SHORT-TERM FLUCTUATIONS IN BENTHIC DIATOM NUMBERS ON AN INTERTIDAL SANDFLAT IN THE WESTERSCHELDE ESTUARY (ZEELAND, THE NETHERLANDS)

During the period March-May 1991, sediment samples were taken every tw o or three days at one intertidal station in the brackish part of the Westerschelde estuary. Quantitative cell counts were made in order to investigate the short-term temporal changes in diatom numbers and asse mblage structure. Throughout the whole sampling period, the diatom ass emblage was dominated by epipsammic diatoms. Three species, Achnanthes delicatula, Opephora cf. perminuta and Catenula adhaerens on average accounted for almost 67% of all valves counted. The epipsammic diatom fraction showed no significant changes in absolute numbers; its specie s composition appeared relatively stable. In contrast, epipelic diatom densities significantly increased towards the end of the study period . Species composition within this fraction was less stable. Multivaria te analysis (Principal Components Analysis), in combination with multi ple regression, indicated that total sky irradiance (on the second and third day preceding sampling) and percentage organic matter were rela ted to the short-term fluctuations of the epipelic diatom fraction.