Functional differences between aquatic herbivores emerge from bioenergetic processes

Primary consumers, also known as grazers, transfer energy fixated by primary production to higher trophic levels. In aquatic environments, two grazer groups are dominant — those living in the open water (pelagic) and those living associated to a surface (benthic). When traits are used to compare both groups, selecting the appropriate traits to compare is a crucial step. Bioenergetic approaches can help standardize trait selection and identify which are most relevant. We showed the bioenergetic traits that differentiate these grazer groups using Dynamic Energy Budget theory. We also explored the life-history implications of expressing such traits. Pelagic grazers reach sexual maturity faster and use more energy for somatic maintenance per unit of structural volume. The energy used in somatic maintenance is also associated to nitrogen excretion, which helps cycle nutrients in the water column. In contrast, benthic grazers generally consume less oxygen per unit of dry weight and are more efficient in producing biomass from assimilated energy. The bioenergetic traits driving differences between grazer groups uncover the emergence of contrasting body sizes, development rates, and life-span. Predictions based on these traits also reveal different ecosystem roles in terms of nutrient cycling and biomass fixation. This bioenergetic approach clarifies how different traits contribute to the relative roles of organisms in an ecosystem. • Trait differences in aquatic grazers go beyond contrasting body sizes. • These differences can only be revealed by the underlying bioenergetics. • Pelagic grazers grow fast with high metabolic excretion. • Benthic grazers grow slow and efficiently fixate biomass. • Bioenergetics can increase the generality of functional approaches.