Patterns of Carbon and Nitrogen Accumulation in Seagrass (Posidonia oceanica) Meadows of the Eastern Mediterranean Sea

The variability in stocks and accumulation rates of organic carbon (C_org), nitrogen (N), and carbonate (CaCO₃) was studied in fifteen Posidonia oceanica meadows spread throughout the South Aegean Sea (Greece). In addition, the abiotic and biotic drivers determining the pattern of variability in the accumulation rates were assessed by exploring the influence of sediment characteristics, seagrass traits, and environmental settings. The meadows supported on average (±STDEV) 14.6 ± 5.0 kg C_org m⁻², 0.47 ± 0.17 kg N m⁻², and 249 ± 210 kg CaCO₃ m⁻² in the top meter of their sediments, with mean accumulation rates over the last 500 years of 33.6 ± 23.6 g C_org m⁻² yr⁻¹, 1.00 ± 0.62 g N m⁻² yr⁻¹, and 405 ± 336 g CaCO₃ m⁻² yr⁻¹ across sites. A redundancy analysis (RDA) explained 70% of the variation in C_org, N, and CaCO₃ accumulation rates, with three sediment characteristics (i.e., sediment C_org:N and C_org:C_inorg ratios and P. oceanica contribution to the sediment C_org pool) emerging as the primary set of factors shaping the accumulation of matter, followed by seagrass traits (i.e., leaf biomass and rhizome elongation) and environmental variables (i.e., suspended organic matter). The high degree of variability within the region emphasizes the need for fine-scale assessments to understand the local conditions influencing sequestration. Our findings underscored the critical role of seagrass meadows in carbon and nitrogen sequestration in the region, urging conservation efforts to protect these ecosystems and prevent potential losses of stored carbon and nitrogen following seagrass degradation.