Human induced climate and land-use change are severely impacting global biodiversity, but how community composition and richness of multiple taxonomic groups change in response to local drivers and whether these responses are synchronous remains unclear. We used long-term community-level data from an experimentally manipulated grassland to assess the relative influence of climate and land use as drivers of community structure of four taxonomic groups: birds, mammals, grasshoppers, and plants. We also quantified the synchrony of responses among taxonomic groups across land-use gradients and compared climatic drivers of community structure across groups. All four taxonomic groups responded strongly to land use (fire frequency and grazing), while responses to climate variability were more pronounced in grasshoppers and small mammals. Animal groups exhibited asynchronous responses across all land-use treatments, but plant and animal groups, especially birds, exhibited synchronous responses in composition. Asynchrony was attributed to taxonomic groups responding to different components of climate variability, including both current climate conditions and lagged effects from the previous year. Data-driven land management strategies are crucial for sustaining native biodiversity in grassland systems, but asynchronous responses of taxonomic groups to climate variability across land-use gradients highlight a need to incorporate response heterogeneity into management planning.

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1. Animals contribute significantly to nutrient cycling through excretion, but most studies consider their effects under relatively benign abiotic conditions. Disturbances such as drought may alter animals' nutrient contributions through shifts in species composition and biomass. Headwater streams are particularly vulnerable to extreme climate events and thus might show rapid changes in stream biota and their ecosystem effects.2. We tested how biomass and subsequent ecosystem effects (nutrient cycling) of an intermittent prairie stream community changed during a drought. We quantified the biomass and contributions to nutrient cycling for assemblages comprising fishes, crayfish, and tadpoles in 12 isolated pools over 3 months encompassing the harshest drought on record for Kings Creek, KS, U.S.A. We predicted that macroconsumer biomass would decline with pool surface area and that differences in macroconsumer biomass and taxonomic composition would lead to different contributions of pool assemblages to nutrient cycling.3. The biomass of pool assemblages declined with decreasing pool size, a pattern apparently driven by mortality, emigration, or metamorphosis. We also observed a change in assemblage structure of drying pools during drought relative to pool size, shifting dominance toward species with more drought-resistant traits.Accordingly, assemblage nitrogen (N) excretion rates declined as pool biomass was reduced, leading to a 58% reduction in N available to epilithic biofilms. Phosphorus (P) excretion rates declined from June to July, but increased in August, as species with high P excretion rates maintained similar proportional biomass and biomass of a non-native fish increased. Molar N:P of pool assemblage excretion declined significantly throughout the drought and coincided with loss of southern redbelly dace (Chrosomus erythrogaster: Cyprinidae). 4. Animal-mediated nutrient cycling was altered by the loss of biomass and stoichiometric traits of taxa that differed in their occurrences and ability to tolerate abiotic conditions during drought. Elevated availability of dissolved N in isolated pools may increase N uptake rates by biofilms during drought conditions, indicating the importance of N excreted by aggregated macroconsumers, especially those with S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Hopper GW, Gido KB, Pennock CA, et al. Biomass loss and change in species dominance shift stream community excretion stoichiometry during severe drought.

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Riparian zones, the interfaces between land and stream, perform vital ecosystem functions including transformation and retention of nutrients and sediment moving across the landscape. Although many studies assess transport through and transformation of materials in riparian zones, less is known about the direct influence of precipitation falling on these zones on material retention and transport. Additionally, few experiments can compare riparian retention to stream‐channel retention. We present a novel experimental approach to assess retention of nitrate entering as precipitation in riparian zones and compare riparian retention and movement of nitrate, other ions, sediments to and within the adjacent stream channel. We simulated an intense precipitation event with 15N‐labelled nitrate as a bioactive solute and bromide as an inert tracer. This method extends tracer release approaches applied to streams worldwide and links it to processes at the aquatic/ terrestrial interface. It further allows determination of movement of materials into streams from bankside precipitation. The riparian zone removed or retained a greater proportion of nitrate than the stream relative to bromide; over half the added bromide reached the stream through a few metres of riparian zone, compared to only 0.2% of the added nitrate. Of the 0.2% that reached the stream, 30% of that nitrate was removed or retained by instream processes after travelling 60 downstream. Roughly 10% of the total 15N addition ended up sequestered in the above‐ground portions of the riparian grasses by the end of the growing season, and very little of it was recovered from the soil. We saw little evidence of bulk transport of other ions or sediment from this riparian soil to the stream. Our data are consistent with the concept of high nitrate retention in vegetated riparian zones, even for nitrate falling directly upon them in the form of atmospheric deposition in precipitation.