Spatial and temporal variation in spectral reflectance: Are seagrass species spectrally distinct?

Abstract

The spectral signatures of the seagrasses Zostera capricorni, Posidonia australis, and Halophila ovalis were investigated to determine whether species could be discriminated by remote sensing. The spectral reflectance of fouled and unfouled leaf samples collected from marine and brackish habitats at three estuaries in southeastern Australia were measured in the field with a spectroradiometer during each season in 1999 and 2000. Seagrass species were spectrally distinct regardless of whether the leaves were fouled by epibionts and despite spatial and temporal variability in the reflectance of each species. The visible wavelengths that penetrate water coincide with the regions of maximum absorption and characteristic reflectance by plant photosynthetic and accessory pigments. Strong and consistent differences in reflectance between species were recorded in the green wavelengths at 530-580 nm with additional discrimination in the regions 520-530 nm and 580-600 nm and at the red chlorophyll absorption trough at 686-700 nm. Species discrimination should be possible in the remote sensing of benthic aquatic vegetation using a hyperspectral sensor that has narrow bands centered on pigment-related spectral features in the visible wavelengths. The detection of statistically significant differences in intraspecific reflectance associated with the year, season, estuary, and habitat of sample collection suggests a potential for monitoring seagrass health and estuarine water quality. This study has produced the first spectral library of aquatic plant species to take into account the range of spectral variability expected for the species under natural conditions. The results provide a sound basis for future mapping of seagrass species in Australia.

A Method to Analyze the Potential of Optical Remote Sensing for Benthic Habitat Mapping

“…We should note that unlike HICO, HyMap does not have bands below 450 nm [51], hence both sensors have wavebands that emphasize the spectral differences between the benthic classes analyzed. The literature has shown that specific wavelength ranges predominantly within 520-680 nm are useful for identifying substrate types, such as healthy coral, bleached coral, seagrass, sand and algae [34,39,50,52,53]. Therefore, it is likely that the difference in bandwidth is the cause where the spectral resolution of HICO and HyMap are 5.7 and 15 nm [51] respectively.…”

“…samples were senescent and less spectrally distinct (using WV2 bands at least), which likely lead to its inclusion into the green algae cluster. Fyfe [50] compared the in-air reflectance of several unfouled and fouled seagrass species and showed that the latter had broader and less reflective peaks between 520-580 nm compared to the unfouled case. It is likely that this seagrass species can be optically separated if their reflectance spectra are collected during their growing season.…”

Retrospective assessment of macrophytic communities in southern Lake Garda (Italy) from in situ and MIVIS (Multispectral Infrared and Visible Imaging Spectrometer) data

“…With adequate repeatability, airborne and satellite sensor data can be used for multi-temporal studies and they provide reliable information for understanding the ecological dynamics of macrophytes, through the analysis of intra-and inter-seasonal patterns or spatial landscape trends or changes (Fonseca et al 2002). Furthermore, some biophysical and physiological vegetation features (i.e., biomass, cover percentages, structural stand complexity and composition and the physiological status of vegetation) can be detected and estimated from specific spectral properties of reflectance spectra (Fyfe 2003;and references therein). In addition, these parameters enable us to assess the environmental status and the biodiversity of the meadows studied, and also use them as input data for ecosystem models (e.g., for determining the habitat for organisms that depend on seagrass) (Bell et al 2006;Duffy et al 2006;Gillanders 2006 Jaeger et al 2004;Schaumburg et al 2007;Oggioni et al 2011) require great efforts to detect the actual presence of macrophyte areas, and such efforts become prohibitive in large, deep lakes.…”

Remote Sensing of Shallow Coastal Benthic Substrates: In situ Spectra and Mapping of Eelgrass (Zostera marina) in the Gulf Islands National Park Reserve of Canada

“…Studies have shown that heavy epiphyte loads can absorb up to 63% of incident light in the peak chlorophyll absorption bands, leading to greater reflectance slopes at 440 nm and 680 nm than at 550 nm [25]. Reflectance in the 575-630 nm region is markedly increased [26] while green light is physically blocked by non-photosynthetic material accumulation [25] and absorbed by non-chlorophyte epiphytes [27]. These effects lead to a flattening of the eelgrass spectral signature and increased variability in green reflectance values [26,28].…”