Eutrophication of the Seas along Sweden s West Coast

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Eutrophication of the

Seas along Sweden’s

West Coast

report 5898 • novEmbEr 2008

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Report to the Swedish Environmental Protection Agency (Naturvårdsverket)

Panel for the

Expert Evaluation of Eutrophication in the Western Swedish Seas

Dr. Donald F. Boesch, Chair

University of Maryland Center for Environmental Science, Cambridge Maryland, USA

Dr Jacob Carstensen

National Environmental Research Institute, Aarhus University, Roskilde, Denmark Dr. Hans W. Paerl

Institute of Marine Sciences, University of North Carolina, Morehead City North Carolina, USA

Dr. Hein Rune Skjoldal

Institute of Marine Research, Bergen, Norway Dr. Maren Voss

Leibniz Institute for Baltic Sea Research, Warnemünde, Germany

November 10, 2008

SWEDISH ENVIRONMENTAL PROTECTION AGENCY

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Orders

Phone: + 46 (0)8-505 933 40 Fax: + 46 (0)8-505 933 99

E-mail: natur@cm.se

Address: CM Gruppen AB, Box 110 93, SE-161 11 Bromma, Sweden Internet: www.naturvardsverket.se/bokhandeln

The Swedish Environmental Protection Agency

Phone: + 46 (0)8-698 10 00, Fax: + 46 (0)8-20 29 25 E-mail: registrator@naturvardsverket.se

Address: Naturvårdsverket, SE-106 48 Stockholm, Sweden Internet: www.naturvardsverket.se

ISBN 978-91-620-5898-2.pdf ISSN 0282-7298 © Naturvårdsverket 2008

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6.1.2. Specific goals and strategies for west coast marine waters 54 6.1.3. The transgenerational reality 55 6.1.4. Climate change and other compounding forces 55 6.2. Measures and Their Implementation 56 6.2.1. Nitrogen controls are essential 56 6.2.2. Phosphorus reductions produce local benefits 57 6.2.3. Greater reductions of agricultural and atmospheric loads are needed 57 6.2.4. Multi-national cooperation is required 58 6.3. Integration of Monitoring, Modeling and Research for Adaptive

Management 58

6.4. Transparency and Accountability 59

7. FINDINGS AND RECOMMENDATIONS 61

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1 Introduction

One of the most serious and challenging environmental problems facing Sweden is eutrophication of its surrounding seas as a result of excessive human emissions of plant nutrients. In 1999 the Swedish parliament (Riksdag) set fifteen environmental quality objectives for the nation, including the objective of Ingen Övergödning, translated as “Zero Eutrophication,” but more literally “No Over-Enrichment”. Specifically, the objective is: “Nutrient levels in soil and water must not be such that they adversely affect human health, the conditions of biological diversity or the possibility of varied uses of land and water.” It is further specified that: “The intention is for this environmental quality objective to be achieved within one generation.” In 2001, the Riksdag established interim targets, strategies and

measures to facilitate reaching the national environmental quality objectives, which were revised in 2005. The Swedish Environmental Protection Agency (SEPA 2007) recently conducted a second in-depth evaluation of the Zero Eutrophication environmental quality objective, including progress in achieving the interim targets.

As part of its continued efforts to assess the state of eutrophication and progress toward its alleviation, the SEPA convened this international expert evaluation of eutrophication in the seas and coastal environments along the west coast of Sweden. It follows an earlier expert evaluation of eutrophication in all Swedish seas, which, while briefly addressing the western seas, focused largely on the Baltic Sea and its coastal environments (Boesch et al. 2006). That evaluation concentrated on the controversies regarding the controls of nitrogen versus phosphorus emissions. The SEPA used that report to develop standpoints to guide its actions to combat eutrophication (SEPA 2006).

This present expert evaluation was charged to evaluate the measures taken so far to achieve the Zero Eutrophication objective for the Danish Sounds, the Kattegat and the Skagerrak and the Swedish coastal environments bordering these waters and to recommend future strategies to counteract eutrophication there. These western seas have important differences from the Baltic Sea, including higher salinity and the influence of tides and the dynamic forces of the North Sea. As in the Baltic Sea, considerations have to be given to the sources and transport processes affecting nutrient delivery into these international seas, including from the Baltic and North Seas.

An expert panel was assembled by the SEPA to perform the evaluation. It consisted of five members, including one each from the neighboring countries of Denmark and Norway. Dr. Donald Boesch of the United States was invited by SEPA to chair the panel. The panel met from 8-13 August, 2008, in Marstrand, an island in the coastal archipelago along Sweden’s southern Skaggerak coast. Dr. Per Jonsson was the SEPA coordinator and Mats Blomqvist assisted the panel in accessing data and information and producing graphics. On 10 August, several Swedish experts met

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with the panel, presenting their recent findings and participating in discussion of issues before the panel. These included Drs. Suzanne Baden, Odd Lindahl, Leif Pihl, Johan Rodhe, and Rutger Rosenberg of Gothenburg University and Dr. Daniel Conley of Lund University. In addition to this consultation, the panel reviewed the findings of more than 100 scientific papers and reports, including very recent publications and national and regional assessments. A draft report was prepared while the panel worked at Marstrand and subsequently refined through

correspondence.

The expert panel specifically considered: the status and sources of anthropogenic emissions of nutrients, including trans-boundary sources; the extent of

eutrophication and the nutrients responsible for it; the effects of eutrophication on the ecosystem and natural resources; the confounding influence of other factors such as climate variability and change and fishing activities; the effectiveness of the present Swedish strategy to counteract eutrophication and prognosis for the future; and the adequacy of scientific research, monitoring and assessment to support its execution.

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2 Physical Setting

2.1 Geography and Bathymetry

The Swedish west coast faces the Öresund, Kattegat and Skagerrak (Figure 1). These are three very different although connected water bodies. The Skagerrak is part of the deep Norwegian Trench which is a glacially excavated valley that runs along the coast of Norway and connects the North Sea with the deep Norwegian Sea to the north. The Skagerrak is about 700 m deep in the inner (eastern) part and there is a steep slope from the Swedish Bohuslän coast down into the deep

Skagerrak. The Kattegat and Öresund in contrast are shallow sea areas that connect the Skagerrak with the Baltic Sea.

The Kattegat is a broad basin about 200 km long and 100 km wide. The boundary between Kattegat and Skagerrak is usually taken as a line from Skagen (north tip of Denmark) to the city of Göteborg on the Swedish coast. To the south, the Kattegat is connected with the Baltic Sea through the narrow strait of Öresund between Sweden and the Danish island of Zealand and through the belts around the island of Funen. This latter connection is broader and topo-graphically more complex, connecting through Samsø Belt, Little and Great Belts, and Fehmarn Belt, and finally across Darss Sill, with the Arkona Basin as the westernmost part of the Baltic Sea. The connection through Öresund has a sill depth of only about 8 m, while the connection through the belts is deeper at about 15 m at Darss

Sill (Gustafsson 2006). Figure 2.1. Seas along the Swedish west coast. The western part of Kattegat is mostly shallow (<20 m deep), with the islands Læsø in north and Anholt in the south-central part. A deeper depression or trench cuts in from Skagerrak on the eastern side with depth >60 m to southeast of Læsø. The central Kattegat (between Læsø and Anholt) has a rugged topography with shallow (<20 m) areas also on the eastern side (e.g. Fladen Grund) except for a narrow, deeper trench running close to the Swedish coast. The southern Kattegat

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(south of Anholt) is mostly moderately shallow (20-40 m), while Laholm Bay on the Swedish side is particularly shallow (<20 m).

