Interactions between benthic macroinvertebrates and saltmarsh plants : consequences for saltmarsh restoration and the policy of managed realignment on the coast of SE England
Over the last half century, the saltmarshes of south east England have undergone an extensive decline, especially the pioneer zone vegetation. These losses have generally been blamed on coastal squeeze resulting from sea level rising against sea walls. There is little evidence to support this hypothesis however, and an alternative hypothesis, based on infaunal invertebrates preventing the establishment of saltmarsh plants was tested. In the managed realignment site at Tollesbury, and the other sites examined, the mudflat fauna was dominated by Nereis (= Hediste) diversicolor and Hydrobia ulvae. In laboratory experiments N. diversicolor and H. ulvae reduced the production of seedlings from seeds of Salicornia europaea agg.. Conversely the presence of S. europaea agg. significantly reduced the normal burrowing activity of N. diversicolor. Invertebrate exclusion experiments established at five sites in south east England facilitated colonisation by saltmarsh plants at some sites (Orplands, the Blythe Estuary, Wallasea Island and Maldon), by excluding large (>3cm) N. diversicolor. However, at the Tollesbury realignment site, the high rate of sediment deposition and the relatively long distance to a source of seeds prevented plant colonisation. This study supports the hypothesis that establishment of saltmarsh vegetation is prevented by infaunal invertebrates,particularly N.diversicolor, which exclude plants through bioturbation, herbivory and granivory. These interactions may help explain the loss of saltmarshes and will reduce the success of future managed realignment schemes which depend upon the colonisation of new intertidal areas by saltmarsh vegetation. Further management of realignment sites will be necessary to encourage saltmarsh development.