Complex interactions of light, nutrients and consumer density in a stream periphyton–grazer (tailed frog tadpoles) system

Abstract

Summary Consumer–resource interactions may be modified by complex environmental interactions. We experimentally manipulated light, nutrients, consumer density (tailed frog tadpoles − Ascaphus truei), and enclosure sizes in two small and steep streams of coastal British Columbia, Canada. We used flow‐through, screened enclosures in situ as experimental units and measured the responses of algae (as chlorophyll a), periphyton (ash‐free dry mass) and growth rates of tailed frog tadpoles using a factorial model design. Light exerted a strong, positive effect on algae and periphyton production with 30–40% higher biomass over shaded treatments, and a 14% higher relative growth rate of tadpoles. Nutrient additions had little to no effect on any measure, alone or in interaction with other factors. Tadpole growth, algae and periphyton standing crop were significantly reduced along a gradient of increasing consumer density. Enclosure size had a large effect on all three measures. Algae and periphyton production were under simultaneous top‐down (tadpole grazing) and bottom‐up control (light); however, the relative importance of these effects was stream‐dependent. At Klondike Creek algae and periphyton production was influenced primarily by grazing, and by light levels at Dipper Creek. Mean growth rate of tadpoles was on average 45% higher at Klondike Creek than at Dipper Creek. Most of the effects of the abiotic treatments and consumer density interacted statistically with stream identity, despite these streams being very similar and < 6 km apart. The strong effect of stream on the outcomes may be a consequence of large temperature differences. This study demonstrated that light, consumer density, stream, enclosure size and nutrients alone and in combination affected algae and periphyton production, and grazer growth rate. While the interactions between consumer and resources were tightly coupled, the relative importance of the driving factors varied significantly between nearby streams.