Monitorización de floraciones de macroalgas verdes en la Bahía de Cádiz mediante técnicas ópticas

Abstract

Optical monitoring techniques are defined (Absorbance, Reflectance in situ, hand glider photographs, remote sensing) of green macroalgae blooms (Ulva spp.) They are present on intertidal inner Bay of Cadiz. Macroalgae show different optical properties such Absorbance and Reflectance. Those properties have defined and classified their spectral signature to differentiate them from other organisms on intertidal areas. Laboratory experiments and field work (in situ reflectance) have been decisive to establish the relationship between the number of layers of algae and NDVI (R2adj = 0.86, P <0.001), the biomass and NDVI (R2adj = 0.69, P <0.001) and the vegetation coverage and NDVI (R2adj = 0.61, P <0.01). Results were also obtained from vegetation coverage and NDVI for three treatments using three different sediment types (sand, mud, sandy mud) R2adj = 0.93, P <0.001 for sand; R2adj = 0.96, P <0.001 for mud; R2adj = 0.94, P <0.001 for sandy mud. Thus was obtained an equation type (vegetation coverage = a + b*NDVI) to estimate the vegetation coverage of macroalgae in a satellite image. Hand glider photographs for January 2009 defined vegetation coverage (1.55 km2) that represent 12% of intertidal (~13 km2) and 5% of inner Bay (~30 km2) Using in situ values of biomass and tissue content of C and N (%) we have been able to obtain the first approximation of total biomass (166.40 Tn), tissue content of carbon (31.18 Tn) and nitrogen (3.68 Tn) and the distribution of macroalgae to the bloom maximum development. Average values for winter and summer have been obtained by satellite image. Values of δ15N (11.5) of macroalgae have indicated that can be N anthropogenic source. Optical methods have been effective for monitoring the level bloom around the bay. Although they have limitations such as the tide or the clouds in the case of satellite images and aerial photographs cannot calculate NDVI. They have been used to obtain the first approximation of blooms state on Bay of Cadiz. Methods can be used without sampling site; however, it would be better to ensure the results doing a calibration.