Indirect Effects of a Shredding Invertebrate On Leaf Breakdown

Shredder-detritus interactions may differ with stream nutrient concentrations. Aquatic invertebrate shredders often break down lignocellulose, making refractory carbon more labile. We predicted increases in labile organic matter (LOM) from shredder processing of detritus would stimulate degradation of refractory organic matter by increasing labile carbon available to microbial decomposers, thus increasing their activity and indirectly increasing litter decomposition. We quantified decomposition rates of maple leaves with two levels of C:P, and with or without shredding Tipula larvae. We predicted greater mass loss of low C:P leaves than high C:P leaves, and in chambers with Tipula. Tipula were fed ad libidum for one week in chambers with mesh bottoms. Leaf disks were placed at the bottom of chambers under mesh, excluding Tipula consumption and allowing heterotrophic microbes. Mass loss was not different between leaf C:P levels (p=0.55) or Tipula treatments (p=0.48); yet mass loss trended to be greatest on high C:P leaves with Tipula, and least on high C:P leaves without Tipula (p=0.07). Shredder influence on decomposition is predicted to be both indirect and direct with strength of indirect effects determined by stream nutrient concentrations.