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JPR Advance Access originally published online on October 9, 2006
Journal of Plankton Research 2007 29(1):47-56; doi:10.1093/plankt/fbl056
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Influence of free-floating plants on the structure of a natural phytoplankton assemblage: an experimental approach

Paula de Tezanos Pinto, Luz Allende and Inés O’Farrell*

Consejo Nacional de Investigaciones Científicas y Técnicas (Conicet) and Lab. Limnología, Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, C1428 EHA, Buenos Aires, Argentina

* Corresponding Author: ines{at}ege.fcen.uba.ar

Received on April 6, 2006; accepted on October 3, 2006


  Abstract

The influence of free-floating plants on the structure of natural phytoplankton was assessed using field mesocosms. Poor underwater light climate (natural macrophyte cover and dark mesh treatments) determined the dominance of shade-adapted, slow-growing filamentous and colonial cyanobacteria [S and Lo functional groups; Reynolds, C. S., Huszar, V., Kruk, C. et al. (2002) Towards a functional classification of the freshwater phytoplankton. J. Plankton Res., 24, 417–428]. Biovolume and chlorophyll a (Chl a) remained low, anoxic conditions prevailed, pH was circumneutral and nutrient levels did not differ significantly. Conversely, the improvement in light availability by removal of floating plants brought about major changes on the phytoplankton and in the physicochemical environment: the S–Lo assemblage was replaced by fast-growing unicellular green algae (X1 and X2) and eukaryotic picophytoplankton (X3) increased, anoxia was reverted and ammonium and phosphate concentrations decreased significantly. We propose that in warm nutrient-rich wetlands, the shading effect posed by floating plants is the driving force in shaping the phytoplankton and in determining a newly described alternative state. Notwithstanding, if the shading constraint is released, a strong phytoplankton enhancement occurs and nitrogen becomes limiting. This experimental approach confirms the behaviour of dominant functional groups as described in nature and thus reinforces the validity and deepens the comprehension of this novel scheme for phytoplankton control.

Communicating editor: K.J. Flynn


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