An algorithm for detecting trophic status (chlorophyll-a), cyanobacterial-dominance, surface scums and floating vegetation in inland and coastal waters

A novel algorithm is presented for detecting trophic status (chlorophyll-a), cyanobacterial blooms, surface scum and floating vegetation in coastal and inland waters using top-of-atmosphere data from the Medium Resolution Imaging Spectrometer (MERIS). The maximum peak-height algorithm (MPH) uses a baseline subtraction procedure to calculate the height of the dominant peak across the red and near-infrared MERIS bands between 664 and 885 nm caused by sun-induced chlorophyll fluorescence (SICF) and particulate backscatter. Atmospheric correction of the MERIS TOA reflectance data for gaseous absorption and Rayleigh scattering proved adequate given the spectral proximity of the relevant bands and the sufficiently large differential spectral signal. This avoided the need to correct for atmospheric aerosols, a procedure which is typically prone to large errors in turbid and high-biomass waters. A combination of switching algorithms for estimating chl-a were derived from coincident in situ chl-a and MERIS bottom-of-Rayleigh reflectance measurements. These algorithms are designed to cover a wide trophic range, from oligotrophic/mesotrophic waters (chl-a < 20 mg m− 3), to eutrophic/hypertrophic waters (chl-a > 20 mg m− 3) and surface scums or dry floating algae or vegetation. Cyanobacteria-dominant waters were differentiated from those dominated by eukaryote algal species (dinoflagellates/diatoms) on the basis of the magnitude of the MPH variable. This is supported by evidence that vacuolate cyanobacteria (Microcystis aeruginosa) possess enhanced chl-a specific backscatter which is an important bio-optical distinguishing feature. This enables these phytoplankton groups to be distinguished from space. An algorithm derived from cyanobacteria-dominant waters had a r2 value of 0.58 for chl-a between 33 and 362 mg m− 3 and an error of 33.7% (N = 17). The operational algorithm for eukaryote-dominant algal assemblages gave a coefficient of determination of 0.71 and a mean absolute percentage error of 60% for chl-a in the range 0.5–350 mg m− 3 (N = 48). A flag based on cyanobacteria-specific spectral pigmentation and fluorescence features was used to identify cyanobacterial-dominance in eutrophic waters. Global applications demonstrate how the MPH algorithm can offer rapid and effective assessment of trophic status, cyanobacterial-dominance, surface scums and floating vegetation in inland and coastal waters.

► A new trophic status (chl-a) detection algorithm for inland and coastal waters.
► Trophic status can be determined from bottom-of-Rayleigh reflectance.
► A new flag for cyanobacteria based on unique pigmentation and fluorescence features.
► Distinction between prokaryotic and eukaryotic-dominant eutrophic waters.
► Surface scum and floating vegetation detection using MERIS red/NIR wavebands.