Environmental drivers of Adirondack stream food web dynamics: Analyzing the results of Bayesian stable isotope mixing models across multiple sites
Forested stream food webs often rely upon allochthonous basal resource inputs in the form of terrestrial detritus. Using the results of Bayesian mixing models from a gradient of sites, we asked how natural variation in environmental factors, such as cover, water temperature, and pH, drive stream food web reliance on autochthonous versus allochthonous resources. In summer 2013, we surveyed basal resources, invertebrates, and fish at eleven study sites on ten southwestern Adirondack streams in a region that has experienced decades of acid deposition. These streams all have low inorganic nutrient levels, but span a natural gradient in environmental variables including: cover, water temperature, pH, TDS, ANC, DOC, and monomeric aluminum. In addition, we analyzed the quantity (chlorophyll a, ash free dry mass, and dry mass) and quality (carbon to nitrogen ratios) of allochthonous and autochthonous basal resources and treated these as additional site-specific environmental variables. We analyzed carbon (δ13C), nitrogen (δ15N), and hydrogen (δ2H) stable isotopes and used Bayesian mixing models to estimate stream fish and invertebrate reliance on autochthonous versus allochthonous basal resources at each site. We then used partial least squares regression to determine which environmental variables drove patterns of allochthony across our all of sites.
Across all macroinvertebrate consumers, we found stream temperature, cover, DOC, and AFDM to be the strongest predictors of allochthony. However, we also found strong taxa-specific patterns that corresponded to invertebrate functional feeding groups. For example, across streams crayfish generally relied on allochthonous resources, but consumed more autochthonous resources when autochthonous primary producer biomass was higher. In contrast, mayflies were generally relied most on autochthonous resources, but consumed more allochthonous resources when DOC was higher. Stream temperature, cover, and AFDM, along with pH, nitrate, and autochthonous primary producer abundance influenced dietary patterns and the degree of allochthony in invertebrate secondary consumers. Interestingly, ANC, and monomeric aluminum had little effect on the invertebrate dietary patterns, but did affect trophic length because fish were excluded from streams with the lowest pH and ANC and the highest monomeric aluminum levels. pH, ANC, and monomeric aluminum, along with DOC, nitrate, autochthonous primary producer abundance, all influenced fish diets and patterns of allochthony. Our results suggest that unique environmental factors control patterns of allochthony at different trophic levels.