不同生长状态珊瑚光谱特征

珊瑚礁生态系统迅速退化是目前重要的生态环境问题之一,应用遥感技术监测大范围珊瑚礁的结构组成和变迁有很大的潜力。珊瑚光谱响应特征受珊瑚生态习性影响,在光学上相似而容易造成混淆误判。采集了西沙群岛大量石珊瑚样品的光谱,对其光谱特征进行分析及成因探讨。通过导数光谱、主成分分析研究了不同生长状态珊瑚的光谱差异,并建立珊瑚生长状态高光谱遥感判别准则。结果表明,珊瑚的光谱特性及其变化均较为复杂,受珊瑚种类和生长环境影响,光谱形状主要由共生藻色素吸收决定的。结合520—530 nm、564—574 nm和600—605 nm的导数光谱可以区分健康珊瑚、白化珊瑚和藻类覆盖的死珊瑚。总体判定准确度优于80%,误判的主要来源是种内珊瑚反射率差异。研究表明珊瑚礁环境高光谱遥感可以定量评估珊瑚状态的变化。

Analysis of spectral characteristics of coral under different growth patterns

Rapid degradation and large-scale coral bleaching of coral reef ecosystems around the world are important ecological and environmental problems. Knowledge of community structure is of great importance in determining the world''s coral reef resources. Remote sensing has great potential for assessing the composition of coral reefs and the extent of reef change. Coral spectral response characteristics are affected by coral ecological habits, which may become confused by optical similarities. High spectral resolution sensors are required to perceive these subtle differences. To map the proportion of live and dead coral, the remote sensing sensor must be able to distinguish between their reflectance spectra. In this paper, we collected samples of various typical coral species with different growth characteristics from reefs near the Xisha Islands (also known as the Paracel Islands). Spectral reflectance of these samples was measured using an AvaField portable spectrometer. Spectral character analysis was carried out based on reflectance spectra, principle components analysis and derivative spectroscopy. Hyperspectral remote sensing criteria were established to distinguish corals with different growth habits. For coral, reflectance is a complex function of pigmentation, material composition structure and morphology. Coral spectral characteristics are highly variable, and are controlled by coral species and the growth environment. The coloration of corals is mainly due to symbiotic photosynthetic dinoflagellates commonly referred to as zooxanthellae. Despite variations in absolute magnitude between species, many healthy species remain a similar shape, and exhibit relatively depressed reflectance in the visible band because of the absorption of symbiotic algal photosynthesis and very rapidly increasing reflectance at wavelengths greater than 675 nm. There are either peaks or shoulders near 575, 600 and 650 nm. Coral bleaching is the result of symbiotic algae loss, which exposes the underlying white skeleton. Reductions in photosynthetic absorption and coral tissue scattering cause a rapid increase in reflectivity in visible bands, especially from 500 to 650 nm. The spectral reflectance of bleached corals has a higher amplitude and flatter shape. Algae-covered dead corals give rise to pigmentation that may be similar to that of healthy coral. Peridinin may be used as an indicator of algae-cover because it is a diagnostic pigment for dinoflagellates. Principal components analysis was employed to determine whether there are statistical differences between healthy, bleached and algae-covered corals. The first principal component explains 90%, 94%and 97%of the variance, respectively. Intra-species variability of the spectral features was not significantly different from interspecies variability. Spectral discrimination of these three states of coral is indeed possible with relative spectral response functions. Derivative spectra showed particular differences between species at specific wavelengths where several chlorophylls and other accessory pigments absorb. The results demonstrated that healthy coral, bleached coral and algae-covered dead coral can be distinguished according to derivative spectra at 522-530 nm, 564-574 nm and 600-605 nm. The overall accuracy is above 80% with the main source of error resulting from intra-species spectral variability. These results show that the application of hyperspectral remote sensing to quantitatively assess the extent of coral bleaching is feasible. The spectral libraries are an important resource, which can be used not only in remote sensing, but also in photosynthetic studies.