Background/Question/Methods Trait-based approaches are being increasingly used to explain the organization of ecological communities. Phytoplankton are an excellent model system to apply these approaches, as the traits that are commonly measured directly define fitness. Here we compare major traits of resource utilization among different species and major taxonomic groups of phytoplankton. We examine the data for significant correlations and trade-offs among parameters and derive these trade-offs from the first principles, such as allometric relationships and enzyme kinetics. We construct functional trait dendrograms and compare them to phylogenetic structure and environmental gradients associated with the species and major taxonomic group distributions. Results/Conclusions Major functional traits of nutrient utilization were significantly correlated. Some trait correlations represented mechanistic trade-offs that defined contrasting strategies of nutrient acquisition. Cell size was a master trait influencing relationships among other functional traits. In addition, phylogenetic relationships also contributed to explaining trait distributions, with major taxonomic groups clustering along the trade-off curves. Functional trait dendrograms based on such traits as cell size, maximum nutrient uptake rates and minimum cellular quotas, were similar to the phylogenetic clustering of species. Moreover, trait dendrograms also reflected differences in the environmental resource levels associated with species and trait distributions. These trait-based approaches can thus be used to explain and predict phytoplankton community composition along major environmental gradients, including community shifts due to global change.