Mangrove Degradation in a Megacity: Linking Tree-Ring δ¹⁵N with Local Ecological Perceptions in Mumbai

Mangrove forests provide critical ecosystem services, including carbon sequestration, coastal protection, and support for local livelihoods. Although global conservation efforts have slowed the rate of mangrove area loss, degradation remains a persistent challenge, particularly in rapidly urbanizing coastal regions.In megacities such as Mumbai, India, mangrove conservation policies are in place. However, intense urban development, population growth, and pollution pressures continue to undermine ecosystem functioning. Nutrient influx from urban sewage has caused pronounced eutrophication, potentially constraining mangrove productivity and carbon storage capacity. At the same time, conservation policies have often been implemented with limited participatory engagement, restricting traditional access to mangrove resources by the indigenous fishing community known as the Kolis. As a result, the perceptions and knowledge of the Koli community remain weakly integrated into mangrove conservation in Mumbai.This study aims to elucidate the temporal progression of mangrove degradation accompanying Mumbai’s urbanization and to examine how the life experiences and environmental perceptions of the Kolis have transformed over this period. We adopted an interdisciplinary approach integrating ecological and social data. Ecological assessments included water quality measurements and nitrogen stable isotope (δ¹⁵N) analysis of tree rings of Avicennia marina, used as a time-integrated indicator of anthropogenic nitrogen. These data were complemented by semi-structured and group interviews with the Kolis, focusing on changes in mangrove use, livelihoods, and environmental conditions. The results show elevated δ¹⁵N values recorded in the tree rings of mangroves growing in close proximity to sewage sources, indicating sustained anthropogenic nitrogen inputs over time. Meanwhile, the Koli communities demonstrated a clear awareness of environmental changes in mangrove forests and reported that fisheries commercialization, urbanization, and environmental policies have substantially altered their relationships with mangrove ecosystems. Importantly, local perceptions of environmental change were found to be largely consistent with the ecological evidence. These results underscore that the local communities, such as the Kolis, play a frontline role in perceiving environmental change, and that their knowledge is essential for effective mangrove conservation in urban coastal areas. This study demonstrates that mangrove degradation in urban coastal areas is reflected in both ecological indicators and local environmental perceptions, highlighting the importance of integrating local knowledge into mangrove degradation assessment and conservation strategies.