Abstract. During the past century, the construction of hydropower dams in the watershed of Lake Brienz has significantly altered the dynamics of turbidity, which has important implications for lake productivity. To assess these effects, we measured in situ carbon assimilation rates and ambient light intensities over 18 months. Based on experimental data, a numerical model was developed to assess gross primary production under present light conditions and those under a hypothetical case without upstream dams. Light conditions for the hypothetical no-dam situation were estimated from pre-dam Secchi depths and simulated no-dam particle concentrations.Current gross primary production is low (~66 gC m -2 yr -1 ), and could increase~44 % if the lake was less turbid. Disregarding nutrient retention in reservoirs, we estimate gross primary production would be~35 % lower in summer and~23 % higher in winter in the absence of reservoirs. The annual primary production (~58 gC m -2 yr -1 ) would decrease~12 % compared to the current primary production with dams. According to model calculations, hydropower operations have significantly altered the seasonal dynamics, but have little effect on annual primary production in Lake Brienz.
A commercial dry diet and both live and frozen zooplankton were compared as food for cultured larvae of Lake Hallwil whitefish Coregonus suidteri during the first 3 weeks after hatching. Whitefish larvae fed live zooplankton grew considerably larger (16.4 mm final length) than those fed frozen zooplankton (14.4 mm), but mortality did not differ significantly (0% versus 3%). Larvae fed the dry diet reached nearly the same length (13.6 mm) as the fish fed frozen zooplankton. However, mortality of fish fed the dry diet was significantly higher (34%) than that of fish fed live or frozen zooplankton. We conclude that diet acceptance is a key factor for whitefish larvae, accounting for the highest growth rates by larvae fed live zooplankton. We also tested the effect of two additional factors on larval mortality: food particle size and water circulation in the rearing tanks. The size of food pellets was reduced from 200–400 μm to 100–200 μm. The water inflow to the tanks was placed below the water surface to increase the time the dry diet was afloat. Our results showed that neither particle size nor inflow placement affected mortality when fish were fed ad libitum. Nevertheless, feed floating times were prolonged significantly when the inflow was placed below the water surface, making it possible to decrease the feeding frequency, reduce food losses, and, thus decrease the amount of daily ration fed.
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