Short term spatial and temporal variation of phytoplankton in a shallow tropical oligotrophic reservoir, southeast Brazil

Locally called IAG pond, system is a small, oligotrophic, shallow, urban reservoir located in the Parque Estadual das Fontes do Ipiranga Biological Reserve, south of Sao Paulo municipality, southeastern Brazil. Study was carried out in two phases to cover the two climatic periods (dry and rainy) that characterize the area. Daily samples were collected on seven consecutive days (dry = 20-26 August 1996; rainy = 22-28 January 1997), three times a day (07:00 h, 13:00 h, and 19:00 h), following the water column vertical profile at reservoir's deepest site (Z(max) = 4.7 m). The phytoplankton's community structure's diurnal variation was compared with environmental variables (thermal structure of the water column, pH, CO2, dissolved oxygen, total phosphorus, and NH4). Analysis of variance (ANOVA) showed significant variation among sampling hours and days but not among depths for most species, thus revealing a tendency towards a homogeneous vertical distribution of phytoplankton in the reservoir. Canonical Correspondence Analysis (CCA) showed a significant relationship between distribution of phytoplankton species and environmental variables used for ordination. Water temperature was the environmental variable that best explained distribution of sample units. Atelomixis was the only possible explanation for how a non-motile alga as, for example, the chlorococcal Oocystis lacustris Chodat and the diatom Cyclotella stelligera Cleve & Grunow, can be so widespread and abundant in the IAG pond during the stratified period.