Long-term changes in intertidal estuarine diatom assemblages related to reduced input of organic waste

A comparison is made of the species composition and density of benthic diatoms of an intertidal brackish mudflat in the Ems-Dollard estuary (Netherlands/Germany) under conditions of high (1977 to 1980) and reduced (1987 to 1993) organic waste input from the river Westerwoldsche Aa. During the observation period (1977 to 1993) the epipelic diatom population on the mudflat was composed of only a limited number of species. From 1977 to 1980, every year broadly the same spatial and temporal patterns of the sediment-inhabiting diatoms were found, viz, an alternating dominance of Navicula salinarum and N. pygmaea. In 1993 this pattern had disappeared and N. phyllepta and N. flanatica, species with a lower tolerance to high ammonium and sulphide concentrations, were the dominant species. This change could be attributed to a reduction of organic waste input. The declining abundance of N. pygmaea was probably also partly caused by the decreased abundance of the nematode Eudiplogaster pararmatus, an indicator of organic enrichment. In 1987 and 1993, the diatom populations on the mudflat showed density maxima in spring ana late autumn, in contrast with the period 1976 to 1977, during which values were high (over 100 mg chlorophyll a m(-2)) during a large part of the year, not showing any spring or autumn bloom. The difference in the development of microphytobenthos biomass was probably caused by the increasing numerical densities of the macrofaunal diatom grazers Nereis diversicolor and Corophium volulator, caused by the reduction of the organic waste.