Effects of the water column on hyperspectral reflectance of submerged coral reef features

Remote sensing technology has many attributes that would be beneficial to monitoring submerged coral reef ecosystems, such as the ability to revisit a large study area repetitively and consistently without the necessity of large teams of field researchers. One limiting factor, however, is the difficulty in accounting for the variable effects of the water column on the optical reflectance characteristics of submerged features. These variable water column effects have been observed as depth, bottom-type, and wavelength dependent; such complex modifications limit the accuracy of remote identification of submerged coral reef features. In a preliminary attempt to examine the extent of the problem in a complex coral reef ecosystem, in situ hyperspectral reflectance measurements were collected in the U.S. Virgin Islands at various depths over different substrate types in water of consistent optical quality. A comparison is made between hyperspectral reflectance measured at the top and bottom of the water column in different water depths over different substrate types in an effort to alert users of remotely sensed data of the potential problems of identifying submerged features without appropriate corrections. While most coral reef remote sensing projects assume vertical and horizontal homogeneity of water optical properties and consider variation in optical reflectance representative of change in bottom type, this preliminary study reveals that the case is more complex and appreciable effort must be made to correct for the effects of the water column for accurate identification of submerged features.