Original paper

Structure and vertical daily dynamics of phytoplankton seasonally collected in a shallow subtropical reservoir in southern Brazil

Ramírez R., John; Jairo Bicudo, Carlos E. de M.

Abstract

This article presents the changes of phytoplankton assemblage in the Garζas' reservoir starting from four diurnal samplings carried out in four different seasons of 1994. The sampling in each season was made at each four hours during a 24 hours period in only one sampling place. 152 taxa were identified. Density varied vertically and daily in fall and spring sampling days. A bloom of Microcystis during the spring diminished richness and diversity; consequently, a complete change in structure was observed. During the other seasons numerical structure was very similar. Evenness values displayed significant differences with depth during the spring only, but significant variation during the daily period was observed in summer, and highly significant differences of this variable were found among sampling days. Vertical variation of diversity was significant during the summer, winter and spring sampling days; diurnal variation was not significant in any sampling day, but there were highly significant differences among year seasons. In spring, increased intensity and decreased frequency in disturbance were related to increased thermal stability and decreased light penetration by biological turbidity as a consequence of Microcystis bloom. It can be concluded that: 1) on spring time a phytoplankton assemblage in steady state, with high homogeneity and slow entropy was present, and 2) because there was no strong changes in the assemblage structure between first three seasons (summer, fall and winter), when the system was mixed, water mixing had the character of an intermediate disturbance with slow intensity and intermediate frequency.

Keywords

brazilsubtropical shallow reservoirphytoplankton assemblage structurephytoplankton ecologyverticaldiurnaland seasonal variation