RELATIVE IMPORTANCE OF INTERNAL SOURCES OF PHOSPHORUS AND ORGANIC-MATTER IN NORTHERN FINNISH RESERVOIRS

Two large reservoirs Lokka (417 km2) and Porttipahta (214 km2) were bu ilt at the upper part of river Kemijoki watercourse in northern Finlan d in the early 1970s. Both reservoirs were built to produce hydropower with wide water level regulation range (Lokka max. 5 m, Pomipahta max . 11 m). Flooded area consisted mainly of peatlands and forests. Water level is lowered during winter, when reservoirs are covered by ice an d the energy need is greatest. Regulation was extensive during the fir st decade and water quality problems occurred. Wintertime oxygen conce ntration was very low and amount of phosphorus was high. This situatio n was caused by degradation of organic matter released from bottom. Or ganic matter was mainly released from peatlands pressured by penetrati ng ice and by breakdown of peat during the summer. Amount of total pho sphorus is quite high compared to natural lakes. Main sources of loadi ng during the first years of the reservoirs were at the littoral zone, where waves and currents caused erosion. The importance of the pelagi c zone becomes more obvious as reservoirs age; bottom area is covered by thick layer of sediment consisting mainly of eroded peat. The botto m sediment forms a store of organic matter and phosphorus, which can b e released during winter, when oxygen concentration is very low and al so 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 ca n be quite important as a transport mechanism of organic matter and nu trients in shallow and open reservoirs.