AME 28:25-36 (2002)  -  doi:10.3354/ame028025

Microbial dynamics associated with leaves decomposing in the mainstem and floodplain pond of a large river

Virginie Baldy1,*, Eric Chauvet1, Jean-Yves Charcosset1, Mark O. Gessner2

1Centre d¹Ecologie des Systèmes Aquatiques Continentaux, CNRS-UPS, 29 rue Jeanne Marvig, 31055 Toulouse Cedex, France
2Department of Limnology, EAWAG, Limnological Research Center, 6047 Kastanienbaum, Switzerland
*Present address: Laboratoire de Biosystématique et Ecologie Méditerranéenne, Institut Méditerranéen d¹Ecologie et de Paléoécologie, Faculté des Sciences et Techniques de St-Jérôme, Case 421 bis, Avenue Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France. E-mail:

ABSTRACT: Aquatic habitats of forested floodplain systems receive large inputs of allochthonous plant litter. We examined the decomposition of, and microbial productivity associated with, leaves of a common floodplain tree, Populus gr. nigra, in the mainstem and floodplain pond of a seventh order river in 2 consecutive years. Litter bags were submerged at both sites, retrieved periodically, and analyzed for litter mass loss, bacterial and fungal biomass, growth rate and production, and sporulation rates of aquatic hyphomycetes. Litter decomposition rates were similar in both sites and years (leaf breakdown coefficients k of 0.0070 to 0.0085 d-1), although microbial dynamics partly differed between sites. Species diversity of aquatic hyphomycetes was lower on leaves submerged in the pond (16 species) than in the river (21 species). Mycelial biomass was also significantly lower in the pond, with values <20 mgCg-1 of detrital C, whereas peaks of 50 and 80 mgC g-1 were reached in leaves in the mainstem. These differences contrast with the comparable fungal productivity at both sites (peak rates of 1.4 mg of mycelial C per g of detrital C per day in both years). This suggests that fungi were equally productive in both habitats but experienced greater losses in the pond. Bacterial numbers and biomass also showed the same basic pattern at both sites, although somewhat higher levels were reached in the pond (maximum of about 1010 cells and 0.5 mg g-1 of detrital C). Bacterial-specific production rates fluctuated between 0.06 and 1.5 d-1 with lower values occurring in the floodplain pond. Although bacteria on leaves were clearly outweighed by fungi in terms of biomass, they accounted for a sizeable fraction of the total biomass (up to 11%), and up to 32% of the total microbial production. Our comparison of bacterial and fungal productivity thus points to a critical role of fungi in litter decomposition in aquatic habitats of river floodplain systems, while suggesting that bacteria must not be overlooked as important agents of litter decompositon in riverine environments.

KEY WORDS: Fungi · Bacteria · Leaf litter · River · Floodplain · Decomposition · Microbial production

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