Light as a first‐order control on ecosystem structure in a temperate stream

AbstractAn emerging issue in ecohydrology is the role of light in fluvial ecosystem dynamics. Here, we investigate how photosynthetically active radiation (PAR) influences the hydrogeomorphology and biogeochemistry of a second‐order temperate stream with varying riparian communities from heavily shaded forest sections to unshaded grass sections. First, in‐stream PAR was compared to submerged aquatic macrophyte distributions along a 1·2‐km reach. The effects of macrophytes on water depth, sediment size, sediment volume, organic matter (OM) accumulation and nutrient uptake were then analysed. Compared with forested sites, non‐forested sites had three times more benthic PAR, which resulted in a quadrupling of macrophyte biomass. This greater biomass at non‐forested sites increased water depth, sediment accumulation and the uptake of soluble reactive phosphorous (SRP). Bed sediment size and OM were not significantly different between forested and non‐forested sites. Finally, we used the above relations to estimate biogeochemical differences between a completely forested reach and a non‐forested reach. Compared with a forested reach, the non‐forested reach accumulated almost twice as much bed sediment and retained more than four times as much SRP. Thus, changes in riparian conditions may create a cascade through which shading drives changes in stream habitat, which in turn drives changes in hydrogeomorphology and biogeochemical cycles. Copyright © 2010 John Wiley & Sons, Ltd.