2.2 Circulation and Water Masses

The North Sea circulation is basically counter-clockwise (Figure 2.2; Otto et al. 1990). Atlantic water from the inflow across the ridge between Scotland and Iceland flows into the North Sea across the northern boundary between Scotland and Norway. A part of this flow comes over the northern North Sea plateau while the rest flows in along the

western slope of the Norwegian Trench. The inflow of Atlantic water over the plateau in the northwestern North Sea continues south with portions being deflected east by shoaling topography in the central North Sea (the Dooley current) and by the Dogger Bank in the

southern North Sea. A portion of this water may also flow south and around the Dogger Bank. Much of the inflow in the Norwegian Trench continues into the Skagerrak where it circulates around and leaves on the northern side along the Norwegian Skagerrak coast. There is also some inflow of Atlantic water through the (English) Channel that continues northeast along the European continent south of the

Dogger Bank. This flow is the main seawater that receives the input of fresh water from the large European rivers including the Seine, Scheldt, Rhine-Meuse, Weser, and Elbe. The fresh water lowers the salinity and gives the flow a distinct

characteristic as a coastal current that flows north as the Jutland Current along the western and northern coasts of Denmark.

The flow of Atlantic water into and through the North Sea is typically of the order 2 Sverdrup (1 Sv = 106_m3_s-1_{). There is a large seasonal variation (by a factor of} 3-5 for flows through various parts of the North Sea), related to the general

intensification of winds in the winter and calmer conditions during summer and also large interannual variability. Thus, the circulation may be particularly great

Figure 2.2. General circulation in the North Sea (OSPAR Commission 2000).

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during winters with a high North Atlantic Oscillation Index (NAO), with many passages of low pressures and strong southwesterly and westerly winds at the entrance region to the North Sea. Roughly half of the total flow through the North Sea circulates through the Skagerrak (Skjoldal 2007). A time series of modeled flux of water

through the Skagerrak for the period 1955-2006 is shown in Figure 2.3.

The Skagerrak experiences the confluence for four different water masses: outflowing Kattegat surface water (KSW, which contains the Baltic outflow), water from the Jutland

Coastal Current (Jutland coastal water), water from the central North Sea (CNSW), and Atlantic water (AW). These water masses have different salinities and densities and when they meet in inner Skagerrak they can be layered one above the other. The least dense water is the Kattegat surface water, with an average salinity around 25 as it leaves Kattegat (Figure 2.4). The next less dense is Jutland coastal water which has salinities typically around 32-33 as it passes off Skagen. The Central North Sea water contains some fresh water mixed in from the coastal zones and typically has salinities between 34.5 and 35, while the Atlantic water has salinities >35.

In the inner Skagerrak these water masses are typically stacked above each other, although there can be short-term and spatial variation in this pattern. The outflow from Kattegat continues north along the Swedish Bohuslän coast, overlying Jutland water, central North Sea and Atlantic water masses. Through mixing and

entrainment, this buoyant coastal current increases its salinity as it continues north into the wide bight of the outer Oslofjord, where it is deflected and flows as the Norwegian Coastal Current along the Norwegian Skagerrak coast and then farther north along the Norwegian west coast. It has been shown that by the time the current passes Arendal, about half way along the Norwegian Skagerrak coast, most of the Jutland water can be accounted for as being present in the deeper part of the upper 30 m of the water column (Skjoldal et al. 1997, Aure et al. 1998).

The Jutland Current is almost always present along the west coast of Denmark, although it can be temporarily halted or reversed by strong northerly winds. This

Figure 2.3. Modelled flux of water through Skagerrak (across a transect between Oksøy in Norway and Hanstholm in Denmark). Time series are for mean flux for the 1st_{and 4}th_{quarters of the year from 1955 to 2004. The}

unit of the flux is Sverdrup (106_m3_s-1_{) and the negative sign indicates flux}

into Skagerrak. Data obtained with the NORWECOM model (Skogen and Søiland 1998) driven by archived meteorological data for the modeled time period.

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leads to an accumulation of water in the German Bight and a stronger flow as the winds slackens or shifts from the south. Thus, the flow into Skagerrak can have a pulsed character leading to spatial variation in the amount and thickness of the submerged Jutland water.

The circulation in the Kattegat is typical estuarine with low-salinity water flowing out from the Baltic Sea as an upper layer, while saltier water flows south as a deeper layer. A large fraction of the deeper water is gradually entrained into the upper layer as it penetrates south through the Kattegat. The upper and deeper layer is usually separated by a pronounced density gradient, or pycnocline. The salinity of the Baltic water in the Arkona Basin is about 8 on average as it approaches the Belts and Öresund. Salinity increases to about 20 in the southern Kattegat. On the further passage north through Kattegat, the salinity of the upper layer increases to an average of about 25 south of Læsø. This corresponds to an entrainment of an amount of water about twice the Baltic outflow, resulting in an increase in the net volume flow by a factor of about 3. In the frontal area north of Læsø, where high salinity Skagerrak water is subducted as a bottom current, surface salinities can rapidly change 5 to 10 due to the mixing of different water masses.

The Baltic outflow is driven by the net freshwater input to the Baltic which is about 16,000 m3_s-1_{. When the Baltic outflow leaves the northern Kattegat at a salinity of} 25 it has increased to a mean flow of about 60,000 m3_s-1_{(0.06 Sv) due to}

Figure 2.4. Annual (1998) mean of salinity and currents in the surface 5 m for the Skagerrak-northern North Sea region as modeled by Albretsen (2007).

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admixture and entrainment of saltier water. About 25 % of the outflow from the Baltic occurs through Öresund while the largest amount (about 75 %) exits through the Belts. Roughly one-quarter of the entrainment occurs in the passage through the Danish Belts, while the remaining takes place in the Kattegat.

Superimposed on the net mean flow through Kattegat there is pronounced short-term variability. The instantaneous flow between the Baltic and Kattegat can be on the order of 0.1 Sv, driven by air pressure differences and effects of winds that lead to sea level changes both in the Baltic and in Kattegat. Thus, the outflow from the Baltic can occur as stronger pulses of 1-10 days duration, interspersed with periods of low or reversed flow in the direction from Kattegat into the Baltic Sea. The higher flow rates during periods of intensified outflows from the Baltic lead to a shallowing and strengthening of the pycnocline in southern Kattegat, while slackening or reversal of the flow leads to a deepening and weakening of the pycnocline. The pycnocline is typically located at about 15 m depth (Andersson and Rydberg 1993) and is very strong with a change in salinity of 5-15 units between the upper and lower layer.

The source of the deep water that flows south in Kattegat and which is

subsequently entrained into the outflowing Baltic water, is water from the North Sea circulation. The average salinity of the inflowing water at 40 m depth in the northern Kattegat is 33.9, decreasing to 33.3 at 40 m in the southern Kattegat (Gustafsson 2000). The water with salinity of about 34 is typically a mixture of Jutland coastal water and water from the central North Sea. Hydrographical data (including nutrients) shows that Jutland water is regularly present as an inter-mediate water layer below the pycnocline in Kattegat, with somewhat saltier water below. The magnitude of the Jutland Coastal Current is around 0.1 Sv as an annual average, based on the freshwater input to the southeastern North Sea (4.5 103_m3 s-1) diluted out to a salinity of 33 (Skjoldal 1993). Due to the seasonality in freshwater input and prevailing wind conditions, the Jutland Current is more voluminous in winter, with a flow of order 0.15 Sv. The inflowing deep water in the Kattegat is about 0.04 Sv as an annual average (to balance the outflow of 0.06 Sv, with about 0.02 Sv coming from the Baltic Sea). Thus, only a fraction of the Jutland coastal water circulates through the Kattegat, the majority being deflected north along the Swedish Bohuslän coast, underlying the outflowing layer of Kattegat surface water.

