Abstract. Standard metabolic rate is
a major functional trait with large inter-individual variability in many
groups of aquatic species. Here we present results of an experimental study
to address variation in standard metabolic rates, over different scales of
organisation and environments, within a specific group of aquatic
macro-invertebrates (i.e. gammarid amphipods) that represent the primary
consumers in detritus food webs. The study was carried out using flow-through
microrespirometric techniques on male specimens of three gammarid species
from freshwater, transitional water and marine ecosystems. We examined
individual metabolic rate variations at three scales: (1) at the individual
level, during an 8 h period of daylight; (2) at the within-population level,
along body-size and body-condition gradients; (3) at the interspecific level,
across species occurring in the field in the three different categories of
aquatic ecosystems, from freshwater to marine. We show that standard metabolic rates vary significantly at all three scales
examined, with the highest variation observed at the within-population level.
Variation in individual standard metabolic rates during the daylight hours
was generally low (coefficient of variation, CV<10 %) and
unrelated to time. The average within-population CV ranged between 30.0 %
and 35.0 %, with body size representing a significant source of overall
inter-individual variation in the three species and individual body condition
exerting only a marginal influence. In all species, the allometric equations
were not as steep as would be expected from the 3∕4 power law, with
significant variation in mass-specific metabolic rates among populations. The
population from the transitional water ecosystem had the highest
mass-specific metabolic rates and the lowest within-population variation. In the gammarid species studied here, body-size-independent variations in
standard individual metabolic rates were higher than those explained by
allometric body size scaling, and the costs of adaptation to short-term
periodic variations in water salinity in the studied ecosystems also seemed
to represent a major source of variation.