Submitted to: Freshwater Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2010
Publication Date: 6/1/2011
Citation: Kohler, T.J., Murdock, J.N., Gido, K.B., Dodds, W.K. 2011. Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms. Freshwater Biology. 56(6):1133-1146. Interpretive Summary: Human activities have greatly altered the physical, chemical, and biological makeup of streams including nutrient concentrations and diversity of organisms. Understanding how these changes alter a stream's ability to retain and process nutrients is unclear. We examined the role of a common grazing fish in regulating algal growth, nutrient storage, and rates of nutrient cycling in experimental streams across a gradient of nutrient additions. Nutrient loading had a much greater effect than fish on algal biomass, significantly increasing biomass. However, both fish and nutrients increased the nitrogen content of periphyton and changed algal composition. The rate of nutrient recycling by fish (through eating algae and excretion) was not affected by nutrient loading rates, possibly decreasing the importance of this nutrient source to algae as nutrient inputs increase. Our results suggest that the biotic element in streams may play an important role to the nutrient cycling in freshwater ecosystems.
Technical Abstract: 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. We characterized 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, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles. Model selection indicated significant interactions between nutrient loading and fish density for algal biomass and composition, and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favored diatom growth over green algae, and decreased periphyton C:N. Increasing grazer density did not affect biomass, but reduced the C:N of overstory, but not understory periphyton. Algal composition of dace diet was similar to available algae, but more diatoms were present in guts. There was no correlation among fish egestion or excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. These results demonstrate that large grazers can have a spatially distinct influence on the nutrient status of primary producers in streams.