Decomposition of allochthonous material is a central process in organic matter budgets of low-order streams. Shredding invertebrates contribute substantially to the decomposition of leaves in northern-temperate-zone streams, but their role in tropical streams is less clear. Hydrology has an overarching control on ecological processes in streams, especially on floodplains, where flooding of the mainstem river may cause backflooding in the tributaries. Decomposition experiments with leaves of Cecropialatiloba, Tessariaintegrifolia, and Symmeriapaniculata were used to assess the importance of hydrology and benthic invertebrates to decomposition in a neotropical floodplain stream. Leaf breakdown rates and leaf-associated aquatic invertebrate assemblages were compared among leaf types in a typical 1st-order stream of the Amazonian whitewater floodplain forest near Leticia, Colombia. The stream hydrograph was influenced by regular rainfall throughout the year and by seasonal backflooding from the Amazon River. Stream discharge was more variable when the Amazon was low (flashy-discharge phase [FDP], September–November) than during the high-water period (backflooded phase [BFP], March–April). Cecropialatiloba and T. integrifolia decomposed much faster than S. paniculata; however, breakdown rates (mean k ± SD = 0.031 ± 0.003, 0.029 ± 0.006, and 0.010 ± 0.003, respectively) were significantly higher during FDP than during BFP (k = 0.009 ± 0.002, 0.009 ± 0.002, and 0.001 ± 0.003, respectively). Thirty-nine taxa of benthic invertebrates (mainly Chironomidae, Trichoptera, and Ephemeroptera) were identified from the litterbags. The tough leaves of C. latiloba were most densely colonized. Most invertebrate taxa (60%) were collectors, whereas shredders were very scarce (0–5%). The proportions of functional feeding groups did not differ significantly between leaf species and hydrologic period. Our results indicated that irregular hydrological pulses in these Neotropical streams had a significantly stronger impact on leaf breakdown than leaf-associated invertebrates.
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