Teledetecion de Habitats Bentonicos en la Bahia de Cadiz, Espana

Abstract

Coastal ecosystems are transition areas which are characterised by a high productivity and biodiversity, making them an important zone for local carbon and nitrogen cycling. Shallow subtidal and intertidal areas are often dominated by benthic macrophytes (marine plants) which provide a number of important ecosystem services, such as trapping particles and improving fisheries. However, continued anthropogenic development in the coastal zone has led to increasing pressure on these crucial ecosystems and their global decline in quality. In order to fully understand the functioning of these systems and protect the valuable resources they represent as part of an integrated coastal management plan, it is crucial to develop techniques capable of identifying and quantifying their physical and ecological properties at the appropriate spatial and temporal scales. In this sense the use of remote sensing techniques in combination with geographical information systems (GIS) is a powerful complement to traditional monitoring techniques. The Bay of Cadiz, located in the southwest of the Iberian Peninsula, is surrounded by heavy urbanisation (a population of about 600 000) which via various pressures is a potential threat to the rich local biodiversity of the bay. This includes “one of the best examples of European Atlantic saltmarshes” (RAMSAR) and 4 species of seagrasses which in turn provide habitat for large number of fauna and flora. Much of the area is protected under natural park status, however little is known about the large scale dynamics of the saltmarshes and seagrass habitats. Thus, the main objective of this work is to use remote sensing techniques to examine spatial and temporal dynamics of benthic habitats within Cadiz Inner Bay. Transects of depth combined with visual identification of habitats were carried out along the SW shore of the bay during the summer of 2009. Five main habitat types were identified; bare sediment, Z. noltii, C. nodosa, C. prolifera and saltmarsh. Hyperspectral reflectance measurements suggested some separation between the optical signatures of each habitat. Satellite images (Quickbird) collected at low tide in April 2005 and Oct 2007, were digitally processed (converted to reflectance, atmospherically corrected and corrected for water depth) using the ground truth measurements. Classification of intertidal habitat types and estimation of the percentage cover of macrophytes was carried within the inner bay. The coverage of macroalgae was the most striking difference between the two images (140 % more coverage in April 2005 than Oct. 2007) which was most likely related to seasonal dynamics. Coverage of saltmarsh and seagrasses appeared to be slightly higher in Oct 2007 (19 and 7 % respectively). Depth correction and classification of the subtidal areas was hampered by the variable sediment content within the bay, however the overall distribution of benthic habitats seemed to fit with the zonation of the seagrasses and C. prolifera. Thus, optical remote sensing techniques were able to detect both temporal and spatial variation in benthic macrophytes within the Bay of Cadiz and are likely to prove effective as a tool for detecting ecosystem change.