Intermittency Mediates Macroinvertebrate And Crayfish Effects On Leaf Breakdown In Temperate Headwater Streams

Changes in climate and water management are expected to result in increased stream drying (i.e., intermittency) in many regions, and these changes could affect ecological processes. Aquatic detritivores process organic matter that supports C cycling. Crayfish can contribute substantially to detrital processing; however, intermittency and biological communities interact to alter detrital pathways and processing rates. Our goals were to evaluate the effect of intermittency on leaf breakdown and to provide empirical evidence regarding whether the role of shredders, including crayfish, changes with intermittency. We hypothesized that different factors would control leaf breakdown in intermittent and perennial stream segments because stream flow and duration affect heterotrophic microbes and macroinvertebrate shredders. We predicted: 1) faster leaf breakdown in perennial segments compared to intermittent segments because of greater processing by microbes and shredders and more physical abrasion from increased discharge; 2) crayfish with access to both detritus and macroinvertebrate prey would function primarily as predators in perennial segments, potentially slowing decomposition; and 3) intermittent segments would have fewer macroinvertebrate shredders and, therefore, crayfish would function as shredders and promote leaf breakdown. To investigate how intermittency and macroinvertebrate assemblages interact to affect leaf breakdown, we used a 53-d experiment with 3 treatments: 1) open bags allowing crayfish and macroinvertebrates access, 2) closed bags with an added crayfish and allowing all macroinvertebrates access, and 3) closed bags allowing only non-crayfish macroinvertebrates access. Leaf breakdown was faster in perennial stream segments likely because of greater macroinvertebrate processing. Because breakdown rates did not differ substantially across treatments, there was no evidence that crayfish suppressed macroinvertebrate shredder processing. However, breakdown rates tended to be higher when crayfish had access to leaf litter, even in intermittent segments where crayfish densities were low. Our results suggest that crayfish were more important than other macroinvertebrates to leaf breakdown in intermittent segments. As hydrologic regimes of perennially flowing streams are modified, increased intermittency could slow decomposition by changing crayfish density and macroinvertebrate assemblages.