Trapping of mangrove propagules due to density-driven secondary circulation in the Normanby river estuary, NE Australia

Abstract

The dispersal of mangrove propagules in the Normanby River estuary is dominated by secondary estuarine circulation. On flood tides, an axial surface convergence forms midstream, generated by a density-driven circulation cell. Floating material including mangrove propagules is trapped in the convergence, and moves upstream. On ebb tides, the circulation cell reverses, and propagules move towards the banks, where water velocities are much smaller than midstream. Over a tidal cycle, a net upstream drift of propagules occurs. This displacement effect occurs throughout the tropical dry season. Buoyant propagules of Rhizophora stylosa, Bruguiera gymnorrhiza, Xylocarpus mekongensis and Heritiera littoralis have been found to be transported upstream at a speed comparable to a net average speed of 3.2 km d-1, found in a drift-drogue experiment. The propagules subsequently accumulate in large numbers in hydrodynamic traps upstream from the convergence and, more importantly, upstream from the mangrove fringe. The hydrodynamic trapping conflicts with the temporal and spatial requirements of propagules to 'find' a habitat suitable for development.