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Advisor(s)
Abstract(s)
Azibo Reservoir (41"34'32, 25"N; 6° 53'51, 63"1JV) is located in Douro catchment.
Reservoir surroundings are occupied with several villages. Extensive farming and cattle grazing
are the most important activities. Furthermore, during the summer the reservoir is used for
recreation. Other uses are water supply and irrigation, yet these activities are not signi ficant.
Consequently, water level fluctuations are of small magnitude, allowing the existence
macrophytes in the reservoir shallow areas. Aguieira Reservoir (40°19'53, 83"N; 8° 11'47, 76"W)
is used to generate hydroelectrical power and to urban water supply. Consequently, accentuated
water level fluctuations occur. The catchment is occupied with intensive forestry, agriculture and
livestock production and with several medium sized towns. Reservoirs were sampled for the
following water quality parameters: Phosphate, nitrate, chlorophyll g, temperature, dissolved
oxygen, conductivity and transparency. Phytoplakton assemblages were characterised. Trophic
state was assessed by computation of Carlson's Trophic State Index. Phosphate, nitrate and
chlorophyll ~were significantly higher in Aguieira, which was classified as eutrophic. Azibo was
considered mesotrophic. Cyanophyceae were abundant in Aguieira. Conversely, in Azibo
phytoplankton assemblage was dominated by Bacillariophyceae and Cryptophyceae. Aguieira
trophic state can be explained by the internal disturbance induced by the intense water level
fluctuations, causing an increment of suspend particulate material in water column. Besides, the
exposure of littoral sediments to cycles of drying and wetting also increases nutrient availability
and prevents the establishment of macrophytes. These phenomena plus the input of
allochthonous nutrients originated from human activities in the catchment area lead to the
increase of phytoplankton biomass. Recreational activities probably are the factors with greatest
influence on Azibo trophic state. Based upon the obtained data, management measures are
suggested to control excessive algal growth and increase ecological integrity of both reservoirs as
proposed in Water Framework Directive
Description
Keywords
Eeservoirs Water level fluctuations Algal growth management
Citation
Geraldes, A.M.; Silva-Santos (2011). Water level fluctuations in reservoirs: A challenge for algal growth control and water quality management. In 2nd IWA Symposium on lake and reservoir management: sustainable approaches to enhance water quality. Granada