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1. Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

   https://doi.org/10.1111/fwb.13509  

   Bonjour, Sophia M. ; Whiles, Matt R. ; Gido, Keith B. ( April 2020 , Freshwater Biology)

   Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.

   Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in‐stream predators such as fishes.

   We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.

   Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.

   There was an increase over time in algal biomass (chlorophyll‐a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.

   Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

    

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

   https://doi.org/10.1111/1365-2435.13289  

   Demi, Lee M. ; Benstead, Jonathan P. ; Rosemond, Amy D. ; Maerz, John C. ; El‐Sabaawi, ed., Rana ( February 2019 , Functional Ecology)

   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.

   Aplain language summaryis available for this article.

    

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3. Mayfly emergence production and body length response to hydrology in a tropical lowland stream

   https://doi.org/10.7717/peerj.9883  

   Gutiérrez-Fonseca, Pablo E. ; Ramírez, Alonso ( January 2020 , PeerJ)

   null (Ed.)

   Background Hydrological impacts on aquatic biota have been assessed in numerous empirical studies. Aquatic insects are severely affected by population declines and consequent diversity loss. However, many uncertainties remain regarding the effects of hydrology on insect production and the consequences of energy transfer to the terrestrial ecosystem. Likewise, sublethal effects on insect morphology remain poorly quantified in highly variable environments. Here, we characterized monthly fluctuation in benthic and emerged biomass of Ephemeroptera in a tropical lowland stream. We quantified the proportion of mayfly production that emerges into the riparian forest. We also examined the potential morphological changes in Farrodes caribbianus (the most abundant mayfly in our samples) due to environmental stress. Methods We collected mayflies (nymphs and adults) in a first-order stream in Costa Rica. We compared benthic and adult biomass from two years’ worth of samples, collected with a core sampler (0.006 m 2 ) and a 2 m 2 -emergence trap. The relationship between emergence and annual secondary production (E/P) was used to estimate the Ephemeroptera production that emerged as adults. A model selection approach was used to determine the relationship between environmental variables that were collected monthly and the emergent biomass. To determine potential departures from perfect bilateral symmetry, we evaluated the symmetry of two morphological traits (forceps and forewing) of F. caribbianus adults. We used Spearman’s rank correlation coefficients (ρ) to examine potential changes in adult body length as a possible response to environmental stress. Results Benthic biomass was variable, with peaks throughout the study period. However, peaks in benthic biomass did not lead to increases in mayfly emergence, which remained stable over time. Relatively constant mayfly emergence suggests that they were aseasonal in tropical lowland streams. Our E/P estimate indicated that approximately 39% and 20% (for 2002 and 2003, respectively) of the nymph production emerged as adults. Our estimated proportion of mayfly production transferred to terrestrial ecosystems was high relative to reports from temperate regions. We observed a strong negative response of F . caribbianus body length to increased hydrology (Spearman: ρ = −0.51, p < 0.001), while slight departures from perfect symmetry were observed in all traits. Conclusion Our two years study demonstrates that there was large temporal variability in mayfly biomass that was unrelated to hydrological fluctuations, but potentially related to trophic interactions (e.g., fish predation). Body length was a good indicator of environmental stress, which could have severe associated costs for mayfly fitness in ecosystems with high temporal variation. Our results highlight the complex ecological and evolutionary dynamics of tropical aquatic insects, and the intricate connection between aquatic and terrestrial ecosystems. 

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4. Thermal traits of freshwater macroinvertebrates vary with feeding group and phylogeny

   https://doi.org/10.1111/fwb.13992  

   Tomczyk, Nathan J. ; Rosemond, Amy D. ; Rogers, Phoenix A. ; Cummins, Carolyn S. ( November 2022 , Freshwater Biology)

   Abstract Functional traits of organisms, especially feeding traits, influence how organisms mediate ecosystem processes. As climate change, landscape modification and industrial waste heat release continue to increase water temperatures, shifts in the composition of feeding traits within aquatic macroinvertebrate communities may alter ecosystem processes. However, it is unclear whether thermal traits of macroinvertebrates vary systematically across functional feeding groups (FFGs; i.e., categories based on feeding ecology such as herbivores, shredders, predators, etc.) or phylogeny. We used previously published datasets on hundreds of macroinvertebrate taxa to evaluate how thermal traits differed across FFGs. We also examined the strength of phylogenetic signal in both FFG and thermal traits, using a new phylogeny of insect taxa. Then, we tested whether phylogenetic patterns offered a plausible explanation for differences in thermal traits among FFGs by comparing phylogenetic and non‐phylogenetic regressions. Shredders tended to have lower temperature preferences, optima and maxima (three of five of the thermal traits evaluated) than other FFGs. Patterns for other FFGs differed by thermal trait, but predators, collector‐gatherers and filterers had some of the highest thermal trait values. FFG explained 40% of the variation in critical thermal maximum, but <12% of the variation in the four other thermal traits. Phylogeny explained 26%–88% of the variation in thermal and feeding traits. For the subset of taxa and trait data that were available, phylogeny explained more than double the variation in thermal traits relative to FFG, but comparison of phylogenetic and non‐phylogenetic regressions highlighted that FFG explained variation in thermal traits that was independent of phylogeny. Our results highlight phylogeny and FFG as predictors of thermal traits in aquatic macroinvertebrates. Our results suggest that warmer water temperatures could favour predators, filterers and collector‐gatherers over shredders. Furthermore, our results confirm that certain orders of macroinvertebrates, such as Diptera, may be better suited to warmer temperatures than other orders, such as Plecoptera. 

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5. Stream network geometry and the spatial influence of aquatic insect subsidies across the contiguous United States

   https://doi.org/10.1002/ecs2.2926  

   Kopp, Darin A. ; Allen, Daniel C. ( November 2019 , Ecosphere)

   Emergent aquatic insects transport aquatic‐derived resources into terrestrial ecosystems but are rarely studied at landscape or regional scales. Here, we investigate how stream network geometry constrains the spatial influence of aquatic insect subsidies in terrestrial ecosystems. We also explore potential factors (i.e., climate, topography, soils, and vegetation) that could produce variation in stream network geometry and thus change the extent of aquatic insect subsidies from one region to another. The stream signature is the percentage of aquatic insect subsidies traveling a given distance into the terrestrial ecosystem, relative to what comes out of the stream. We use this concept to model the spatial extent (area) and distribution (spatial patterning) of aquatic subsidies in terrestrial ecosystems across the contiguous United States. Our findings suggest that at least 8% of the subsidies measured at the aquatic–terrestrial boundary (i.e., the 8% stream signature) are typically transferred throughout the entire watershed and that variation in this spatial extent is largely influenced by the drainage density of the stream network. Moreover, we found stream signatures from individual stream reaches overlap such that the spatial extent of the 8% stream signature often includes inputs from multiple stream reaches. Landscape‐scale stream network characteristics increased the area of overlapping stream signatures more than reach‐scale channel properties. Finally, we found runoff was an important factor influencing stream network geometry suggesting a potential effect of climate on aquatic‐to‐terrestrial linkages that have been understudied.

    

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