Effects of hydropower operation and oligotrophication on internal processes in Lake Brienz

awarded by a ETH jury with the Otto Jaag prize 2007

ABSTRACT

Since the 1980’s, local fishermen have been complaining about the declining fishing yield in Lake Brienz. A preliminary review indicated that the local whitefish (Coregonus fatioi) are suffering from undernourishment due to low phytoplankton and zooplankton densities. Two main hypotheses were put forth to explain the low level of biomass and fish food production: (1) during the last 30 years constructions of sewage treatment plants in the catchment have drastically reduced the phosphorus input into the lake. As phosphorus is the limiting nutrient, algae growth must have decreased accordingly. (2) Increasing hydropower production has lead to a continuous supply of suspended glacial particles which are transported with the melt water to the lake. The suspended particles reduce the light penetration, thus limiting algae growth to the top few meters of the lake.

Methods
Based on extensive measurements and numerical model calculations the present Ph.D. thesis assesses and quantifies the effects of the above mentioned anthropogenic interferences on primary production in Lake Brienz. In a first step the suspended particle budget, under consideration of two particle size classes, was established for both present conditions and hypothetical conditions without upstream hydropower dams (no-dam conditions). Subsequently the effects of suspended solids on the light regime of the lake were assessed by direct observations of beam and in situ light attenuation. Based on in situ C-assimilation rates, light attenuation, and the established suspended particle budgets, gross primary production was assessed for present conditions and estimated for hypothetical no-dam conditions. In the last step, the assembled information of the previous steps was implemented in a numerical, biogeochemical lake model predicting plankton densities both for present and no-dam conditions. The calibrated model was used to perform predictions for various hypothetical scenarios with increased phosphorus input and altered surface turbidity. By comparing the different scenarios with each other, the causes for the biological changes observed in Lake Brienz were identified.

Results
In summary, damming drastically diminishes particle fluxes which leads to lower particle supply during summer, but higher loads in winter due to hydropower production. As a consequence, light attenuation is now half during summer and almost double in winter compared to no-dam conditions. This result is consistent with pre-dam measurements of Secchi depths in the early 1920’s. Under current light conditions gross production is extremely low (~66 gC m-2 yr-1) in comparison with other oligotrophic lakes not affected by surface turbidity (e.g. production in neighboring downstream Lake Thun is estimated to be 45% higher). Under a hypothetical light situation without dams gross production in Lake Brienz would be about 36% lower in summer but up to 17% higher in winter. Enhanced nutrients supply increases the nutritious value of algae, thus stimulating zooplankton growth. Phytoplankton growth, however, hardly changes as the top-down control by zooplankton is enhanced. Annually integrated productivity is only slightly influenced by damming induced changes in water turbidity, since algae growth is strongly limited by phosphorus availability. However, in winter and spring the hydropower production leads to increased turbidity, therefore the spring production peak is delayed. Due to higher algae growth in summer phosphate is continuously consumed, thus preventing a second production
peak in fall.

Conclusions
In summary, oligotrophication was identified as the main cause for declining zooplankton densities during the last 30 years. However, with ongoing oligotrophication the effects of light limitation become more important. While the results of the model calculations suggest that oligotrophication is the primary cause for declining fish yields, they also suggest that the effects of oligotrophication are amplified by damming induced changes in turbidity.

The Lake Brienz Project in the local press:

Throughout 2006/2007

Results are published in peer reviewed scientific journals:

Effects of dams on sediment budget:

http://onlinelibrary.wiley.com/doi/10.1029/2005WR004751/abstract

Effects of changing turbidity on primary production in the lake:

http://link.springer.com/article/10.1007%2Fs00027-007-0873-6

Comparison of oligotrophication versus effects of dams:

http://onlinelibrary.wiley.com/doi/10.1029/2007WR005868/abstract

Effects of turbidity on light regime of the lake:

http://link.springer.com/article/10.1007%2Fs00027-007-0876-3#page-1

Nutrient budget of the lake:

http://link.springer.com/article/10.1007%2Fs00027-007-0871-8#page-1

Effects of dam on sedimentation of suspended particles

http://link.springer.com/article/10.1007%2Fs00027-007-0875-4#page-1

31 March 2006

For dissemination purposes the results of the Lake Brienz project are summarized in a comprehensive leaflet (in German):

2. February 2012
The Bernese newspaper Der Bund reports on fishing yields in Lake Brien.
http://www.derbund.ch/bern/kanton/Kraftwerke-trueben-den-Brienzersee-im-Winter/story/21390377?track

10. September 2012
The Swiss National Council and the Council of States reject calls for artificial lake fertilization referring to the results of our study:
http://www.parlament.ch/ab/frameset/d/s/4905/386647/d_s_4905_386647_386669.htm