Seagrasses are highly productive marine plants that form meadows providing important ecological services, such as oxygen production, water filtration and habitat for marine creatures. They also store approximately 15% of the organic carbon in the ocean, despite covering only a small fraction of the seafloor. In the last decades, seagrass meadows have been declining worldwide at alarming rates, potentially shifting these systems from carbon sinks to carbon sources. This widespread mortality alters coastal carbon cycling through mechanisms are not yet fully quantified.
In this study, we estimate the rates and quantities of carbon and associated chemical components released during seagrass decomposition. We conducted laboratory experiments simulating different seagrass parts remineralization under aerobic conditions. Surprisingly, these experiments showed that alkalinity increases during seagrass decay, even though alkalinity increase is unexpected in an open system, where CO2 exchange with the atmosphere is allowed and seawater is supersaturated with respect to CaCO3.
We suggest that this alkalinity increase is driven by mineral dissolution at the leaf surface, where respiration creates localized acidic microenvironments. To test this hypothesis, we repeated the experiments with the addition of powdered olivine and aragonite. In these experiments alkalinity increased further, strengthening the hypothesis that seagrass remineralization enhances mineral dissolution.
Our calculations indicate that this process retains approximately 10% of the carbon that would otherwise outgas to the atmosphere. These findings demonstrate that seagrass carbon cycling is more complex than a simple sink–source framework and highlight a mineral-mediated mechanism that is not currently accounted for in carbon flux estimates from decaying seagrass meadows.