Influence of Seasonal Variations of Temperature and Light on the Growth Rate of Cultures and Natural Populations of Intertidal Diatoms

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Abstract
Cultures of 4 benthic diatom species (Navicula salinarum, Amphiprora cf. paludosa, N. arenaria and Gyrosigma spencerii) were kept at the surface of a mudflat in order to measure division rates. During incubations in the period of Dec.-June, the mean temperature was the predominant factor regulating cell division rate. Immersion of the mudflat with very turbid tidal water was an important factor only at a low-level mudflat station during winter and early spring. Strong temperature fluctuations during incubations, with periods of frost in winter and periods of insolation during hot summer weather, did not interrupt culture growth. High temperatures and high irradiance values are unlikely to be the direct cause of the summer minimum in population density that is frequently observed in the estuary. After the winter minimum, the increase in population density was measured at 3 levels of the mudflat. At the high-level station, the density increased in Jan. at still very low temperatures, whereas at the mid- and low-level stations, this increase started in March. This observation indicates that the emersion time of the mudflat and hence its exposure to full daylight is an important factor in the spring bloom development. The density of the diatom populations in the 0.5 cm top layer of the sediment increased only at a very low rate (with maximum rates of .apprx. 0.1 doublings per day. Low photosynthetic rates of natural diatom populations in the sediment core are the cause of these low doubling rates of natural populations on the mudflat. Dispersal of diatom cells into the sediment is probably responsible for the decreased photosynthetic rates, and consequently, for the low doubling rates, of sediment-inhabiting diatoms.