Phytoplankton seasonal variation in a shallow stratified eutrophic reservoir (Garças Pond, Brazil)

This study aimed at describing the phytoplankton dynamics and structure in a shallow eutrophic reservoir, the Garças Pond, located in the Parque Estadual das Fontes do Ipiranga (23°38'40.6'' S, 46°37'28.0'' W), in the Municipality of São Paulo, southeast Brazil. Samples were collected monthly from January to December 1997 in five depths (subsurface, 1 m, 2 m, 3 m, and 20 cm above the bottom) in the pelagic zone (Z max = 4.7 m). Abiotic variables studied were: water temperature, turbidity, transparency, conductivity, pH, dissolved oxygen, alkalinity, inorganic carbon, and N and P dissolved and total forms. Altogether 236 phytoplankton taxa distributed among 10 classes were identified. Phytoplankton seasonal and vertical variation was related to shifts in the water chemical features as a consequence of a warm-wet season with stratified water column (phase 1, January-March and September-December) alternating with a cool-dry season with mixed water column (phase 2, April-August). There were shifts in cyanobacterial dominance over the entire year. During phase 1, Raphidiopsis/Cylindrospermopsis was one of the most important taxon. During phase 2, Raphidiopsis/Cylindrospermopsis biomass decreased, whereas richness and diversity increased and diatoms were relatively abundant. In September, when the water column was markedly stratified, a cyanobacterial bloom (Sphaerocavum brasiliense) occurred. Changes in water chemical variables caused by the bloom allowed recognition of a phase 3, in which pH and chlorophyll a, TP and CO₃ ²⁻ concentration reached their highest values. According to Reynolds and collaborators' functional groups approach, phase 1 was marked by groups S/W1/W2/H1/Y, phase 2 by groups K/L M /L O /D/P/X1/F, and phase 3 by group M. This sequence was corroborated by canonical correspondence analysis (CCA) results.