Seasonal shifts in the importance of bottom–up and top–down factors on stream periphyton community structure

Abstract

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.

Composición y estructura de la comunidad de algas perifíticas del río Alvarado y algunos tributarios (Tolima, Colombia)

“…A pesar de no presentar diferencias significativas a nivel espacial es importante mencionar que la localidad racv presentó la mayor densidad en la cuenca del río Alvarado. Esto se puede atribuir, principalmente, a la moderada velocidad del caudal (0,262 m 3 /s) [35], [36], y a la influencia de algunos parámetros fisicoquímicos, como, por ejemplo, coliformes totales [36] y fósforo [37], que favorecen el establecimiento y desarrollo de un gran número de organismos. Por otra parte, la baja densidad observada para la localidad qche se debe, probablemente, a la intervención antrópica, producto de la extracción mineral observada en campo durante los muestreos realizados, la cual influye drásticamente en el establecimiento de la comunidad fitoperifítica.…”

Do Electrofishing Activities Disrupt Stream Biofilm Standing Stocks? An Assessment from Two Headwater Streams in Western Oregon

“…Although the current study cannot clearly separate the effects of the electrical current from those that are associated with wading, we hypothesize that the effects of electrofishing activities on biofilms that were documented here are due primarily to wading in the stream. We hypothesize this because studies have used electricity in streams explicitly in research on aquatic biofilm to exclude grazers and thereby quantify top‐down control on benthic biofilms (Opsahl et al 2003; Lourenço‐Amorim et al 2014; Beck et al 2019). In those studies, electricity is not identified as a factor limiting primary production.…”

“…In one example, electrified tiles where grazers were excluded had biofilm standing stocks that were two to three times greater than those on control tiles, with the highest biofilm abundance occurring closest to the electrical wires (Opsahl et al 2003). Beck et al (2019) evaluated the potential effect of electrical fields on biofilms in their study and found some differential responses among a few taxa in the algal community but no significant overall effects of the electrical field on biofilm ash‐free dry mass.…”

Taxonomic Shift Over a Phosphorus Gradient Affects the Stoichiometry and Fatty Acid Composition of Stream Periphyton

“…Furthermore, the nutritional quality of periphyton may indirectly influence the strength of top‐down control. Grazer behavior is known to affect periphyton community structure through selective feeding and nutrient regeneration (Mulholland et al 1991, Rosemond et al 2000, Beck et al 2019); fluctuations in nutrient availability are likely to interact with top‐down control by affecting grazer behavior, as in, e.g., compensatory feeding (Iannino et al 2019). Future studies should focus on how long‐term bottom‐up effects would alter grazer growth and behavior, and how this would in turn influence periphyton biomass and nutritional quality.…”