Ecological Applications, 18(2), 2008, pp. 511–526 Ó 2008 by the Ecological Society of America EXPERIMENTAL FLOODS CAUSE ECOSYSTEM REGIME SHIFT IN A REGULATED RIVER CHRISTOPHER T. ROBINSON 1 AND URS UEHLINGER Department of Aquatic Ecology, EAWAG, 8600 Duebendorf, Switzerland, and Institute of Integrative Biology, ETH Zu ¨rich 8092 Switzerland Abstract. Reservoirs have altered the flow regime of most rivers on the globe. To simulate the natural flow regime, experimental floods are being implemented on regulated rivers throughout the world to improve their ecological integrity. As a large-scale disturbance, the long-term sequential use of floods provides an excellent empirical approach to examine ecosystem regime shifts in rivers. This study evaluated the long-term effects of floods (15 floods over eight years) on a regulated river. We hypothesized that sequential floods over time would cause a regime shift in the ecosystem. The floods resulted in little change in the physicochemistry of the river, although particulate organic carbon and particulate phosphorus were lower after the floods. The floods eliminated moss cover on bed sediments within the first year of flooding and maintained low periphyton biomass and benthic organic matter after the third year of flooding. Organic matter in transport was reduced after the third year of flooding, although peaks were still observed during rain events due to tributary inputs and side slopes. The floods reduced macroinvertebrate richness and biomass after the first year of floods, but density was not reduced until the third year. The individual mass of invertebrates decreased by about one-half after the floods. Specific taxa displayed either a loss in abundance, or an increase in abundance, or an increase followed by a loss after the third year. The first three flood years were periods of nonequilibrium with coefficients of variation in all measured parameters increasing two to five times from those before the floods. Coefficients of variation decreased after the third year, although they were still higher than before the floods. Analysis of concordance using Kendall’s W confirmed the temporal changes observed in macroinver- tebrate assemblage structure. An assessment of individual flood effects showed that later floods had ;30% less effect on macroinvertebrates than early floods of similar magnitude, suggesting that the new assemblage structure is more resilient to flood disturbance. We conclude that the floods caused an ecosystem regime shift that took three years to unfold. Additional long-term changes or shifts are expected as new taxa colonize the river from other sources. Key words: ecosystem regime shift; experimental flooding; flood disturbance, resilience or resistance; macroinvertebrates; river flow regime; temporal scale. INTRODUCTION Flow is arguably one of the most important master variables of river ecosystems (Poff et al. 1997, Hart and Finelli 1999, Malard et al. 2006). Most aquatic organisms have evolved characteristic biological traits adapted to the natural flow regime of free-flowing rivers (Lytle and Poff 2004). For example, fishes have specialized morphologies that reflect flow regimes in different types of rivers (Welcomme 1985, Poff and Allan 1995), macroinvertebrates have evolved life history traits adapted to temporary waters (Williams and Hynes 1977, Williams 1996, Stanley et al. 1997), and larger biota such as alligators and turtles have evolved life cycles in rivers that experience an annual flow pulse (Junk et al. 1989, Robinson et al. 2002). Floods are an inherent feature of the natural flow regime of many rivers that impose strong ecological and evolutionary constraints on flowing water organisms (Poff et al. 1997). For instance, floods are important drivers in the relationship of disturbance and produc- tivity gradients on biodiversity patterns in rivers (Biggs et al. 1999, Cardinale et al. 2006), and many benthic macroinvertebrates have evolved life cycles to account for predictable seasonal floods such as those associated with spring runoff (Robinson and Minshall 1998). The effects of single flood events, such as reductions in organism densities or standing stocks, on aquatic flora and fauna, whether predictable or unpredictable, have also been well-documented (Resh et al. 1988, Boulton et al. 1992, Matthaei et al. 1997, Robinson et al. 2003). In contrast, there is a paucity of information on the ecological and evolutionary influence of experimental floods on river ecosystems (Robinson and Uehlinger 2003). Besides being excellent tests of ecological and evolutionary theory, this kind of information is espe- Manuscript received 30 May 2007; revised 31 July 2007; accepted 14 August 2007. Corresponding Editor: C. Nilsson. 1 E-mail: robinson@eawag.ch 511