THE MIGRATORY BEHAVIOUR OF DIATOM ASSEMBLAGES IN A LABORATORY TIDAL MICRO-ECOSYSTEM EXAMINED BY LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY

Abstract

Low temperature scanning electron microscopy (LTSEM) was superior to freeze-drying and ambient SEM in preserving the fine structure of diatom assemblages inhabiting cohesive sediments. The accumulation of diatoms at the surface of the sediment, maintained in a tidal micro-ecosystem, was examined for the first time using LTSEM techniques. LTSEM revealed a low standing assemblage present on the sediment surface when exposed before dawn. There was a linear increase in the numbers of diatoms at the surface when photon flux densities increased above 0.5 μEm⁻²s⁻¹. The first diatoms to migrate to the surface were species of Navicula and Nitzschia. After 4h there was a synchronous appearance of large numbers of Scoliopleura tumida at the surface. Data on the depth distribution of diatoms in the sediment, derived from sediment samples that had been frozen and then fractured, support the hypothesis that light is the factor controlling the onset of diatom migration and that different diatoms have different thresholds triggering their movement.