Homeostasis and nutrient limitation of benthic autotrophs in natural chemostats

Abstract

To explore how gradients in nutrient availability influence the stoichiometry of freshwater autotrophs, we sampled algal and submerged vascular plant tissues, as well as water for C : N : P ratios in 41 of north Florida's spring‐fed rivers, which are ideal natural laboratories because of unparalleled temporal stability in chemistry, temperature, and flow. Water chemistry across springs spanned a molar inorganic N : P gradient from 0.6 to 89.4, while repeated measurements within springs suggest low variation (coefficient of variation [CV] < 20%) over interannual timescales. We observed significant interspecific differences in stoichiometry suggesting different nutrient requirements, but there was negligible evidence that variation in nutrient supply influenced taxa presence or absence. Most importantly, we observed no plasticity in tissue stoichiometry for any taxa; homeostatic regulation coefficients (H) ranged from 4.8 to undefined (slope of log‐resource vs. log‐tissue ratios ≤ 0), suggesting strict homeostasis in this setting. To explore the paradox of plasticity in chemostat experiments vs. homeostasis in these natural chemostats, we adapted the Droop model to better represent benthic systems (i.e., where hydraulic and biomass turnover rates can be different) and to allow light limitation. Model results suggest increasing hydraulic turnover induces homeostasis and releases autotrophs from growth limitation by nutrients (i.e., inducing light limitation) over a large range of nutrient concentrations and element ratios, suggesting the resource supply rate in comparison with ecosystem demand, not concentration or element ratios, is the primary determinant of nutrient limitation, and thus a dominant control on autotroph stoichiometry. Our results suggest deviation between observed and taxa‐specific optimal stoichiometry may diagnose nutrient limitation.