Experimental N and P additions alter stream macroinvertebrate community composition via taxon‐level responses to shifts in detrital resource stoichiometry

Abstract

Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities, partly by altering stoichiometric imbalances between consumers and their food. Testing relationships between resource stoichiometry and consumer assemblage structure requires ecosystem‐level manipulations that have been lacking to date. We analysed patterns of macroinvertebrate community composition in five detritus‐based headwater streams subject to experimental whole‐stream N and P additions that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1, 128:1) over 2 years, following a 1‐year pre‐treatment period. We predicted that shifts in leaf litter stoichiometry would drive overall patterns of community composition via greater responses of shredders to enrichment than other taxa, as shredders dominate primary consumer biomass and experience larger consumer–resource elemental imbalances than other functional groups in stream ecosystems. Specifically, we expected litter C:P to be a significant predictor of shredder biomass given the greater relative imbalances between shredder and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment were related to other taxon‐level traits, including body size and stoichiometry, larval life span and growth rate. Whole‐community composition shifted similarly across the five streams after enrichment, largely driven by increased shredder and predator biomass. These shifts were limited to the autumn/winter seasons and related to decreased leaf litter C:P, highlighting important links between the quality of seasonal litter subsidies and community phenology. Among 10 taxa that drove structural shifts, two declined while other taxa from the same functional/taxonomic groups responded positively, suggesting that specific life‐history traits may determine sensitivity to enrichment. Increases in total shredder biomass, and in biomass of several common shredders, were associated with lower litter C:P. Body C:P did not predict shredder response to enrichment. However, weak negative relationships between shredder response and body size, and larval life span, suggest that small‐bodied and short‐lived taxa may be more responsive to shifting resource stoichiometry. Moderate anthropogenic increases in N and P availability affect resource stoichiometry and can alter animal communities, influencing additional food web and ecosystem properties. We provide support for ecological stoichiometry as a framework for predicting such outcomes based on changes in the elemental composition of resource pools. A plain language summary is available for this article.

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

“…However, elevated concentrations as low as 7 µg/L SRP triggered changes in resource nutrient content in our study (which was the 2‐year average SRP concentration in the target 11 µg/L SRP stream; Manning et al, 2016), indicating very little P, combined with N enrichment, can result in changes in food resource nutrient content. Increases in total organic matter flows to primary consumers were largely driven by leaf‐shredding macroinvertebrates, which exhibited the greatest response to nutrient enrichment among macroinvertebrate FFGs (Demi et al, 2019). Nutrient enrichment produced clear but modest shifts in the trophic basis of primary consumer production relative to pre‐enrichment conditions but did not alter the contribution of dominant food types (leaf detritus and fungi) to production.…”

“…The flow of animal prey to predators did not systematically change following nutrient enrichment and was unrelated to total flows of organic matter to primary consumers, which were dominated by leaf litter. The lack of a systematic response in flows to predatory macroinvertebrates may reflect shifts in the potential prey assemblage towards a greater prevalence of large‐bodied, predator‐resistant taxa (Davis et al, 2010a; Davis, Rosemond, Eggert, Cross, & Wallace, 2010b; Demi et al, 2019). For example, an earlier analysis of macroinvertebrate community composition in the study streams revealed that the communities shifted towards greater biomass of the common, large‐bodied shredders Pycnopsyche (Trichoptera), Tallaperla (Plecoptera) and Tipula (Diptera; Demi et al, 2019).…”

“…Although increased N and P availability often stimulates activity at the base of food webs, the extent to which those changes alter food web properties at higher trophic levels is less predictable and likely mediated by the effect of nutrient concentrations on the composition, quantity and quality of basal resource pools (Boersma et al, 2008; Cebrian, 2004; Cebrian et al, 2009). Such nutrient‐induced changes to basal resources have been shown to alter the taxonomic composition (Demi, Benstead, Rosemond, & Maerz, 2019; Evans‐White, Dodds, Huggins, & Baker, 2009; Moe et al, 2005), productivity (Cross, Wallace, Rosemond, & Eggert, 2006; Ott et al, 2014) and trophic structure of consumer communities (Davis, Rosemond, Eggert, Cross, & Wallace, 2010a; Worm, Lotze, & Sommer, 2000). Understanding how basal resource pools respond to variation in nutrient supply, particularly the relative availability of N and P (i.e.…”

Can Constructed Wetlands be Wildlife Refuges? A Review of Their Potential Biodiversity Conservation Value

“…Eutrophic CWs, along with oligotrophic downstream environments, contribute to the maintenance of regional biodiversity [72]. Moreover, the unequal purification effect of CWs, with respect to different contaminants, can change regional stoichiometric characteristics, thus having potential impacts on regional biodiversity [73,74].…”

Spatiotemporal Variations in Seston C:N:P Stoichiometry in a Large Eutrophic Floodplain Lake (Lake Taihu): Do the Sources of Seston Explain Stoichiometric Flexibility?

“…The C:N:P ratio of suspended particulate matters, hereafter "seston", is closely tied to biogeochemical cycles and food webs in aquatic ecosystems [1][2][3]. Since A. C. Redfield first observed that C:N:P ratios of seston in the offshore surface ocean are similar to the ratios of dissolved nutrients in the deep ocean, the classical Redfield ratio (C:N:P = 106:16:1) is routinely used to interpret and local scale.…”