Vertical zonation on rocky shores in the Severn estuary

Zonation of algae and invertebrates was examined on rocky shores in the Severn estuary from 1975 to 1978. These shores were characterised by an absence of macro-algae from MLWS and below. Some littoral algae showed reductions in abundance during the study. In particular, Ascophyllum nodosum and Fucus serratus almost disappeared from the mid shore at one site after 1976, and no significant recolonization had occurred by 1979. Animal species in general showed predictable and constant zonation patterns, but Littorina rudis, Patella vulgata and Elminius modestus all exhibited decreased upper vertical limits in the upper reaches of the estuary.

Three approaches were taken to interpret the zonation patterns and their changes. (i) Exposure to wave action was estimated. Decreasing exposure levels may account for the decline in upper vertical limits of species in the upper estuary. (ii) A survey of silt, salinity and temperature was carried out over complete tidal cycles. High silt concentrations may partially explain the absence of macro-algae below MLWS. (iii) Preliminary experiments compared the effects of grazers in the estuary with those in fully marine conditions. These experiments point to some of the factors which are responsible for the lack of algal recolonization on the shore from which the dominant fucoids disappeared after 1976, but the situation is complex and involves algal reproductive biology as well as grazer activity.