Woody debris is related to reach‐scale hotspots of lowland stream ecosystem respiration under baseflow conditions

P.J Blaen,S Krause,D.M Hannah,J.P Zarnetske,A Jäger,A.S Ward,J Lewandowski,C Mendoza‐Lera,M.J Kurz,J Lee‐Cullin,N.M Schmadel,J.D Drummond,M.J Klaar,S Folegot,T Datry,A.M Milner,J.L.A Knapp

doi:10.1002/eco.1952

Abstract

AbstractStream metabolism is a fundamental, integrative indicator of aquatic ecosystem functioning. However, it is not well understood how heterogeneity in physical channel form, particularly in relation to and caused by in‐stream woody debris, regulates stream metabolism in lowland streams. We combined conservative and reactive stream tracers to investigate relationships between patterns in stream channel morphology and hydrological transport (form) and metabolic processes as characterized by ecosystem respiration (function) in a forested lowland stream at baseflow. Stream reach‐scale ecosystem respiration was related to locations (“hotspots”) with a high abundance of woody debris. In contrast, nearly all other measured hydrological and geomorphic variables previously documented or hypothesized to influence stream metabolism did not significantly explain ecosystem respiration. Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems (this study) and smaller upland streams (most previous studies) under baseflow conditions. As such, these findings have implications for reactive transport models that predict biogeochemical transformation rates from hydraulic transport parameters, for upscaling frameworks that represent biological stream processes at larger network scales, and for the effective management and restoration of aquatic ecosystems.

Highlights

In‐stream processing by aquatic communities constitutes an important control on the export of nutrients and organic matter from landscapes (Mulholland et al, 2008) and the atmospheric evasion of greenhouse gases from water bodies (Cole et al, 2007)
Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems and smaller upland streams under baseflow conditions
To address these research gaps, we investigated the relationships between spatial patterns in hydrological stream solute transport and channel geomorphology and stream ecosystem respiration dynamics in a relatively slow‐flowing lowland stream containing abundant woody debris features

Summary

| INTRODUCTION

In‐stream processing by aquatic communities constitutes an important control on the export of nutrients and organic matter from landscapes (Mulholland et al, 2008) and the atmospheric evasion of greenhouse gases from water bodies (Cole et al, 2007). We hypothesized: (a) that solute patterns of reaches within this stream, such as transient storage, advection, and dispersion, would be related to in‐stream woody debris and the overall reach channel morphology; and (b) that the presence and abundance of in‐stream woody debris would increase rates of reach‐scale ecosystem respiration. To test these hypotheses, we combined conservative and reactive fluorescent tracers to assess spatial variability in both physical transport processes and ecosystem respiration across subreaches with varying morphology and wood‐debris conditions nested within a 1‐km lowland stream reach

| METHODS

| RESULTS

Findings

| DISCUSSION AND IMPLICATIONS