Comparison of simple techniques for estimating chlorophyll a concentration in the intertidal zone using high spectral-resolution field-spectrometer data

We compared 3 simple techniques for estimating the concentration of microphytobenthos chlorophyll a at the sediment surface using high spectral-resolution field spectra. The techniques are based on relationships established between chlorophyll a (ch1 a) absorption at similar to673 nm estimated from field reflectance spectra and ch1 a concentration in field samples. For ease of application, the best approach should provide an estimate that is independent of the physical properties of the sediment such as grain size, moisture content, etc. and of sediment type and illumination conditions. To achieve as comprehensive a picture as possible, tidal flats from the French shore of the eastern English Channel that differed in sediment type and dynamics were studied at low tide and at various times of the year in order to cover seasonal blooms and illumination variations. The preliminary results show that all 3 methods are fairly robust (R-2 > 0.8), with very little difference between them, indicating little influence of the measurement conditions on reflectance in the wavelength range used (570 to 720 nm) and great stability of the instrument (a portable field spectrometer) used in the field. Validation of the 3 approaches was made with further, more recently acquired data set, and a very close correlation (R-2 > 0.93, slope similar to1) was found between measured and estimated ch1 a concentrations for all 3 techniques at the range of chlorophyll concentrations encountered that time at the Baie de Somme site (50 to 200 mg m(-2)). The influence of sampling strategy and sediment optical properties with respect to spectral signatures must, however, be investigated in more detail. More measurements are required to fully validate the relationships found and to ensure that they are generally applicable to most intertidal flat environments.