Chlorophyll a concentration as an index of sediment surface stabilisation by microphytobenthos?

In this study the potential of predicting critical erosion shear stresses from microphytobenthos chlorophyll a surface concentrations in the upper mm of the sediment surface was investigated. The results should provide a better basis for estimating sediment surface stability on large scales by means of optical remote sensing. To achieve a comprehensive picture, tidal flats in the German and Danish Wadden Sea, which differed in tidal range, wave exposure, sediment type and microphytobenthos composition, were studied. The selection of sampling sites was biased towards water-free and visually undisturbed sediment surfaces. The relationship between critical erosion shear stresses and chlorophyll a concentration was found to be highly site-specific. Most of the variation within the data set cannot be explained by the chlorophyll a concentration and the surface sediment types present. Instead, the data suggest distinct dependencies on other site specific features, such as the intensity of sediment surface reworking, microphytobenthos assemblage composition and the endobenthic macrofauna present. It is concluded that to estimate critical erosion shear stresses from chlorophyll a concentration by means of optical remote sensing techniques requires extensive and repeated site specific calibrations since their validity is limited by the patchy and sporadic occurrence of some of the determinant parameters.