The chemical and ecological status of Finnish surface waters has considerably improved since the 1970s. However, some of the stressors that affect our lakes and rivers have proved difficult to control, and some new pressures have also appeared. While point source stressors such as municipal and industrial waste waters are nowadays well managed, diffuse loadings from agriculture and forestry have been more difficult to control. Similarly, acid deposition and lead fallout have diminished in Europe, but the effects of climate change are becoming more evident. New pollutants such as microplastics and nanoparticles are receiving more attention, and our ever-increasing standard of living also works to increase pressures on inland waters.
The Ministry of the Environment has identified nutrient inputs from agriculture as the main threat to surface water quality in Finland. However, forestry and peat extraction can be significant stressors, especially for headwater lakes. These activities typically co-occur in peaty watersheds, and considerable public debate has lately centered on the effects of peat extraction, in particular. However, the magnitudes of these activities are vastly different, with 4.8 M ha of peatlands drained for forestry and 0.06 M ha in active peat production. It is exceptionally difficult to separate the effects of peat extraction and peatland forestry, because the compositions of the loadings are too similar. From a watershed management point of view, however, it is essential to know where to direct the management efforts.
Sediment-derived time series potentially provide a way to separate the effects of forestry and peat production on lakes if a reference lake setup is utilized. In this arrangement, the reference lake records the regional history of stressors and changes in the watershed from early settlement, agriculture, and forestry to acid rains, summer home development, and recent climate change. The impacted lake then has the additional stressor of peat production, the timing of which is known. Ideally, the water and sediment quality histories of the two lakes should be rather similar until the effects of the additional stressor start to be seen in the impacted lake.
GTK conducted a paired lake study on the impacts of peat extraction in central Finland with the Universities of Jyväskylä and Turku. The study consisted of sediment echo sounding, coring at several locations in each lake, radiometric dating of the sediments (C-14, Pb-210, Cs-137, Am-241), and chemical and biological (chironomids, diatom algae) analyses. The results showed that peat extraction has not resulted in the deposition of thick layers of organic sediment in the impacted lake, as had been anticipated. In fact, the increments in sediment thickness since the start of peat production were similar in both lakes. As this may be the result of sedimentation conditions, the recent sediment accumulation rates were also compared with background values at the same coring site. The results confirmed that the accumulation of carbon or dry matter had not increased more in the impacted lake than in the reference lake.
The sediment records displayed marked changes in both lakes starting from the late 19th century. The lakes became more eutrophic and humus-rich due to the first loggings, settlements, and agriculture. The greatest changes occurred in the 1960s and 1970s, when the region was drained for forestry. The onset of peat production was also visible as a transient peak in mineral erosion and a short-lived shift in the chironomid fauna.
Perhaps the most interesting features of the sediment records were the recent changes that occurred as late as in the 2000s. Water quality has deteriorated and eutrophication has increased, especially in the reference lake. These alarming signs could be due to climate change, summer home development, or drainage network maintenance. Recent research also suggests that old (>30 yrs) drainage networks on peatlands may start to discharge more dissolved nutrients due to peat mineralization and mire surface subsidence.
These signs of deteriorating water quality highlight the need to continue our water management efforts. Sediment records of environmental change can provide valuable insights for the planning and design of management actions.
T. Kauppila, T. Ahokas, L. Nikolajev-Wikström, J. Mäkinen, M. H. Tammelin & J. J. Meriläinen 2016. Aquatic effects of peat extraction and peatland forest drainage: a comparative sediment study of two adjacent lakes in Central Finland. Environmental Earth Sciences 75:1473.
Text: Tommi Kauppila
Tommi Kauppila works as a Research Professor of Mine and Industrial Environments in the Industrial Environments and Recycling unit of the Geological Survey of Finland. His research focuses on the effects of mining and other industrial activities on especially the surface waters.