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.