Effect of light and sediment grain size on the vertical migration of benthic diatoms

Using chlorophyll fluorescence, the vertical migration of benthic diatoms responding to light intensity and affected by sediment grain size was studied.Minimal fluorescence (F o ) of surface sediment was measured by imaging pulse amplitude modulated (Imaging-PAM) fluorometer, and used to monitor diatom biomass variation in surface sediments.The test diatoms, Amphora coffeaeformis (C.Agardh) Kützing and Cylindrotheca closterium (Ehrenberg) Reimann & Lewin, migrated to the sediment surface under irradiance from 50 to 500 µmol photons m -2 s -1 .However, the diatoms exhibited no evident increase of surface biomass under dark conditions, and even showed slightly decrease of surface biomass under irradiances over 1,000 µmol photons m -2 s -1 .The light intensity inducing the maximum surface migration of A. coffeaeformis was 100 µmol photons m -2 s -1 , while the light intensity producing the same effect for C. closterium was 250 µmol photons m -2 s -1 .C. closterium showed higher motility than A. coffeaeformis.Faster diatom surfacing was observed in larger grain size sediments (125-335 µm) than smaller ones (63-125 µm).This study confirmed the significant influence of light as a main triggering factor behind migration, indicated the distinct effect of different sediment grain size, and highlighted the species-specific migratory ability.