Bioavailable DOC: reactive nutrient ratios control heterotrophic nutrient assimilation—An experimental proof of the macronutrient-access hypothesis

Abstract We investigate the "macronutrient-access hypothesis", which states that balance between stoichiometric macronutrient demand and accessible macronutrients controls nutrient assimilation by aquatic heterotrophs. Within this hypothesis, we consider bioavailable dissolved organic carbon (bDOC), reactive nitrogen (N) phosphorus (P) to be heterotrophic assimilation. Here, N P are sums of inorganic (nitrate-N, nitrite-N, ammonium-N), soluble-reactive (SRP), (bDON) (bDOP). Previous data from various freshwaters suggests yet clear experimental support is missing. assessed hypothesis in a proof-of-concept experiment for waters four small agricultural streams. used seven different bDOC:reactive ratios, induced levels alder leaf leachate addition. With these treatments stream-water specific bacterial inoculum, conducted 3-day with three independent replicates per combination stream water, treatment, sampling occasion. extracted matter (DOM) fluorophores measuring excitation-emission matrices subsequent parallel factor decomposition (EEM-PARAFAC). true bioavailability DOC, DON, DOM as concentration difference beginning end each experiment. Subsequently, calculated bDOC bDON concentrations based on EEM-PARAFAC fluorophores, compared their bioavailability. Due very low DOP concentrations, determination uncertainty was high, assumed negligible part P. For bDON, measurements agreed same fractions indirectly (bDOC r 2 = 0.96, p < 0.001; 0.77, 0.001). Hence could predict fluorophores. The ratios (sum DIN) (equal SRP) exerted strong, predictable control uptake (R 0.80 0.83). To define zones C:N:P (co-)limitation assimilation, novel ternary-plot approach combining our literature ranges biomass. found zone maximum (C:N:P approx. > 114: 9:1), 170:21: 1) co-limitation 204:14:1). “macronutrient-access hypothesis” links ecological stoichiometry biogeochemistry, may importance many freshwater ecosystems. However, only starting point needs corroborated further experiments more sites, in-situ studies, DOC sources.