A complete halt on all anthropogenic activities and human movement due to COVID-19 lockdown has provided a great opportunity to assess the impact of human activities on coastal marine ecosystems. The current study assessed the concentration of the metals in water samples of the largest brackish water lagoon of Asia; the Chilika lagoon in the state of Odisha, India between pre-COVID-19 and post-COVID-19 lockdown scenarios. Monthly water samples (n=30 stations) from 0.3 m depth were collected from three sectors of the lagoon seasonally; pre-monsoon, monsoon, and post-monsoon. In addition to various physical parameters [pH, salinity, alkalinity, (DO) dissolved oxygen, (TDS) total dissolved solids, and (EC) electrical conductivity] the collected water samples were analysed for 18 trace metals (Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sr, Th, Tl, U, V). Most of the physical parameters showed a significant variation between pre-and post-COVID-19 scenarios, except for pH and DO. The concentration of five metals (Be, Cd, Co, Ni, and Pb) remained below detection limits in all water samples. The impact of COVID-19 lockdown on the concentration of the metal in the water samples was noticed along with the three sectors of the lagoon. However, eight metals (Al, As, Cr, Fe, Mn, Th, U, and V) were significantly different between the COVID-19 scenarios and the remaining five metals were not statistically significant. The mean concentration of Al, As, Fe, Th, and V were higher in the pre-COVID-19 scenarios, whereas only Cr and Mn were higher in the post-COVID-19 scenarios. The mean concentration of U was similar among both COVID-19 scenarios, even though there were seasonal and sectoral differences. The seasonal influence of riverine influx was more evident on metal concentration during the monsoon season, whereas the difference between sectors was more prominent during the post-monsoon season. An increased number of correlations between physical parameters and metal concentration were observed in the post-monsoon season and post-COVID-19 scenario. This study provides evidence that the imposition of COVID-19 lockdown reduced metal influx in the water column and improved the water quality of the Chilika lagoon. Our results can be used as baseline for metal concentration in surface waters of the lagoon.

Despite saltmarshes being considered as nature-based solutions (NbS) towards climate change mitigation, India’s saltmarsh ecosystems are least studied. This study quantifies the seasonal variation in carbon stocks mono-specific saltmarsh species (Porteresia coarctata) and its potential to play an important role in India’s climate change mitigation plans. Seasonal (pre-and post-monsoon) sampling of P. coarctata surface water, biomass and 30 cm sediment core was collected across four locations on the east coast of India to quantify sediment dry bulk density (DBD), organic matter (OM%), organic carbon (Corg%), Corg stocks of sediment and biomass, total carbon (C%) and nitrogen (N%) and stable isotopes of δ13C and δ15N. In general, the sediment DBD, OM and Corg of P. coarctata meadows was higher in post-monsoon and was influenced by salinity and pH changes. Isotope (δ¹³C) modelling of various sediment C sources suggested, particulate organic matter (POM) contribution was highest (0.04-0.79%) followed by P. coarctata (0.01–052%) and other macrophytes across our study locations. The seasonal variation of δ¹³C showed increased contribution of marine derived particulate and dissolved organic matter into P. coarctata meadows in post-monsoon season. Heavier δ¹⁵N values were observed in post-monsoon season suggesting anthropogenic input, that was utilized by P. coarctata to increase its above and below-ground biomass and shoot density. The combined ecosystem (30 cm sediment + biomass) Corg stocks of P. coarctata was 1.7-fold higher in post-monsoon (4021.20± 917 Mg C) compared to pre-monsoon (2297.36 ±647 Mg C) season among the four locations with a sediment Corg contribution >70%. Based on the International Panel for Climate Change Tier II assessment the P. coarctata meadows (443 ha) can help in avoiding the pre and post-monsoon emissions of 8431.34 and 14757.84 Mg CO₂ respectively. The combined price of the total CO₂ equivalent stored in P. coarctata meadows in pre- and post-monsoon is US$ 14.50 and US$ 25.38 million respectively. Further studies quantifying the NbS potential of P. coarctata mono-specific and mixed meadows of India’s coast is needed along with integration of saltmarsh ecosystems into India’s National Action Plan on Climate Change.

The bioindicator potential of seagrass ecosystems in coastal trace metal monitoring is low for tropical island ecosystems. This study evaluated the bioindicator potential of six seagrass species exposed to anthropogenic trace metal pollution in the Andaman and Nicobar Islands (ANI) of India. Sediment and seagrass biomass, samples were analyzed for trace metals (Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Zn) from four locations of ANI exposed to anthropogenic contamination. Geo accumulation Index values indicated moderate trace metal contamination in seagrass sediment. Seagrass trace metal accumulation from sediment was both species-specific and location- specific for ANI. Small seagrass species such as H. ovalis, H. beccarii and H. uninervis accumulated the maximum concentration of six trace metals (Cr, Cu, Fe, Ni, Pb, Zn) in their tissues compared to big seagrass species like T. hemprichii and E. acoroides of ANI. This study indicates leaves of small seagrass and roots of big seagrass species can serve as short- and long-term bioindicators respectively, of coastal trace metal contamination in tropical islands of ANI.