Consumer-resource interactions: Expanding ecological stoichiometry beyond C:N:P across environmental gradients in Neotropical streams

Background/Question/Methods

Consumer−resource stoichiometry focusing on both macro- and micronutrients is receiving increasing attention. Deviations in elemental ratios in consumers can be driven by variation in the abiotic environment. Much of our knowledge in micronutrient stoichiometry stems from terrestrial systems, and regulation and variation of micronutrients and macronutrients other than carbon, nitrogen, and phosphorus are less well understood in freshwater ecosystems. Here, we expand the study of ecological stoichiometry in freshwaters beyond C, N, and P to investigate patterns of variation in other essential macronutrients (N, P, K, Ca, Mg) and micronutrients (Fe, Mn, Na, Si) in consumers and basal resources.  In six Panamanian streams, we sampled consumers (invertebrates) and their potential food resources (seston, algae, leaf detritus, leaf biofilm, and fine benthic organic matter) along an 1800 m elevation gradient and macro- and micronutrient gradient (low to high soil and water P, Na, Si, Mg), during the dry season (January- April) over multiple years. 

Results/Conclusions

Macro- and micronutrient concentrations varied systematically between and within biotic and abiotic components of streams among sample sites as a result of edaphic conditions, water chemistry, trophic level, and taxonomy.  Taxonomic identity influenced elemental concentrations and varied among trophic levels and sites.  Nitrogen, P, K, Fe, and Mg concentrations were higher in consumers and their resources in sites with high dissolved P concentrations compared to sites with low dissolved P concentrations.  The greatest differences in macro- and micronutrient concentrations in consumers occurred in species in the shredder and predator functional feeding groups.  In addition, stoichiometric homeostasis varied among taxa along the P-concentration gradient. Our results suggest that edaphic conditions influence/regulate the ecological stoichiometry of consumer-resource interactions in aquatic systems and may aid in understanding the ecological impacts of multi-element nutrient limitation on individuals and on ecosystem function. Macro- and micronutrient studies linking multidimensional biogeochemical properties of ecosystems are necessary to advance our understanding of ecological stoichiometry in a changing world.