Eutrophication processes in a shallow, macrophyte-dominated lake - Nutrient loading to and flow through Lake Luknajno (Poland)

Abstract

Lake Luknajno, a shallow (max. depth 3 m) and relatively large (630 ha) water body situated in the Great Masurian Lakes system (Poland) is strongly affected by its agricultural watershed. The total volume of drainage waters pumped into the lake in 1993 amounted 2.84 x 106 m3 which is equivalent to 65% of the lake volume. In spring and autumn drainage waters were extremely rich in nitrogen (especially nitrates) and the N:P ratio reached 57 in spring. In summer, drainage waters contained relatively more phosphorus so the N:P ratio decreased to nearly 10. Nutrient concentrations in lake water followed the changes in inflowing waters. High concentrations of nitrates and ammonia were noted in lake water in spring but summer concentrations of these ions were close to zero. Soluble reactive phosphorus varied between 10 and 20 μg P l-1 throughout the season. A constant outflow of nitrogen to the lower Lake Sniardwy was assumed based on a permanent concentration gradient between waters of these two lakes. Lake Luknajno is a hard water lake. Co-precipitation of phosphorus with calcium carbonate is likely to occur though no significant P accumulation in bottom sediments was found. Sorption of phosphorus on sediments as measured under experimental conditions has minor effects on P cycling. Bottom sediments composed mainly of calcareous gyttja do not accumulate either organic matter or mineral forms of nutrients.