Variation in Detrital Resource Stoichiometry Signals Differential Carbon to Nutrient Limitation for Stream Consumers Across Biomes
Stoichiometric ratios of resources and consumers have been used to predict nutrient limitation across diverse terrestrial and aquatic ecosystems. In forested headwater streams, coarse and fine benthic organic matter (CBOM, FBOM) are primary basal resources for the food web, and the distribution and quality of these organic matter resources may therefore influence patterns of secondary production and nutrient cycling within stream networks or among biomes. We measured carbon (C), nitrogen (N), and phosphorus (P) content of CBOM and FBOM and calculated their stoichiometric ratios (C/N, C/P, N/P) from first- to fourth-order streams from tropical montane, temperate deciduous, and boreal forests, and tallgrass prairie, to compare the magnitude and variability of these resource types among biomes. We then used the ratios to predict nutritional limitations for consumers of each resource type. Across biomes, CBOM had consistently higher %C and %N, and higher and more variable C/N and C/P than FBOM, suggesting that microbial processing results in more tightly constrained elemental composition in FBOM than in CBOM. Biome-specific differences were observed in %P and N/P between the two resource pools; CBOM was lower in %P but higher in N/P than FBOM in the tropical montane and temperate deciduous forest biomes, while CBOM was higher in %P but similar in N/P than FBOM in the grassland and boreal forest biomes. Stable 13C isotopes suggest that FBOM likely derives from CBOM in tropical and temperate deciduous forest, but that additional non-detrital components may contribute to FBOM in boreal forests and grasslands. Comparisons of stoichiometric ratios of CBOM and FBOM to estimated needs of aquatic detritivores suggest that shredders feeding on CBOM are more likely to experience nutrient (N and/or P) than C limitation, whereas collector–gatherers consuming FBOM are more likely to experience C than N and/or P limitation. Our results suggest that differences in basal resource elemental content and stoichiometric ratios have the potential to affect consumer production and ecosystem rates of C, N, and P cycling in relatively consistent ways across diverse biomes.
Keywordsnitrogen phosphorus benthic organic matter elemental ratios macroinvertebrate lotic aquatic ecosystem threshold elemental ratio
We appreciate the help of Matthew Bosiak, John Brant, Walter Dodds, Claire Ruffing, Geoffrey Schwaner, and Katherine Swan in the collection of field samples, and thank Jason Coombs, along with Tom Maddox and Emmy Deng of the University of Georgia Analytical Chemistry Lab for assistance in sample processing. Chao Song, Ford Ballantyne, and Mary Freeman provided guidance on statistical analyses. Logistical support was provided by the US Forest Service Coweeta Hydrologic Laboratory and Luquillo Experimental Forests and LTER sites, and the Konza Prairie and Bonanza Creek LTER sites. Members of SCALER, Dr. Stuart Bunn, and two anonymous reviewers provided constructive comments that greatly improved this manuscript. Funding was provided by a National Science Foundation (NSF) Grant (EF-1064998) to ADR and JSK and an NSF Research Experiences for Undergraduates supplement to ADR and JSK. Field collections were also supported by NSF Grants EF-1065286 (LUQ), EF-1065255 (KNZ), and EF-1065055 (CPC). Partial support during manuscript preparation was provided to KJF from EF-1702506 and ICER-1517823.
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