The average residence time of water in Kattegat is typically 2-3 months if calculated on the basis of flushing time [the time needed for the net flow of 0.06 Sv to replace the volume of water in Kattegat (0.5 1012_m3_{)]. More detailed} information on residence time for different parts of Kattegat and the Belts is presented by Gustafsson (2000). The residence time for the outflowing surface layer in Kattegat is typically about 1 month, while the inflowing deep layer can have residence time of several months. The residence time varies with local metorological conditions, both seasonally and inter-annually.

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The buoyant coastal current that flows north along the Swedish Bohuslän coast consists of the outflowing Kattegat surface water overlying Jutland water, floating on top of the central North Sea and Atlantic waters that circulate through Skagerrak (Rohde 1996). This deeper circulation is strong, typically of order 0.5-1 Sv, while the coastal current may be of order 0.15 Sv (0.06 Sv Kattegat outflow and 0.1 Sv Jutland Current). The transport time for the coastal current to flow along the Bohuslän coast is typically a few weeks (assuming a net current speed of 20 cm s-1_{). The average salinity of the surface layer of the coastal current is around 28 in} northern Bohuslän (Koster), showing admixture of some of the underlying Jutland water into the surface layer.

2.3 Swedish Coastal Waters

The Swedish west coast is characterized mostly by scattered skerries and small bays and fjords that communicate openly and effectively with the offshore waters both in Kattegat and in particular in Skagerrak. The coastal waters (defined as waters within the baseline) have been divided into regions based on typology according to the methodology given by the EU Water Framework Directive. The typology is based mainly on water salinity, exchange and residence time, and bottom substrate. The areas are from south to north: the coast along the Öresund, the coast along southern Kattegat including Skälderviken and Laholm Bay, the coast along northern Kattegat, the coast along Skagerrak, and the fjord systems north of Gøteborg including Havstensfjord and Gullmarfjord. In addition, the inner coastal water in many areas has been identified as a separate water type, being less exposed as habitats than the outer skerries and coast. The residence times of bottom water within the different regions are mostly of the order of some days (<9 days), except for the fjords where it is typically >40 days. The inner coastal water also typically exhibits short residence time (<9 days), although it can be somewhat longer (10-39 days) in some areas (SEPA 2008b).

The openness of the coastal areas and short residence times of water within them mean that their general conditions are determined by the physicochemical properties of the offshore waters. Exceptions to this are some of the west coast fjords where narrow and shallow sills may limit the water exchange. This is particularly the case in Koljöfjord, which has shallow sills, but Gullmarsfjord and Havstensfjord also have relatively shallow sills that reduce the rate of renewal of the bottom water, e.g. for the Gullmar Fjord a mean residence time for the water below the sill of one month is reported (Lindahl 1989). This renewal takes place mainly in the winter period when the water outside the sill is coldest and relatively high salinities occur because freshwater discharge is low and mixing is high in the coastal water bodies. The longer residence time of deeper water makes these fjords more susceptible to local influence. Nevertheless, the fjords are also influenced from the outside in that the water above the sill may be effectively exchanged and the fjords act as sedimentation basins for fall-out from the production and organic load of the euphotic zone.

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3 Nutrient Sources and Trends

3.1 Sources

3.1.1 Contributions of countries, the atmosphere, and the North and Baltic seas

The most important human sources of the nutrients responsible for eutrophication, nitrogen (N) and phosphorus (P) for the Kattegat and Skagerrak region are from land discharges and atmospheric deposition. The land-based inputs are driven by the amount of freshwater discharge that varies between 12 and 29 km3_yr-1 (Håkansson 2007). Most freshwater input comes from Sweden but the highest loadings of N and P come from Denmark. The largest single Swedish nutrient source is the River Göta Alv, the sixth largest river draining the greater Baltic Sea catchment; however, most of its load is transported northwards affecting the Skagerrak. Aside from inputs to the North Sea, only a small catchment of Germany drains into the Belt Sea region, with no nutrient discharge directly into the Öresund or Kattegat.

From Sweden, the Kattegat receives 20,800 t yr-1 and the Skagerrak 1,800 t yr-1 of diffuse nitrogen loads (Table 3.1). Agricultural land covers only approximately 12% of the catchments, which are 55% forested. Of these diffuse loads, 55 % of the nitrogen emanates from agricultural land and 15% from N-deposition on lakes and other inland open waters that drain to the coast (i.e. indirect deposition)

(Håkansson 2007). When N-retention is considered the overall input into the Kattegat is naturally reduced by 12,000 t yr-1_{before it enters the coastal sea. Point} sources deliver much less nitrogen to the Kattegat and Skagerrak with only 6,700 t yr-1 and 500 t yr-1, respectively (Table. 3.1).

Table 3.1. Gross loads given in t N yr-1_{for 2006 and normalized for mean runoff}

(Håkansson 2007).

Diffuse sources Point sources Agricultural land Forests and clear cut areas Open land Deposition on water Urban water Unconnected dwellings and WWTP Kattegat 20,800 8,100 2,400 5,900 700 6,700 Skagerak 1,800 900 400 100 100 500 Sum 41,400 7,200

Phosphorus gross loads from Sweden are 910 t yr-1_{and 180 t yr}-1_{to Kattegat and} Skagerrak and the diffuse input from agricultural land was again by far the largest share with 56% and 66%, respectively. Point sources only contributed 270 t yr-1 and 30 t yr-1. Assuming the retention estimates reliably reflect the natural processes the inputs into the Kattegat are roughly halved due to natural processes.

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From Denmark, the total nitrogen and total phosphorous loads discharged into the Kattegat, Öresund and Belt Seas were 51,800 and 1,500 t yr-1_{in 2006 (Ærtebjerg} 2007), but they should be considered in the framework of the overall decreasing trends (see below). Diffuse loads make up 60% for phosphorus and about 80-90% in the case of nitrogen; most comes from agricultural activities. Both nitrogen and phosphorus discharges show declining long-term trends with phosphorus loads decreasing more substantially than nitrogen (Carstensen et al. 2006). Nutrient yields (inputs per unit area) are still higher in Denmark than in Sweden. Norway contributes approximately 22,000 t yr-1_{of N and 750 t yr}-1_{of P to the} Skagerrak. These loads are mostly of anthropogenic origin 80% of the P and 50% of the N (based on 1993 data, Skjoldal et al. 1997). These nutrients largely enter in the Outer Oslofjord area and they contribute to very minor degree to the nutrient load in Swedish waters.

As reviewed earlier, the outflow of water from the Baltic Sea at the surface is compensated with inflowing deep water through the Kattegat, Belt Sea and the Öresund. The exchange of water and nutrients imposes considerable variability in nutrient concentrations depending additionally on large scale climate variations (e.g. the NAO). In a model and data evaluation study Rasmussen and Gustafsson (2003) estimated that net transports were directed from the Baltic Sea towards the Skagerrak with high inter-annual and decadal variability. They also point out that there are decadal changes in these fluxes and that the Kattegat imported inorganic P from the Skagerrak. The exchange of water and nutrients between the Skagerrak and the North Sea is extremely high with an average of 4,300 kt TN yr-1_{and over} 400 kt TP yr-1_{, but there is a net export of 179 kt N and 15 kt P from the Skagerrak} to the North Sea (Håkansson 2007). These fluxes are difficult to compare to the nutrient input from land but may contribute significantly to the nutrient budget. Atmospheric deposition brings another 40-45 kt N y-1 into the Kattegat/Skagerrak region (Håkansson 2007). A modelling study estimated the long-term mean input and demonstrated high variability in N deposition (Spokes et al. 2006). The mean input was estimated to be 70 mg N m-2 d-1, which is equivalent to a nitrogen concentration of 0.5 μm L-1_{when mixed into a 10 m water column.}

3.1.2 Contributions from the Jutland Coastal Current

The open North Sea is dominated by exchange with the North Atlantic Ocean, but the coastal regions receive large amounts of nutrients from western European rivers. The riverine input of nutrients increased up to the 1980s, particularly for N (as nitrate), resulting in N/P (atomic) ratios of 30-35 for the total annual inputs of N and P to the southeastern North Sea in 1990 (Skjoldal 1993, NSTF 1994). The loadings of nutrients from these rivers have declined as a result of pollution reduction measures, beginning in the 1980s for P and 1990s for N (Figure 3.1). Flow-adjusted P loadings have declined by more than half, while equivalent nitrogen loads have declined by about 20%. As a result, the N:P ratios in the river

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discharges and in coastal waters near the rivers have increased to well above the Redfield ratio of 16 on a molar basis (McQuatters-Gallop et al. 2007; Philippart et al. 2007).

