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WATER SOURCE QUALITY ANALYSES AS A TOOL IN MIRE RESTORATION – IMPACTS OF RESTORATION ON WATER QUALITY T. SALLANTAUS Pirkanmaa Regional Environment Centre, Nature Conservation Department Tampere, Finland, Tapani.Sallantaus@ymparisto.fi INTRODUCTION Finland has a comprehensive network of protected areas representing different biotopes. The establishment of this network has been greatly aided by different protection programmes. When peatlands are concerned, the most important are the Mire Conservation Programme from the years 1979 and and the Programme for the Development of National Parks and Nature Reserves, which came out first in 1978.
The accomplishment of the programmes has been gradual, when privately owned lands are concerned, and neither of these programmes is yet fully accomplished. Before the conservation the areas have been utilized mainly as forestry land. For these reasons, altogether about 40 000 ha of mires in the Finnish nature conservation network have been drained by landowners before final inclusion to nature reserves.
As a rule, these areas need active restoration measures to eventually regain their original biodiversity. In those peatlands in which the conservation is based on the Mire Conservation Programme, the drained areas are usually a smaller part of a mire complex, and the restoration is important from the hydrological point of view. The main goal is a hydrologically sound ecosystem. Therefore, hydrological understanding is essential in the repair of these protected but disturbed sites. In addition, water quality data of different water sources of a disturbed mire ecosystem are a valuable tool in planning actual restoration measures. Potential diversity hot spots can be identified, diversity of water chemistry can be preserved, potential problems e.g. due to excess nutrients in the potential inflows identified.
In the National Parks, also mires, which have been fully ditched and have turned into forests, are restored. Promoting landscape diversity is one of the aims. Also restoring naturally wooded nutrient rich peatland sites is considered important, since these are rare in pristine condition in southern Finland, and many threatened peatland and forest species require these wet habitats.
By the end of the year 2004, approximately 10000 ha of mires had already been restored in Finland. One of the pioneer large scale restoration projects was "Restoration of active raised bogs, aapa mires and bog woodland in Natura 2000 sites", by the Forest and Park Service, Western Finland. The project received funding from Life Nature, and was conducted in 1996-1999. Seitseminen National Park was one of the 17 target areas in the project, and the largest one. In this area, hydrological monitoring was also conducted, and the collection of data presented here was initiated during this project. Today, about all of the more than 1000 ha of drained mires in Seitseminen are already restored.
The mires of Seitseminen are mainly rather unfertile bogs or very poor fens, and the original tree stand consisted mainly stunted pines (Leivo & al.
1989). Therefore, also data from wooded swamps from very southern Finland are added, to get a wider scope. Main emphasis is given to the short term water quality impacts of restoration measures, since these may pose some problems within the mire ecosystem to e.g some sensitive mire species still persisting in spite of the disturbances. Water quality changes may also cause some unwanted changes in the recipient watercourses.
MATERIALS AND METHODS Study areas. Seitseminen National Park is situated in western Finland, in the municipalities Kuru and Ikaalinen, Pirkanmaa, 61o, 58' N, 23o 22' E. It was founded in 1982, and covers an area of 4200 hectares. The area is about 160 – 200 m above sea level, and belongs to the very southern part of the middle boreal zone in Finland (Kalela 1961). The annual precipitation is 666 mm on average (1961-1990, uncorrected, Finnish Meteorological Institute), mean annual temperature about 3 oC.
The bedrock is mainly coarse granite or granodiorite, and the soils are fairly acidic and poor in nutrients. The area is situated somewhat higher than most of the southern Finland and this is reflected in the slightly higher precipitation and cooler climate compared with areas of the same latitude.
Since also the terrain is fairly flat, mires are abundant, covering about half of the total area of the park. Oligotrophic and ombrotrophic mire site types dominate (Leivo et al. 1989). Before drainage, these mires were fairly open compared with the mires of southern Finland on average, or growing usually only stunted pines. About 60 % of the mires had been drained before foundation of the park (Heikkil & Lindholm 1994) – today, nearly all of these are already restored.
The hydrological monitoring was started in the spring 1997 and took place in 5 catchments. Two of these are small lakes, Lake Srkijrvi and Lake Pitkjrvi, surrounded by mires which were to be restored during the project.
