Stream ecosystem responses to forest leaf emergence in spring

Streams in deciduous forests undergo marked transitions from light-replete to light-limited ecosystems every spring when leaves emerge on streamside t rees. During the course of leaf emergence and enlargement, shade from leave s on streamside trees can reduce photosynthetically active radiation (PAR) falling on the streambed from > 1000 to < 30 mu mol .m(-2).s(-1). In this s tudy, we examined the effects of leaf emergence at multiple levels in two h eadwater streams in eastern Tennessee. Primary production estimated from bo th photosynthesis-irradiance measurements of periphyton in the laboratory a nd whole-steam diurnal oxygen measurements decreased dramatically over the course of canopy closure. Monthly carbon fixation estimates for periphyton in White Oak Creek declined from 354 mug C/cm(2) in April to 66 mug C/cm(2) in June. while carbon fixation in Walker Branch declined from 495 to 168 m ug C/cm(2). Periphyton photosynthesis became increasingly efficient at low irradiances (alpha increased more than threefold) as ambient streambed irra diances declined, but this increase in efficiency only partially compensate d for the photon scarcity caused by riparian shade. Ecological photosynthet ic efficiency (percentage of incident PAR energy fixed by photosynthesis) e stimated from static models. whole-stream measurements, and ambient PAR was a negative exponential function of incident PAR, increasing from < 0.3% to 2% during canopy closure. This increase was attributable to (1) inefficien t use of the relatively high irradiances before leaf emergence, and (2) gre ater photoefficiency (increased alpha) at low irradiances after leaf emerge nce. Nutrient concentrations (dissolved nitrate and phosphate) in both stre ams increased coincident with leaf emergence, implying a cascade of shade e ffects through primary producers to abiotic components of the ecosystem. Sh ade effects also propagated to higher trophic levels: growth rates of grazi ng snails (Elimia clavaeformis) in both streams decreased substantially fro m April to June, consistent with modeled decreases in the productivity of t heir food resource (periphyton). Snail growth rates were almost zero in Whi te Oak Creek and were negative in Walker Branch during summer when streambe d PAR was lowest. The multilevel effects of leaf emergence reported in this study accentuate the importance of light variation in aquatic ecosystems a nd illustrate the close coupling between streams and their surrounding terr estrial ecosystems.