Long-Term Analysis of Water Quality in Chilika Lagoon and Application of Bio-optical Models for Cyclone Impact Assessment

Abstract

A comprehensive analysis of sediment and phytoplankton dynamics in Chilika lagoon by synthesizing various remote sensing datasets is presented in this study. The goal of the study was to monitor and analyze the spatio-temporal variability of total suspended sediment (TSS) and chlorophyll-a (chl-a) concentration and associated environmental forcings in the coastal lagoon. NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance cloud free data was used to develop a TSS and chl-a model. Finally, a case study showing implication of satellite based TSS and Chl-a models to assess the impacts of natural hazards such as cyclones on water quality of Chilika Lagoon is presented. This case study is based on comparing the effect of two anniversary very severe cyclonic storms (VSCSs): category-5 Phailin (12 October, 2013) and category-4 Hudhud (12 October, 2014) that impacted the lagoon. Analysis for 14 years (2001–2014) using MODIS 8-day composites (MOD09Q1) data indicated that the seasonal variability of TSS is dominant in all the three sectors of the lagoon compared to inter-annual variability. The main reason for large variations in the northern sector is the shallow depth and intrusion of large sediment discharge from Mahanadi River from the northern side, which is the largest fresh water distributary for Chilika Lagoon. Anniversary cyclone impact analysis revealed that Phailin’s impact on Chilika Lagoon and its watershed resulted in unprecedented levels of precipitation and runoff before-during-after the landfall, which shattered the typical sectorial turbidity gradient. Exponential increase in turbidity because of a combination of run-off and wind driven re-suspension of fine sediments resulted in strong attenuation of light in water column post-Phailin. Limited light condition coupled with enhanced flushing rate due to flooded river and increased freshwater discharge reduced the Chl-a concentration after the passage of Phailin. In contrast, relatively farther landfall location, trajectory away from the lagoon, relatively lower wind intensity and short duration of stay of VSCS Hudhud, led to lesser precipitation and surface runoff compared to Phailin. Consequently, lagoon did not experience a drastic increase in turbidity and light attenuation. Sufficient light availability, stable wind, reduced flushing all favored the phytoplankton growth after passage of Hudhud and thus, Chl-a concentration increased almost threefold in all the sectors of the lagoon. The approach used in this study can be applied to other cyclone-prone coastal areas. Coupling of satellite based observation with modelling output from systems such as Giovanni can improve monitoring program implemented in numerous coastal estuaries and lagoons.