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Phosphate fertilization of Lake Ringedalsvatnet – an oligotrophic hydroelectric reservoir

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Eirik Fjeld, Sigurd Rognerud


The oligotrophic hydroelectric reservoir Lake Ringedalsvatnet (Odda municipality, S.W. Norway) was in June 2013 fertilized with phosphate as an experimental measure to stimulate primary production and the quality of Brown trout. The fertilization raised the epilimnic concentration of phosphate from about 1 to 4 mg P/L. Developments in phytoplankton and zooplankton biomass and composition were followed throughout the growth season. Diet, length, weight, condition, flesh color and stable isotopes (δ13C and δ15N) in fish were also recorded. Phytoplankton biomass (0–10 m) was approximately doubled from July to September, but the maximum level was relatively low (0.18 mm3 /L). In the littoral zone, there was a significant growth of periphytic algae. Zooplankton-biomass (0–10 m) increased from 17 mg/m3 in July to a maximum but moderate level of 90 mg/m3 in August. Seasonal fish growth (back calculated) was significantly better than previous years. For age groups 5 and 6 years (average length 18.3 and 20.1 cm), which were the dominant age groups in the catches, the average growth of 3.3 and 1.5 cm, i.e. 90% and 50% greater than in 2012. Fish diet was first dominated by surface insects, but the littoral water flea Eurycercus lamellatus became an important prey item for age groups 5 and 6 years in the fall. The fish's condition factor in 2013 was good with an average k-factor of 1.1. The proportion of fish with bright red and red flesh color rose markedly throughout the season, probably due to the increased importance of E. lammellatus in the diet. To conclude, the lake fertilization led to a moderate increase in the production of phytoplankton, but a more marked increase in the amount of littoral periphytic algae, which created a favorable habitat for E. lammellatus. The Brown trout population experienced a significantly improved seasonal length growth and condition factor due to the measure.