Influence of water volume reduction on the phytoplankton dynamics in a semi-arid man-made lake: A comparison of two morphofunctional approaches.

Gustavo G Braga,Vanessa Becker

doi:10.1590/0001-3765202020181102

Abstract

Significant reductions in the water levels of lakes are influenced by droughts and freshwater demands, especially in semi-arid regions, where hydric stress is greater. The aim of this study was to investigate the dynamics of phytoplankton during two different water volume periods resulting from an extended drought in a semi-arid lake. Another objective was to compare two functional approaches to test which one of these best captures phytoplankton variability as a function of environmental variability. Multivariate analyses performed using the Reynolds Functional Groups (RFG) and Morphology-Based Functional Groups (MBFG) classification schemes indicated two periods, high and a low water volume. The results demonstrated the importance of light availability on phytoplankton assemblages as these two periods showed significant differences in water transparency and phytoplankton composition. During extended droughts the reduction in water volume enhances the development of bloom-forming cyanobacteria through the limitation of light in a eutrophic man-made lake. Moreover, both functional traits approaches demonstrated the effect of light availability on phytoplankton assemblage composition and can be applied in similar systems. However, the RFG classification provides more information and allows a more detailed description of the algal assemblages.

Highlights

Physical and chemical changes in water columns associated with climatic circumstances largely determine changes in the composition of phytoplankton assemblages in lakes and reservoirs (Tundisi 1990)
The study was divided into two distinct periods defined by water level fluctuation: (I) high water volume period, marked by water volumes above 50% of the total capacity and by moderately wet and normal hydrological conditions; and (II) low water volume period, characterized by water volumes lower than 50% of the total capacity and hydrological conditions varying from normal to extremely dry (Figs. 2 and 3)
Seasonal reduction in the water volume of a reservoir related to drought periods is considered an important environmental condition, especially for phytoplankton dynamics

Summary

Introduction

Physical and chemical changes in water columns associated with climatic circumstances largely determine changes in the composition of phytoplankton assemblages in lakes and reservoirs (Tundisi 1990). In ecosystems with large variations in water levels, the annual and inter-annual variability of phytoplankton abundance and composition can be strongly influenced by peculiar hydrological regimes (Harris & Baxter 1996) and by changes in mixing regimes (Naselli-Flores & Barone 2005). A number of studies demonstrate that water level fluctuations can affect the abundance, biomass, composition, and diversity of pelagic primary producers through changes in light availability and nutrients (Kangur et al 2003, Naselli-Flores & Barone 2005, Leira & Cantonati 2008, Costa et al 2016). Considerable attention should be given to managing these water resources

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