Consequently, while inputs of both nutrients are clearly declining, there remains surplus N to be exported in coastal currents along the coast to the north. As a result, the more distal portions of the shallow Wadden Sea, where N is imported from theses coastal water masses and phosphorus is efficiently recycled from sediments, continues to be affected by eutrophication (van Beusekom et al. 2005). While the nutrients from these riverine sources that potentially reach the Skagerrak and Kattegat region by transport with the Jutland Coastal Current (JCC) have likely declined, the decline in N is probably much less than the decline of P.

Around 1990 the Rhine and Elbe had nitrate concentrations of 500 μM or higher in winter. Inputs from these rivers lead to concentrations in the German Bight often exceeding 40 μM, more than twice the concentrations 20 years earlier (Figure 3.2). The European rivers are particularly enriched with nitrate, leaving an estimated surplus amount of 300,000 tons of nitrogen when phosphorus was depleted by the spring growth of phytoplankton (NSTF 1994). The N-enriched JCC flows into the inner Skagerrak, where, as described in Section 2.2, it submerges under the less-dense water flowing from the Kattegat. Around 1990 it was estimated that the JCC transported an annual amount of about 400,000 tons of nitrogen of anthropogenic origin into Skagerrak (NSTF 1994).

Figure 3.1. Trends in specific N and P loads (mean annual load/mean annual discharge) from the Rhine-Maas and Elbe-Weser rivers (van Beusekom et al. 2005).

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Most of the Jutland coastal water reaching the Skagerrak is advected north along the Swedish west coast and farther along the Norwegian Skagerrak coast. The winter (January-April) nitrate concentrations in the upper 30 m of the Norwegian Coastal Current (NCC) doubled between the 1970s and the early 1990s, apparently as a result of the increased concentrations in the Jutland coastal water (Skjoldal et al. 1997, Aure et al. 1998). This was associated with organic enrichment and lowered oxygen concentration in basins of fjords along the Norwegian Skagerrak coast, reflecting increased sedimentation and oxygen consumption rates (Skjoldal et al. 1997). There was also a declining trend in the oxygen concentrations at intermediate depths and salinities in the NCC in the autumn, starting around 1970 (Johannessen and Dahl 1996).

The observed decrease in nitrate concentrations in the German Bight from the 1990s is reflected in the Norwegian Coastal Current where the mean nitrate concentration in the upper 30 m is now reduced roughly half way back to the situation in the 1970s (Aure and Magnusson 2008).

It is likely that this same situation, reflecting transport of nutrients with the Jutland Coastal Current, has also affected the Swedish Skagerrak coast and, to some degree, the Kattegat. Some portion of the Jutland coastal water is advected south as an intermediate layer below the pycnocline in Kattegat (Section 2.3). In the

southern Kattegat, high nitrate concentrations and high N/P just below the

pycnocline in late April in most years suggest the presence of Jutland coastal water (Figure 3.3). This water would be entrained into the outflowing water and enriches the upper layer in spring and early summer with a surplus of nitrogen relative to phosphorus relative to the Redfield ratio. This could result in phosphorus limitation in spring and early summer, with the surplus nitrogen (mainly as nitrate) still used by phytoplankton nourished by recycling of P. Later in summer the situation would

Figure 3.2. Mean nitrate concentrations in January–April at Helgoland in the German Bight (Aure and Magnusson 2008).

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likely change to one with predominant N limitation as nitrogen is depleted and phosphorus is efficiently recycled.

3.1.3 Point sources and atmospheric deposition

The nutrient inputs from Denmark and Sweden are dominated by diffuse inputs which come from agricultural land due to loss of nutrients applied in excess of the nutrients removed in crops and animal products. By the late 1970s all urban populations in Sweden have been connected to waste water treatment plants (WWTPs), which have constantly improved waste treatment and nutrient removal (Bernes 2005). The Rya WWTP in Göteborg is one such example, built in 1974 and amended with a nitrogen removal step in 1987. The nutrient release has decreased from over 600 t P and almost 3000 t N in the 1970s to less than 100 t P and 1500t N nowadays. Point sources from Sweden make up less than 15% of the total nitrogen inputs; however, a substantial improvement for diffuse nitrogen inputs has not yet been reached although progress has been made. This is not only the case in Sweden but is true for all riparian states along the Kattegat and

Skagerrak region.

Figure 3.3. Nitrate concentrations (upper) and N/P ratios (based on nitrate and inorganic phosphate) at 20 and 50 m depth in southern Kattegat in late April from 1988 to 2007. Data from Institute of Marine Research, Bergen, Norway.

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Atmospheric deposition is an important source of N in coastal waters. It is suggested that N deposition from air may support ongoing phytoplankton blooms in summer but the N supplies and concentrations are considered too low to initiate a phytoplankton bloom (Carstensen et al. 2004, Spokes at al. 2006).

Figure 3.4. Total nitrogen (left, blue bars) and total phosphorus (right, yellow bars) loads of Swedish rivers and flow of rivers (red line) to the Kattegat, Skagerrak and Öresund from 1969 to 2007. Note the different scaling for the N and P load and the mean flows (SMHI database).

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3.1.4 Trends in source inputs

Swedish river N loads into the Kattegat, and Skagerrak have increased through the 1970s to the mid 1980s, but do not show any significant trend over the past 20 years (Fig 3.4). In the Öresund region the situation is more dynamic with fewer clear trends, but overall high loads. Loads clearly depend on the flow rate so that changes in precipitation may be directly translated into changes of N and P inputs into coastal waters. P loads from Swedish rivers are still more variable and have not seen the trend development that can be seen for N inputs. Input into the Kattegat is much higher than that into the Skagerrak and Öresund. Data from 1995 to 2005 did not have statistically significant trends and suggested only slight decreases if any in N and P inputs into the Kattegat and Skagerrak (Håkansson 2007).

Nutrients inputs from Denmark show a drastic decrease from the late 1980s until present (Fig. 3.5). There is no similar trend apparent in the data from Swedish sources.

Trends in total N and P loads from the Baltic Sea can be investigated from changes in surface water concentrations at a station in the Arkona Sea (Figure 3.6). There has been a slight decline in TN concentrations and a more precipitous decline in TP since the mid-1980s. However, there was a substantial increase in TP observed over the past three years, which could be related to P releases from internal sources as a result of hypoxia (Vahtera et al. 2007). Chlorophyll concentrations have increased as well (not shown in the figure) for reasons that have not been

investigated, but could be related to blooms of nitrogen-fixing cyanobacteria that respond to such increases in P.

0 20000 40000 60000 80000 100000 120000 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Tot al ni tr oge n ( to ns y r-1) 0 1000 2000 3000 4000 5000 6000 Tot al phos phor us ( tons y r-1) TN TP

Figure 3.5. Trend in nutrient loads from Danish rivers into the Kattegat from 1989 to 2006. (http:/www.dmu.dk; Ærtebjerg 2007).

