Differential Impact of Anniversary-Severe Cyclones on the Water Quality of a Tropical Coastal Lagoon

摘要

Mapping spatio-temporal dynamics of suspended load in a lagoon before-during-after a cyclone is crucial for monitoring sudden nutrient enrichment and associated processes such as algal blooms and siltation. However, not all cyclones produce similar impact on a coastal lagoon, some trigger algal blooms after passage while others just increase the overall turbidity. Asia's largest brackish water lagoon, Chilika Lagoon, India was hit by two anniversary-severe cyclones Phailin (12 October 2013) and Hudhud (12 October 2014) recently. Their impacts were analysed with respect to physical, biological and meteorological factors which favour or restrict a phytoplankton bloom after the passage of a storm. Moderate-resolution imaging spectroradiometer surface reflectance data were used to examine the spatio-temporal variability in total suspended sediment (TSS) and chlorophyll-a (Chl-a) concentration pre-and post-cyclone. Comparative results revealed that Phailin was associated with higher rainfall, wind speed and surface runoff compared with Hudhud. These factors contributed to higher TSS concentration in all sectors of the lagoon post-Phailin compared with post-Hudhud. Extreme TSS, limited light, high rainfall and runoff and increased flushing rate post-Phailin restricted the likelihood of a phytoplankton bloom in the lagoon, a commonly reported phenomenon after the passage of a cyclone. In contrast, sufficient light availability due to lower TSS, low runoff and flushing and stable wind supported a phytoplankton bloom post-Hudhud. The observed differential impacts were linked to the individual cyclone's characteristics such as differences in landfall locations, wind speed, trajectory after the landfall, differences in rainfall rate and surface runoff and speed of passage.