Beyond biomass: Resource effects on primary production and consumer nutrient assimilation in streams

Abstract

Abstract Primary production in aquatic ecosystems is strongly controlled by resource availability (bottom‐up). At the same time, grazers exert top‐down pressure on algae. The effects of both resources and grazers on algal communities have been investigated extensively for both benthic and planktonic systems. However, most studies focus exclusively on net effects on algal standing stock, while the underlying ecological functions and processes often remain unknown. Here, we tested the effects of the two resources, light (L) and the limiting nutrient phosphorus (P), on both primary production (net oxygen production) by benthic algae (periphyton) and matter assimilation by the next trophic guild (particularly nitrogen assimilation by grazers). For this, we grew natural periphyton communities in stream mesocosm flumes together with two native grazer taxa and 15N as a tracer. Both resources resulted in an initial increase in net primary production, but this effect equalised after 36 days in the more mature communities. While both resources still showed strong positive effects on algal biovolume, the biovolume‐specific net primary production decreased with resource level and was lowest in the double elevated (+L, +P) treatment. The response of nitrogen assimilation by grazers was more closely related to the response of periphyton quality (stoichiometric and taxonomic composition) than to its biomass. In addition, light affected the nitrogen assimilation of both grazers negatively. These findings demonstrate that the responses of ecological functions, here primary production and assimilation by grazers, are not necessarily proportional to algal biomass and are thus not easily predictable from the common use of biomass as a proxy for functions.