Effects of upstream hydropower operation and oligotrophication on the light regime of a turbid peri-alpine lake

Abstract.: Anthropogenic activities in catchments can alter the light regimes in downstream natural waters, affecting light attenuation and the perceived optical properties of the waters. We analyzed the effects of upstream hydropower operation and oligotrophication on light attenuation and reflectance in Lake Brienz (Switzerland). For this purpose, we reconstructed its light regime for the pre-dam condition and for periods of 4-fold increased primary productivity, based on direct observations of light and beam attenuation as well as concentrations of optically active compounds, especially observed and simulated mineral particle concentrations. Based on our assessment, light attenuation before the construction of upstream dams was double the current value during summer and nearly half in winter. This result is consistent with pre-dam measurements of Secchi depths in the early 1920s. Using a simple optical model, a significant increase in reflectance since the 1970s was estimated, assuming a 4-fold decrease of optical active organic compounds within the lake. As reflectance is perceived by human eyes as turbidity, this may explain subjective reports by local residents of increasing turbidity in recent year

Effects of alpine hydropower operations on primary production in a downstream lake

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 18months. 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 Brien

Effects of upstream hydropower operation and oligotrophication on the light regime of a turbid peri-alpine lake

ISSN:1015-1621ISSN:1420-905

Effects of alpine hydropower operations on primary production in a downstream lake

ISSN:1015-1621ISSN:1420-905

Effects of alpine hydropower operations on primary production in a downstream lake

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 (similar to 66 gC m(-2) yr(-1)), and could increase similar to 44% if the lake was less turbid. Disregarding nutrient retention in reservoirs, we estimate gross primary production would be similar to 35% lower in summer and similar to 23% higher in winter in the absence of reservoirs. The annual primary production (similar to 58 gC m(-2)yr(-1)) would decrease similar to 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