Propiedades ópticas y teledetección de macrófitos marinos en la Bahía de Cádiz, España

Abstract

The optical properties of marine macrophytes within Cádiz Inner Bay (Cymodocea nodosa, Zostera marina, Zostera noltii and Caulerpa prolifera) were examined in detail. C. prolifera had a higher total absorption and specific absorption than the seagrass species, suggesting the macroalgae was more efficient at capturing light. In the field, in situ reflectance (R) spectra of different intertidal biotopes were investigated. R spectra of ‘bare’sediment suggested the presence of benthic microalgae. Differences were observed between bare sediment and macrophyte R spectra. Changes in R related to increasing % cover of seagrasses could be easily summarised using the normalised difference vegetation index(NDVI). In order to examine the effect of the water column, we logged bottom reflectance during the incoming tide and calculated the effective attenuation of light. This calculation was repeated using radiance values extracted from the remote sensing image in an area where the depth was know in detail. Red and near infra-red light was attenuated more strongly than blue or green, except in areas with high turbidity. Rearrangement of the equations describing attenuation through the water column, allowed us to derive a prediction of the water depth throughout the rest of the image. In shallow areas variation in turbidity, bottom type and optical effects, and in deep areas, high turbidity limited the application of the model. Using the depth prediction and the calculated attenuation coefficients we were able to correct the majority of the image for the influence of water-column attenuation. The corrected image was classified using the reflectance spectra of different biotopes, to produce a first approximation of the spatial distribution of benthic macrophytes within Cádiz Inner Bay. In conclusion, remote sensing appears to be a promising technique for the mapping of marine macrophytes within shallow, intertidal systems such as Cádiz Bay.