Tapani Sallantaus

Brook Water Quality and Background Leaching from Unmanaged Forested Catchments in Finland

The water chemistry of 21 outlet brooks in undisturbed, forested catchments (0.074–38 km2) in Finland was monitored during three years (1997–1999) in order to estimate the background levels and leaching of total organic carbon (TOC), iron, nitrogen and phosphorus, and examine the relationships between catchment characteristics and brook water quality in pristine forested areas. The studied catchments are located throughout Finland except the northernmost parts of the country. The concentrations of TOC were relatively high, on average 20 mg L-1. Annual leaching of TOC ranged from 3000 to 10 000 kg km-2. The average total nitrogen concentration and annual leaching were 430 μg L-1 and 140 kg km-2, respectively. The average total phosphorus concentration and annual leaching were 15 μg L-1 and 5.4 kg km-2, respectively. On a national level temperature and discharge conditions were the most important variables for predicting total phosphorus, total nitrogen and TOC concentrations. Both total nitrogen and total phosphorus concentrationsdisplayed positive correlation with temperature, and thus the concentrations were higher in the south than in the north. Nitrogen was mainly organic and showed strong correlation with TOC. On the catchment level, both TOC and total nitrogen concentrations and export had a strongpositive correlation with the abundance of Norway spruce (Piceaabies Karsten) and a strong negative correlation with the abundance of Scots pine (Pinus sylvestris L.). Nitrate concentrations and leaching were related to average site type. The more fertile the average site type was in the catchment, the higher the nitrate concentrations and export were.

Restoration of nutrient-rich forestry-drained peatlands poses a risk for high exports of dissolved organic carbon, nitrogen, and phosphorus

Restoration impact of forestry-drained peatlands on runoff water quality and dissolved organic carbon (DOC) and nutrient export was studied. Eight catchments were included: three mesotrophic (one undrained control, two treatments), two ombrotrophic (one drained control, one treatment) and three oligotrophic catchments (one undrained control, two treatments). Three calibration years and four post-restoration years were included in the data from seven catchments, for which runoff was recorded. For one mesotrophic treatment catchment only one year of pre-restoration and two years of post-restoration water quality data is reported. Restoration was done by filling in and damming the ditches. Water samples were collected monthly-biweekly during the snow-free period; runoff was recorded continuously during the same period. Water quality was estimated for winter using ratios derived from external data. Runoff for non-recorded periods were estimated using the FEMMA model. A high impact on DOC, nitrogen (N) and phosphorus (P) was observed in the mesotrophic catchments, and mostly no significant impact in the nutrient-poor catchments. The DOC load from one catchment exceeded 1000kg (restored-ha)-1 in the first year; increase of DOC concentration from 50 to 250mgl-1 was observed in the other mesotrophic treatment catchment. Impact on total nitrogen export of over 30kg (restored-ha)-1 was observed in one fertile catchment during the first year. An impact of over 5kg (restored-ha)-1 on ammonium export was observed in one year in the mesotrophic catchment. Impact on P export from the mesotrophic catchment was nearly 5kg P (restored-ha)-1 in the first year. The results imply that restoration of nutrient-rich forestry-drained peatlands poses significant risk for at least short term elevated loads degrading the water quality in receiving water bodies. Restoration of nutrient-poor peatlands poses a minor risk in comparison. Research is needed regarding the factors behind these risks and how to mitigate them.

Nitrogen and phosphorus concentrations in discharge from drained peatland forests are increasing

The current understanding, based on previous studies, is that increased discharge nutrient concentrations from boreal peatlands drained for forestry return to similar levels as those of pristine peatlands within about 20years after their drainage. As an implicit consequence of this finding, it has been assumed that there are no long-term increasing trends in nutrient exports from these peatlands after the establishment of forestry. We analysed discharge total nitrogen (TN) and phosphorus (TP) concentration data from 54 catchments with undrained pristine peatlands and 34 catchments with drained peatlands using data with considerably longer drainage history than in previous studies. Our results agree with previous studies in that discharge TN and TP concentrations in areas drained 20-30years ago did not differ much from those in pristine sites. However, we also observed that the TN and TP concentrations were increasing with years since drainage of these catchments. Discharge TN and TP concentrations were over two times higher in areas drained 60years ago when compared with more recently drained areas. Our results challenge the current perceptions by showing that forestry-drained peatlands may contribute to water eutrophication considerably more than previously estimated.

Natural and Restored Wetland Buffers in Reducing Sediment and Nutrient Export from Forested Catchments: Finnish Experiences

One of the water quality management practices in forested catchments is to construct wetland buffers between managed areas and recipient water courses. Wetland buffers can be constructed simply by routing runoff from forested areas to natural peatlands and wetlands, or by rewetting lower sections of drained peatlands by filling in or blocking the drainage ditches. The use of natural and restored wetland buffers for reducing nutrient and sediment export from forested catchments, particularly catchments dominated by forestry-drained peatlands, has been studied actively in Finland during the last 15 years. The studies have shown highly variable retention capacity for wetland buffers with different site characteristics and under different environmental conditions. In favorable conditions, high amounts of sediments and adhered mineral elements may be deposited within peat and surface vegetation of the buffer. Dissolved nutrients can be retained biologically into plant and microbial biomasses and chemically into peat. In contrast, nitrogen can also be lost into the atmosphere in gaseous form. In this literature review, we summarize the results of the experiments established on natural and restored wetland buffers in forested catchments in Finland to clarify the different processes and factors controlling their nutrient and sediment retention capacity. We also discuss the limitations and possible negative consequences of using wetland buffers for managing water quality in forested catchments.