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Atmospheric deposition of nitrogen is rising globally (Duce et al. 2008), but declining within the region included in this evaluation. Atmospheric releases of ammonia from agriculture in Sweden have diminished by about 18% between 1995 and 2005. It was determined that 55% of the decline in the release of ammonia between 1990 and 2001 was linked to a reduction in the numbers of animals and 45% due to directed measures to reduce emissions (SEPA, 2007). Increasing trends in atmospheric deposition of nitrogen over the past decades have been reversed and should continue to decline as further controls are implemented. Atmospheric concentration and depositions of all nitrogenous compounds measured at the island of Anholt in the Kattegat has decreased from 1989 to 2006, corresponding to a reduction of 22% in annual depositions (Ærtebjerg 2007).

3.2. Nutrient Status and Trends in Coastal

Waters

3.2.1 Concentrations and dynamics

The surface TN and TP concentrations in the Skagerrak and Kattegat in winter are much higher than in the Baltic Proper. Close to the coast, nutrient levels are elevated over background levels. Concentrations follow a distinct annual cycle in the surface waters and inorganic nutrients are fully consumed during spring and summer. Below the thermocline, large inventories of nutrients are still present in summer, especially in the deeper parts of the Skagerrak. In the Kattegat there is close coupling between recycling from sediments into the water column due to the shallowness of the system.

Along a transect away from a WWTP discharge in the Danafjord, a rapid decline in nutrient concentrations from over 30 to 5 µmol L-1_{of NO}

3 (10 to 2 µmol L-1 of 1970 1975 1980 1985 1990 1995 2000 2005 2010 8 12 16 20 24 TN (µ g L -1) 0.4 0.5 0.6 0.7 0.8 0.9 TP (µ g L -1 ) TN TP

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NH4) over only 20 km has been described (Selmer and Rydberg 1993).

Concentrations reach those typical for salinity of 25 in open waters, implying rapid uptake and mixing processes. However, the removal of nutrients may partly be achieved by deposition of organic matter in sediments, from which nutrients are partly released again.

N:P ratios in coastal surface waters often deviate from the Redfield ratio of 16, but are adjusted over the annual cycle of production and recycling. Nutrient

imbalances, however, may support growth of harmful algal blooms (HABs, Richardson 1997), but not cyanobacteria blooms as they do in the Baltic Sea (see Section 4). A surplus of nitrate or phosphate can also be found in the coastal water close to WWTP and river inputs. Moreover, dissolved organic matter, carried by outflows of the Baltic Sea or rivers, may add to the imbalances in nutrient supply depending on their state of degradation and susceptibility to biodegradation. Removal process of N and P can furthermore change nutrient availability.

Denitrification and anammox rates are significant in Aarhus Bay, but may be lower along the Swedish west coast due to a different sediment type (Thamdrup and Dalsgard 2002). Indirect estimates from the Laholm Bay suggest rates of 1.87 mmol m-2_d-1_{(Rydberg and Sundberg 1988). High removal rates of nitrate and} ammonia in the surface water close to the outlet of the WWTP are suggested (Selmer and Rydberg 1993).

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3.2.2 Trends

Trends in nutrient concentrations in coastal waters are more

difficult to evaluate than those for loads because the effects of currents and stratification column need to be considered. Data suggest that nutrient concentra-tions in the upper 15m (in

summer that is the layer above the pycnocline) in the Kattegat increased in the 1970s and started decreasing slightly from the mid 1980s (Figure 3.7). Similar declines are noted in deeper waters. This decrease is more pronounced for P than for N. Both nutrients are still at concentra-tions greater than those preceding the acceleration of enrichment in the 1970s. Declines in TP and TN are also evident throughout the water column both in the Kattegat and south-eastern Skagerrak (Figure 3.8). Vertical time series make clear the importance of surface depletion and benthic regeneration of nutrients in the Kattegat, particularly for P.

Declines in nutrient and chlorophyll a concentrations and primary production have been observed in the Göta Älv estuary as a result of advanced wastewater treatment (Rydberg 2008). In Denmark, nutrient concentrations have significantly declined in coastal and open waters in response to measures taken reducing the inputs of nutrients from land (Carstensen et al. 2006).

1980 1985 1990 1995 2000 2005 2010 T ot al n itr og en (µm ol /l) 0 10 20 30 40 1980 1985 1990 1995 2000 2005 2010 T ota l p hos pho ru s (µm ol/l) 0,0 0,5 1,0 1,5 2,0 2,5 3,0

Figure 3.7. Trends in TN (top) TN (bottom) and since 1982 in surface waters (<15 m) at Anholt in the Kattegat (SMHI data).

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Figure 3.8. Variations of the concentrations of total nitrogen and total phosphorus over time and with depth at stations in the southeastern Skagerrak and central Kattegat since the 1980s (SMHI database).

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It is stressed that not only the absolute concentrations of nutrients are important, but also the ratios of nitrogen to phosphorus. Most nutrient sources, including river discharges, waste waters, and the Jutland Coastal Current, have increasing N:P ratios because phosphorus has been controlled more efficiently than the nitrogen. Atmospheric deposition is naturally greater for N than for P and the JCC can have N:P ratios up to 50. Although N and P deposition inputs from Denmark have large and different inter-annual variations, nutrient concentrations in coastal and open waters seem to be below the Redfield ratio most of the time (Carstensen et al. 2006). Increasing N:P ratio of nutrients in the input sources and in the coastal waters may potentially affect the composition of the phytoplankton community (see Section 4).

The sediment pool of nutrients is very large (Rydberg and Sundberg 1988, Conley et al. 2007). Hypoxic and anoxic conditions in the bottom water have occurred and resulted in large scale die-offs of the benthic fauna. This again affects the nutrient sequestration at the sediment-water interface and may lead to higher storage of nutrients and organic matter in the sediments, which can readily be released by resuspension or events of hypoxia, and less denitrification (Conley et al. 2002).

3.3.3 Budget aspects

Different numbers and extrapolations have been used to put the nitrogen sources in perspective. One budget for nitrogen (Figure 3.9) suggests that the Kattegat receives half as much N from direct

atmospheric deposition as from land. However, the lateral transport through the Belt Sea to the Kattegat and Skagerrak is less than

that estimated by Rasmussen and Gustafsson (2003). The nutrient fluxes to the Skagerrak, particularly for DIN, are clearly influenced by continental river water and average DIN flux can be as high as 350,000 kt yr-1_{(Rydberg et al. 1996).} While Rydberg et al. suggested that little of this nutrient load reaches the Swedish west coast or the Kattegat, Norwegian monitoring results by IMR in Norway have shown consistently elevated nutrient concentrations (particularly nitrate) in the coastal water masses along the Swedish and Norwegian Skagerrak coasts in the period from winter to early summer (Skjoldal 1993, Skjoldal et al. 1997, Aure et al.

Fig 3.9. Budget of biologically active nitrogen for the Kattegat (units kt yr-1₎

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1998, Aure and Magnusson 2008). Nutrient concentrations in the rivers draining into the North Sea have been decreasing (van Beusekom et al. 2005) and this flux may further decrease. The extent to which these nutrients contribute to primary production is difficult to evaluate but should largely depend on the timing. In winter, primary production is low but the above mentioned concentrations have been observed in spring at highest river runoff when the spring bloom is starting. It may be assumed that the nutrients in surface waters and to a certain extent at the thermocline are fully consumed by phytoplankton (Rydberg et al. 2006).

