Effect of Macrograzers (Campostoma spp. and Faxonius spp.) on Periphyton in Ozark Streams with Considerations Given to Macrograzer Biomass, Phosphorus, and Season: Mensurative and Manipulative Studies

Nutrient and benthic algal biomass relationships can guide numeric nutrient criteria development in lotic systems. However, herbivorous macrograzers may confound this relationship by reducing the slope of the positive relationship between nutrients and periphyton biomass in streams. I conducted a mensurative field study to determine if stoneroller and crayfish abundance related to algal biomass at varying nutrient concentrations and manipulated macrograzer presence with electrical exclosures in streams to examine macrograzer effects on algal biomass and understand whether these effects on periphyton varied with total phosphorus (TP) or season. Macrograzer density was quantified across a TP gradient (n=15 streams; range = 0.009-0.100 mg TP/L) in August 2015 and manipulative exclosure experiments were completed in a subset of these streams in late summer 2016 (n=5, range = 0.0025-0.140 mg TP/L) and winter 2017 (n=3, range 0.001-0.49 mg TP/L). Multiple linear regression of mensurative field data revealed a statistically-significant positive relationship between macrograzer abundance, nutrient levels, and algal biomass. In the manipulative experiments, macrograzer presence did not influence algal biomass. Benthic algal biomass did vary over time in 3 of the 5 stream, which may be due to stream flow permanence or intermittence. Macrograzer effect on algal biomass was not significant and did not depend on TP or season. The positive relationship between macrograzers on algal biomass measures in the mensurative abundance study was not corroborated by the manipulative experiment, but indirect positive effects of macrograzers may have been equal to direct negative consumptive effects making grazed and ungrazed algal biomass equal. Many previous studies observed a negative influence of macrograzers on filaments lengths of benthic algae with mixed effect on chlorophyll a and ash-free dry mass measures (algal biomass measures), but no or positive relationships were found in the present study. Effects abundance and exclosure study may indicate indirect stimulatory effects of macrograzers on stream algal biomass where high abundances of macrograzers and TP may produce systems dominated by autotrophic processes. These studies highlight the importance of considering indirect effects of macrograzers on periphyton biomass