Invertebrate seedbanks: rehydration of soil from an unregulated river floodplain in the south-eastern U.S. LUSHA M. TRONSTAD, BRYAN P. TRONSTAD AND ARTHUR C. BENKE Aquatic Biology Program, Department of Biological Sciences, University of Alabama, Tuscaloosa, AL, U.S.A. SUMMARY 1. We investigated the responsiveness of aquatic invertebrates to rehydration of floodplain soil in a south-eastern U.S. river floodplain. Non-inundated soil divots containing invertebrate seedbanks were collected from three floodplain elevations with different inundation histories (inundated for 2, 38, and 78% of a year), and subjected to rehydration (experimental inundation) in aquaria for 10 weeks. 2. Before rehydration, samples were collected to assess the initial density and composition of invertebrates in non-inundated soil at each elevation. After rehydration, benthic samples were collected biweekly and emergence of aquatic insects was collected weekly from aquaria. 3. The surviving invertebrates were classified as aquatic, semi-aquatic and terrestrial. Some aquatic invertebrates survived the non-inundated period in an active state (e.g. Chironomidae and Ceratopogonidae), while others appeared to be dormant (e.g. Heptageniidae, microcrustaceans and Chironomidae). The response for several inverte- brates (e.g. Chironominae, Oligochaeta, Collembola and terrestrial invertebrates) depen- ded on the length of rehydration and inundation history. 4. The capacity of aquatic invertebrates to survive and recover was greatest at the most frequently inundated site. Despite variation in floodplain environments, both active and dormant invertebrates persisting in non-inundated floodplain soil contributed to the floodplain assemblage during flooding. Thus, maintaining the connection between river and floodplain appears to be important in sustaining the invertebrate seedbank and its capacity to recover during inundation. River channelisation and regulation that severs this connection may result in a loss of diversity and abundance. Keywords: experimental inundation, floodplain, invertebrate dormancy, invertebrate seedbank, soil hydration Introduction The floodplain is a critical component in the natural functioning of low gradient rivers, which can be inundated for weeks to months each year. A river’s connection to its floodplain is essential for the exchange of nutrients and transport of fine particulate organic matter (Junk, Bayley & Sparks, 1989). Addi- tionally, the floodplain provides important habitat for many invertebrates (Benke, 2001). The life histories of aquatic organisms may depend on the frequency and duration of inundation, which varies within a flood- plain. Floodplains are often inundated at least once during the year, depending on elevation of the floodplain compared with the river channel and floodplain morphology (e.g. channels connected to river versus elevated depressions; Junk et al., 1989; Smock et al., 1992; Benke et al., 2000). Low floodplain elevations with connections to the river may be Correspondence: Lusha M. Tronstad, Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071-3166, U.S.A. E-mail: tronstad@uwyo.edu Freshwater Biology (2005) 50, 646–655 doi:10.1111/j.1365-2427.2005.01351.x 646 Ó 2005 Blackwell Publishing Ltd