Seasonal dynamics of periphytic algae in the vicinity of the hydroelectric plant in the Pasłęka River (north-east Poland)

This study of the periphytic algal differentiation of epiphyton, epilithon and algae on the dam was carried out above and below the hydroelectric plant on the Pasłeka River in the vegetative seasons of 2001–2003. During this period there was varying mean monthly precipitation and therefore varying water levels. The damming of this river caused a decrease in water discharge above the plant and an increase below it. The highest mean biomass was recorded for periphytic algae on the dam and the lowest for epiphyton. The highest nutrient concentrations (Ntot, PO4 and Si) favored epiphyton growth on sites with slow discharge, and this was supported by positive correlations between algal biomass and nutrients. The substratum type and water velocity at sites with rapid water discharge exerted greater regulation on the growth of epilithon and the algae on the dam than any nutrient. Differences were observed in the biomass of periphytic algae when varying precipitation regulated water levels. The maximum biomasses were as follows: (1) epiphyton at low water in 2003, (2) epilithon at a medium water level in 2001 and (3) periphytic algae on the dam at high water in 2002. A decrease in all studied periphytic algal biomass was noted with increased precipitation from April to November. This was related to lower nutrient concentrations and light availability monitored by water transparency, rather than to a low water level. This indicated that the current velocity and precipitation induced water level indirectly regulated the growth of these algae during the vegetative season.