Periphyton response to nutrient enrichment in a stoneroller-dominated world

We compared the effects of phosphorus (P) enrichment and grazing by central stonerollers on standing stocks and stoichiometry of periphyton.  Herbivorous fish are known to interact strongly with benthic algal communities.  However, few studies have examined how fish alter periphyton response to nutrient enrichment.  Twelve artificial streams were randomly assigned to one of three different P concentrations (8, 20 and 100 ppb PO4-P) and dosed appropriately.  Within pools of each stream we established grazed and un-grazed areas using fish exclusion cages.   AFDM was limited by grazing on day 14 but this effect diminished by the end of the study (day 28).  Chlorophyll a was enhanced by grazing on both sampling dates.  Neither biomass measure responded to P enrichment under grazing pressure.  Grazing enhanced periphyton carbon (C), nitrogen (N), and P content on both sampling dates.  Periphyton N and P content responded to P enrichment in grazed treatments.  Molar C:P and N:P ratios responded strongly to P enrichment, regardless of grazing treatment.  Our results support the use of stoichiometric measures as robust tools for assessing nutrient enrichment in streams.