Distribution of seagrass and other macrophytes

Seagrass beds are of great ecological significance in shallow bays, estuaries and coastal lakes but are frequently under threat from dredging, eutrophication and other consequences of human activity. Ocean colour sensors such as SeaWiFS are of little use for monitoring these ecosystems because of their low spatial resolution. Remote sensors such as Landsat Thematic Mapper, although spectrally limited, do have potential for this purpose because of their high ground-level resolution —30 m in the case of Landsat TM. With the help of in situ measurements of reflectance spectra of various seagrass and macroalgal species, and of Kd(l) spectra at the sampling sites, Dekker et al. (2005) were able to use atmospherically corrected Landsat TM images to retrospectively monitor four seagrass species over a 14-year period in Wallis Lake - a shallow coastal tidal lake on the East coast of Australia. A substantial reduction in the area of Zostera beds was observed, but little change in the area occupied by Posidonia, Ruppia and Halophila. Sagawa et al. (2008), used data from the IKONOS satellite sensor, which has bands in the blue, green, red and infrared, and a resolution of 4 m, to map seagrass beds in Funakoshi Bay, Honshu Island, Japan. Phinn et al. (2008) used remote sensing to map seagrass species, cover and biomass in shallow (<3.0 m depth) waters in Moreton Bay (Australia). Ground-truth validation was achieved with underwater photography, and sampling of the benthos, along transects. The airborne CASI hyperspectral sensor was found to perform best, followed by Quickbird 2 (spacecraft at 450 km altitude; blue, green, red, near-infrared bands; 2.4 m ground-level resolution), followed by Landsat TM.

In the narrow shallow perimeter off rocky shores, extensive beds of large brown algae frequently occur, and are of great ecological significance. Simms and Dubois (2001) used satellite images recorded by Landsat TM (blues, green and red bands) and the SPOT HRV sensor (green and red bands), together with scuba diving inventories and in situ reflectance data, to follow seasonal variation in kelp (Laminaria longicruris) beds in the Baie des Chaleurs (Gulf of St Lawrence, Quebec, Canada).

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