Experimental nutrient enrichment of forest streams increases energy flow to predators along greener food-web pathways

Nutrient enrichment is a key stressor of lakes and streams globally, affecting the relative availability of important basal resources such as algae and detritus. These effects are controlled by responses of autotrophic and heterotrophic microorganisms that subsequently affect primary consumers and higher level predators. Despite the potential for propagation of these bottom-up effects, few studies have examined how nutrients affect green (autotrophic) versus brown (heterotrophic) energy pathways to predators via changes in the quantity or type of prey consumed. We studied the pathways by which nutrient enrichment affected two predatory salamander species (Desmognathus quadramaculatus and Eurycea wilderae) using detailed diet analyses before and during 2-year nutrient additions to five headwater forest streams. The streams were continuously enriched with different concentrations of dissolved nitrogen (N) and phosphorus (P), creating relatively greater N or P concentrations and distinct N:P ratios (2:1, 8:1, 16:1, 32:1 and 128:1) in each stream. Nutrient addition resulted in greater prey number, size and biomass consumed by D.quadramaculatus, an effect driven more by P than by N additions. Some of these effects were greater in the second year of enrichment and were greater for larger individuals. Shifts in the prey composition of D.quadramaculatus included increases in algivores and decreases in detritivores, tracking observed treatment effects on basal resource quantity (e.g. algivore abundance in guts was related to algal biomass, which increased with enrichment, and detritivore abundance in guts was related to detrital standing stocks, which declined with enrichment). For E.wilderae diets, there was limited evidence for increased prey size and number, or for alteration of prey composition with enrichment despite evidence of increased larval growth. We hypothesise that body size differences between the two salamander species partially explain their different dietary responses to enrichment. Our results show that nutrient addition, primarily of P, affected the quantity and composition of predator diets in our nutrient-poor streams. These effects on diet were consistent with concurrent studies showing that P enrichment resulted in faster growth of salamanders and occurred partly via effects on algal biofilm or green food-web pathways, despite the dominance of detrital or brown resources in our heavily shaded forest stream sites. Thus, nutrient enrichment can promote algae- versus detritus-based energy-flow pathways in nominally light-limited stream ecosystems, with associated changes in food-web characteristics and function.