Seascape connectivity has low influence on seagrass blue carbon in tropical oligotrophic Islands

Seagrass meadows are important blue carbon sinks, yet significant knowledge gaps exist in understanding the role of seascape connectivity in influencing carbon (C) and nitrogen (N) stocks. This study addressed this existing knowledge gap, by quantifying the sediment and seagrass ( Thalassia hemprichii ) biomass C and N stocks, and utilized stable isotope non-connected modelling to assess the contribution of various sources to the sediment C pool in non-connected (with other seagrass) and connected (with mangroves) meadows in the Andaman and Nicobar Islands, India. We observed that non-connected meadows sediment contained 3.7-fold higher total N, and enriched δ 13 C and δ 15 N values compared to connected meadows. Contrastingly, the sediment in connected meadows exhibited higher organic matter, total C, and more depleted δ 13 C and δ 15 N. Surficial sediment C stocks (3.50 ±1.78 Mg C ha -1 ) were higher in connected meadows, while N stocks (1.21 ± 0.71 Mg N ha -1 ) were higher in non-connected meadows. Higher N availability could be led to higher density and biomass N stocks in non-connected meadows. Stable isotope mixing modelling indicated that the average contribution of T. hemprichii biomass was highest (0.69 ± 0.13%) to the sediment C pool, followed by other seagrass biomass in non-connected meadows. In connected meadows, the mean contribution of T. hemprichii biomass (0.68 ± 0.10%) was also higher than mangrove C sources. This study highlights that the influence of seascape connectivity for cross habitat subsidies of C and organic matter is lower than connected seagrass meadows in intertidal oligotrophic island ecosystems of the Indian Ocean region. • Non-connected seagrass meadows store higher total N than connected meadows • Connected meadows have higher sediment carbon (C) stocks than non-connected meadows • Non-connected meadows have higher sediment nitrogen stocks than connected meadows • Stable isotopes modelling indicated the highest contribution of Thalassia hemprichii biomass to sediment C pool • Cross habitat subsidies of C are lower intertidal oligotrophic seagrass meadows