Studying Coral Reef Health & BioDiversity
The calcium carbonate structures of tropical coral reefs protect coastlines from storms, wave damage, and erosion; create habitats for the world’s greatest marine biodiversity (fish, marine mammals, invertebrates, and algae); provide nurseries for many marine species; play roles in carbon and CO2 cycles; are major protein sources for many local populations; and are vital for sustainable economies of many societies. Coral reefs are not only vulnerable to changes in their environment, but also are likely to respond rapidly to such changes. Oceanic and especially continental margin reefs are in peril from both acute impacts of rapidly growing local human populations and from more diffuse regional to global atmospheric, oceanographic, and climatic changes. The degradation of reefs will also have significant economic impact due to declining fisheries and increased coastal storm damage. Acropora species (elkhorn and staghorn coral) have been the dominant reef builders in the Caribbean and in Florida over the past 500,000 years. However, since the 1980s, Acropora species have declined by 97% due to global warming, disease, increasing hurricane impact, and direct human damage. In May 2006, both Acropora palmata (elkhorn coral) and Acropora cervicornis (staghorn coral) were listed as threatened under the Endangered Species Act.
Coral Reef Health
Worldwide since the 1980’s, coral reefs have experienced increasing frequencies, intensities and distributions of bleaching events, diseases, microbial blooms and other sources of mortality. Many coral reef systems are deteriorating biologically and physically and some are “in crisis,” with such symptoms as: loss of hard corals, increased abundance of algae, replacement of corals by algae, and increasing bleaching, disease, predation and microbial outbreaks. The broad extent of such events appears linked to increasing sea temperatures, rising CO2 and falling pH associated with climatic changes that may make them more sensitive to increasing local human influences as well as to regional impacts of land-based erosion with microbial intrusions and dusts. Local human impacts include population increases, shoreline development, increased sedimentation and pollution from riverine plumes, trampling by tourists and divers, ship groundings, overfishing, fishing with poisons and explosives, sewage and other waste disposal, and construction. At one extreme, bleaching events are often temporary in nature, but may cause high coral mortality within days to weeks. Other causes of mortality may be almost undetectable, extending over months to years (e.g., disease), yet contribute equally to reef decline. This range of time and spatial scales is indicative of the issues involved in monitoring change in coral reefs. Corals may have natural resistance or resilience to some levels of coral bleaching and disease. Corals may be able to endure bleaching or may be able to recover from a bleaching event.
Corals have a symbiotic relationship (photosymbiosis) with the endosymbiotic algae, zooxanthellae. Zooxanthellae use light and the nitrogenous and CO2 wastes of corals to facilitate the deposition of calcium carbonate (CaCO3) that forms the coral’s skeleton and accumulates as the hard reef structure. Coral reefs are restricted to shallow (usually clear and well-lit) waters. Under low light, in deeper waters, or in the absence of zooxanthellae, corals are sparse or absent due to their inability to deposit CaCO3 at rates higher than the losses due to bioerosion of skeletons. With sustained sea surface temperatures of >1ºC above normal ranges for sustained periods of time, zooxanthellae are often expelled, resulting in coral bleaching. Anomalous high sea surface temperatures during El Niño events have resulted in massive bleaching. Coral diseases common in the Caribbean, such as white band disease or white pox of elkhorn coral, involve bacteria that also cause the zooxanthellae to be expelled and result in bleached patches. Further, recent surveys suggest that Caribbean corals are in regional decline with up to 80% of hard corals being lost since the 1970’s.
Why is NASA Studying Coral Reefs?
Previous studies of coral reef degradation usually involved significant diving time to assess disease and change, and to take measurements of coral density. Such surveys of large reefs may incur prohibitive costs, time and effort, and are often impractical in remote areas. Remote sensing platforms allow large reef areas to be viewed more or less simultaneously and are the only practical solution for repeated monitoring of reef systems with sufficient frequency to detect major ecological phenomena on different scales. Differences in bottom reflectance must be analyzed quickly to map the reefs and especially to examine relevant optical properties of corals and other major components. Such analyses are crucial to future coral research: identification of spectral features indicative of degradation in reefs could lead to better ecological assessment and forecasting of detrimental conditions of the reef environment worldwide.
We have conducted three airborne imaging missions over coral reefs using the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) hyperspectral sensor and the Cirrus high resolution Digital Camera System (Cirrus DCS).