Nutrient release rates and ratios by two stream detritivores fed leaf litter grown under elevated atmospheric CO2

We examined how nutrient release by two common stream detritivores, Asellus and Gammarus, was affected by the consumption of aspen leaf litter from trees grown under elevated CO2. We measured excretory release of dissolved organic carbon (DOC), ammonia (NH4), and soluble reactive phosphorus (SRP) from consumers fed senesced leaves of Populus tremuloides (trembling aspen) trees grown under elevated (720 ppm) and ambient (360 ppm) CO2. Contrary to predictions based on ecological stoichiometry, elevated CO2 leaves caused greater NH4 and SRP release from both animals but did not affect the release of DOC. Elevated CO2 leaves reduced DOC : NH4 and DOC : SRP ratios released from Asellus but did not affect these ratios from Gammarus. Both animals showed lower NH4 : SRP release ratios after eating elevated CO2 leaves. A mass balance model of consumer N and P release demonstrated that increased excretion rates likely resulted from reduced absorption efficiencies (and unchanged or higher digestive efficiencies) in these aquatic detritivores. Our results indicate that changes in leaf biochemistry resulting from elevated atmospheric CO2 will strongly affect the ability of stream consumers to retain important biogenic elements. Increased release rates of NH4 and SRP are another indication, along with reduced growth and reproduction, that litter produced under elevated CO2 has strong effects on key physiological processes in detritivores with potentially strong consequences for nutrient cycling in streams of forested regions.