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

Abstract

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.

Experimental N and P additions relieve stoichiometric constraints on organic matter flows through five stream food webs

“…This pattern was attributed to increased consumption of algal grazing taxa (i.e. Maccafertium ; Bumpers et al, 2015, Bumpers, Rosemond, Maerz, & Benstead, 2017), which is consistent with our finding of increased grazer biomass (Demi et al, 2019), as well as diatom consumption and flows during the 2 years of enrichment. These results highlight the importance of shifts in individual material flow pathways for higher‐level consumers.…”

Litter P content drives consumer production in detritus‐based streams spanning an experimental N:P gradient

“…As such, the specific mechanisms by which macroinvertebrate predators are released from nutrient limitation deserve further investigation (but see Bumpers et al. for salamander responses).…”

“…However, biomass and production of predatory macroinvertebrates did not increase in all five streams following nutrient enrichment and was not directly related to primary consumer production in this study, even though predator response to nutrient enrichment is often attributed to shifts in prey availability (Deegan andPeterson 1992, Cross et al 2006). As such, the specific mechanisms by which macroinvertebrate predators are released from nutrient limitation deserve further investigation (but see Bumpers et al [2017] for salamander responses).…”

Differential responses of macroinvertebrate ionomes across experimental N:P gradients in detritus-based headwater streams

“…These inconsistencies along with extensive elemental variation in our study animals suggest that other factors such as sex (Back and King 2013 ; Goos et al 2017 ) or nutrient storage (Bertram et al 2008 ) could also play an important role in shaping ionomic composition in these populations. Variation in food preference (or simply stochastic resource availability) is also plausible for Maccaffertium because, while functioning predominately a detritivore in this study, its diatom consumption increased with P enrichment and higher algal biomass in our study streams (Bumpers et al 2017 ; Demi et al 2020 ). Diatom supplementation may further explain the high Si concentrations in this genus and potentially of other trace elements such as Co, Fe, Al and Cd as well.…”

“…First, macroinvertebrate community production increased with stream N and P enrichment (Demi et al 2018 ), but production did not change symmetrically across all taxa as Tallaperla and Maccaffertium production increased more on average than Pycnopsyche (Supplementary Table 3). As Tallaperla and Maccaffertium are the two most common prey items by biomass for an important salamander species in these streams ( Desmognathus quadramaculatus ), changes in their production and/or elemental composition under nutrient enrichment could feed back to influence nutrition and production of these vertebrate predators (Bumpers et al 2017 ). Secondly, previous work has documented negative relationships between body K and Ca composition and excretion in four common taxa in Coweeta streams, including Maccaffertium , which suggests that taxonomic changes in macroinvertebrate communities may alter nutrient cycling in these streams in a stoichiometric manner (Webster and Patten 1979 ).…”