Variations in modelled and measured hyperspectral remote sensing reflectance
Measurements of hyperspectral surface reflectance, with a spectral range of 350-800nm and sampling interval of 3.3nm, were made in Scottish coastal waters, the Bristol Channel and the Atlantic Ocean. Analysis of the shape of these spectra by normalisation and differentiation revealed three prominent features: (1) the magnitude of the integral between 400-455nm, (2) the width of a trough occurring between 560-615nm, and (3) the peak to trough height between 660-750nm. The characteristics of these features were not determined by individual seawater constituents, but they proved useful as a tool for water type classification. The sign of the integral between 400-455nm discriminated between open ocean and coastal waters, and coastal sub-types could be distinguished by applying cluster analysis to the other three features. The hyperspectral data were integrated over appropriate bandwidths to generate multi-band surface reflectance values which were used to assess the performance of remote sensing algorithms in coastal water. All the chlorophyll algorithms tested (SeaWiFS OC4V4, MODIS Chlor_a_2 and Chlor_a_3, and MERIS OC4E) overestimated the values measured in situ. The MODIS algorithm for absorption by phytoplankton, αphyto(675), performed poorly, but the MODIS algorithm for the absorption by coloured dissolved organic material, αCDOM(400), provided accurate values of the absorption coefficient (R² = 0.91). Algorithm performance was improved when turbid stations, identified using cluster analysis, were removed. Hyperspectral radiometry was also used to investigate variations in chlorophyll fluorescence line height (FLH) with chlorophyll concentration, solar irradiation and seawater composition. FLH and chlorophyll α concentration were not correlated in the coastal waters sampled and variations in the photosynthetically available radiation (PAR), CDOM and suspended sediment concentrations affected the magnitude of FLH observed. A study of (FLH / Chl) under natural, fluctuating irradiances allowed the onset of adaptive non-photochemical quenching to be monitored in situ.