Wednesday, June 6, 2007 - 9:15 AM
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The Bottom-up Effect of Nutrient Availability on Fish and Macroinvertebrate Stoichiometry and Nutrient Excretion

Ryan A. McManamay and Jackson R. Webster. Department of Biological Sciences, Virginia Tech, 2119 Derring Hall, Blacksburg, VA 24061

Resource nutrient availability can influence fish and macroinvertebrate consumers by: 1) altering body nutrient composition, or 2) altering nutrient excretion. This study was conducted to determine whether riparian canopy cover and background nutrient concentrations influenced macroinvertebrate and fish nutrient excretion. The study was conducted on six stream sections in North Carolina and Virginia. Streams were selected based on riparian density and water nitrogen and phosphorus concentrations. Forested streams ranged from 89 – 99% canopy cover while open streams ranged from 1 – 24%. Seven fish taxa and eleven macroinvertebrate taxa were collected and nutrient excretion was quantified. Macroinvertebrates of fish gut contents were identified to order. Consumer excretion varied with taxa. Predatory fish generally had higher N:P excretion rates in forested streams compared to open streams. This trend could possibly be explained by a higher number of stream vs. terrestrial insects consumed in forest reaches. Scrapers and collector/filterers generally had the highest nutrient excretion rates while shredders and predators had the lowest. Crayfish had the highest N:P excretion rate most likely due to lower body N:P. Heptageniid mayfly ammonia excretion was negatively related to in-stream NO3- concentration (r2 = 0.40) possibly reflecting plasticity in epilithon stoichiometry.