Revealing the superiority of dissolved organic carbon fluxes from submarine groundwater discharge in karst seagrass beds, Morinje Lagoon, Croatia

Underwater ecosystems, particularly those rich in seagrass, are characterized by their robust carbon sequestration capabilities and serve as a natural solution for mitigating climate change. Submarine groundwater discharge (SGD) acts as a conduit for transporting substances, such as dissolved organic carbon (DOC), from land to coastal areas. The SGD-derived DOC is pivotal in the carbon cycle of seagrass beds, although its influence on this cycle is not yet fully understood. In this research, a radon (²²²Rn) mass balance model was utilized to measure the SGD and SGD-derived DOC fluxes in a typical karst seagrass bed located at Morinje Lagoon in Croatia. The SGD was estimated to be (17.6 ± 4.4) cm/d, and the SGD-derived DOC was (155 ± 39) mmol/(m²·d). Furthermore, the carbon burial rate in the seagrass bed sediment was calculated to be 18.1 mmol/(m²·d). The ratio of SGD-derived DOC to carbon burial was 8.6, surpassing that of other coastal blue carbon ecosystems (0–1.1). Through excitation emission matrix fluorescence (EEM) and parallel factor (EEM-PARAFAC) analysis, it was determined that fluorophore dissolved organic matter (FDOM) components, predominantly humic-like (FDOMH) and protein-like (FDOMP), contribute 80% and 20%, respectively. In addition to vertical burial fluxes as a means of carbon sequestration, the SGD-derived DOC, especially FDOMH, has the potential for long-term storage in the ocean, underscoring the seagrass bed’s role as a dependable carbon sink.