PATTERNS OF DISTRIBUTION OF DIATOM ASSEMBLAGES ALONG ENVIRONMENTAL GRADIENTS IN THE SEVERN RIVER ESTUARY,CHESAPEAKE BAY,MARYLAND1

Distribution patterns of diatoms from natural substrates and surface waters of the Severn River, Maryland, are related to salinity and seasonal gradients through time and space by minimum spanning tree cluster analysis and principal components analysis. Assemblages were found to be distributed in a complex but orderly fashion, with primary discontinuties occurring between seasons, and between planktonic and attached forms. Within seasons, benthic groupings are distributed as continua along a horizontal salinity gradient, with one discontinuity at a location with a mean salinity of approximately 7%o. Downstream from this discontinuity, most species do not follow the changing salinity patterns from season to season, and are considered euryhaline for a salinity range of 8–16%o. Upstream from this discontinuity there is a relatively distinct and persistent assemblage of oligohaline indicators. Evidence derived by comparing this study with similar studies from Oregon estuaries indicates that similar distribution patterns, community structures and taxa can be expected to occur in estuaries on the east and west coasts of North America.