Two large reservoirs Lokka (417 km2) and Porttipahta (214 km2) were built at the upper part of river Kemijoki watercourse in northern Finland in the early 1970s. Both reservoirs were built to produce hydropower with wide water level regulation range (Lokka max. 5 m, Porttipahta max. 11 m). Flooded area consisted mainly of peatlands and forests. Water level is lowered during winter, when reservoirs are covered by ice and the energy need is greatest. Regulation was extensive during the first decade and water quality problems occurred. Wintertime oxygen concentration was very low and amount of phosphorus was high. This situation was caused by degradation of organic matter released from bottom. Organic matter was mainly released from peatlands pressured by penetrating ice and by breakdown of peat during the summer. Amount of total phosphorus is quite high compared to natural lakes. Main sources of loading during the first years of the reservoirs were at the littoral zone, where waves and currents caused erosion. The importance of the pelagic zone becomes more obvious as reservoirs age; bottom area is covered by thick layer of sediment consisting mainly of eroded peat. The bottom sediment forms a store of organic matter and phosphorus, which can be released during winter, when oxygen concentration is very low and also other transport mechanisms (convection, diffusion) exist. Nowadays, when the regulation amplitude is only 2-3 m/yr, water quality is more stable. In that situation the wind-caused resuspension of sediment can be quite important as a transport mechanism of organic matter and nutrients in shallow and open reservoirs.

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