Impact of reservoir filling on phytoplankton succession and cyanobacteria blooms in a temperate estuary

Previous studies in the Guadiana upper estuary showed a relationship between nutrient ratios and phytoplankton succession, which involved a diatom spring bloom followed by cyanobacteria dominance during summer. Silica depletion was thought to be the trigger of cyanobacteria summer blooms. The recently constructed Alqueva dam is expected to affect nutrient loadings and phytoplankton succession, namely a decrease in Si concentration and enhancement of cyanobacteria blooms. The aim of this study was to assess changes in phytoplankton composition, nutrient concentrations and ratios, and light availability during the filling up of the dam, during 2002 and 2003. Water samples were collected in three stations along the upper estuary from March 2002 to October 2003. Several environmental variables were determined and phytoplankton composition was studied using inverted and epifluorescence microscopy. Results from 2002 to 2003 did not follow the expected phytoplankton succession. With the exception of the upriver station, the diatom spring bloom did not occur despite high Si concentrations. Cyanobacteria blooms, composed mainly of Oscillatoria, occurred several times throughout the year. Freshwater flow was lower than before dam construction and saline waters intruded in previously freshwater locations. Contrary to previous findings, cyanobacteria growth in the Guadiana estuary was not related to silica depletion, but seemed to be favoured by low freshwater flow, increased salinity, low light intensity and high nitrogen concentration.