Distribution of diatom species on an estuarine mud flat and experimental analysis of the selective effect of stress

Abstract

The distribution patterns of seven dominant diatom species on an estuarine mud flat transect were related to salinity gradients and organic pollution. The temporal distribution of species can be explained partly by the seasonal variation in irradiance and temperature, and partly by the large discharges of organically polluted fresh water during the late autumn. The selective effects of stress factors, such as low or high salinity, high concentrations of ammonia and free sulphide, and high temperatures were studied by measuring: (1) the tolerance of natural diatom populations isolated from the sediment; (2) the tolerance and capacities of unialgal cultures: and (3) the effects of the stress factors on the species composition of the populations in sediment samples incubated in the laboratory. A low salinity (1‰), but not a high salinity (20‰), markedly altered the species composition of natural diatom populations kept in the laboratory. Navicula salinarum Grunow and N. cf. cryptocephala Kützing outcompete other species only at a salinity of 1‰, which is below the optimum salinity for growth of these species. High concentrations (2–4 mM) of ammonia inhibited the photosynthesis of N. phyllepta Kützing and N. flanatica Grunow and decreased the cell numbers of these species in mixed populations kept in the laboratory. N. salinarum and Gyrosigma fasciola (Ehr) Cleve were relatively ammonia-tolerant and reached their highest abundance during periods of large discharges of organically polluted water. The populations on the mud-flat stations that had black, sulphide-containing layers 1 mm below the surface of the sediment, were dominated by the relatively sulphide-tolerant Navicula salinarum and N. pygmaea Kützing. High values of irradiance and temperature were a selective factor explaining the absence of N. flanatica in summer. Uptake of organic substrates and the inhibitory effect of high population densities are discussed in regard to their possible modifying influence on the distribution of diatom species on the mud flat.