Understanding the effect of temperature on phytoplankton ecophysiology is key to predict the effects of global warming on the productivity of aquatic ecosystems. More than a century ago, Jensen stated, through the fallacy of the average, that the response of a given system to constant conditions is seldom equal to its mean response to variable ones. We thus hypothesized that a thermally fluctuating environment may cause changes in phytoplankton growth, stoichiometry and metabolic rates that differ from the effect of constant warming. To test this hypothesis, we exposed five phytoplankton species (Emiliania huxleyi, Micromonas commoda, Skeletonema costatum, Synechococcus sp. and Thalassiosira rotula) to a 2x2 full-factorial design with two temperatures (control vs. warming) and two thermal regimes (constant vs. fluctuating). We found that thermal fluctuation stimulated photosynthesis whereas constant warming reduced it when compared to control conditions. Biomass-based growth rates under constant warming increased in E. huxleyi, M. commoda and Synechococcus but decreased in S. costatum and T. rotula. In general, thermal fluctuations tended to cause a decrease in growth rates, or a reduction of the positive effect of warming. The carbon to nitrogen ratio often increased under warming, but tended to decrease under thermal fluctuation. We conclude that fluctuations alter the effect of temperature on the metabolism, growth and stoichiometry of phytoplankton, but the observed responses are different depending on the species.

Primary Presenter: Osvaldo Tascón, University of Vigo (osvaldo.tascon@uvigo.gal)

Authors:

Osvaldo Tascón-Peña, University of Vigo  (osvaldo.tascon@uvigo.gal)

Marco J. Cabrerizo, University of Vigo  (marcojc@uvigo.es)

Emilio Marañón, University of Vigo  (em@uvigo.es)