Migratory benthic fishes may induce regime shifts in a tropical floodplain pond ROGER P. MORMUL*, SIDINEI M. THOMAZ † , ANGELO A. AGOSTINHO † , CLAUDIA C. BONECKER † AND NESTOR MAZZEO ‡ *Po ´s-graduac ¸a ˜o em Ecologia de Ambientes Aqua ´ticos Continentais – PEA, Universidade Estadual de Maringa ´ – UEM, Maringa ´ – PR, Brazil † Departamento de Biologia, Nu ´ cleo de Pesquisas em Limnologia, Ictiologia e Aquicultura – Nupelia, Universidade Estadual de Maringa ´– UEM, Maringa ´ – PR, Brazil ‡ Departamento de Ecologı ´a & Evolucio ´n, CURE-Facultad de Ciencias, Universidad de La Republica, Maldonado, Uruguay SUMMARY 1. Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid- and clear-water conditions. Here, we investigate whether such shifts in a tropical floodplain pond may be related to the effect of the flood pulse regime on the community structures of fish and macrophytes. 2. Using a long-term data set, we demonstrate how benthic fish migration together with colonisation by submerged plants affected the transition from a turbid to a macrophyte-dominated state in a floodplain pond without top-down control. 3. In our study, the turbid state occurred mostly during low water phases and was largely characterised by high values for the biomass of benthic fish, chlorophyll-a and total phosphorous. 4. During the period of rising water levels, the migration of benthic fish out of the pond occurs simultaneously with the establishment of submerged plants, while water turbidity decreases along with phytoplankton and nutrient concentrations, inducing a clear-water phase. However, when submerged plants are absent and fish migration is low, a transient state is generated. 5. We suggest that, in contrast to temperate ponds and shallow lakes, where the main driving mechanisms establishing alternative states are related to cascading effects via the food chain, in tropical ponds and shallow lakes it is resuspension of sediments by benthic fish that plays the most significant role in establishing alternative states. However, the effect of the flood pulse regime plays an important role in the temporal dynamics of fish community structure by controlling benthic fish migration. Keywords: abrupt shifts, alternative states, Egeria, fish migration, flood pulse Introduction Shallow lakes are commonly characterised by alternative stable states, oscillating between a macrophyte-dominated state and a turbid state (dominated by a high abundance of phytoplankton and ⁄ or suspended matter concentra- tion) (e.g. Scheffer et al., 1993, 2001). Several studies examining alternative states have been based on correla- tions and models using long-term data obtained in the field (e.g. Bro ¨ nmark et al., 2010; Gelo ´s et al., 2010) or on experiments carried out in mesocosms (e.g. Mazzeo et al., 2010), while others have discussed the use of theoretical models (e.g. Scheffer & Jeppesen, 2007; Attayde et al., 2010). These contrasting yet complementary approaches have provided fundamental insights and new hypotheses (e.g. Schooler et al., 2011) that have supported the alter- native states phenomenon as a convincing theory. Most studies investigating regime shifts have been carried out in temperate regions, and few of these studies have included shifts occurring in river–floodplain systems Correspondence: Roger Paulo Mormul, Po ´ s-graduac ¸a ˜o em Ecologia de Ambientes Aqua ´ticos Continentais – PEA, Universidade Estadual de Maringa ´ – UEM, Av. Colombo 5790, 87020-900 Maringa ´ – PR, Brazil. E-mail: roger.mormul@gmail.com Freshwater Biology (2012) 57, 1592–1602 doi:10.1111/j.1365-2427.2012.02820.x 1592 Ó 2012 Blackwell Publishing Ltd