Temperature and stoichiometric dependence of phytoplankton traits

Understanding the links between intraspecific trait variability and environmental gradients is an important step toward unravelling the mechanisms that link species performance to environmental variation. Here, we performed a comparative, experimental study to investigate variability of cellular traits in three prokaryotic and three eukaryotic freshwater phytoplankton species along gradients of temperature and nitrogen:phosphorus ratio (N:P) in laboratory microcosms. Temperature and N:P strongly affect phytoplankton growth and are changing due to climate change and eutrophication. Metabolic theory and allometric scaling predict that smaller organisms should be favored at higher temperatures through improved metabolic uptake partly due to greater surface area to volume ratios. In addition, chlorophyll a (chl a) concentration should increase due to higher chlorophyll synthesis in response to light limitation at higher cell densities. We found that cell volume both increased and decreased with temperature, whereas intermediate N:P yielded higher growth rates and more extreme conditions yielded bigger cell volumes. Species growth responses to these gradients were distinct and not related to phylogenetic differences. Meaningfully coupled traits like the chl a fluorescence and cell volume shifted consistently and can improve our understanding of individual cell responses to abiotic drivers. This study showed that intraspecific trait variability of freshwater phytoplankton harbors potential for short term acclimation to environmental gradients. Finally, the high trait variability in some species has strong implications for their ecology and the accuracy of predictions where responses may differ when based on mean or fixed trait values. Over the course of the last decade, trait‐based approaches have gained popularity for many applications in community ecology, spanning the spectrum of describing interspecific trait variation (Makkonen et al. 2012, Auger and Shipley 2013, Civitello et al. 2015), understanding the functional roles of traits for species performance (Edeline et al. 2007, Litchman et al. 2010, Fernandes et al. 2011, Ebeling et al. 2014, Peres‐Neto et al. 2017), and predicting functional responses based on species traits (Haddad et al. 2008, Kearney and Porter 2009, Guittar et al. 2016, Krasnov et al. 2016). Traits are commonly used to compare the differences among species that relate to a higher level process or the fitness of an organism and are thus often called functional (Litchman et al. 2007, Violle et al. 2007). Trait variations within and