Seagrass meadows store significant carbon stocks at a global scale, but land-use change and anthropogenic activities can alter the natural process of organic carbon (C_org) accumulation. Here, we assessed the carbon accumulation history of two seagrass meadows in Zanzibar (Tanzania) that experienced different degrees of disturbance. The meadow at Stone Town has been highly exposed to urban development during the 20^th century, while the Mbweni meadow is located in an area with relatively low impacts but historical clearing of adjacent mangroves. The results showed that the two sites had similar sedimentary C_org accumulation rates (22–25 g m^-2 yr^-1) since the 1940s, while during the last two decades (~1998 until 2018) they exhibited 24–30% higher accumulation of C_org, which was linked to shifts in C_org sources. The increase in the δ¹³C isotopic signature of sedimentary C_org (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and C_org accumulation capacity of the meadow after the relocation of a major sewage outlet in the mid–1990s. In contrast, the decrease in the δ¹³C signatures of sediment C_org in the Mbweni meadow since the early 2010s was likely linked to C_org transport from mangrove/terrestrial material run-off following the mangrove deforestation. This study exemplifies two different pathways by which land-based human activities can alter the carbon storage capacity of seagrass meadows (i.e. sewage waste management and mangrove deforestation) and showcases opportunities for management of vegetated coastal C_org sinks

Seagrass meadows are considered efficient sinks of Blue Carbon. They capture CO₂ by an effective photosynthetic uptake as well as by trapping large amounts of carbon originating from adjacent systems, which in turn can be stored in the sediments. Such import of allochthonous carbon will be partly degraded in the system, increasing the overall community respiration and thus CO₂ production and at the same time add to sediment carbon accumulation. Additionally, tropical seagrass meadows can contain a high proportion of calcareous organisms, which (by the pH suppressing the effect of calcification) can further increase the CO₂ partial pressure of the seawater if resulting CO₂ is not internally used. The scarce literature on actual CO₂ fluxes over submerged vegetation in coastal marine areas is reporting partly contrasting data over how coastal areas in general shall be counted in carbon budgets. To better understand the CO₂ cycle within a tropical seagrass system, we constructed a simple carbon flux simulation model in which we evaluated the possible fluxes of carbon within the meadow and with regards to the surrounding seascape. We measured air-water CO₂ fluxes in seagrass meadows with different plant community compositions (i.e. mixtures of seagrass and calcifying macroalgae) using field measurements, estimated water column productivity, and extracted data for primary productivity, plant composition, and calcification from previous studies in the same area and, traced organic carbon (C_org) sources in seagrass sediment by measuring bulk stable isotope signals of carbon (δ¹³C) in order to feed the model with the best available data. When needed we supplemented with published data from other regions. The measured fluxes indicated a net efflux of CO₂ over the meadows, from sea to air. The fluxes changed both in rate and direction over the day, and were significantly related to plant community composition and environmental conditions, where pH had the strongest influence on CO₂ fluxes. Downward fluxes were found only over vegetation in the afternoon. The isotope signals of carbon revealed a strong input of carbon from other habitats. The outcome of the simulation model suggests that highly productive seagrass meadows can generate a net CO₂ flux from the water column to the atmosphere since the plants’ demand for CO₂ to a large extent is covered by a major internal cycling of CO₂, which is from degradation of autochthonous and allochthonous material as well as from CO₂ released from calcification. The calculated accumulation3of sedimentary carbon is however larger than the flow to the atmosphere, indicating that these systems can still be carbon sinks.