Remote Sensing and GIS is very useful tool for mapping historical changes in coastal system and to predict future conditions of coastal ecosystem. Benthic cover is one of the important feature of coastal ecosystem, it plays significant role in maintaining coastal activities and its environment.
Effective mapping of benthic cover provides many important information for maintain and development of coastal ecosystem.
Mapping Changes in Benthic cover
Coral reefs need our attention because of the high biodiversity and their key role in the tropical marine biosphere. These fragile ecosystems are under growing pressure from land-based sources of pollution as the result of increasing deforestation, soil loss, and excess use of fertilizers and pesticides.
Coastal coral reefs, like other marine coastal ecosystems are increasingly exposed to growing loads of nutrients, sediments and pollutants discharged from the land.
The deteriorating water quality from marine pollution and coastal development adds pressure to coral reefs due to which coral reef communities change to increasing proportion of macroalgae.
The efforts to accurately monitor and assess the changes in coral reef community composition by incorporating satellite and airborne imagery is now increasing in the scientific community at a variety of spatial and temporal scales.
On the contrary for the small islands, the use of an airborne platform may be the most appropriate for mapping and monitoring the coral reefs. Aerial photographs and digital image processing techniques have been successfully applied for mapping the optically clear and shallow reef waters.
The reflectance properties of coral reef, seagrass and algal communities could be applied to differentiate and map the benthic cover.
Figure : Aerial photograph of the Kuroshima Island
Interpreted Information from Mapped Benthic Cover
- The classified data (below figure) revealed considerable changes in the benthic cover adjoining the coastal area during 1977 to 2005.
- The benthic cover change analysis results indicated the increase in pavement area (6.97%) especially in the northern part of the Island.
- However, coral cover reduced by 10.49% during 1977 to 2005 due to increase in algae (4.75%) and seagrass (1.10%).
- The spatial distribution of benthic cover indicates that maximum increase in seagrass and algae was in northern, western and southeastern part of the Kuroshima Island.
- The overgrowth of algae and seagrass may result in a loss of fish and invertebrate biodiversity, as a loss of habitat heterogeneity occurs compared to that presented by the live coral.
Figure: Benthic cover change analysis from 1977 to 2005
Analysis From Mapped Benthic Cover
The anthropogenic sources on land have been identified as the major controlling factor for the phase shifts in reef ecosystem. At local scale causal links between reef degradation and the diffuse pollution with changing land use practices have often been more difficult to demonstrate, because of a lack of historic data.
Therefore, this study is quite important to monitor such kind of changes and impacts on surrounding reefs.
The landscape development intensity (LDI) index, an indicator of human activity was also calculated from land use/cover data and used to identify the watersheds which may have more impact on changing coral condition. Hence to quantify the LDI, first watersheds were delineated for the study area.
Although, there is no perennial river in Kuroshima Island, but using digital elevation model of 1m spatial resolution, six different watersheds (W1 to W6) were delineated in the study area.
These watersheds were used to measure the human disturbance gradient at each watershed level and to investigate the impact of land driven activities on adjacent reef areas.