The Seasonal Succession Patterns of Diatom Species on an Intertidal Mudflat: An Experimental Analysis

The seasonal succession of seven dominant diatom species on a brackish intertidal mudflat in the Ems-Dollard estuary, The Netherlands, was found to follow simple recurring patterns in four successive years. Two species were especially abundant on the mudflat, Navicula salinarum being dominant in the cold seasons and N. pygmaea blooming mainly in summer. The wax and wane of these two species and their co-occurrence were analysed in experiments with oneor two-species cultures and with natural samples of sediment and algae, incubated in the laboratory. The importance of the following selection mechanisms was demonstrated: 1) temperature dependence of cell division, 2) interspecific competition in dense populations, 3) vertical zonation of species in micro-layers of the mud, 4) occurrence of stress factors e.g. desiccation, and 5) grazing by herbivorous meiofauna. The response of the two Navicula species generally agreed with their seasonal occurrence; e.g. N. pygmaea required higher temperatures for rapid cell division, was insensitive to temperatures over 30'C and survived partial desiccation more completely than N. salinarum. The relative resistance of N. pygmaea to grazing contributed to its abundance in summer. Parts of the seasonal succession patterns could be simulated using natural populations in experimental systems incubated under various combinations of temperature, irradiance level and exposure to desicccation.