Quantifying organic carbon burial rates and stocks in seagrass meadow sediments influenced by sargassum-brown tides

Seagrass meadow sediments are efficient organic carbon (C_org) sinks and can store C_org for hundreds of years. The temporal variation of C_org burial rates and stocks over recent decades at nearshore seagrass meadows in the Puerto Morelos Reef Lagoon, Mexico, was evaluated in ²¹⁰Pb-dated sediment cores from nearshore meadows dominated by Thalassia testudinum. The sediments were predominantly sandy (>52% sand) rich in carbonate grains (11.8-12.5% C_inorg) with minor C_org (0.24-1.12%) and N_org (0.02-0.13%) concentrations. The C:N ratio (9.4-13.0) indicated that marine-derived C_org was prevalent. C_org stocks in the upper 30 cm sediment were 15.9 ± 3.0-24.8 ± 4.6 Mg ha^-1. Sedimentary mass accumulation rates (MAR) (0.7-1.5 g cm^-2 yr^-1) were higher than those previously recorded in seagrass sediments from the reef lagoon and other parts of the world. The highest MAR values, recorded in 2015 (±0.13) and 2018 (±0.03), coincided with the peak sargassum influx years. MAR and C_org burial rates (11.4-133 g m^-2 yr^-1) were correlated (r² = 0.76), indicating that the massive influxes of sargassum have accelerated C_org burial rates in the region since 2015. This study marks the initial evaluation of the interaction between the massive influx of sargassum, MAR, and C_org burial rates in seagrass sediments, potentially laying the groundwork for future extended monitoring initiatives.