Aquatic grazers reduce the establishment and growth of riparian plants along an environmental gradient G. F. (CISKA) VEEN* ,† , JUDITH M. SARNEEL*, LONE RAVENSBERGEN*, NAOMI HUIG*, JOS  E VAN PAASSEN*, WINNIE RIP ‡ AND ELISABETH S. BAKKER* *Department of Aquatic Ecology, Netherlands Institute of Ecology, Wageningen, The Netherlands † Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Ume  a, Sweden ‡ Waternet, Amsterdam, The Netherlands SUMMARY 1. The establishment of riparian plants is determined by abiotic conditions and grazing, although it is usually presumed that the former are most important. We tested the impact of aquatic grazers on the survival and growth of establishing riparian plants and whether the impact of grazing interacts with abiotic conditions. 2. We conducted an experiment across 10 Dutch wetlands, covering a large range of water depth and nutrient availability. We introduced 1-year-old plants of an emergent (common reed, Phragmites australis) and a floating (water soldier, Stratiotes aloides) species in individual enclosures (n = 5 per site) that excluded predominantly waterbirds, which were the most abundant grazers, and on adja- cent unprotected plots. Survival and growth were measured during one growing season. 3. Grazing reduced growth (as biomass) of Phragmites and Stratiotes by a mean of 25 and 60%, respectively. Grazing decreased survival of Stratiotes, but not of Phragmites. Shallow water, water-level fluctuations, eutrophic conditions and enough light favoured both growth and survival of Phragmites. Growth of Stratiotes was unaffected by these factors, but they reduced its survival. For both species, grazing effects on biomass were consistent across environmental conditions, but for Phragmites, graz- ing effects on survival were influenced by abiotic conditions. 4. We conclude that aquatic grazers significantly reduce the establishment and growth of macro- phytes in the riparian zone over a wide range of environmental conditions. Keywords: herbivore, macrophyte, plant–herbivore interactions, trophic gradient, wetland ecology Introduction Natural riparian zones, the transition between aquatic and terrestrial ecosystems, harbour a high floral and faunal diversity and can play a key role in the function- ing of both aquatic and terrestrial ecosystems (Naiman, Decamps & Pollock, 1993; Naiman & Decamps, 1997; Nilsson & Svedmark, 2002). Riparian zones are habitats that cover large environmental gradients. Consequently, macrophyte establishment, community composition and diversity are strongly driven by abiotic conditions (Scheffer et al., 2001; Bornette & Puijalon, 2011; Bakker et al., 2013). However, the importance of abiotic factors does not exclude the possibility that grazing can struc- ture riparian plant communities. Aquatic grazers, particularly waterfowl and fish, affect the biomass (e.g. Hilt, 2006; Rodriguez-Perez & Green, 2006; Hidding et al., 2009) and community composition (Pipalova, 2002; Hidding et al., 2010) of submerged aquatic vegetation in lakes and ponds. Whereas water- fowl can also affect the growth of emergent macrophytes (Chaichana, Leah & Moss, 2011; Dingemans, Bakker & Bodelier, 2011), data on the impact of grazing by birds or mammals on emergent plants are scarce and on floating macrophytes are absent to our knowledge. The impact of grazers on the establishment and growth of macrophytes could interact with abiotic conditions, for example, water depth (Taft et al., 2002; Coops, Vulink & van Nes, 2004) and nutrient availability, although evidence for such interactions is currently lacking (Gayet et al., 2011). Correspondence: Ciska Veen, Department of Aquatic Ecology, Netherlands Institute of Ecology, P.O. Box 50, 6700 AB, Wageningen, The Netherlands. E-mail: c.veen@nioo.knaw.nl 1794 © 2013 John Wiley & Sons Ltd Freshwater Biology (2013) 58, 1794–1803 doi:10.1111/fwb.12168