Contrasting bio-optical characteristics of coastal water prior to and in the aftermath of a tropical super cyclone

Abstract

ABSTRACTApparent and inherent optical properties were measured in the coastal region off Visakhapatnam (Bay of Bengal) during pre-monsoon season (January–May) of successive years, in-between of which the (October 2014) Hudhud super cyclone had landfall across. For 5 months after the cyclone in the region up to 50 m isobath, or 15 km from the coast and 6 months after the cyclone in the region up to 30 m isobath, or 5 km away, the particle back-scattering coefficient (bbp) and remote-sensing reflectance (Rrs) were anomalously higher throughout the spectral region (400–700 nm) than during other times. The bbp spectral slope coefficient γ is abnormally higher than predicted from the bbp(680)–γ model relationship, and Rrs(685) is also abnormally higher than the significant Rrs(685)–chlorophyll-a (chl-a) relationship would predict. These exceptional properties are attributed to the presence of little absorbing and highly reflecting sub-micrometre-sized particles in fine suspension that leached from the coastal deposits emplaced in the study region from the nearby harbour during the cyclone. The optical properties recovered subsequently. In the period when the reflectance and scattering spectra are not recovered, the Rrs(685) is no longer a proxy of chl-a. Remote-sensing algorithms for phytoplankton abundance of the coastal region during this period need correction.