Ecosystems

Figure 4‑3. Chloride concentrations in Lake Bolundsfjärden during the period 2002–2007, and measure-ments of the sea level in the Baltic Sea. Red dashed lines mark the gradual decrease in Cl concentration due to dilution by the supply of freshwater from the catchment (from /Tröjbom et al. 2007/).

2002 2003 2004 2005 2006 2007

Standardized time series ^{Sea level}Chloride - surface

Chloride - bottom

carried out. Thorough descriptions and evaluations of the input data (site-specific and generic) and of the methods used are presented in /Nordén et al. 2008/ for the limnic ecosystem and in /Wijnbladh et al. 2008/ for the marine ecosystem.

Terrestrial ecosystems Vegetation

The terrestrial vegetation is strongly influenced by the characteristics of the Quaternary deposits and by human land use. The area is fairly young as an effect of the land uplift and the low topogra-phy /Söderbäck (ed) 2008/. The location at the sea coast makes the seashore a prominent feature in the east along with conifer forests, shallow lakes, mires and some agriculture land. Below follows a brief description of three major vegetation types within the regional model area, see /Löfgren (ed) 2008, chapter 4/ for more details.

Forests cover 73% of the land area at Forsmark and are dominated by Scots pine (Sw. tall) and Norway spruce (Sw gran) growing mainly on wave-washed till. The spruce becomes more abun-dant in areas with a deeper soil cover in combination with mesic-moist conditions. Outcrops are not prevalent in the Forsmark area, making pine forest on acid rocks quite scarce. The calcareous soil material provides nutrient-rich conditions, characterised by herbs and broad-leaved grasses, along with a number of orchid species, and predominant humus forms of mull and intermediate moder types /Lundin et al. 2004/. The deciduous tree species are dominated by silver birch (Sw. våtbjörk), common alder (Sw. klibbal) and rowan (Sw. rönn), but also Norway maple (Sw. skogslönn) and ash (Sw. ask) are fairly common. The Forsmark area has a long history of forestry, currently revealed as a fairly high percentage of younger and older clear-cuts in different successional stages in the landscape. Silver birch is the dominate species in many of the earlier successional stages until it is replaced by young Norway spruce or Scots pine, depending on soil type and/or management.

Wetlands occur frequently and cover 10–20% of the area in the three major catchments, and up to 25–35% in some sub-catchments /Johansson et al. 2005/. A major part of the wetlands consists of coniferous forest swamps or open mires. The mires are characterised by a high calcareous influence, which makes the extremely to intermediately rich fen types common in this area /Göthberg and Wahlman 2006, Jonsell and Jonsell 1995/. Although not yet so numerous, bogs are also present in the inland areas, and are continuously created due to land uplift and mineral and nutrient leaching processes. Other important wetland types are the freshwater shores (wet meadows or marshes) and riparian deciduous forest swamps along streams. The latter are inundated by water at least once a year and affected by overbank sedimentation. The flat topography in the area promotes the occurrence of small floodplains that are flooded during high-flow periods /Carlsson et al. 2005/.

Such areas may be of importance for the retention of different substances that otherwise would be transported by the water directly to the sea.

Agricultural land covers hardly 5% of the land area at Forsmark and is mainly located in the south-eastern part of the candidate area. The agricultural land consists of arable land and grasslands.

It provides food for humans, either directly as crop production or as production of fodder for animals. Grasslands are used for browsing cattle (pastures) or for hay-making (meadows) and are mainly found close to settlements. Some arable land and to a large extent semi-natural grasslands (intensively used grassland with a long management tradition) in the area have been abandoned following the nation-wide general regression of agricultural activities during the past 60 years. The standard yield of the dominant grain species, barley, is 2,800 kg per ha (based on information from the year 2002).

Fauna

The fauna has been investigated in a number of different surveys, see section 4.2 in /Löfgren (ed) 2008/. The aims of the surveys were to 1) describe which species or functional groups are present in the area, 2) establish reliable density estimates of larger animals and birds, and quantify important pools/fluxes to be used for the ecosystem models, and 3) establish a baseline for the now ongoing monitoring programme that can be used to relate different kinds of disturbance to wildlife population changes. The fauna is more difficult to associate with specific habitats than vegetation, but for some species or functional groups an attempt has been made to distribute their consumption in the

landscape, either by using their habitat preferences or their feeding preferences, or both. Density and biomass estimates for all mammals, birds, amphibians and reptiles are presented in section 4.2 in /Löfgren (ed) 2008/, together with calculations of their production, consumption, egestion and respiration. The calculations are based on the field metabolic rate for each species.

Limnic ecosystems

The Forsmark area today contains 25 lakes. Most of them are small (lake areas range from 0.01–0.75 km²). They are all very shallow (the “median lake” has an average depth of 0.3 m and a maximum depth of 1 m) and are classified as oligotrophic hard-water lakes, i.e. they contain high calcium levels, but low levels of nutrients, as phosphorus is precipitated together with the calcium.

This kind of lake is common in the region, i.e. along the coast of northern Uppland, but is very uncommon in Sweden in general.

Due to the shallow depth, all lake bottoms are reached by sunlight, and vegetation occurs at all depths. The dominant vegetation is stoneworts (Chara sp.). At the top of the bottom sediment, algae and cyanobacteria are often found in thick layers. These two groups of primary producers dominate the biomass and primary production, making phytoplankton biomass and production less important.

