A remote sensing approach for assessing the invasion of Najas marina in Madu Ganga Estuary, Sri Lanka

The present study was carried out to determine the distribution of Najas marina, an invasive floral species in the Ramsar site of Madu Ganga Estuary in Sri Lanka using ASTER satellite data. Cloud free ASTER imageries of Madu Ganga Estuary with 15 m resolution were atmospherically corrected using the FLAASH in ENVI software. The NDVI was calculated and unsupervised classification was applied for the study site for each image and the distribution maps of N. marina were developed from 2007 to 2014. Methodology was validated using in situ data, which were collected in 2014 with monthly intervals parallel to ASTER overpass. The derived distribution maps indicated that N. marina was distributed in about 31% of the estuary in April 2014. The highest densities were mostly found in bay areas and peripheral areas. Maps developed for December 2007, December 2009 and December 2013 indicated that there is a temporal variation in the distribution of N. marina over the years. The overall distribution of N. marina has decreased from December 2007 to December 2009 and increased from December 2009 to April 2014. Low water levels and stagnation of water appears to be conducive for the variation of this species. These factors should be taken into consideration when managing the invasion of N. marina in this economically and ecologically important estuary.