Diatom zones across intertidal flats and coastal saltmarshes in britain

The distributional patterns of diatom assemblages across the upper intertidal zone are examined in a number of British coastal sites which differ in morphological and hydrological conditions. The relationships between diatom assemblage composition and a number of environmental variables, including vascular vegetation cover, organic content and particle size of the surface sediments, and ground altitude which is related to the frequency and duration of tidal inundation are investigated. Three diatom groups common to most sites are identified. The first group is associated with the high marsh environment, where tidal inundation is infrequent and of short duration, organic content of the sediments is high and vascular vegetation cover is dense. The second group is related to low marsh conditions characterized by longer duration and higher frequency of tidal inundation, much lower organic content and variable density of vascular vegetation cover. In this zone, water salinity and particle size of the surface sediments are weakly correlated to the composition of the diatom assemblages. The third group occurs on tidal flats where vascular vegetation is absent and organic content of the sediments is very low. Overall, the composition of diatom assemblages in the intertidal zone are usually a direct function of ground altitude with the duration and frequency of intertidal exposure as the most important factors.