Aquatic Botany 116 (2014) 19–26 Contents lists available at ScienceDirect Aquatic Botany jou rn al hom ep age: www.elsevier.com/locate/aquabot Avoidance of hydrological disturbance by aquatic vegetation in the ﬂoodplain of a large upland river Antoine A. Keruzoré ∗ , Nigel J. Willby Biological and Environmental Sciences, School of Natural Science, University of Stirling, Stirling FK9 4LA, Scotland, UK a r t i c l e i n f o Article history: Received 18 February 2012 Received in revised form 18 December 2013 Accepted 10 January 2014 Available online 23 January 2014 Keywords: Macrophyte Backwater Connectivity Biomass Flood a b s t r a c t Most studies suggest that ﬂoods remove substantial plant biomass due to mechanical forces applied during peak ﬂows, thus contributing to the structuring of riverine vegetation. Effects on biomass were tested via an in situ experiment in the backwater of a large upland river in Scotland where frequency of connection to the main channel during ﬂoods controlled exposure to potential mechanical disturbance. Four macrophyte species (Potamogeton natans L., Myriophyllum alterniﬂorum DC., Ranunculus ﬂammula L. and Mentha aquatica L.) were grown in trays and exposed to ﬂoods of different amplitude. Trays were distributed between an adjacent non-ﬂooded control and the intermittently ﬂooded backwater that dif- fered principally in exposure to ﬂoods. The four taxa combined post-ﬂooding biomass was surprisingly insensitive to ﬂoods, including two large events with recurrence intervals of 10–30 years. The four species showed different responses to ﬂooding but only M. aquatica experienced a signiﬁcant but small biomass reduction relative to control. Differences in biomass between control and backwater were mostly non- signiﬁcant and did not vary with ﬂood amplitude or spatially within the ﬂooded backwater, with the exception of amphibious species that were disproportionately affected in the most disturbed upstream part. The macrophyte assemblage was generally more sensitive to winter than summer ﬂoods. This study indicates that macrophytes can limit signiﬁcant biomass loss during major ﬂoods, and that this as with other disturbances, is likely to promote species coexistence. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In freshwater ecosystems, water movement (waves or currents) can inﬂuence macrophyte communities both directly and indi- rectly. The hydrodynamic forces exerted on rooted plants are the principal direct effect of water movement (Bornette and Puijalon, 2010). The consequences for plants can be multiple, ranging from no damage, partial or complete breakage, to uprooting, and depend on the magnitude of the hydrodynamic forces and the size and capacity of the plant to resist breakage and uprooting forces (Schutten et al., 2005). While some species may reduce the hydro- dynamic forces they sustain through reconﬁguration, others may break the above ground organs and thus protect the underground organs from uprooting (Usherwood et al., 1997). Water movements also have indirect effects on macrophytes by scouring or depositing sediment, resulting in uprooting or burial of plants or the associ- ated seedbank (Combroux and Bornette, 2004), through traction of ∗ Corresponding author. Tel.: +44 1786 466542; fax: +44 1786 467843. E-mail addresses: antoine.keruzore@gmail.com (A.A. Keruzoré), n.j.willby@stir.ac.uk (N.J. Willby). woody debris, and by the re-suspension of sediments leading to elevated turbidity. In rivers ﬂoods are considered a major disturbance and key fac- tor in structuring and modifying morphology and associated biota (Ward et al., 1999; van Geest et al., 2003; Gurnell et al., 2012). Floods entail extreme and fast-changing ﬂow conditions and are characterised by timing, frequency, duration and amplitude in both discharge and velocity. A naturally dynamic hydrological regime promotes lateral instability and drives the formation or destruc- tion of ﬂoodplain habitats, including oxbows and palaeo-channels (Amoros and Bornette, 2002). In this paper we use the term ‘backwaters’ to refer to former river channels in which upstream connection to the main stem is progressively lost through alluvial sediment and woody debris deposition (Petts and Amoros, 1996). At base ﬂow, the most commonly experienced ﬂow condition in back- waters, only a downstream connection to the river corridor exists (van der Nat et al., 2003) thus providing standing water conditions in an otherwise ﬂuvial environment. This restricted connectivity to the main river where ﬂows are stronger and more variable presents ideal conditions for aquatic plant colonisation and growth. Indeed backwaters accumulate macrophytes along large rivers and rep- resent a major reservoir of ﬂoodplain biodiversity (Abernethy and http://dx.doi.org/10.1016/j.aquabot.2014.01.005 0304-3770/© 2014 Elsevier B.V. All rights reserved.