Carbon dioxide and methane emissions from eutrophic, vegetated coastal lagoons potentially offset carbon accumulation...

Vegetated coastal lagoons provide carbon dioxide (CO₂) uptake and are often inventoried as blue carbon (C) sinks. We measured greenhouse gas (GHG) fluxes, CO₂, methane (CH₄), and nitrous oxide (N₂O), at the water-air interface in dominant habitats including oyster aquaculture, eelgrass, and bare sediments of two temperate, eutrophic coastal lagoonal systems in Rhode Island (RI), and estimated sediment C accumulation in eelgrass habitats. We examined whether system GHG emissions at the water-air interface offset estimated eelgrass sediment C accumulation. Although highly variable, we often measured net CO₂ and CH₄ emissions from eelgrass and other habitats, which offset some or all of the estimated C accumulation in eelgrass sediments. At Potter Pond the monthly (May - October 2023) mean CO₂ equivalent (CO₂e) over a 100y horizon at the bare habitat (-0.0071 ± 0.63 ug m^-2 s^-1) was a net sink, while the mean CO₂e-100y at eelgrass (4.74 ± 0.91 ug m^-2 s^-1) and oyster (2.30 ± 0.71 ug m^-2 s^-1) habitats were net sources. At Pt. Judith Pond the mean CO₂e-100y across months and among habitats ranged from 2.49 ug m^-2 s^-1 at the oyster habitat to 5.08 ug m^-2 s^-1 at the eelgrass habitat. High CO₂ and CH₄ emissions might be attributed to the abundance of labile macroalgae, which decomposes rapidly, and to mineralization of legacy C in sediments associated with declining eelgrass. GHG emissions in eutrophic, vegetated lagoons might offset sediment C accumulation and cause an overestimation of blue C sinks.