Subaqueous hydrochory: open-channel hydraulic modelling of non-buoyant seed movement SCOTT H. MARKWITH* AND DAVID S. LEIGH† *Department of Geosciences, Florida Atlantic University, Boca Raton, FL, U.S.A. †Department of Geography, University of Georgia, Athens, GA, U.S.A. SUMMARY 1. Subaqueous transport may be a significant dispersal and migration mechanism of non-buoyant seeds of aquatic and riparian plants, and also secondary transport of seeds once they have lost buoyancy, but the efficiency of this difficult to observe process is largely unexamined. This study uses hydraulic modelling to establish the discharges that move the non-buoyant seeds of Hymenocallis coronaria as bedload or suspended load; uses stream gauge data to examine the frequency of effective discharges from late June to late September, the seed maturation and germination period; and the potential transport distance of the seeds. 2. The results show that the majority of non-buoyant seeds of H. coronaria can be transported as bedload through entire modelled stream reaches of lengths 10.8, 18 and 14.4 km with the 0.5 year return interval flow. Bedload apparently has the ability to move seeds over great distances, and may be a substantial factor determining the genetic structure, demography and dynamics of populations and communities. How- ever, prolonged movement of non-buoyant seeds in suspension appears to be quite rare. 3. Although insect mediated pollination and biochory occur concurrently with bedload transport, bedload transport alone may be sufficient to account for the established gene flow rate of H. coronaria. The potential transport distance of many of the seeds exceed that between populations, and migration may occur more frequently than the species’ generation time. 4. This is the first known study to use open-channel hydraulic modelling and sediment transport analysis to determine the effectiveness of non-buoyant seed transport. This method of analysis shows promise for application in other contexts, and especially where flow management is a critical issue for maintenance of rare species. Keywords: bedload transport, hydraulic modelling, macrophytes, Shoals spider-lily, suspended load Introduction Hydrochory, the movement of seeds by water, is a dispersal and migration mechanism of aquatic and riparian plant species with a long documentation record (Ridley, 1930). In recent years the process of hydrochory has often been investigated in terms of its influence on community structure and species diversity in riparian and wetland areas (Jansson, Nilsson & Reno ¨fa ¨lt, 2000; Middleton, 2000; Nilsson et al., 2002; van den Broek, van Diggelen & Bobbink, 2005; Jansson et al., 2005), and on population dynam- ics of individual species of general and conservation interest (Schneider & Sharitz, 1988; Johansson & Nilsson, 1993; Craddock & Huenneke, 1997; William- son, Costa & Minte-Vera, 1999; Griffith & Forseth, 2002; Hampe, 2004). A common theme throughout the vast majority of the aforementioned investigations is a paramount concern with buoyancy in the ability of streams to Correspondence: Scott H. Markwith, Department of Geosciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, U.S.A. E-mail: smarkwit@fau.edu Freshwater Biology (2008) 53, 2274–2286 doi:10.1111/j.1365-2427.2008.02054.x 2274 Ó 2008 The Authors, Journal compilation Ó 2008 Blackwell Publishing Ltd