Environmental context differentially influences nitrogen and phosphorus uptake in streams

Shannon L. Speir, Jennifer L. Tank, Matt T. Trentman, Waterborne's Martha M. Dee (Gerig), and Arial J. Shogren

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Abstract: In headwater streams, local environmental context, including underlying substrate, light availability, and hydrology, can set the template for biofilm colonization and growth, which can then alter nutrient uptake. However, understanding the influence of abiotic drivers in controlling nutrient uptake in natural systems, as well as partitioning the relative contributions of autotrophs vs heterotrophs, remains challenging. Here, we used 4 experimental streams (50 m in length) to identify the relative influence of benthic substrate, light, and streamflow on NO₃-N, NH₄-N, and soluble reactive P (SRP) uptake over 3 experimental phases. We conducted short-term nutrient additions in the 4 streams, yielding a total of 240 nutrient uptake measurements across the 3 solutes. During phase I, we documented the effects of benthic substrate on nutrient uptake and found that substrate differences in biofilm accumulation (as chlorophyll a) resulted in differences in only SRP uptake velocities. In phase II, we used whole-stream shading to partition the relative contributions of autotrophs and heterotrophs to nutrient uptake in streams with and without sand. In streams with sand present, autotrophs dominated areal uptake for dissolved inorganic N (NO₃-N + NH₄-N) and SRP. In phase III, we simulated a storm event and measured declines in organic matter, autotrophic metabolism (as gross primary production), and uptake velocity for all solutes post-disturbance; however, we did not observe substrate-driven differences in disturbance response. Our results demonstrate that both substrate and light availability influence the contributions of autotrophs and heterotrophs to N and P uptake. Understanding the roles of these interacting drivers is critical for predicting instream nutrient cycling and for restoration and management efforts that aim to prevent loss to downstream systems by maximizing nutrient removal.