The lakes are surrounded by reed belts, which are extensive around smaller lakes.

The dense stands of Chara harbour various kinds of benthic fauna and also function as refuges for smaller fish. Common fish species are perch and roach, as well as tench and crucian carp. This last species survives low oxygen levels and is the only fish species present in the smaller lakes, where oxygen levels can be very low during winter.

The streams in the Forsmark area are very small and mostly resemble man-made ditches. In some stretches, the riverbanks are about two metres deep. Long stretches of the streams are dry during summer, but they may function as passages for migrating spawning fish during wetter conditions, especially in the more downstream areas.

An extensive compilation of all the data concerning limnic ecosystems in the Forsmark area is presented in /Nordén et al. 2008/. In that report, details about the ecosystem models and mass balance methodology, as well as results for lake ecosystems in the Forsmark area, are discussed.

The delimitations between different habitats in all lakes larger than 0.5 ha in the Forsmark area were determined in a study that also included identification of the watershed. An extensive amount of biological data, including biomass data on phytoplankton, bacterioplankton, zooplankton, benthic microphytes, benthic bacteria, benthic fauna, macrophytes and fish have been collected from Lake Eckarfjärden, one of the larger lakes in the area /Nordén et al. 2008, section 3.9.1/. Some of these functional groups have been sampled several times per year over two or three years. Moreover, also primary production of phytoplankton and benthic microphytes has been measured several times over two years.

Biological data from other large lakes in the area as well as from some of the smaller lakes (which often are in a later successional stage than the larger ones) are also available, although these data are not very extensive. Data on water chemistry are available for both large and small lakes as well as from streams in the area /Nordén et al. 2008, section 3.8/. Data on the chemical composition of biota are available for a limited number of functional groups from three different lakes /Nordén et al.

2008, section 3.10/.

Marine ecosystems

Forsmark is situated at the Bothnian Sea, a semi-closed part of the Baltic Sea with large net fresh-water supply, separated by narrow connections from the Baltic Proper in the south. The freshfresh-water

The marine area at Forsmark consists of a funnel-shaped bay (Öregrundsgrepen), separated from the open sea in the east by the Gräsö island. The area consists of a shallow (depth often less than 10 m) and sparse archipelago along the coast and a deeper trough in the eastern part, the Gräsö Trough, with a maximum depth of 58 m, Figure 4-4. Generally, the bottom in the area has a gentle slope.

Most investigations and studies of the marine ecosystem have been performed in the coastal area around the Forsmark power plant. The following description concerns this area, but also the open sea in the Öregrundsgrepen. Generally, physical characteristics, such as light and temperature, have been monitored continuously for several years at a few monitoring stations. Chemical characteristics have been monitored monthly or bi-weekly in three to five stations, whereas sediment conditions and biota have been surveyed once or twice, but with a higher spatial resolution.

The coastal water in the Forsmark area has an average light penetration depth of 5.5 m, which is low compared with the value recorded at the national monitoring station located further out in the Bothnian Sea (8.7 m). The light penetration varies, however, and the inner, less exposed, areas have Secchi depths (sea water transparency) of 2–3 m.

Due to the wide open boundary towards north, Öregrundsgrepen is strongly influenced by the large-scale circulation and dynamics of the Baltic Sea, especially by the southbound coastal current, with seasonal density fluctuations due to variations in the collective discharge to the Bothnian Sea from the major rivers in northern Sweden and Finland. The local freshwater discharge to Öregrundsgrepen is moderate with the streams Forsmarksån and Olandsån discharging about 3 and 6 m³/s,

respec-Figure 4‑4. The marine area at Forsmark and the sub-basins studied in Öregrundsgrepen. Increasing dark blue indicates increasing depth /Wijnbladh et al. 2008/.

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tively, as an annual average. Water exchange is estimated for modelling purposes for the 28 sub-basins in the area and presented as Average Age (AvA) of the water parcel. AvA values are small, less than one day, for all but two basins.

The dominance of shallow coastal areas enhances the importance of benthic communities and, hence, the marine biota in the area is dominated by benthic organisms. The benthic zone can be separated into soft and hard bottom communities and into photic and aphotic zones. Although the light penetration is rather low, most of the area is within the photic zone (receiving more than 1% of surface irradiance). However, the light intensity decreases rapidly and, therefore, most benthic organisms depending on light, i.e. macroalgae, vascular plants and benthic microalgae, are concentrated along the coast. On the soft bottom substrate area, rooted plants and the algae Vaucheria dominate, whereas bottoms with till or bedrock host brown, green and red algae.

Detrivores, snails and mussels feeding on dead material dominate both hard and soft bottom substrates. The fish community is dominated by the marine species herring (Sw. strömming) in the pelagic area, whereas limnic species, especially eurasian perch (Sw. aborre), dominate in the coastal areas and in the secluded bays. In open sea, the zooplankton community is dominated by meso-zooplankton, especially copepods. Concentrations of chlorophyll in seawater indicate that the phytoplankton biomass in the Forsmark area is lower than that generally found in the national monitoring programme. The measured biomass varies between 30–180 µg/L and the plankton is dominated by ciliates, cryptophytes and diatoms.

An extensive compilation of all available data from the marine ecosystem in the Forsmark area is presented in chapter 3 in /Wijnbladh et al. 2008/. That report also presents details about the ecosystem models and mass balance calculations for marine ecosystem basins in the Forsmark area.