The effects of bioturbation and herbivory by the polychaete Nereis diversicolor on loss of saltmarsh in south-east England

1. The saltmarshes of south-east England are eroding rapidly. Field and laboratory experiments were used to test the hypotheses that: (i) at the mudflat-saltmarsh boundary there are two alternative states, one dominated by pioneer zone vegetation that excludes burrowing infauna, and the other dominated by infaunal invertebrates that exclude vegetation; and (ii) the major cause of the loss of saltmarshes in south-east England is internal creek erosion, which is exacerbated by bioturbation and herbivory by the infaunal polychaete Nereis diversicolor. 2. In laboratory experiments Nereis ate the seeds and seedlings of Salicornia spp., and Salicornia deterred burrowing by Nereis. In field experiments, at Tollesbury in Essex, UK, exclusion of Nereis from the sediment surface increased the density of Salicornia, but only when a source of seeds was close by. In the Tollesbury saltmarsh the plants and Nereis had mutually exclusive distributions within a vertical zone of overlap. The recently vegetated area of the managed realignment site at Tollesbury contained no Nereis, but Nereis colonized areas where Salicornia had been removed. These observations and data support the first hypothesis. 3. Much of the loss of the Tollesbury saltmarsh is by lateral erosion of the internal creeks. Physical factors alone cannot be responsible for this erosion because experimental exclusion of Nereis led to sediment accretion. These results support the second hypothesis. Creek erosion may create a positive feedback where creek enlargement leads to faster tidal currents and further erosion until creeks have widened to their new equilibrium morphology. 4. Synthesis and applications. We conclude that the infauna are a major cause of reduction in area of saltmarsh vegetation and this has implications for the management of saltmarsh restoration. These results call into question the assumption that saltmarsh erosion in south-east England is due to sea level rise and coastal squeeze, and demands reexamination of the role of management realignment in the regeneration of saltmarshes. Reducing the rates of saltmarsh creek erosion, by exclusion of the infauna, and/or by reducing current velocities in the saltmarsh creeks, would reduce the need to replace eroded marshes by managed realignment, and would reduce future erosion of existing sea walls by wave action.