Application of Remote Sensing and GIS in Coral Reef Assessment
Impact assessment of Human activities on reef ecosystem
Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral. The corals have a symbiotic, or mutually beneficial, relationship with the zooxanthellae. These algae live inside the coral polyp’s body where they photosynthesize to produce energy for themselves and the polyps.
The coral polyps, in turn, provide a home and carbon dioxide for the algae. Zooxanthellae also provide the coral with their lively colors as most coral polyp bodies are clear and colorless without zooxanthellae.
Coral reefs cover less than 1% of the ocean floor, but they are among the most productive and diverse ecosystems on Earth. About 25% of all known marine species rely on coral reefs for shelter, food and breeding. Coral reefs are the primary habitat for more than 4,000 species of fish and thousands of other plants and animals.
Corals are found all over the world’s oceans, the biggest coral reefs are found in the clear, shallow waters of the tropics and subtropics. The largest of these coral reef systems, the Great Barrier Reef in Australia, is more than 1,500 miles long (2,400 kilometers).
Coral Reefs Importance
Reefs play an important role in protecting the shoreline from storms and surge water. Coral reefs support an incredible diversity of fish, lobster, stone crab, snapper and grouper, all directly rely on the reef for spawning and habitat.
Other fisheries, such as tuna, dolphin and other pelagic species, rely on the reef indirectly, though the bait fish that they consume.
Most corals act as water filters, by consuming the particulate matter suspended in the water column. This contributes to enhanced quality and clarity of our near shore waters.
Coral reefs and their associated habitats are important resources for coastal countries as well as for earth’s environment.
The economy of coastal countries depends on the marine resources as reefs sustain the second largest aquarium export industry in the world. However, these coastal zones are experiencing intense and sustained environmental pressures from a range of driving forces including natural and human induced activities.
The main stressors affecting the coastal ecosystem are sediments and nutrients from the terrestrial ecosystem, which occurs through river transport and via drainage basin network.
Coral reefs have been rapidly deteriorating because of population growth, coastal development, influx of tourism, coral and live rock harvesting, agriculture, deforestation etc.
Such impacts, often acting synergistically with natural impacts, can result in a decline in ecosystem productivity and stability. Left unchecked, this will ultimately lead to a reduction in income and resources for coastal communities and other stakeholders that rely on fishing and marine environment.
This need the assessment of driving forces to provide an understanding of socio-economic changes on ecosystem and monitoring of coral reef status for better management of these important marine resources.
In the most common approaches for the monitoring of coral reef environments, field sampling requires large numbers of transects data to monitor the extent of the reef environment, which can be costly and labor intensive.
Remote sensing technology is an ideal mapping tool as it can cover the complete spatial extent of a large area and be collected on a repeated basis/temporal.
Instead of this it can identify a number of environmental variables associated with habitat that are potential indicators of marine resources distribution and abundance such as coral reefs, algae and seagrass.
Monitoring and management of tropical benthic habitats require accurate and timely information on the composition and condition of the habitat. The monitoring of coral reef environments, is possible through remote sensing technology as it can cover the complete spatial extent of a management area and be collected on a repeated basis.
Remote Sensing can identify a number of environmental variables associated with habitat that are potential indicators of marine resources distribution such as coral reefs, algae and seagrass. However, field sampling requires large numbers of transects data to monitor the extent of the reef environment, which can be costly and labor intensive.
Remote Sensing and GIS for Coral Reef Assessment
In the Coral Coast area of Fiji, the change in coral cover was assessed using Landsat ETM+ data. To investigate the effect of nutrients on coral cover, different indicators of human activities could be used to measure the expected anthropogenic pressure in coastal area. This requires the rating to criteria deemed relevant to determine expected anthropogenic pressure.
The weights for each of the criteria could be assigned according to their quantity of discharge and distance from the coastline using GIS. An ordinal scale can be used for each criterion to reflect relative importance to the level of impact.
Where, P is expected anthropogenic pressure, C is criteria for rating 1 to n, W is weight for criteria 1 to n. To quantify the impact of human induced activities on the coral community, the buffer zones are created from the coastline, and on the basis of intensity of impact, the rates are allocated to each of the criteria.
For estimation of nutrient discharge from socio-economic activities affecting Coral coast area, different variables have been used.
These variables include discharge from village/town, livestock farms, hotels and agriculture land. For the purpose of anthropogenic pressure estimation, we created different zones on the basis of distance from the coastline using GIS environment. These zones include Zone 1 (0-3 km), Zone 2 (3-6 km), Zone 3 (6-9 km) and Zone 4 (9-17 km).
Criteria for Reef Assessment
The nutrients from the terrestrial ecosystem are transported through rivers and via drainage basin network therefore, ordinal ranking procedure was adopted to measure the impact of discharge from different sources. Initially ranking has been assigned to each variable based on distance from river in each zone. Therefore, 1 to 4 ranks has been defined to river based distance category (<500m, 500m-1km, 1km -1.5km and >1.5km).
On the basis of proximity and impact, river based distance category <500m was assigned higher rank (4), whereas for the distance >1.5km lowest rank (1). Then criteria rating were calculated for each of the variable (villages/farms/hotels/agriculture land) on the basis of distance for each zone using following equation:
Defining Weights
For defining the weights to each variable, two criteria have been used. One is distance from the coast and another one is discharge from each source (villages/ Livestock farms/hotels/ agriculture land).
Therefore, the weights assigned on the basis of distance from the coast varies between 2 to 7, indicating that for 0-3 km distance weight is high (7) and for distance >9km, the weight is low (2). The weights assigned for second criteria i.e. for discharge from each source (villages/ Livestock farms/hotels/ agriculture land), varies between 2 to 9.
The high weight (9) was assigned to the source with higher discharge whereas, low weight (2) assigned to the source responsible for less discharge to the coast.
Interpretation
For the Coral coast of Fiji the anthropogenic pressure was estimated for the year 1999 and 2003 on the basis of discharge and distance from the source. Result indicates that anthropogenic pressure was maximum within the distance of 0-3 km.
In 1999 the discharge from the villages (239.4), beef farms (186.2), dairy farms (157.5), piggeries (122.5), hotels (84) and agriculture land (56) was higher within 0-3 km distance zone from the coastline.
Therefore, the expected anthropogenic pressure was also higher (3.66) within this zone. In year 2003, the discharge increased from each of the source except agriculture land which was reduced due to low consumption of fertilizer.
However, increase in discharge from villages (252), piggeries (129.5), beef farms (196), dairy farms (159.6) and hotels (112) was responsible for high expected anthropogenic pressure (3.86) during 2003.
Results of quantification of nutrient discharge shows maximum input from coastal villages, which rely on onsite wastewater disposal in comparison to other activities and most affected area is within 3km from the coast. Water and reef quality tended to decrease with immediate vicinity of villages/resorts, but improved with increasing distance from sewage discharge locations.
Benthic cover /benthic habitat Change Analysis
The data revealed considerable changes in coral cover of the Coral Coast area from 1999 to 2003 period On the Coral Coast high levels of algae, dominated by sargassum are found in the inner reef flat area at many locations especially near hotels and coastal villages.
The change detection analysis of both the satellite images (ie. 1999 and 2003) show that coral cover reduced by 26% while algae and sargassum cover increased by 119% and 53% respectively.
The overgrowth of algae may also result in a loss of fish and invertebrate biodiversity. The increased nutrient inputs from villages, piggeries, beef farms, dairy farms and hotels discharges have led to a ‘phase shift’ to algal dominated reefs in the Coral Coast area.
Superb article and aptly shows the application of remote sensing data.
Thanks for appreciation.