Ecological stoichiometry incorporates the balance of carbon (C) and nutrients (e.g., nitrogen [N], phosphorus [P]), and has been used to assess consumer nutrition limitation via threshold elemental ratios (TERs). To date, basal resource nutrient content and stoichiometric ratios have been calculated and synthesized from many ecosystem compartments, including terrestrial leaves and litters, soils, and marine and lacustrine primary producers and seston. Largely missing from these syntheses is an assessment of basal resource stoichiometry within stream networks and across streams from different biomes. Our study sought to assess whether detrital basal resource nutrient content and stoichiometry in streams differs across biomes and from basal resources in other ecosystems, and to qualitatively assess the potential for nutrient limitation in stream detritivores. We collected coarse [CPOM] and fine [FPOM] particulate organic matter from four stream networks spanning tropical forest, temperate deciduous forest, grassland, and boreal forest biomes, then analyzed percent C, N, and P and calculated stoichiometric ratios (C:N, C:P, N:P) for each resource. We used mixed effects models to test whether the magnitude and variability of nutrient content and stoichiometry differed across stream networks, and used previously published estimates of detritivore TERs to assess the potential for nutrient limitation in our streams.
Results/Conclusions
Across biomes, CPOM had significantly higher C:N (F1,235=574.02, p<0.001) and C:P (F1,235=336.80, p<0.001) than FPOM, and CPOM ratios were significantly higher than basal resources from terrestrial, lacustrine, and soil ecosystems. In contrast, FPOM C:N and C:P were similar to soils and lake seston. Overall, 90% of CPOM samples had C:N ratios that exceeded the estimated TERC:N of shredding macroinvertebrates, and 59% of samples had higher C:P than estimated shredder TERC:P, though biomes ranged widely in the proportion of samples exceeding the TERC:P. In contrast, FPOM C:N and C:P ratios were consistently lower than estimated TERs for macroinvertebrate collector-gatherers. This suggests that consumer-resource mismatches could be widespread, but differ between functional feeding groups and biomes. Aquatic consumers that feed on CPOM may more often experience nutrient (N and/or P) limitation, while FPOM consumers may be more vulnerable to C limitation. While our collections were from relatively undisturbed watersheds, changes in detrital stoichiometry in response to nutrient enrichment may alter stream detritivore nutrient limitation, and in turn, nutrient cycling and energy flow. Considering the implications of consumer-resource stoichiometric imbalances on carbon and nutrient cycling is an important next step in understanding the biogeochemical implications of stoichiometric differences of basal resources in streams.