Effects of intertidal microphytobenthos migration on biomass determination via laser-induced fluorescence

Laser-induced fluorescence (LIF) spectra of intertidal microphytobenthos (MPB) communities were obtained in the laboratory with a 532 nm pulsed Nd:YAG laser. The laser-induced chlorophyll (chl) fluorescence emission spectra of MPB in mud and sand sediments were characterized by a band in the red region with a maximum at ca. 685 nm. Biomass accumulation on the surface of the mud due to cell migration caused a shift to longer wavelengths (up to 5 nm) of the red emission maximum and the development and increase of an emission shoulder at the far-red region (maximum at ca. 732 nm), probably owing to increased re-absorption of chl fluorescence within the denser microalgae biofilm. Direct relations were observed between MPB biomass proxies (normalized difference vegetation index [NDVI] and phytobenthos index [PI]) and fluorescence intensity. LIF was used to track migratory rhythms of epipelic benthic microalgae in muddy sediments, which are absent in epipsammic communities in sand: progressive accumulation of biomass occurred at the sediment surface during diurnal low tide periods and was followed by a rapid downward migration before tides began to cover the sampling site. When exposed to high light, surface biomass decreased in migratory biofilms, indicating that diatom cells avoid photoinhibitory light levels. This phenomenon is known as behavioral photoprotection. For the first time, LIF was applied to study intertidal MPB communities to adequately describe surface biomass, which included changes due to migration.