Relevance of carbon burial and storage in two contrasting blue carbon ecosystems of a north-east Pacific coastal lagoon.

Coastal vegetated ecosystems constitute very productive habitats, characterized by efficient Corg sequestration and long-term preservation in sediments, so they play an important role in climate change mitigation. The temporal evolution of Corg content, stocks and burial rates were evaluated in seagrass and salt marsh habitats in San Quintin Bay (northeast Pacific, Mexico) by using 210Pb-dated sediment cores. Salt marsh cores were characterized by fine-grained sediments, higher salinities, lower terrigenous input and lower mass accumulation rates (MAR: 0.01–0.03 g cm−2 yr−1) than seagrass cores (MAR: 0.02–3.21 g cm−2 yr−1). Accumulation rates in both habitats steadily increased throughout the past century most likely because of soil erosion promoted by land use changes in the surroundings. The Corg stocks were highest in salt marsh cores (12.2–53.6 Mg ha−1 at 10 cm depth; 259–320 Mg ha−1 at 1 m depth) than in seagrass cores (5.7–14.4 Mg ha−1, and 80–98, Mg ha−1, respectively), whereas Corg burial rates were considerably lower in salt marsh (13–60 g m−2 yr−1) than in seagrass (9–144 g m−2 yr−1) habitats, and the temporal variations observed in Corg burial rates were mostly driven by changes in the accumulation rates. The overall Corg stock (485 ± 51 Gg C) for both habitats together was comparable to the carbon emissions of a major city nearby. Our results highlight the need to protect these environments as relevant carbon reservoirs.