Three catchments, Saukkolamminsuo, Tuulimensuo and Ksikivenlamminsuo, are lake-less basins and had a high degree of mires needing restoration. The percentage of drained mires varied 28-46, and these have been restored during the monitoring; the main parts in 1997-1999, but in the case of Ksikivenlamminsuo, also in 2000 and in Pitkjrvi, some parts were restored in the year 2003. The lakes are rather small, 8 and 9 ha, about m deep, and with a catchment area about 200 ha. This means that average retention time is only about 0,3 years. The lake-less basins are about 50 ha in size, and in these areas the percentage of restored mires varies 36-44.
Most of the mires had been drained in the sixties and had clearly responded to drainage; however, they were still in the transitional stage towards real forest vegetation. All the areas had been fertilized in the past, too.
The trees were mostly removed before filling of the ditches – the fillings took place mainly in the late autumn.
The watersheds of Seitseminen have also a reference basin, Pehkusuonoja, in Kuru, which is a forest drained basin and has been intact during the project. Therefore, variation of runoff water quality due to natural reasons can be detected from these results.
Nuuksio National Park is situated in southern Finland, in the municipalities Espoo, Kirkkonummi and Vihti, in the southern part of the southern boreal zone, and is an important recreational area for Helsinki.
The park was founded in 1994. The studied mire area, Mustakorpi, about 80 m a.s.l., is in Vihti, and represents mainly rather fertile spruce swamps, originally mainly "herb-rich hardwood-spruce swamps" (see Ruuhijrvi & Lindholm, this volume), which have greatly changed due to forest drainage. Today, these support a spruce stand exceeding even cubic meters per hectare, and the vegetation is similar to moist grass-herb forest. All the trees were left to the site in spite of the restoration. Only small patches of dead wood have been formed after damming of the ditches in the fall 2001. In addition, during the damming, the excavator also imitated wind throws, and created some extra dead wood to this site which was in active forestry until very recently. Monitoring was started in April 2001, 6 months before restoration. The watershed is about 50 ha, of which about 20 % consists of restored mires.
There is also one monitored lake in southern Finland receiving waters from similar restored spruce mires, 20 % of the catchment: Lake Vh Ruuhijrvi in Evo, Lammi, 154 m a.s.l. The mires were restored in 2000, and these are slightly less fertile than those in Nuuksio, originally mainly Vaccinium myrtillus spruce mires. The results from this site are also briefly described.
Monitoring. Measuring weirs were constructed into two of the brooklets both in Seitseminen and Nuuksio. During the 9 study years in Seitseminen and 5 years in Nuuksio, the lake-less brooklets were sampled for water quality about 11 times every year on average, the lakes 3 - 4 times per year. Runoff was recorded by the weirs at the time of sampling. A thorough list of water quality variables (about 20) was analyzed in the laboratory of Pirkanmaa Regional Environment Centre. Only the results of organic carbon in the waters are presented in detail over here, as well as some new results of total phosphorus and lake responses to restoration. Total phosphorus results already presented by Sallantaus (2004), are cited and discussed.
RESULTS Before the restoration, the water quality in the monitored sites was characterized by high organic matter content, relatively low pH and fairly low nutrient concentrations. Organic carbon content in the waters was usually above 20 mg/l, in the brooklets occasionally above 40 mg/l, and pH varied 4,1–5,4. The highest pH values as well as alkalinities were observed in Nuuksio, Mustakorpi, as well as the lowest organic carbon concentrations. The mean phosphorus concentration was about 20 µg/l in all the areas. Suspended solids were very low, below 2 mg/l for 95 % of the time, even immediately after restoration with only a few slightly elevated exceptions, and therefore total organic carbon concentrations (TOC) represent mainly dissolved organic carbon, coloured humic substances.
The most notable change in the water quality, caused by restoration, was the increase in phosphorus concentrations. The restoration measures in the catchments increased the annual mean phosphorus concentrations at least five-fold in all the catchments. Mean annual flow weighted values between 113-142 µg/l were recorded in Seitseminen, nearly 100 µg/l in Nuuksio. The excess annual leaching, calculated per restored mire area, was even above 100 mg/m2 and up to 300 mg/m2 in excess as a sum of years (Sallantaus 2004).
