Soil nutrient content and water level variation drive mangrove forest aboveground biomass in the lagoonal ecosystem of Aldabra Atoll

Seabird nutrient subsidies enrich mangrove ecosystems and are exported to nearby coastal habitats

“…Foliar N:P ratio in our study was 11.7 ± 2.45 (Mean ± SD), suggesting mangroves in our study are N-limited, which is typical for fringe mangroves influenced by frequent tidal flushing 7,44 . N limitation at our study site is further indicated by the equally low N:P ratios documented in previous analyses on mangrove soil 45 and lagoonal sediment porewater on Aldabra 46 . At our seabird sites, uptake of seabird-derived nutrients alleviated mangrove nutrient limitations of both N and P, observed by reductions in foliar C:N and C:P ratios of R. mucronata .…”

“…Altogether, this suggests that seabird-derived nutrient enrichment is unlikely to be causing declines in mangroves on Aldabra. Soil nutrient content is a main driver of mangrove aboveground biomass on Aldabra 45 . Therefore, by fertilizing mangroves where they nest, seabirds likely enhance mangrove productivity in their breeding areas, further promoting suitable nesting habitat, likely creating a positive-feedback loop for seabird populations 10 .…”

“…Aldabra’s mangroves support the highest biomass of herbivorous fish and the highest abundance of sharks in Seychelles 68,69 . Furthermore, Aldabra’s mangroves comprise one of Seychelles’ largest blue carbon ecosystems 45 . By improving mangrove nutrient status and health, we show that seabirds nesting on Aldabra boost biodiversity and ecosystem services.…”

“…We assessed forest characteristics in each quadrat following methods previously employed on Aldabra 45 . We recorded the species of all trees >2 m height and measured their diameter at breast height (DBH, 1.3 m height) using a measuring tape, following guidelines for trees with anomalies such as prop roots 81 .…”

“…We recorded the species of all trees >2 m height and measured their diameter at breast height (DBH, 1.3 m height) using a measuring tape, following guidelines for trees with anomalies such as prop roots 81 . We counted the number of seedlings and used an inclinometer (Suunto PM-5/66 PC, Vantaa, Finland) to measure tree height to the nearest ± 0.5 m. We applied the allometric equation by Chave et al, 82 to determine aboveground biomass (ABG) following Constance et al, 45 . Aboveground biomass (ABG, kg) per tree was obtained from the wood density ( ρ , g.cm −3 ), the DBH (D, cm), and height (H, m).…”