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

Remote sensing technology has many attributes that would be beneficial to m onitoring submerged coral reef ecosystems, such as the ability to revisit a large study area repetitively and consistently without the necessity of la rge teams of field researchers. One limiting factor., however, is the diffi culty in accounting for the variable effects of the water column on the opt ical reflectance characteristics of submerged features. These variable wate r column effects have been observed as depth, bottom-type, and wavelength d ependent; such complex modifications limit the accuracy of remote identific ation of submerged coral reef features. In a preliminary attempt to examine the extent of the problem in a complex coral reef ecosystem, in situ hyper spectral reflectance measurements were collected in the U.S. Virgin Islands at various depths over different substrate types in water of consistent op tical quality. A comparison is made between hyperspectral reflectance measu red at the top and bottom of the water column in different water depths ove r different substrate types in an effort to alert users of remotely sensed data of the potential problems of identifying submerged features without ap propriate corrections. While most coral reef remote sensing projects assume vertical and horizontal homogeneity of water optical properties and consid er variation in optical reflectance representative of change in bottom type , this preliminary study reveals that the case is more complex and apprecia ble effort must be made to correct for the effects of the water column for accurate identification of submerged features.