A budget provided in Håkansson (2007) shows very high nitrogen exchange rates of over 4000 kt N yr-1_{from North Sea to the Skagerrak and vice versa and a net} input of 231 kt N yr-1_{from the Belt Sea and Öresund into the Kattegat. These} numbers have to be considered when overall reductions are considered for the Swedish nutrient input from land based sources. Differences between the annual averages of net supply and export of nutrients to the Kattegat-Skagerrak region for the period 1985-2002 indicate a “change” or uptake of 214 kt N and 5 kt P y-1 (Håkansson 2007). In 2001-2002, the net supply of N from land, atmosphere and the Baltic Sea to the Skagerrak and Kattegat was estimated at 300 kt N yr-1_{and of} this about 225 kt N yr-1_{are assumed to be exported to the North Sea and the rest} removed by denitrification. This external load is approximately five times higher than all land based N-loads from Sweden for 2006. The overall reduction of land based sources is higher for TP than it is for TN. A lowering of primary production rates is to be expected and presumably also a change in the N:P ratios (Rydberg et al. 2000, addressed in section 4.1.2). The construction of a reliable budget is thus rather difficult due to major uncertainties inherent in the large and variable volumes of water exchanged between the Baltic Sea, Kattegat and Skagerrak and the North Sea to the west.

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4 Ecosystem Responses

4.1 Phytoplankton Production

4.1.1 Phytoplankton

Planktonic primary production in the Kattegat-Skagerrak region of the Swedish west coast ranges from oligo- to mesotrophic (100-250 g C m-2 y-1) in open waters to meso- to eutrophic (200-350 g C m-2_y-1_{) in estuarine and near-shore regions,} with monthly mean rates fluctuating from >700 mg to 2000 mg C m-2_d-1_{based on} 14_{C uptake measurements (Richardson and Heilmann 1995, Lindahl et al. 1998,} Rydberg et al. 2006, Lindahl 2002). These rates show strong geographic gradients, with highest rates present in fjords, embayments and river mouth regions and lowest rates in open sea regions. These gradients appear to follow gradients in natural (oceanic) import and anthropogenic nutrient sources (Lindahl 2002, Johan Rodhe presentation to panel). The phytoplankton community is dominated by diatoms, which typically form spring blooms (late February-April) and large dinoflagellates (e.g. Ceratium), followed by flagellates and smaller dinoflagellates that dominated from late spring through summer and autumn months (Heilmann et al. 1994). In contrast to the Baltic Sea, filamentous and colonial cyanobacteria appear to largely be absent from the phytoplankton community in west coast waters. Likely reasons for this will be discussed later.

While seasonal light availability and temperature regimes play important roles in determining phytoplankton successional patterns, nutrient availability and excesses determine the spatial distribution, magnitude and duration of phytoplankton biomass and blooms. Dominant nutrient sources, such as the Jutland Coastal Current, Baltic Sea outflow and local riverine inputs strongly modulate phyto-plankton primary production and biomass (as cell counts and chlorophyll a). This has been shown for both seasonal blooms and more sporadic blooms of potentially harmful taxa, such as the dinoflagellates (e.g. Dinophysis, Gymnodinium,

Alexandrium, Prorocentrum), haptophytes (e.g. Chrysochromulina) (Aksnes et al. 1989) and prymnesiophytes (Prymnesium parva, Prymnesium spp.) (Håkansson 2007). The absolute loads and concentrations as well as ratios of nutrients supplied play roles in determining the structure and abundance (biomass) of phytoplankton communities. This suggests that the loading rates, concentrations and relative proportions of key nutrients (nitrogen, phosphorus and silicon) are important determinants of observed patterns in primary productivity, phytoplankton biomass, composition and successional patterns.

Seasonal and inter-annual variability in nutrient supplies plays an important role in explaining variability in phytoplankton community biomass and compositional responses. This linkage can be shown both in terms of the extent to which major coastal currents are advected, dispersed and distributed in the west coast region

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(i.e. Baltic outflows, JCC and other currents) and the manner by which fluctuations in freshwater discharge via rivers impacts nutrient delivery to estuarine and coastal waters. Typically, high rainfall seasons and years, during which the delivery of nitrogen and phosphorus loads is elevated, lead to relatively high rates of primary production and maximum phytoplankton standing stocks. The interactions of offshore, riverine, atmospheric and Baltic Sea nutrient inputs, together with vertical stratification, control the magnitude and temporal and spatial extent of phyto-plankton biomass and blooms. This is true for both diatoms and highly motile flagellate/dinoflagellate species.

4.1.2 Nutrient limitation

Observational and experimental data indicate that the rates of supply, total loads and resultant concentrations of both nitrogen and phosphorus play key roles in determining the biomass and composition of planktonic primary producers. However, as is the case in most coastal marine ecosystems, the oversupply of nitrogen drives the overall eutrophication of Swedish west coast waters (Box 4.1). As with many other estuaries and continental shelf waters that have been studied, Swedish west coast waters exhibit a continuum of salinity (Figure 2.4) and nutrient gradients resulting from the interactions of freshwater runoff and coastal and oceanic circulation features. Spatial and temporal patterns of nutrient surpluses and depletions result in differential availabilities of N and P along these gradients (Figure 3.8). Typically, the more riverine and upper estuarine regions exhibit excess N relative to P supplies, while more saline coastal and seaward regions tend to have the lowest N supplies relative to P supplies. Loss of N due to denitrification (Rydberg and Sundberg 1988), more rapid turnover of available P in surface waters, and release of P from sediments due to iron sequestration by sulfide (Blomqvist et al. 2004) all contribute to the shift from N surplus to P surplus along the continuum.

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Box 4.1. Evidence for the Role of Nitrogen in Marine Eutrophication

Many studies conducted over the past 50 years have shown that N enrichment is a primary causative agent of marine eutrophication (Dugdale 1967, Ryther and Dunstan 1971, Nixon 1995, Smetacek et al. 1991, D’Elia et al. 1986, Vollenweider 1992). Evidence includes:

• In situ evidence of the spatial and temporal relationship of N inputs vs. primary

production responses (Paerl and Piehler 2008);

• Nutrient addition bioassays where N enrichment has been shown to stimulate primary production (Dugdale 1967, D’Elia et al. 1986, Fisher et al. 1992, 1999, Paerl and Bowles 1987, Pennock et al. 1994, Oviatt et al. 1995, Piehler et al. 2004); • Paleoecological studies showing that historic increases in anthropogenic nutrient

(N-dominated) loading led to eutrophication (Cooper and Brush 1993, Kemp et al. 2005);

• Uptake studies which have shown that at ambient concentrations and supply rates, N limitation is widespread (Harrison and Turpin 1982, Harrison et al. 1987, Syrett 1981);

• Correlative budgetary studies in which N supply rates were directly related to daily or annual rates of primary production in diverse coastal ecosystems (Nixon 1986, 1995);

• Stoichiometric analyses showing that, relative to carbon (C), phosphorus (P), and silicon (Si), N often falls below the nutrient supply ratio needed to sustain balanced plant growth (i.e. Redfield ratio of 105:16:1 for C:N:P; Redfield, 1958, Smith 1990); • Case studies (e.g., Kaneohe Bay, Chesapeake Bay, Neuse River-Pamlico Sound,

Long Island Sound, Narragansett Bay, Baltic Sea, coastal North Sea, northern Adriatic Sea, northern Gulf of Mexico) have shown that increasing N loads are directly linked to accelerated eutrophication (Smith et al. 1981, Nixon 1995, Fisher et al. 1999, Elmgren and Larsson 2001, Boesch et al. 2001, Boesch 2002, Paerl et al. 1998, 2004, Rabalais 2002).

Receiving waters exhibit varying sensitivities to N and other nutrient (P, Fe, Si) loads that are controlled by their size, hydrologic properties (e.g. flushing rates and residence times), morphologies (depth, volume), vertical mixing characteristics, geographic and climatic regimes and conditions. The magnitude and distribution of N in relation to other nutrient loads can vary substantially. In waters receiving very high N loads relative to requirements for sustaining primary and secondary production, other nutrient limitations may develop. This is evident in estuarine and coastal waters downstream of rivers draining agricultural regions highly enriched in N, such as the Po, Rhine, Yangtze and Mississippi, Ganges and Nile rivers (cf. Rabalais 2002, Nixon 2003). Excessive N loading may saturate in-shore primary

production, leading to either P and Si co-limitation or exclusive P and Si limitation (Dortch and Whitledge 1992. Lohrenz et al. 1999, Conley 2000, Sylvan et al. 2006), but farther offshore or down drift, chronic N limitation remains (Smetacek et al. 1991, Rabalais et al. 1996). These more distant waters can support additional N-driven eutrophication (Smetacek et al. 1991, Codispoti et al. 2001).

