Linking timescales of nutrient physiology and ecosystem stoichiometry
Over ecological timescales of organismal growth and population change, biota assimilate nutrients in relatively constant proportions, but variability in resource supply and assimilation can de-couple element cycles at finer timescales (minutes to days). This fine-scale variability can provide information about the physiological processes, such as uptake and growth kinetics, that link organismal stoichiometry and ecosystem-scale nutrient dynamics. A theoretical model suggests that the magnitude, timing, and shape of diel nutrient variability depends on both physiological traits and resource availability. In this study, we evaluate the specific hypothesis that autotrophic assimilation varies over diel timescales only when inorganic nutrients are not limiting to growth. To do so, we use fine-scale nutrient chemistry and whole-ecosystem metabolic rates (from streams and rivers) to compare expected and observed magnitude of diel nutrient variation, along a continuum of nutrient supply relative to demand.
Consistent with model predictions, the magnitude of diel variation increased with metabolic rate, but decreased under low nutrient conditions. This pattern supports the hypothesis that resource supply mediates the temporal covariation of metabolism and nutrient uptake. Under nutrient replete conditions, our data suggest that virtually all nutrient assimilation occurs during limited times of day, rather than continuously. In contrast, our model, which assumes strictly passive regulation of uptake by enzyme kinetics, predicts far more muted diel variation. This mismatch between observed and modeled nutrient dynamics supports the hypothesis that active regulation of uptake and growth influence fine-scale patterns of nutrient cycling. This inference is consistent with other studies showing widespread diel variation in gene activity.