Water velocity limits the temporal extent of herbivore effects on aquatic plants in a lowland river

  • Kevin A. Wood
  • Richard A. Stillman
  • Ralph T. Clarke
  • Francis Daunt
  • Matthew T. O’Hare


The role of herbivores in regulating aquatic plant dynamics has received growing recognition from researchers and managers. However, the evidence for herbivore impacts on aquatic plants is largely based on short-term exclosure studies conducted within a single plant growing season. Thus, it is unclear how long herbivore impacts on aquatic plant abundance can persist for. We addressed this knowledge gap by testing whether mute swan (Cygnus olor) grazing on lowland river macrophytes could be detected in the following growing season. Furthermore, we investigated the role of seasonal changes in water current speed in limiting the temporal extent of grazing. We found no relationship between swan biomass density in 1 year and aquatic plant cover or biomass in the following spring. No such carry-over effects were detected despite observing high swan biomass densities in the previous year from which we inferred grazing impacts on macrophytes. Seasonal increases in water velocity were associated with reduced grazing pressure as swans abandoned river habitat. Furthermore, our study highlights the role of seasonal changes in water velocity in determining the length of the mute swan grazing season in shallow lowland rivers and thus in limiting the temporal extent of herbivore impacts on aquatic plant abundance.


Carry-over effects Mute swan Cygnus olor Plant senescence Trophic interactions Water velocity Waterfowl grazing and herbivory 



We thank the Freshwater Biological Association and the other riparian landowners for allowing access to the study river reaches and for logistical support. The Environment Agency provided daily water discharge data. Pete Scarlett and Lucy Mulholland kindly assisted with macrophyte species identification and data collection, respectively. Two anonymous reviewers provided valuable feedback on an earlier version of this manuscript. This study was funded by the Natural Environment Research Council (NERC) through a Centre for Ecology & Hydrology Algorithm studentship awarded to KAW (NEC3579).


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Copyright information

© Springer International Publishing Switzerland 2016

Authors and Affiliations

  1. 1.Wildfowl & Wetlands Trust, SlimbridgeGloucestershireUK
  2. 2.Centre for Ecology & HydrologyEdinburghUK
  3. 3.Department of Life & Environmental Sciences, Faculty of Science & TechnologyBournemouth UniversityPooleUK

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