., 2003).

These results were supported by modelled fluxes from theACDEP atmospheric deposition model (Hertel et al., 2003)and a hydrodynamical model of the Baltic Region(Gustafsson, 2000). Model results showed that nitrogen inputthrough entrainment is highly episodic and carries more nitro-gen to the surface layer during short-term pulses (!1 week)than atmospheric deposition events do (Carstensen et al.,2004). This analysis of the modelled fluxes combined withthe observed blooms also showed that a substantial portionof the blooms were generated in the frontal area of the north-ern Kattegat through entrainment mainly and thereafter trans-ported to other parts of the Kattegat and adjacent seas.Atmospheric deposition fluxes were correlated with the en-trainment fluxes, although with a minor nitrogen contribution(10e20%). Thus, atmospheric deposition plays a minor directrole in combination with entrainment mechanisms for the for-mation of blooms in the Kattegat, although the possibility oflarge local deposition needs to be further investigated using

Table 2

Changes in summer primary production in the Kattegat Sea for different

emission reductions scenarios

Scenario Percentage change in primary production

1999 2000 2001

No Danish emissions �4.0 �3.2 �6.0

�50% NH3 emissions �4.0 �4.2 �4.2

�50% NOx emissions �5.0 �4.2 �5.0

�50% NH3 and NOx emissions �8.6 �7.6 �8.8

461L. Spokes et al. / Environmental Pollution 140 (2006) 453e462

high-resolution modelling. Atmospheric deposition may con-tribute to the sustaining of algal blooms once initiated.

Atmospheric inputs make an important contribution to theoverall nitrogen economy of the Kattegat both directly andvia their contribution to fluvial inputs and lateral transportfrom the Baltic Sea and hence to the total eutrophication pres-sure. Although bloom events are rare, they can be very damag-ing and it is clear that reductions in the overall nitrogen stockare likely to reduce bloom intensity. Reductions of atmosphericnitrogen inputs need to consider both nitrate and ammoniaand possibly organic nitrogen (Cornell et al., 2003). The atmo-spheric chemistry of nitrate and ammonia are very differentdespite their close correlation in the atmosphere (Spokes andJickells, in press). In particular, ammonia is much more rapidlydeposited than NOx because this component must first be oxi-dised to nitric acid and nitrate aerosols before significant depo-sition can occur. Existing monitoring and modelling effortsprobably underestimate the role of ammonia deposition, partic-ularly in coastal waters close to emission sources and high res-olution physical modelling is required to correctly describe theatmospheric turbulence in coastal areas which governs ammo-nia deposition. The differing sources and atmospheric chemis-try of oxidised and reduced nitrogen means that regulation ofammonia inputs requires management of local, primarily agri-cultural, emissions, while regulation of nitrate requires Europe-an wide regulation of combustion sources. Since both inputs areof similar magnitude, substantial reductions of atmosphericnitrogen inputs to the Kattegat, and other coastal seas require,in practice, strategies to reduce both inputs.

MEAD has demonstrated that marine and atmospheric sci-entists working together with a combined modelling and mea-surement campaign can tackle complex problems that crosstraditional scientific boundaries and deliver results with directpolicy implications. The specific results of MEAD are ofcourse directly relevant to the Kattegat, but the approach adop-ted and the general principles developed can be applied to anycoastal sea.

Acknowledgements

MEAD was funded by the EU under contract numberEVK3-CT-1999-00014. We thank all who helped with field-work, particularly the crew of the Skagerrak.

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