With the purpose of finding out whether different landscape occupation could affect water quality in two reservoirs
of distinct age and subjected to the same climatic influence, several factors were investigated in a study lasting
from January 2000 till December 2001. Total phosphorus, orthophosphate, chlorophyll a concentrations and water
colour were determined monthly in winter and biweekly in summer, in two reservoirs located in the Portuguese
part of River Douro catchment. Complementarily, variables such as nitrate, ammonium ion and ammonia gas, as
well as water temperature, dissolved oxygen, conductivity, and transparency were measured. Trophic state of both
reservoirs was assessed by computation of Carlson’s Trophic State Index. The potential allochthonous sources of
phosphorus and nitrogen to both reservoirs were identified and estimated. Differences between reservoirs were
found for conductivity, water temperature, transparency and water colour. According to Carlson’s Trophic State
Index both reservoirs were classified as meso-eutrophic from winter to the beginning of summer, and as eutrophic
from this period onwards. Intrinsic factors such as age of reservoir, organic matter inputs from decomposition of
flooded terrestrial vegetation and exposure of littoral sediments to cycles of drying and wetting, as well as extrinsic
factors such as grazing and frequent land fires, can explain the trophic state of S. Serrada Reservoir. Allochthonous
sources of nutrients originated from agriculture and grazing in the catchment area, and recreational activities in the
reservoir probably are the factors with greatest influence on Azibo Reservoir trophic state. Based upon the obtained
data, management measures are suggested to prevent further eutrophication and water quality degradation in both
reservoirs.
