Chlorophyll a and intertidal epilithic biofilms analysed in situ using a reflectance probe

Abstract

Spectrometry of biofilms in microhabitats (e.g. rock crevices) is difficult using natural light. An alternative approach was evaluated using a reflectance probe with an artificial light source to measure the abundance and pigment composition of intertidal biofilms in situ. Measurements made by the probe were compared with those made under sunlight. To determine whether the probe could be used to measure amounts of chlorophyll a (chl a, as an index of biomass), reflectance spectra (350-1050 nm) were acquired from microalgae grown on sandstone discs. Spectra were acquired under natural light and using the probe (artificial light) with or without an intervening layer of plastic. Two chlorophyll indices (a ratio of reflectances at 750 and 672 nm and the Phytobenthos Index) were used to estimate amounts of chl a from the spectra. Strong linear relationships were found between laboratory measures of chl a and both chlorophyll indices (R 2 ranging from 0.85 to 0.94). Spectra of natural intertidal biofilms were used to determine whether there were any differences between spectra measured under natural and artificial light in terms of the shapes of their curves and the wavelengths and heights of absorption peaks caused by photosynthetically active pigments. No differences in shapes of curves were found between spectra measured under natural and artificial light. The wavelength positions of absorption peaks were similar between spectra measured under sunlight and artificial light but there were differences in their height. Taken together, results show that the reflectance probe presented a reliable and rapid method for providing quantitative and qualitative information about intertidal biofilms in situ. © Inter-Research 2011.