Resource quality and stoichiometric constraints on stream ecosystem functioning

Authors

  • SALLY HLADYZ,

    1. Environmental Research Institute, Department of Zoology, Ecology & Plant Science, University College Cork, Lee Maltings Building, Prospect Row, Cork, Ireland
    2. CSIRO Land and Water, Murray-Darling Freshwater Research Centre, Wodonga, Vic., Australia
    Search for more papers by this author
  • MARK O. GESSNER,

    1. Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and Institute of Integrative Biology (IBZ), ETH Zurich, Dübendorf, Switzerland
    Search for more papers by this author
  • PAUL S. GILLER,

    1. Environmental Research Institute, Department of Zoology, Ecology & Plant Science, University College Cork, Lee Maltings Building, Prospect Row, Cork, Ireland
    Search for more papers by this author
  • JEŚUS POZO,

    1. Department of Plant Biology and Ecology, Faculty of Science and Technology, The University of The Basque Country, Bilbao, Spain
    Search for more papers by this author
  • GUY WOODWARD

    1. Environmental Research Institute, Department of Zoology, Ecology & Plant Science, University College Cork, Lee Maltings Building, Prospect Row, Cork, Ireland
    2. School of Biological and Chemical Sciences, Queen Mary University of London, London, U.K.
    Search for more papers by this author

Dr Guy Woodward, School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, U.K. E-mail: g.woodward@qmul.ac.uk

Summary

1. Resource quality and stoichiometric imbalances in carbon : nutrient ratios between consumers and resources can influence key ecosystem processes. In many streams, this has important implications for food webs that are based largely upon the utilization of terrestrial leaf-litter, which varies widely among litter types in its value as a food source for detritivores and as a substrate for microbial decomposers.

2. We measured breakdown rates and macroinvertebrate colonization of leaf-litter from a range of native and exotic plants of differing resource quality and palatability to consumers [e.g. carbon : nitrogen : phosphorus (C : N : P) ratios, lignin and cellulose content], in a field experiment. We also measured C : N : P ratios of the principal leaf-shredding invertebrates, which revealed strong stoichiometric imbalances across trophic levels: C : N and C : P ratios typically differed by at least one order of magnitude between consumers and resources, whereas N : P imbalances were less marked. Application of the threshold elemental ratio approach, which integrates animal bioenergetics and body elemental composition in examining nutrient deficiency between consumers and resources, revealed less marked C : P imbalances than those based on the simpler arithmetic differences described above.

3. Litter breakdown rates declined as nutrient imbalances widened and resource quality fell, but they were independent of whether resources were exotic or native. The principal drivers of total, microbial and invertebrate-mediated breakdown rates were lignin : N, lignin : P and fungal biomass, respectively. However, multiple regression using orthogonal predictors yielded even more efficient models of litter breakdown, as consumers responded to more than one aspect of resource quality. For example, fungal biomass and litter C : N both influenced invertebrate-mediated breakdown.

4. Large stoichiometric imbalances and changes in resource quality are likely to have serious consequences for stream ecosystem functioning, especially when riparian zones have been invaded by exotic plant species whose chemical composition differs markedly from that of the native flora. Consequently, the magnitude and direction of change in breakdown rates and, thus, resource depletion, will be driven to a large extent by the biochemical traits (rather than taxonomic identity per se) of the resident and invading flora.

Ancillary