PRIMARY RESEARCH PAPER Dynamics of microbial planktonic food web components during a river flash flood in a Mediterranean coastal lagoon David Pecqueur • Francesca Vidussi • Eric Fouilland • Emilie Le Floc’h • Sebastien Mas • Cecile Roques • Christian Salles • Marie-George Tournoud • Behzad Mostajir Received: 3 December 2010 / Revised: 12 April 2011 / Accepted: 4 May 2011 / Published online: 15 May 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Episodic river flash floods, characteristic of Mediterranean climates, are suspected to greatly affect the functioning of microbial food webs. For the first time, the abundance, biomass and diversities of microbial food web components were studied before and during 4 consecutive days after a flash flood that occurred in November 2008, in the surface waters of five stations along a salinity gradient from 20 to 36 in the Thau lagoon. Eukaryotic pico- and nanophyto- plankton were discharged from the river into the lagoon and increased by 30- and 70-fold, respec- tively. Bacteria increased by only 2-fold in the lagoon, from around 4–8 9 10 6 cells ml -1 , probably benefiting from river nutrient input. Chlorophyll a increased 4-fold, and pigment biomarkers showed that the dinophyceae, prasinophyceae and prymnesi- ophyceae were sensitive to the flood perturbation, whereas the bacillariophyceae, cryptophyceae and chlorophyceae were resistant and/or transported to the lagoon from the river. Predator responses were more complex as total heterotrophic flagellate abun- dance decreased slightly, whereas those of specific naked ciliates increased, particularly for Uronema sp. The flood also induced a specific change in diversity, from a community dominated by Strobilidium Handling editor: Sigru ´n Huld Jo ´nasdo ´ttir D. Pecqueur (&) Á F. Vidussi Á E. Fouilland Á E. Le Floc’h Á S. Mas Á C. Roques Á B. Mostajir Laboratoire d’Ecologie des Syste `mes Marins co ˆtiers (ECOSYM), Unite ´ Mixte de recherche (UMR) 5119, Universite ´ Montpellier 2, Centre National de la recherche Scientifique (CNRS), Institut de Recherche pour le De ´veloppement (IRD), Institut franc ¸ais de recherche pour l’exploitation de la mer (IFREMER), Universite ´ Montpellier 1, Montpellier, France e-mail: david.pecqueur@univ-montp2.fr D. Pecqueur Á F. Vidussi Á E. Fouilland Á E. Le Floc’h Á S. Mas Á C. Roques Á B. Mostajir ECOSYM, Universite ´ Montpellier 2, Place E. Bataillon, Case 93, 34095 Montpellier, France E. Fouilland Á E. Le Floc’h Station Me ´diterrane ´enne de l’Environnement Littoral, ECOSYM, 2 Rue des Chantiers, 34200 Se `te, France S. Mas Á B. Mostajir Centre d’e ´cologie marine expe ´rimentale MEDIMEER (Mediterranean center for Marine Ecosystem Experimental Research), UMS 3301, Universite ´ Montpellier 2, Centre National de la recherche Scientifique (CNRS), Montpellier, France S. Mas Á B. Mostajir Station Me ´diterrane ´enne de l’Environnement Littoral, MEDIMEER, 2 Rue des Chantiers, 34200 Se `te, France C. Salles Á M.-G. Tournoud Hydrosciences, UMR 5569 CNRS, Universite ´ Montpellier 2, IRD, Montpellier, France C. Salles Á M.-G. Tournoud Maison des Sciences de l’Eau, 34095 Montpellier Cedex 5, France 123 Hydrobiologia (2011) 673:13–27 DOI 10.1007/s10750-011-0745-x