Use of hyperspectral remote sensing reflectance for detection and assessment of the harmful alga, Karenia brevis

We applied two numerical methods to in situ hyperspectral measurements of remote sensing reflectance [R.sub.rs] to assess the feasibility of remote detection and monitoring of the toxic dinoflagellate, Karenia brevis, which has been shown to exhibit unique absorption properties. First, an existing quasi-analytical algorithm was used to invert remote sensing reflectance spectra, [R.sub.rs]([lambda]), to derive phytoplankton absorption spectra, [a.sub.[phi].sup.Rrs]([lambda]). Second, the fourth derivatives of the [a.sub.[phi].sup.Rrs]([lambda]) spectra were compared to the fourth derivative of a reference K. brevis absorption spectrum by means of a similarity index (SI) analysis. Comparison of reflectance-derived [a.sub.[phi]] with filter pad measured [a.sub.[phi]] found them to agree well ([R.sup.2] = 0.891; average percentage difference, 22.8%). A strong correlation ([R.sup.2] = 0.743) between surface cell concentration and the SI was observed, showing the potential utility of SI magnitude as an indicator of bloom strength. A sensitivity analysis conducted to investigate the effects of varying levels of cell concentrations and colored dissolved organic matter (CDOM) on the efficacy of the quasi-analytical algorithm and SI found that [a.sub.[phi].sup.Rrs]([lambda]) could not be derived for very low cell concentrations and that, although it is possible to derive [a.sub.[phi].sup.Rrs]([lambda]) in the presence of high CDOM concentrations, CDOM levels influence the [a.sub.[phi].sup.Rrs]([lambda]) amplitude and shape. Results suggest that detection and mapping of K. brevis blooms based on hyperspectral measurements of [R.sub.rs] are feasible. OCIS codes: 010.4450, 280.0280.