Phytoplankton in an Amazonian flood-plain lake (Lago Batata, Brasil): diel variation and species strategies

Phytoplankton diel variations in an Amazonian flood-plain lake (Lago Batata) were studied quarterly for 1 year, according to the hydrometric level fluctuation of the Trombetas river (39.7-47.0 m over sea level), to which the lake is permanently connected. Hydrological and hydrographic conditions determined mixing patterns of the water column and phytoplankton distribution. When the lake is deep (>4.0 m), sparse phytoplankton communities are found at the bottom, by thermal discontinuities, and may remain so for a short duration (days). When the lake is shallow (<4.0 m), mixing of the waters ensures a nearly even distribution of organisms throughout the water column. The weak stability of stratification is a major influence on the distribution of phytoplankton population densities in this shallow hushed lake. The highest phytoplankton densities during the second part of the day, mentioned in the literature, are observed only during decreasing water level. During the other phases of the hydrological cycle (increasing, high and low water levels), no differences in phytoplankton population density were observed throughout the diel cycle. Assemblages, or groups of descriptor species of systems having similar features, have recently been proposed. Assemblages of flushing-tolerant nano-algae (X-1) and of mixing-dependent species (T, S) have shown that the physical properties of the environment were often the strongest determinants of the phytoplankton strategies.