Ocean and coastal areas are buffers against global warming by absorbing 23% CO2 from the atmosphere (Sabine et al., 2004; DeVries et al., 2017). Significantly, coastal and oceanic vegetated ecosystems (such as wetlands, corals, mangroves, seagrass meadows, and kelps), so-called “blue carbon ecosystems” (B.C.E.), have been paid attention to absorb and sequester carbon dioxide from the atmosphere (Nellemann et al., 2009). Recently, freshwater carbon ecosystems, such as lakes, ponds and reservoirs, have also been revealed to contribute to atmospheric carbon sequestration. However, variations in climate patterns, biodiversity spectrums, and anthropogenic activities (e.g., non-sustainable resource extraction, land-based pollution and habitat degradation) across the ecosystems could alter the fragile balance of B.C.E. as carbon sinks or sources (Malhi et al., 2020; Abbass et al., 2022). Also, higher spatiotemporal carbon flux variation was observed in coastal and estuary ecosystems due to intensive anthropogenic activities, natural biological processes, and fluctuating terrestrial loads (Asmala et al., 2016, Asmala et al., 2020). Consequently, understanding the factors and processes that drive the spatiotemporal variations in Dissolved Organic Matter (D.O.M.) quantity and quality under changing environmental conditions would benefit the sustainable use of natural resources in freshwater, coastal and oceanic ecosystems. To contribute to the S.D.G. Goals in Climate Action (Goal 13) and Life Below Water (Goal 14), this Research Topic focuses on carbon dynamics, e.g., GHG emission/storage (blue carbon), as well as environmental stresses such as acidification, fishery resource and pollution (e.g., eutrophication, dead zone and metal pollution) in freshwater, coastal and oceanic ecosystems. Eleven research articles were collected on this Research Topic, as summarized below.

Shallow coastal waters have attracted due to their strong carbon sequestration capacity. Macroalgae culture is considered a natural-based solution for marine carbon sequestration. Research Topics revealed that carbon sequestration by kelp culture is applicable; 5% of DOC released by S. japonica was transformed to refractory DOC, and an estimated 1–2% of the net primary production of cultured kelp was sequestrated as refractory DOC. Also, tidal flats are expected to have the same carbon sequential potential as shallow coastal areas. However, the complicated temporal and spatial variation of carbon dioxide (CO₂) makes it difficult to accurately estimate the air-water CO₂ fluxes (fCO₂). For example, the size of an Submerged Aquatic Vegetation (S.A.V.) patch significantly affects flows and carbon sequestration. In contrast to small-scale phenomena, the higher fCO₂ values in the cyclonic and non-eddy regions were revealed due to the upwelling, increasing the surface fCO₂. Additionally, physical processes (such as the residence time and water depth) drive CO₂ and O₂ temporal and spatial patterns; otherwise, biological processes (benthic algae biomass and respiration) may also affect variability within lakes, ponds and reservoirs.

From the water quality perspective, nutrient pollution threatens the seagrass community and may adversely affect their carbon sequestration potential, diminishing the carbon sequestration potential of seagrass ecosystems. Higher nutrient loading elevated labile organic carbon content (e.g., free amino acids and soluble sugars). Contrary, refractory organic carbon compositions of seagrass tissues (i.e., cellulose-associated organic matter) decreased with increasing nutrient loading. Also, an urbanized enclosed bay has environmental problems, such as hypoxia, which degrades carbon dioxide absorption from the atmosphere to the water surface.

As a necessity of new research, the importance of “blue carbon” (marine carbon) in the global quest to achieve carbon neutrality has been well understood. However, freshwater carbon has yet to be considered as a climate change mitigation approach, although freshwater areas (5.0 million km²) are more extensive than coastal areas (1.8 million km²) and are expected to have more significant potential for carbon sequestration. Therefore, we need more investigations on both not only marine carbon but also freshwater carbon. The results of this study provide new and deeper insights into coastal carbon dynamics to propose climate change mitigation in future climate and to achieve the S.D.G.s.

J-WT and KN structured this Research Topic. All guest editors have edited and reviewed the editorial articles, and approved the submitted version.