Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms

Abstract

1. Anthropogenic activities in prairie streams are increasing nutrient inputs and altering stream communities. Understanding the role of large consumers such as fish in regulating periphyton structure and nutritional content is necessary to predict how changing diversity will interact with nutrient enrichment to regulate stream nutrient processing and retention. 2. We characterised the importance of grazing fish on stream nutrient storage and cycling following a simulated flood under different nutrient regimes by crossing six nutrient concentrations with six densities of a grazing minnow (southern redbelly dace, Phoxinus erythrogaster) in large outdoor mesocosms. We measured the biomass and stoichiometry of overstory and understory periphyton layers, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles to evaluate whether fish were selectively foraging within or among habitats. 3. Model selection indicated nutrient loading and fish density were important to algal composition and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favoured diatom growth over green algae and decreased periphyton C : N. Increasing grazer density did not affect biomass and reduced the C : N of overstory, but not understory periphyton. Algal composition of dace diet was correlated with available algae, but there were proportionately more diatoms present in dace guts. We found no correlation between fish egestion ⁄ excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. 4. Large grazers and nutrient availability can have a spatially distinct influence at a microhabitat scale on the nutrient status of primary producers in streams.

Increasing fish taxonomic and functional richness affects ecosystem properties of small headwater prairie streams

“…Indeed, significant treatment 9 habitat interactions were apparent for four of the five ecosystem response variables. Moreover, Kohler et al (2011) showed that diet of dace in experimental stream mesocosms reflects the riffle algal community composition more than the pool community composition. Macroinvertebrate density also was greater in riffles than pools with two grazers present.…”

“…On the basis of previous experiments by our group quantifying trophic interactions and nutrient dynamics in prairie streams (Gido, Dodds & Eberle, 2010;Kohler et al, 2011), we hypothesised that effects of increased fish richness and corresponding compositional changes on measured ecosystem properties would depend on the relative importance of direct versus indirect effects (Fig. On the basis of previous experiments by our group quantifying trophic interactions and nutrient dynamics in prairie streams (Gido, Dodds & Eberle, 2010;Kohler et al, 2011), we hypothesised that effects of increased fish richness and corresponding compositional changes on measured ecosystem properties would depend on the relative importance of direct versus indirect effects (Fig.…”

Quantifying the top-down and bottom-up effects of a non-native grazer in freshwaters

“…Understanding how these anthropogenic stressors act synergistically in rivers and streams is required to predict how nutrient enrichment will interact with changing biodiversity to regulate instream nutrient dynamics (Kohler et al 2011).…”

The Stream Biome Gradient Concept: factors controlling lotic systems across broad biogeographic scales

“…Information comparing effects of stream consumers on ecosystem structure and function across biomes is limited. Grazing fishes can have strong effects on periphyton communities and nutrient cycling in desert (Grimm 1988) and grassland streams (Power et al 1985, Murdock et al 2010, Kohler et al 2011) that have relatively high rates of autochthonous production. The role of shrimp and crayfish in litter processing probably increases in forested tropical and temperate streams (Huryn andWallace 1987, Crowl et al 2001).…”