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RELATIVE IMPORTANCE OF TOP-DOWN AND BOTTOM-UP INFLUENCES ON STREAM PERIPHYTON – GRAZER SYSTEM (the tailed frog – Ascaphus truei)
MICHELINE A. KIM and JOHN S. RICHARDSON
Abstract. Removal of the riparian canopy due to forest disturbance can alter light and nutrient regimes in small streams, which in turn may influence grazer-periphyton interactions. To investigate the mechanisms by which forest disturbance affects grazer-periphyton interactions, we conducted an experiment to test for effects of light, nutrients, and grazing by tailed frog tadpoles (Ascaphus truei) on periphyton standing crop. Light, nutrients, and tadpole density were manipulated in replicated, in situ, flow-through enclosures in two streams using a complete block design to test for effects on periphyton biomass and individual tadpole relative growth rates.
Light exerted a strong, positive effect on periphyton production resulting in a 30-40% increase in periphyton biomass over the shaded treatment. Tadpole grazing significantly decreased periphyton biomass at both streams. Mean growth rate of tadpoles was 45% higher at Klondike Creek than at Dipper Creek. Light had a significant positive effect on tadpole growth resulting in a 14% increase over the shaded treatment. Tadpole density had a negative effect on tadpole growth and accounted for a greater percentage of the overall variation than did light. Periphyton production was under simultaneous control by light and tadpole grazing, however the relative importance of these top-down vs. bottom-up effects was context-dependent. At Klondike Creek periphyton production was influenced primarily by grazing, however at Dipper Creek periphyton production was more strongly influenced by light availability. The strong, positive effect of light on periphyton production and tadpole relative growth rate indica tes a tight trophic coupling between grazers and their algal food resource, and a potential positive indirect effect of light on grazers.
The decrease in tadpole growth rates with increasing tadpole density indicates that tadpoles are subject to intraspecific density-dependence, consistent with food-limitation. This study demonstrates that both bottom-up and top-down forces may act in unison to control periphyton production and grazer growth rate, and that the relative importance of these factors may vary significantly between nearby streams.
Key words: Ascaphus; grazer-periphyton interactions; density dependent interactions; light; nutrients; periphyton; headwater stream; tadpole; top-down vs. bottom-up forces.