Watershed Land Use and Seasonal Variation Constrain the Influence ofRiparian Canopy Cover on Stream Ecosystem Metabolism

摘要

Ecosystem metabolism is an important determinant of trophic structure, nutrient cycling, and other critical ecosystem processes in streams. Whereas watershed- and local-scale controls on stream metabolism have been independently investigated, little is known about how controls exerted at different scales interact to determine stream metabolic rates, particularly in urban streams and across seasons. To address this knowledge gap, we measured ecosystem metabolism in four urban and four reference streams in northern Kentucky, USA, with paired closed and open riparian canopies, during each of the four seasons. Gross primary production (GPP), ecosystem respiration, and net ecosystem production (NEP) were all best predicted by models with season as a main effect, but interactions between season, canopy, and watershed varied for each response. Urban streams exhibited higher GPP during most seasons, likely due to elevated nutrient loads. Open canopy reaches in both urban and forested streams, supported higher rates of GPP than the closed canopy which reaches during the summer and fall, when the overhead vegetation shaded the closed reaches. The effect of canopy cover on GPP was similar among urban and forested streams. The combination of watershed and local-scale controls resulted in urban streams thatalternated between net heterotrophy (NEP <0) and net autotrophy (NEP>0) at the reach-scale during seasons with dense canopy cover. Thisfinding has management relevance because net production can lead to accumulationof algal biomass and associated issues like nighttime hypoxia. Our studysuggests that although watershed urbanization fundamentally alters ecosystemfunction, the preservation and restoration of canopied riparian zones canprovide an important management tool at the local scale, with the strongestimpacts on stream metabolism during summer.