Data from: Seasonal shifts in the importance of bottom-up and top-down factors on stream periphyton community structure

We examined the importance of temporal variability in top-down and
bottom-up effects on the accumulation of stream periphyton, which are
complex associations of autotrophic and heterotrophic microorganisms.
Periphyton contributes to primary production and nutrient cycling and
serves as a food resource for herbivores (grazers). Periphyton growth is
often limited by the availability of nitrogen and phosphorus, and biomass
can be controlled by grazers. In this study we experimentally manipulated
nutrients and grazers simultaneously to determine the relative
contribution of bottom-up and top-down controls on periphyton over time.
We used nutrient diffusing substrates to regulate nutrient concentrations
and an underwater electric field to exclude grazing insects in three
sequential 16-17 day experiments from August to October in montane
Colorado, USA. We measured algal biomass, periphyton organic mass, and
algal community composition in each experiment and determined densities of
streambed insect species, including grazers. Phosphorus was the primary
limiting nutrient for algal biomass, but it did not influence periphyton
organic mass across all experiments. Effects of nutrient additions on
algal biomass and community composition decreased between August and
October. Grazed substrates supported reduced periphyton biomass only in
the first experiment, corresponding to high benthic abundances of a
dominant mayfly grazer (Rhithrogena spp.). Grazed substrates in the first
experiment also showed altered algal community composition with reduced
diatom relative abundances, presumably in response to selective grazing.
We showed that top-down grazing effects were strongest in late summer when
grazers were abundant. The effects of phosphorus additions on algal
biomass likely decreased over time because temperature became more
limiting to growth than nutrients, and because reduced current velocity
decreased nutrient uptake rates. These results suggest that investigators
should proceed with caution when extending findings based on short-term
experiments. Furthermore, these results support the need for additional
seasonal-scale field research in stream ecology.