Linking carbon storage with land use dynamics in a coastal Ramsar wetland

Coastal wetland ecosystems make an important contribution to the global carbon pool, yet their extent is declining due to aquaculture-related land use changes. We conducted an extensive investigation into the carbon stock and area coverage of macrophytes in a tropical coastal Ramsar wetland, Kolleru in Andhra Pradesh, India. A total of 72 quadrats of size 1 × 1 m² were laid in the wetland, 19 species of macrophytes were collected and analyzed for carbon content using a CNHS analyzer. To assess changes in the wetland macrophytes, Normalized Difference Vegetation Index (NDVI) was estimated using Landsat time series data from 1975 to 2023. The importance value index (IVI) of macrophytes scored highest for the Ipomoea aquatica (41.4) and the lowest for Ottelia alismoides (1.9). Non-metric multidimensional scaling (NMDS) significantly (r = 0.1905, p = 0.0361) revealed a clear separation of macrophytes in ordination space. ANOVA indicated highly significant (p < 0.0001) variations in the carbon content of aboveground and belowground components of macrophytes. Among the different macrophytes, the highest carbon content was found in Phragmites karka (0.6 g. g^-1) and the lowest was recorded in Utricularia stellaris (0.2 g. g^-1). On an average, emergents in the Kolleru wetland sequester 1525 ± 181 g C m^-2 yr^-1, rooted floating species sequester 858 ± 101 g C m^-2 yr^-1, submerged macrophytes sequester 480 ± 60 g C m^-2 yr^-1, and free-floating macrophytes sequester 221 ± 90 g C m^-2 yr^-1. Land cover mapping revealed a decrease in spread of aquatic vegetation from 225.2 km² in 1975 to 100.6 km² in 2023. Although macrophytes are vital carbon sinks, the wetland conversion into fishponds has resulted in a loss of 55.3 % of carbon storage. Therefore, immediate restoration of macrophyte cover is vital for the proper functioning of carbon sequestration and mitigation of climate change impacts.