SEASONAL AND SPATIAL VARIATION IN EPIPELIC DIATOM ASSEMBLAGES IN THE SEVERN ESTUARY

Abstract

The species composition of epipelic algal assemblages on intertidal mudflats was investigated at three sites in the Severn estuary, England, between March 1990 and February 1991. Diatoms were the dominant algal group, comprising greater than 95% of living cells in 100 out of 106 lens tissue samples. Euglenoid algae were abundant at one site during the summer months. Although over 60 diatom taxa were identified, only 15–20 species occurred regularly, with 12 species being dominant in the samples. Upper and middle shore diatom assemblages were dominated by Navicula pargemina during the spring and autumn, and by Nitzschia epithemioides during the early summer months. High relative abundances of Rhaphoneis minutissima were recorded at lower shore stations throughout the year and on upper and middle shores during the winter months. These lower shore assemblages contained high proportions of epipsammic species (Coscinodiscus sp., Cymatosira belgica). Principal component scores (obtained from assemblage composition data) were correlated with position on shore, season, temperature and sediment carbohydrate and chlorophyll a concentrations. The relative abundance of N. epithemioides was positively correlated with temperature and sediment carbohydrate and chlorophyll a concentrations, whereas the relative abundance of R. minutissima, Coscinodiscus sp. 1 and Navicula flanatica were correlated negatively with these variables. The diversity (H′) of diatom assemblages was greater on the lower shores, with upper shore assemblages having low diversity values. Upper and middle shore assemblages tended to be dominated by single taxa (e.g. N. epithemioides and N. pargemina). No single measured variable explained the distribution of diatom species in this study. Differences in species composition, diversity and seasonality of diatom assemblages across mudflats appear to be related to the gradients of environmental stress and disturbance between the upper and lower shores.