Examination of ecological status and recipient capacity of Lake Lygne, Hægebostad

Sammendrag

Hægebostad municipality is planning a new waste-water treatment plant at Eiken close to Lake Lygne. Knowledge of the ecological status and recipient capacity for effluents from the treatment plant was therefore important for decisions regarding the treatment process (mechanic/chemical vs chemical/biological). Total phosphorus (P) and chlorophyll was measured at four depths on five occasions during the summer 2014. Additional information and data were collected from several national sources in order to establish water type and ecological status according to the Norwegian water regulations. The lake has good ecological condition based on fish, invertebrates and the measured chlorophyll concentrations. Data for P and total nitrogen support this classification, whereas the measured Secchi depth indicated poor condition. No other data indicated poor condition, so we therefore decided not to include Secchi depth in the classification. Based on estimated mean lake P concentration (6.7 μg P/L) and calculations with the P-load model of Rognerud et al (1979), the present P-load is 5.8 tonnes P/year. A little less than half of this is from natural sources based on the reference condition for this water type (3 μg P/L). The difference in P-load resulting in a condition close to the good/moderate limit (10 μg P/L) and the present condition is about 3 tonnes P/year. This extra load will not deteriorate the present ecological condition. The lake is now (from the autumn 2014) accessible for anadromous fish (Atlantic salmon and sea trout), thanks to construction of a passage through a major fish obstacle downstream of the lake. The lake condition with regard to eutrophication will not have any negative impacts on salmon. However, the lake and major inlet stream are acidic, mean pH 5.3 and 5.0, respectively, and the Ca-concentration of the inlet is extremely low (<0.3 mg/L). Liming may thus be necessary to avoid physiological stress for salmonids in this water.