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

An 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 (C) 2010 John Wiley & Sons. Ltd.