The highest leaching values were recorded after a time lag; a warm period is needed, before concentrations really start to rise and the peak leaching occurs during high flows of the following fall and spring. Thereafter, a more or less rapid recovery towards pre-restoration values takes place. In Mustakorpi, the phosphorus concentrations are still around 100 µg/l, nearly 4 years since restoration, because rewetting was not properly started in the dry years 2002-2003. A new increase in concentrations took place after a wet period in late summer 2004.
The increase in organic carbon concentrations is more moderate compared with phosphorus. However, the catchments of Seitseminen show an initial increase of about 50 – 100 %, and thereafter a slow recovery. However, a 20 - 30 % increase seems to persist even 5 – 6 years from restoration (Table 1), the only exception being Lake Srkijrvi, which has been near the prerestoration values, about 20 mg/l, for most of the time. A short calibration period especially in case of Srkijrvi, and the wet years 1998 and complicate the interpretations.
In case of Nuuksio, Mustakorpi, the increase in organic carbon concentration was more than three-fold, and this increase has persisted more than 3 years (also in 2002, based on colour and COD values; some missing values of TOC have been replaced by COD values multiplied by 0,7). In this area, the proportion of restored mires is the lowest, about half of that in Seitseminen. On the other hand, the mires are clearly minerogenic mires, and both the vegetation and the peat quality have greatly altered due to drainage.
Concentrations of total organic carbon (TOC, mg/l) in the studied catchments in 1997-2004. In 2002, TOC was not measured. Pre-restoration values are in italics.
Area Year 1997 1998 1999 2000 2001 2003 Seitseminen:
Mustakorpi 22 63 In all the 3 studied lakes, phosphorus concentrations rose at least by 100 µg/l. The phosphorus was mainly dissolved, reactive phosphate phosphorus, readily available for e.g. algae. In the studied lakes in Seitseminen, there was a clear shortage of nitrogen from the point of view of algal growth and therefore, chlorophyll a concentrations remained low, below 10 µg/l, biological uptake of phosphorus was low, and the consecutive removal of phosphorus from the water phase in the form of sedimenting algae was unefficient. In Lake Vh Ruuhijrvi, which has a much higher pH and more fertile soils surrounding it than in the lakes in Seitseminen, the increase in total phosphorus resulted in high concentrations of chlorophyll a, up to 110 µg/l. Algal growth and resulting biological removal of P from epilimnion were efficient. As a result of decaying algae in the hypolimnion, together with the high TOC concentrations in the lake, above 40 mg/l, oxygen status of the lake was very poor in several occasions, even in the summertime after heavy rains. In the winter 2003 and in August 2004, the oxygen saturation was below 10 % even in 1 m depth and 0% in all the other measured depths in this 7,4 m deep, 10 ha lake.
DISCUSSION AND CONCLUSIONS The vegetation of a mire is determined by abiotic, to a large extent by hydrological and hydrochemical factors. Well studied catchments give us perspective to understand these regularities (see e.g. Tahvanainen et al. 2002, Laine et al. 2004). Such model studies of undisturbed mires are useful also in the planning of restoration measures in different cases.
The results presented here complicate the situation, if mire areas to be restored are situated in a protected catchment with original nature values present in the areas below. The water quality is greatly modified when passing through a rewetted mire, at least in those cases, in which drainage has substantially altered the vegetation and peat properties. Especially aquatic life may be endangered, but the same processes may also affect vegetation.
In addition to phosphorus mobilization (Sallantaus 2004), the mobilization of organic carbon, mainly dissolved, coloured organic matter, is an important factor. In unfertile mires, the relative increase in organic carbon concentrations in watershed scale is relatively small, but in minerotrophic mires with a large tree stand and greatly altered peat, large amounts seem to be mobilized.
This can be understood in the light that the amount of water getting into contact with the peat, altered by drainage, is greatly increased by restoration especially in minerotrophic mires with a large watershed. In the drained state, waters from the watershed are cut away already by the first ditch, in a rewetted state these waters should pass through the mire, as in the natural state.
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