The abundance, composition and
dynamics of zooplankton were followed in two
reservoirs of the River Douro catchment. The
Serra Serrada Reservoir is subject to marked
fluctuations in water levels. The highest values of
total phosphorus, soluble reactive phosphorus,
nitrate, water colour and chlorophyll a were found
during the minimum level phase. Rotifera was
dominant except in late summer and autumn when
the cladoceran Ceriodaphnia quadrangula or the
copepod Tropocyclops prasinus replaced them as
the dominant zooplankton. Among the rotifers
the most common taxa were Keratella cochlearis,
Conochilus sp. and Asplanchna priodonta. Maximum
rotifer density was about 80,000 ind m–3 in
2000, 200,000 ind m–3 in 2001 and 100,000 ind
m–3 in 2002. Among the crustacean zooplankton
C. quadrangula achieved densities of up to
45,000 ind m–3 and T. prasinus, up to 80,000 ind
m–3. Canonical correspondence analysis revealed
a strong contribution of the variation in the stored
water volume, temperature, total phosphorus,
chlorophyll, nitrates, and water transparency to
the observed, significant association between
zooplankton assemblage and environmental variables.
In the Azibo Reservoir, fluctuations in water
level are smaller. Only total phosphorus,
cholorophyll and conductivity varied seasonally.
Cladocera and Copepoda were dominant during
the whole study period. The most abundant taxa
were Ceriodaphnia pulchella, Daphnia longispina,
Diaphanosoma brachyurum, Bosmina longirostris
and Copidodiaptomus numidicus. Cladocera
achieved densities of up to 25,000 ind m–3 and
Copepoda up to 15,000 ind m–3. Rotifera in
general reached densities of up to 6,000 ind m–3.
On the basis of canonical correspondence analysis
only temperature and conductivity were significantly
associated with zooplankton assemblage.
