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 years.

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

“…The right side of equation (7) describes the empirical relation between measured K 0 and s d . Jaun et al (2007) determined k 1 = 0.22 m -1 and k 2 = 0.69 for intense turbidity periods (May to September) and k 1 = 0.11 m -1 and k 2 = 0.61 for reduced turbidity periods (October to April). For a given K 0 , the corresponding light profile in the lake for the instant global radiation is given by:…”

“…Moreover, the operation of the hydropower reservoirs has decreased turbidity in Lake Brienz in the summer through particle retention, and increased turbidity during winter through hydropower production . Based on these findings, Jaun et al (2007) concluded that the altered particle dynamics led to a doubling of light attenuation during winter and a reduction of~50 % during summer.…”

“…We used recorded Secchi depths (s d ) from Flück (1926) to reconstruct no-dam light conditions. Light attenuation coefficients (K 0 ) and s d can be correlated according to Jaun et al (2007):…”

“…) denoting suspended particle concentration in the upper 50 m of the water column in Lake Brienz (Jaun et al, 2007). Hence, primary production under typical no-dam light conditions was estimated using predicted K 0 (equation 9) and the fits of equation (2) …”

Lake Brienz Project: An interdisciplinary catchment-to-lake study

“…Based on the particle budget in Lake Brienz (Finger et al, 2006) and past Secchi depth records, Jaun et al (2007) reconstruct the light attenuation for the last decades and the early 1920s. As a result of the hydropower operation, Lake Brienzs PAR attenuation was~twice as strong in summer under pre-dam conditions, whereas it is now~twice as strong in winter compared to the pre-dam conditions.…”