Surface properties of stabilizing coastal dunes: combining spectral and field analyses

The coastal dunes of Israel have been undergoing a process of stabilization since 1948. One of the major features of this process is a change in the surface properties of the dunes - the development of a biological soil crust (BSC), and a change in the properties of the sand grains themselves. In Ashdod, at the southern coastal plain of Israel, sand properties that include the BSC, their fines (silt and clay) content and free iron-oxide (indicating their degree of rubification) have been analysed in detail using field and lab spectroscopy methods. In addition, sand erosion and deposition were measured using erosion pins to determine their effect on the presence of the above-mentioned factors. It was found that the BSC over these dunes is comprised of green algae that differs in its reflectance spectra from cyanobacterial crust, especially in the blue band. The crust was found to be particularly developed in the stable areas (mainly the interdunes) and on the north-facing slopes rather than on the south-facing slopes. A positive correlation was found between the crust fines and chlorophyll content, with stable areas showing more developed BSC. The stable areas showed also a lower albedo and slightly more developed reddish colour, indicating a slightly higher rate of rubification. This study demonstrates that the intensity of sand erosion/deposition rates affects soil properties, with the BSC being the fastest to react to the stabilization process (months to several years), followed by the content of fine particles (several years to a decade), whereas the rubification process is a much weaker marker and may need much longer time periods to develop (decades to centuries)