Short-term desalination of Pulicat lagoon (Southeast India) due to the 2015 extreme flood event: insights from Land-Ocean Interactions in Coastal Zone (LOICZ) models

Abstract

Introduction:We investigated the magnitude and duration of desalination of Pulicat-a coastal lagoon ecosystem connected to the Bay of Bengal on the South-eastern coast of India-during the 2015 South India flood event which was a period of high-magnitude precipitation and riverine flooding. Methods:We estimated freshwater runoff into the lagoon using flow accumulation models for a period of 55 days (November 1 to December 25, 2015) using daily gridded precipitation data from the Global Precipitation Measurement and a digital elevation model. Using the estimates of freshwater runoff, direct precipitation and observed salinities, we simulated water and salinity fluxes of the lagoon using the Land-Ocean Interactions in the Coastal Zone model. Further, we also used Monte Carlo simulation to estimate the uncertainty in system salinity, the residual salinity at the boundary and the freshwater residence times in the lagoon. Results:We estimated that a high volume (~760 × 10 6 m 3 ) of relatively low salinity waters (residual salinity = 23.47 psu) had been exported from the lagoon to the Bay of Bengal during the period which is likely to have caused a strong dip in the daily salinity profile of the coastal sea. We contend that the lagoon experienced~40% desalination due to the 2015 event with a freshwater residence time of 18.5 days. Conclusions:The study highlighted the short-term, high-magnitude desalination undergone by Pulicat lagoon during the 2015 South India floods. Considering the high residual and exchange volumes obtained from the study, we conclude that Pulicat could be a major exporter of relatively low salinity waters to the Bay of Bengal during monsoons.

Spatial and temporal analyses of salinity changes in Pulicat lagoon, a transitional ecosystem, during 1996–2015

“…Thus, it should be noted that any dilution in salinity would be facilitated through direct surface mixing with the rainwater. The presence of sandy shoals in this portion of the lagoon (Sanjeevaraj, 2011;Santhanam & Natarajan, 2018) contributes to the uneven mixing of the tidal water from the south, with the freshwater from the Western portion under the influence of river Kalangi and the hypersaline water from the stagnant eastern sector of the lagoon. Hence, the impact of secondary salinization is not easily visible in central sector as it was in the case of the southern sector.…”

“…The data from 2015 support this observation on a possible regime shift. The salinity dilution in the aftermath of the event was quantified to be approximately 40% (Santhanam & Natarajan, 2018) and concurrently, a high-magnitude diatom bloom comprising most exclusively of the Biddulphia sp. has recently been reported closer to the Kalangi influence zone in the Western sector and the Araniar influence zone in the Southern sector .…”

“…We used the salinity dataset for the POE season of 2015 to illustrate the salinity distributions of the lagoon for a recent period under maximum deviation from the normal mean (e.g. Santhanam & Natarajan, 2018). This dataset is used as a standalone dataset for studying the change in the spatial trends as a whole during the period 1996-2015.…”

“…From the perspective of coastal systems of Southeast India, monsoonal exchanges with low-saline waters of lagoons and influx from riverine estuaries have long been suspected as causes for the reduction of the long-term average salinity of the adjacent Bay of Bengal (BoB; e.g. Jensen, 2001;Santhanam & Natarajan, 2018;Vinayachandran, Jahfer, & Nanjundiah, 2015). Such lagoon ecosystems also exhibit high internal salinity variabilities between critical phases of shifts across the seasonal gradients.…”

Nutrient dynamics and budgeting in a semi-enclosed coastal hypersaline lagoon

“…Pulicat is reported to have the highest salinity of 90 during a severe drought in 1975 (Raman et al 1977(Raman et al ), 44 in 1999(Raman et al and 46 in 2008 (Santhanam and Amal Raj 2019). The most recent event of lagoon desalination by 40% was reported by Santhanam and Natarajan (2018) during the flood events of 2015. Such salinity fluctuations in Pulicat lead to severe consequences on the lagoon biota, thereby altering the lagoon biogeochemical attributes (Liu et al 2009).…”