Eutrophication can exert feedbacks on internal N cycling, altering the availability of N and subsequent eutrophication potential. Numerous studies have shown organic matter loading, sedimentation and the extent of bottom hypoxia can regulate key N transformations, including nitrification and denitrification (Henricksen and Kemp 1988, Smith and Hollibaugh 1989, 1998, Seitzinger and Giblin 1996, Heggie et al. 1999, Boynton and Kemp 2000, Fear et al. 2005). These feedbacks can significantly affect N availability, and hence subsequent eutrophication potential (Smith and Hollibaugh, 1998; Eyre and Ferguson, 2002). For example, in the Baltic Sea the extent of hypoxia formation is thought to control denitrification rates and hence the ability of the system to depurate itself of fixed N (Elmgren and Larsson 2001, Vahtera et al. 2007). Lastly, top down effects such as grazing, and removal of grazers by overfishing (Jackson et al. 2001) can significantly alter the flux, availability, utilization and manifestation of N and other nutrient inputs.

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Not surprisingly, bioassays that have been conducted along such gradients in this region show a greater tendency for P limitation toward the fresher end of this continuum, while N limitation tends to be more dominant in the more distal and more saline regions (Granéli et al. 1990, Elmgren and Larsson 2001, Ærtebjerg et al. 2003, Spokes et al. 2006). As with many other large coastal transitional systems (e.g. Chesapeake Bay, Neuse Estuary-Pamlico Sound system, the Dutch Delta region, the Nile Delta, Mississippi Delta, Danube River plume, and the Yangtze Delta), N and P co-limited conditions can also exist and at times prevail (Paerl et al. 1990, Rudek et al. 1991, D’Elia 1987, D’Elia et al. 1986, Fisher et al. 1992, Nixon 2003, Kemp et al 2005, Paerl and Piehler 2008). The Swedish west coastal region appears to fall in line with many other such coastal continua with fairly predictable spatial and temporal gradients in N and P limitation and co-limitation that reflect the combined influence of land-based, human-dominated inputs together with oceanic inputs of these nutrients.

Patterns and trends in nutrient limitation can also be inferred from examining nutrient distributional data over time at key monitoring locations in the west coast waters (Figures 3.7 and 3.8) At locations near river mouths and in brackish

estuaries, depletion of DIP is experienced earlier and more widely than depletion of DIN. More saline estuarine and near-shore locations demonstrate the highest incidence of depletion of both DIP and DIN; while offshore, mid-Kattegat and Skagerrak regions tend to show the highest incidences of strong DIN depletion, while DIP remains detectable at quite low concentrations (Johan Rodhe

presentation to panel). At offshore stations, evidence suggests that DIN depletion tends to occur more rapidly than DIP depletion during and following the spring bloom, suggesting that N limitation develops during the course of the bloom. There is considerable variability in the timing and magnitude of these patterns. Most likely, this reflects the extent to which N is supplied by the external sources such as the Jutland Coastal Current, Baltic Sea outflow and the North Sea, as well as land runoff and atmospheric deposition. During summer months, both DIN and DIP remain depleted in the euphotic, upper mixed layer; however, in contrast to near-undetectable DIN concentrations, DIP concentrations remain detectable and DIN:DIP molar ratios are typically <5, indicating more effective recycling of P and significant and persistent N limitation. Bioassays conducted during this period have confirmed N limitation (Granéli et al. 1990; Spokes et al. 2006).

During early spring periods of sufficient N and or P availability, silicon may play an increasingly important role in limiting growth of the dominant diatoms.

Bioassays have not specifically indicated Si limitation; however very few bioassays have been conducted in these waters and they have largely focused on late spring and summer periods when diatoms would typically not dominate. The potential for Si limitation has been shown for the Baltic proper (Humborg et al. 2000) and additional bioassays during the early spring bloom period are needed to examine the importance of Si limitation and co-limitation, especially with N.

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4.1.3 Climatic factors

Climatic variability plays an important role in determining the nature and extent of nutrient limitation of primary production in near-shore and off-shore waters. Firstly, variability in temperature influences the metabolism and growth optima of various phytoplankton groups. In particular, those groups that exhibit relatively slow growth rates at low temperatures, including some dinoflagellate and

cyanobacterial species, may be favored by a general warming of the water column (Reynolds 2006, Paerl and Huisman 2008, BACC Author Team 2008).

Presumably, these species may compete more effectively with diatoms under a regional warming scenario (although here are other physical constraints, including persistent mixing and high flushing rates that would prevent cyanobacterial dominance, see Section 4.1.4). Another product of warming will be intensification of thermal stratification. Density stratification of the waters of the Kattegat and Skagerrak is dominated by vertical salinity gradients, therefore increased surface water warming will likely play a relatively small role in enhancing stratification. However, stronger stratification would favor highly motile flagellate and

dinoflagellate species that can migrate between the pycnocline and well-mixed surface waters. These species are capable of effectively sequestering DIP and (using alkaline phosphatases) organically-bound phosphorus at depth and storing assimilated P as polyphosphates for use in the lighted surface waters (cf. John and Flynn 2000, Reynolds 2006). The combination of effective P uptake and storage is likely to enhance the reliance on DIN (and potentially dissolved organic nitrogen, DON) availability to optimize bloom formation. Stated differently, the scenario of surface water warming, combined with stronger stratification (and calmer weather) should enhance N limitation, especially in off-shore waters.

Climate warming models project elevated amounts and more episodic delivery of precipitation for northern Europe (Christensen et al. 2007), with potential impacts on the delivery of diffuse nutrients from the catchments to estuarine and coastal waters (Bernes 2003; Graham 2004, BACC Author Team 2008). The ramifications for nutrient limitation are uncertain; however, larger freshwater discharge events are likely to enhance delivery of nutrients to receiving waters. This would have a proportionately larger effect on N as opposed to P delivery, because DIN is more soluble and more effectively leached from soils than DIP (McDowell and Sharpley 2001, Toth et al. 2006). If so, P limitation should increase at riverine-estuarine locations and delivery of N to more distal waters should increase, possibly enhancing primary production, biomass and bloom formation in these largely N-limited waters. Accompanying this might be an increased potential for harmful (toxic, hypoxia generating and food-web altering) phytoplankton blooms, which are known to be strongly stimulated by increased N supplies in coastal marine waters (Anderson and Garrison 1997, Paerl 1997, Paerl and Whitall 1999). It would seem highly unlikely that the enhanced N load accompanying more frequent and intense storm (and runoff) events will cause these ecosystems to switch from N to P limitation or co-limitation, as stoichiometric analyses of these waters indicate

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low DIN:DIP ratios, except where enriched with N from the Jutland Coastal Current.

