Are cyanobacterial blooms trophic dead ends?
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Cyanobacterial blooms induce significant costs that are expected to increase in the near future. Cyanobacterial resistance to zooplankton grazing is one factor thought to promote bloom events. Yet, numerous studies on zooplankton ability to graze upon cyanobacteria have been producing contradictory results and such a puzzle might arise from the lack of direct observations in situ. Our objective was to track, using fatty acid (FA) and fatty acid stable isotope analyses (FA-SIA), the fate of cyanobacterial organic matter in the food web of a lake subjected to summer blooms of Planktothrix rubescens. A metalimnetic bloom of P. rubescens occurred in Lake Bourget (France) during the study period (May–November 2009). The bloom was especially rich in α-linolenic acid, 18:3(n-3), but none of the considered zooplankton taxa exhibited spiking content in this particular FA. FA-SIA revealed, however, that over a quarter of 18:3(n-3) in small zooplankton (<500 μm) was provided by P. rubescens while large cladocerans (>500 μm) did not benefit from it. P. rubescens 18:3(n-3) could be tracked up to perch (Perca fluviatilis) young of the year (YOY) to which it contributed to ~15 % of total 18:3(n-3). Although transferred with a much lower efficiency than micro-algal organic matter, the P. rubescens bloom supported a significant share of the pelagic secondary production and did not constitute, sensu stricto, a ‘trophic dead end’. The cyanobacterial bloom also provided perch YOY with components of high nutritional values at a season when these are critical for their recruitment. This cyanobacterial bloom might thus be regarded as a significant dietary bonus for juvenile fish.
KeywordsLake Food web Fatty acid Fish Planktothrix rubescens
Comments from two anonymous reviewers on a previous version of this manuscript were much appreciated. This work is part of a project funded by the Rhône-Alpes region (CIBLE project) and the French National Institute for Agronomical Research (Projet Innovant). J.F. Humbert provided useful comments on a previous version of the manuscript. We thank Jean-Christophe Hustache, Michel Colon, Leslie Lainé and Raphaël D’Elbee for their technical help during field sampling and laboratory analysis. We also would like to thank Sébastien Cachéra (CISALB) and Gérard Paolini (the technique cell of Bourget) for their logistical support during field sampling.
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