Field-based remote sensing of intertidal epilithic chlorophyll: Techniques using specialized and conventional digital cameras

Information on the amount of chlorophyll (as an index of micro-algal abundance) on rock surfaces is essential for many reasons, including studies of grazing and its role in structuring intertidal assemblages. Many methods are destructive, error-prone and expensive. Remote sensing allows non-destructive, inexpensive and quantitative measurements to be made of chlorophyll in situ. One specialized and two inexpensive commercially-available digital cameras (Fuji IS1 and Sony DSC-V1) are evaluated for estimating amounts of chlorophyll on rock surfaces. To compare measurements from different images, they are calibrated, using reflectance standards of different brightness. To test the calibration, images of a natural rock platform were acquired under variable solar illumination and camera-exposure times. Analyses before and after calibration showed that the method was effective.A range of quantities of micro-algae was grown on sandstone disks in an aquarium over different intervals of time. Red and NIR reflectance images were obtained from the cores. For each core, the amount of chlorophyll was determined spectrophotometrically and estimated from the images using the Ratio Vegetation Index (RVI) and Normalised Difference Vegetation Index (NDVI). Each of these was linearly related to the measured chlorophyll, with r2 ranging from 0.78 to 0.9. These techniques can be applied to the study of intertidal and freshwater benthic habitats.