4.1.4 Why N2 fixation does not compensate for N limitation

Geochemists have pointed out that, theoretically, nitrogen (N2) fixation should compensate for N-limitation in the world’s oceans and seas (c.f., Doremus 1982, Tyrell 1999) as well as inland waters (Schindler et al. 2008). According to this argument, P availability (which is assumed to control N2 fixation) is ultimately limiting primary production. In the world’s oceans, this argument operates over geological time scales and requires predictable and consistent biology (i.e., N2 fixation is solely and consistently controlled by new P inputs; Doremus 1982, Tyrell 1999). However, the theory does not seem to be compatible with biological time scales and the complex environmental controls of N2 fixation beyond

phosphorus availability (Paerl 1990). In many estuarine and coastal systems, N2 fixation does not automatically “turn on” when P is adequate and N is limiting. Experimental data indicate that other factors, including N:P supply ratios, iron (Fe) limitation, organic matter availability, physical constraints such as turbulence, advective processes and residence time, irradiance, and potentially “top down” consumption processes control N2 fixation (Howarth 1988, Paerl 1990, Paerl and Fulton 2008). As a result, this argument has limited application to managing coastal eutrophication. Here we elaborate on these alternative restrictions on N2 fixation; most of them are applicable to Swedish west coast waters.

Trace metal (Mo) and iron (Fe) limitation have been identified as potential factors controlling N2 fixation potentials in marine ecosystems (Howarth and Cole 1985, Rueter 1988, Paerl et al. 1994) because these metals are cofactors in the enzyme complex, nitrogenase, which mediates N2 fixation (Paerl 1990). Molybdenum was suggested as limiting N2 fixation under increasingly-saline conditions, based on the observation that sulfate (SO4-2), which is abundant in seawater and is a structural analogue of the dominant source of Mo, molybdate (MoO4-2), might competitively inhibit uptake of molybdate (Howarth and Cole 1985). Subsequent studies have found this not to be the case, even at very high salinities exceeding those found in the Kattegat-Skagerrak regions, i.e. molybdenum availability exceeds demands in these waters (Collier 1985, Paulsen et al. 1991). Therefore, there is little reason to believe that N2 fixation might be controlled by molybdenum availability in Swedish west coast waters. These waters have also been found to be quite rich in biologically-available iron (Croot et al. 2002). Accordingly, we conclude that it is unlikely that the paucity in N2 fixation in these waters is due to iron-limited conditions.

For some time, it has been argued that salinity itself might be a barrier to the establishment of N2 fixers in coastal and open ocean environments, because growth of dominant freshwater N2 fixing genera, including Anabaena and Aphanizomenon, can be shown to be inhibited by salinities exceeding a few salinity units

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either the establishment or activities of all cyanobacterial diazotrophs. A wide variety of active N2-fixers, including the genus Nodularia, common to the Baltic Sea, has been observed in the plankton and benthos of estuarine, coastal and open ocean environments, and even hypersaline lakes and lagoons (Potts 1980, Paerl 2000, Moisander et al. 2002a). Therefore, salinity cannot explain the scarcity of N2 fixers in Swedish west coast waters.

Turbulence exerts a strong impact on phytoplankton growth and structural integrity (Fogg 1982, Reynolds 1987). Increased levels of turbulence may inhibit growth of diazotrophs (Fogg 1982; Paerl 1990). Aquatic environments with persistent elevated turbulence may have a lower abundance of active N2-fixing heterocystous cyanobacteria. In laboratory experiments where shear rates representative of surface wind-mixed conditions were applied to bloom-forming cyanobacteria (Anabaena, Nodularia), Kucera (1996) and Moisander et al. (2002b) showed that rates of N2 fixation and photosynthesis can be suppressed by strong turbulence. The negative impacts of elevated shear could be due to: 1) breakage or weakening of cyanobacterial filaments, specifically at the delicate heterocyst-vegetative cell junction, causing O2 inactivation of nitrogenase in heterocysts (Fogg 1969), and 2) disruption of bacterial-cyanobacterial associations (Paerl 1990).

In the Baltic Sea there are mid-summer periods of relaxed winds as well as stable fronts during summer months. These are often the times and locations where cyanobacterial blooms occur (Kononen et al. 1996) as was particularly evident during the warm, still conditions that prevailed during the summer of 2005 (Vahtera et al. 2007). Another major difference is that along the west coast the nutricline is located with the pycnocline around 15 m, whereas in the Baltic Sea the nutricline is deeper. This means that along the west coast there will be more pulses of nutrients being entrained into the surface layer as opposed to in the Baltic Sea, where the surface layer remains deficient in N for longer periods. These nutrient pulses favor diatoms and dinoflagellates such as Ceratium that are typically abundant around the pycnocline. The dinoflagellates can use their motility to exploit both nutrients and light. The horizontal movement of water is also much stronger along the west coast than in the Baltic Sea, due to both strong variations in barotrophic and baroclinic differences.

Interestingly, despite the absence of planktonic N2 fixation in these turbulent systems, cyanobacteria and bacteria potentially capable of N2-fixation can be found in these systems, but they are most often confined to the benthos, submersed surfaces and in epiphytic communities (Paerl et al. 2000). Molecular studies, based on the analysis of the N2 fixing gene nifH, indicate that a diverse taxonomic potential exists for N2 fixation in these waters (Affourtit et al. 2001, Jenkins et al. 2004). However, N2 fixation activity is generally absent or present at ecologically-insignificant rates, and if it does occur, it is usually confined to sedimentary or biofilm habitats. A number of studies have suggested various physical and

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estuaries, especially in the water column. The relatively turbulent properties of estuarine waters, which include strong wind mixing, horizontal advection, tidal mixing, and high rates of small-scale shear, may restrict the establishment and proliferation of diazotrophic cyanobacterial and bacterial communities (Moisander et al. 2002b; Paerl 1996). In particular, persistent vertical mixing of near-surface waters prevents dominance by buoyant filamentous diazotrophic cyanobacterial bloom genera (e.g., Anabaena, Aphanizomenon, Nodularia, Trichodesmium). Further, the growth of these N2-fixing species that typify Baltic Sea blooms is inhibited in higher salinities because nitrogenase activity is limited by higher sulfate concentrations (Stal et al. 2003).

Residence time (flushing rates) can also play an important role in determining the degree to which diazotrophic cyanobacteria are present and dominate N-limited aquatic ecosystems. Even though N2 fixing cyanobacteria can form massive surface blooms in many lakes and quiescent marine ecosystems, growth rates of key bloom-forming genera (Nodularia, Aphanizomenon, Anabaena) are generally much lower (doubling times of 2-3 days) than those of non-N2 fixing eukaryotic groups such a diatoms, flagellates and even dinoflagellates (doubling times of 0.5-1 day). Therefore, in rapidly flushed estuaries and coastal sounds with low residence time that experience N limitation or P enrichment, bloom-forming cyanobacteria often do not compete effectively because growth rates cannot effectively keep up with flushing rates. As a result, they fail to exert dominance and more rapidly-growing taxa prevail. Relatively slow growth rates are often exploited by lake and reservoir managers to control and prevent cyanobacterial blooms, by keeping these systems flushed during periods of optimal cyanobacterial growth (summer), thereby promoting dominance by fast- growing and more desirable eukaryotic groups (Reynolds 1987).

Water residence time in the surface layer of the Kattegat and Skagerrak is in the order of one month (Gustafsson 2000, Johan Rodhe presentation to panel). Such short residence times and highly dynamic horizontal advective conditions prevent the establishment and buildup of cyanobacterial bloom populations and may help explain their absence on seasonal and multi-annual time scales. In contrast, the Baltic Proper has a residence time on the order of 25 years, accompanied by per-manent stratification and strong fronts. These are ideal conditions for the establish-ment and persistence of cyanobacterial bloom populations (Kononen et al. 1996). In summary, estuarine and coastal waters have a diverse genetic potential for N2 fixation, which under favorable conditions (e.g., mid-summer stratified conditions in the Baltic Sea) can be readily expressed. However, more often, persistently wind mixed surface waters, readily flushed and nutrient-pulsed conditions in these environments represent physical and chemical barriers to N2-fixers, thus restricting their dominance and bloom potentials. This, combined with the fact that estuarine and coastal systems are frequent sites of active denitrification and phosphorus

Eutrophication of the Seas along Sweden s West Coast