A novel method for characterizing harmful algal blooms in the Persian Gulf using MODIS measurements

Abstract

Biophysical properties of water undergo meaningful variations under red tide (RT) outbreak. A massive Cochlodinium polykrikoids RT began in the eastern Persian Gulf (PG) in October 2008 and extended to the northern PG in December 2008. It killed large fish and hampered marine industries and water desalination appliances. Yet monthly averages of satellite-derived Chl-a (Chlorophyll-a), nFLH (normalized Fluorescence Line Height), and Kd490 (diffuse attenuation coefficient at 490nm) have not been compared in the PG. MODIS (MODerate Resolution Imaging Spectroradiometer) sensor provides global coverage, with short revisit time, and accessible, well validated ocean color products. This study compares the behavior of MODIS Chl-a, nFLH, and Kd490 in both normal and RT conditions. In doing so, their color maps are shown during normal and RT periods. Then, monthly variations of these products are shown as time-series between 2002 and 2008. HOCI (Hybrid Ocean Color Index) is defined by integrating these products to detect RT affected areas. The results gained from 100 locations in the PG show that HOCI >0.18mWcm−2μm−1sr−1mgm−4 and nFLH >0.04mWcm−2μm−1sr−1 discriminates non-bloom waters from algal blooms. Rrs(443)/Rrs(412)>1 is a proper statement to separate Trichodesmium erythtraeum from Noctiluca millaris, Noctiluca scintillans, and diatoms. Rrs(667)/Rrs(443)>1 can differentiate Cochlodinium polykrikoids from T. erythtraeum, N. millaris, N. scintillans, and diatoms as well. So, the combination of HOCI and Rrs(667)/Rrs(443) ratio is useful for detection and quantization of C. polykrikoids.