Leaf-litter decomposition and microbial responses to light and macroinvertebrate consumer manipulations in experimental streams
Light availability and macroinvertebrate consumers are strong bottom-up and top-down controls, respectively, in aquatic ecosystems. However, these 2 factors are rarely examined in combination, particularly regarding detrital heterotrophic processes such as organic matter decomposition. We conducted separate experiments manipulating light and presence of the omnivorous shrimp Macrobrachium ohione (S. I. Smith, 1874) and the shredder-detritivore caddisfly Pycnopsyche sp. to test effects of light availability and consumer presence on decomposition of Liriodendron tulipifera L., a fast-decaying leaf species, in outdoor experimental streams. We measured litter algal, fungal, and bacterial biomass and production, enzymatic activity, and decomposition rates over 6 wk during each experiment. Both experiments demonstrated positive algal responses to light; during the Macrobrachium experiment, light also increased fungal growth rates and bacterial biomass. During the Pycnopsyche experiment, light reduced bacterial production rates and beta-glucosidase activity. Despite these strong light effects on microbial heterotrophy, litter decomposition rates did not respond to light availability, indicating minimal algal priming effects, and decomposition only increased in the presence of Pycnopsyche, regardless of light treatment. In both experiments, consumer presence did not elicit strong microbial responses, with the exception of the timing of algal and bacterial colonization of litter during the Macrobrachium experiment. Under the conditions of our study, our results affirm that light elicits a strong bottom-up control on microbial interactions during litter decomposition. In turn, consumer top-down effects depend on feeding mode and are more likely independent from light availability. These findings advance understanding of the interplay between light availability and foodweb structure in shaping aquatic ecosystem processes.
Link to Published Version
Brady, C. M., Bonney, J., Francoeur, S. N., Halvorson, H. M., & Kuehn, K. A. (2021). Leaf-litter decomposition and microbial responses to light and macroinvertebrate consumer manipulations in experimental streams. Freshwater Science, 40(2), 340–353. https://doi.org/10.1086/714516