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 trees. During the course of leaf emergence and enlargement, shade from leaves on streamside trees can reduce photosynthetically active radiation (PAR) falling on the streambed from >1000 to <30 μmol·m⁻²·s⁻¹. In this study, we examined the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Primary production estimated from both photosynthesis–irradiance measurements of periphyton in the laboratory and 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 μg C/cm² in April to 66 μg C/cm² in June, while carbon fixation in Walker Branch declined from 495 to 168 μg C/cm². Periphyton photosynthesis became increasingly efficient at low irradiances (α increased more than threefold) as ambient streambed irradiances declined, but this increase in efficiency only partially compensated for the photon scarcity caused by riparian shade. Ecological photosynthetic efficiency (percentage of incident PAR energy fixed by photosynthesis) estimated 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) inefficient use of the relatively high irradiances before leaf emergence, and (2) greater photoefficiency (increased α) at low irradiances after leaf emergence. Nutrient concentrations (dissolved nitrate and phosphate) in both streams increased coincident with leaf emergence, implying a cascade of shade effects through primary producers to abiotic components of the ecosystem. Shade effects also propagated to higher trophic levels: growth rates of grazing snails (Elimia clavaeformis) in both streams decreased substantially from April to June, consistent with modeled decreases in the productivity of their food resource (periphyton). Snail growth rates were almost zero in White Oak Creek and were negative in Walker Branch during summer when streambed PAR was lowest. The multilevel effects of leaf emergence reported in this study accentuate the importance of light variation in aquatic ecosystems and illustrate the close coupling between streams and their surrounding terrestrial ecosystems.