Limnological variations of a deep reservoir in periods with distinct rainfall patterns,Variações na limnologia de um reservatório profundo em períodos com diferentes padrões de precipitação
This study intends to provide information on how precipitation
variation may impact environmental parameters (total phosphorus, water temperature,
pH, conductivity, dissolved oxygen, Secchi depth, chlorophyll a and Anabaena abundance)
and crustacean zooplankton composition and abundance. Methods: Samples were
collected monthly from October 2000 to September 2002 and from October 2007
to September 2009 at one single sampling station located at the deepest point of the
reservoir. All data were obtained in the euphotic zone. Results: The highest values of
total phosphorous concentrations were reported together with the maximal values of
precipitation. Conversely, conductivity decreased during the wet periods. Anabaena
presence was only detected between September and December 2001. Daphnia longispina
total abundance showed large inter-annual differences. A clear seasonal pattern was
inferred for Diaphanosoma brachyurum and Ceriodaphnia pulchella. Both species are
thermophilic and begin to be detected in early summer when the water temperature is
around 15 °C. Copidodiaptomus numidicus abundance did not present a clear seasonal
pattern. Conclusions: Some of the observed shifts in environmental parameters
(Conductivity, total phosphorus concentrations and Anabaena abundance) resulting in
changes in zooplankton abundance are induced by rainfall variation. The most affected
species were those with a lack of seasonality (C. numidicus) or species occurring mainly in
winter/early summer (Daphnia longispina). However, a wide range of physical, chemical
and biotic interactions is likely to influence zooplankton abundance. Therefore, caution
is advised when relating the observed variation exclusively with rainfall intensity. In fact,
some of the observed shifts were induced by other factors varying subtly in a seasonal or
inter-annual pattern, independently of rainfall intensity.
