Mangroves are critical blue carbon ecosystems. Measurements of methane (CH₄) emissions from mangrove tree stems have the potential to reduce uncertainty in the capacity of carbon sequestration. This study is the first to simultaneously measure CH₄ fluxes from both stems and soils throughout tidal cycles. We quantified carbon dioxide (CO₂) and CH₄ fluxes from mangrove tree stems of Avicennia marina and Kandelia obovata, which have distinct root structures, during tidal cycles. Tree stems of both species served as net CO₂ and CH₄ sources. Compared to fluxes in the soils, the mangrove tree stems exhibited remarkably lower CH₄ fluxes but no difference in CO₂ fluxes. The stems of A. marina exhibited an increasing trend in CO₂ flux from low to high tides. However, CH₄ fluxes showed high temporal variability, with the stems of A. marina functioning as a CH₄ sink before tidal inundation and becoming a source after ebbing. In contrast, the stems of K. obovata showed no consistent pattern in the CO₂ or CH₄ fluxes. Based on our findings, the stem CH₄ fluxes in A. marina may vary by up to 1200 % when considering tidal influence, compared to when tidal influence is ignored. Therefore, sampling only during low tides might underestimate stem CO₂ and CH₄ fluxes on a diurnal scale. This study highlights the necessity of considering tidal influence and species when quantifying greenhouse gas (GHG) fluxes from mangrove tree stems. Further study is needed to explore the underlying mechanisms driving the observed flux variations and improve the understanding of GHG dynamics in mangrove ecosystems.

Mangroves play an important role in carbon sequestration. However, mangroves can be sources of greenhouse gas (GHG) emissions. In this study, methane (CH₄) emissions and related soil properties were determined in multiple mangroves in Taiwan, including Kandelia obovata and Avicennia marina mangroves. K. obovata possess prop roots, whereas pneumatophores are found in A. marina. Our results showed that mangrove soils were significant sources of CH₄ emissions, which should be accounted for in mangrove carbon budgets. In particular, CH₄ emissions in the A. marina mangroves were approximately 50- to 100-fold those of the K. obovata mangroves and the adjoining mudflats. Multiple regression analyses indicated that the soil salinity and pH in K. obovata mangroves and the soil redox potential and organic content in the mudflats were the key factors affecting CH₄ emissions. However, the pneumatophore density alone explained approximately 48% of the variation in CH₄ emissions in the A. marina mangroves. More pneumatophores resulted in higher CH₄ emissions in the A. marina mangroves. Thus, compared with the assessed soil properties, the contribution of pneumatophores to the transportation of CH₄ from soil was more significant. In addition to soil properties, our results demonstrated that the root structure may also affect GHG emissions from mangroves.

Tropical seagrass beds are productive coastal ecosystems that are important blue carbon sinks and crucial habitats and feeding grounds for consumers at high trophic levels. To understand how energy sustains the ecosystem from seagrass production, we constructed an Ecopath trophic model to reveal the possible pathways of energy flow in the tropical seagrass beds around Dongsha Island, South China Sea. The model indicates that Dongsha seagrass beds were developing but well-structured ecosystems. The productive seagrasses were rarely directly consumed by herbivores and, ultimately, flowed into detrital pools. Detritus was the main food source used to support diverse consumers in the food web. Nevertheless, the low cycling rate (2.74%) suggests that most detritus was not reused or exported and was stored as a large organic carbon pool in the sediments. The detritus-feeding invertebrates are keystone groups in the Dongsha seagrass beds, as they recycle energy from detritus and transfer to top predators such as sharks. The predation of top predators affected the biomasses of other compartments, leading to strong top-down control via a trophic cascade effect.

Greenhouse gas (GHG) emissions, particularly methane (CH₄) and nitrous oxide (N₂O), from mangroves can challenge their carbon sequestration capacity. This study measured GHG emissions from the tree stems of four mangrove species (Avicennia marina, Kandelia obovata, Lumnitzera racemosa, and Rhizophora stylosa) along the western coast of Taiwan. All studied species emitted CO₂, CH₄, and N₂O at the stem-atmosphere interface, with A. marina exhibiting the highest GHG fluxes. Notably, stem N₂O emissions from A. marina significantly contributed (1.2 to 100.1%) to total ecosystem fluxes, whereas stem CH₄ emissions were relatively low (-3.8 to 2.6%). This study is the first to demonstrate seasonal variations in stem N₂O flux in mangroves, mainly influenced by temperature, as shown by the structural equation model. The results indicate that stem CH₄ and N₂O emissions can significantly offset soil carbon burial rates (0.19 to 889.28%), particularly in A. marina. First-order estimates suggest global mangrove stem-mediated emissions of 5.94 [0.88-168.65] Mg CH₄ year^-1 and 8.66 [2.46-36.19] Mg N₂O year^-1, presented as median [Q1-Q3] values, although these estimates carry some uncertainties. Nonetheless, these insights underscore the importance of including stem emissions in carbon budget assessments for mangrove ecosystems, highlighting their essential role in mitigating climate change.