Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

Abstract

Summary We conducted a paired large‐scale predation experiment over 32 months in two streams being seasonally shaded by deciduous riparian trees, using the benthivorous fish species gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) as top predators. The biomass of benthic grazers and periphyton in the presence/absence of fish was measured and the periphyton production was compared with the consumption rates using a model‐based approach. A three‐level trophic cascade from benthivorous fish via benthic grazers to periphyton was evident from the field experiment. Integrated over the whole study period, fish reduced the biomass of benthic grazers and indirectly increased the periphyton biomass. Scenario analyses, using a simple dynamic model, indicated top‐down control of periphyton to be strongest during autumn, when periphyton growth was light‐limited, and weaker in the spring, when periphyton growth was not light‐limited. The seasonal light supply variation was caused by shading due to deciduous riparian trees during the vegetation period. This asymmetry in temporal processes weakened the top‐down control in a natural benthic community. Even though grazer biomass is naturally reduced in summer, due to the emergence of the most abundant species (mayflies), a grazer biomasses high enough to reduce the spring periphyton peak could not be sustained by the low summer periphyton growth. We suppose that the temporal decoupling of grazer biomass from periphyton biomass might be caused by the very short generation time of the primary producers (days) compared with the long generation time of the primary consumers (mostly 1 year).