We investigate the "macronutrient-access hypothesis", which states that the 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) and reactive phosphorus (P) to be the macronutrients accessible to heterotrophic assimilation. Here, reactive N and P are the sums of dissolved inorganic N (nitrate-N, nitrite-N, ammonium-N), soluble-reactive P (SRP), and bioavailable dissolved organic N (bDON) and P (bDOP). Previous data from various freshwaters suggests this hypothesis, yet clear experimental support is missing. We assessed this hypothesis in a proof-of-concept experiment for waters from four small agricultural streams. We used seven different bDOC:reactive N and bDOC:reactive P ratios, induced by seven levels of alder leaf leachate addition. With these treatments and a stream-water specific bacterial inoculum, we conducted a 3-day experiment with three independent replicates per combination of stream water, treatment, and sampling occasion. Here, we extracted dissolved organic matter (DOM) fluorophores by measuring excitation-emission matrices with subsequent parallel factor decomposition (EEM-PARAFAC). We assessed the true bioavailability of DOC, DON, and the DOM fluorophores as the concentration difference between the beginning and end of each experiment. Subsequently, we calculated the bDOC and bDON concentrations based on the bioavailable EEM-PARAFAC fluorophores, and compared the calculated bDOC and bDON concentrations to their true bioavailability. Due to very low DOP concentrations, the DOP determination uncertainty was high, and we assumed DOP to be a negligible part of the reactive P. For bDOC and bDON, the true bioavailability measurements agreed with the same fractions calculated indirectly from bioavailable EEM-PARAFAC fluorophores (bDOC r² = 0.96, p < 0.001; bDON r² = 0.77, p < 0.001). Hence we could predict bDOC and bDON concentrations based on the EEM-PARAFAC fluorophores. The ratios of bDOC:reactive N (sum of bDON and DIN) and bDOC:reactive P (equal to SRP) exerted a strong, predictable stoichiometric control on reactive N and P uptake (R² = 0.80 and 0.83). To define zones of C:N:P (co-)limitation of heterotrophic assimilation, we used a novel ternary-plot approach combining our data with literature data on C:N:P ranges of bacterial biomass. Here, we found a zone of maximum reactive N uptake (C:N:P approx. > 114: < 9:1), reactive P uptake (C:N:P approx. > 170:21: < 1) and reactive N and P co-limitation of nutrient uptake (C:N:P approx. > 204:14:1). The “macronutrient-access hypothesis” links ecological stoichiometry and biogeochemistry, and may be of importance for nutrient uptake in many freshwater ecosystems. However, this experiment is only a starting point and this hypothesis needs to be corroborated by further experiments for more sites, by in-situ studies, and with different DOC sources.

我们调查了“常量营养素获取假说”，该假说指出化学计量常量营养素需求和可获得的常量营养素之间的平衡控制着水生异养生物的营养同化。在这个假设中，我们认为生物可利用的溶解有机碳 (bDOC)、活性氮 (N) 和活性磷 (P) 是可用于异养同化的常量营养素。此处，活性 N 和 P 是溶解的无机 N（硝酸盐-N、亚硝酸盐-N、铵-N）、可溶性反应性 P (SRP) 和生物可利用的溶解有机 N (bDON) 和 P (bDOP) 的总和。来自各种淡水的先前数据表明了这一假设，但缺少明确的实验支持。我们在对来自四个小型农业河流的水的概念验证实验中评估了这一假设。我们使用了七种不同的 bDOC：反应性 N 和 bDOC:反应性 P 比率，由七种水平的桤叶渗滤液添加诱导。通过这些处理和溪水特异性细菌接种物，我们进行了为期 3 天的实验，每个溪水、处理和采样时机的组合进行了三个独立的重复。在这里，我们通过测量激发-发射矩阵和随后的平行因子分解 (EEM-PARAFAC) 来提取溶解有机物 (DOM) 荧光团。我们评估了 DOC、DON 和 DOM 荧光团的真实生物利用度，作为每个实验开始和结束之间的浓度差异。随后，我们根据生物可利用的 EEM-PARAFAC 荧光团计算了 bDOC 和 bDON 浓度，并将计算出的 bDOC 和 bDON 浓度与其真实生物利用度进行了比较。由于 DOP 浓度非常低，²  = 0.96，p < 0.001；bDON r ²  = 0.77，p < 0.001)。因此，我们可以基于 EEM-PARAFAC 荧光团预测 bDOC 和 bDON 浓度。bDOC:reactive N（bDON 和 DIN 的总和）和 bDOC:reactive P（等于 SRP）的比率对活性 N 和 P 吸收（R ² = 0.80 和 0.83）。为了定义异养同化的 C:N:P（共）限制区域，我们使用了一种新的三元图方法，将我们的数据与 C:N:P 细菌生物量范围的文献数据相结合。在这里，我们发现了一个最大活性氮吸收区（C:N:P 大约 > 114: < 9:1）、活性磷吸收（C:N:P 大约 > 170:21: < 1）和活性氮和 P 共同限制养分吸收（C:N:P 约 > 204:14:1）。“大量营养素获取假说”将生态化学计量学和生物地球化学联系起来，可能对许多淡水生态系统的养分吸收很重要。然而，这个实验只是一个起点，这个假设需要通过更多地点的进一步实验、原位研究和不同的 DOC 来源来证实。