Mangroves grow in high-salinity environments with low soil water potential (Ψ_s), where high light intensity and strong winds increase the vapor pressure deficit (VPD), causing physiological drought and high transpiration demand (Δw), which limits carbon dioxide (carbon gain) for photosynthesis. This study explored how mangroves optimize their carbon-gain-to-water-loss ratio (water-use strategies) to maximize carbon gain during both dry and rainy seasons. We also calculated the relative costs of key leaf traits and compared them with those of terrestrial forests under the carbon gain optimization model. The results revealed that (1) with increasing Δw, terrestrial forests primarily adjusted leaf hydraulic conductance (K_leaf), while mangroves altered the difference in water potential (ΔΨ); (2) as Ψ_s decreased, π_tlp of both terrestrial forests and mangroves increased; (3) terrestrial forests developed a more balanced distribution of leaf trait costs between osmotic pressure (46.7 ± 0.2%) and stomata (43.3 ± 1.2%), whereas mangroves had the highest cost in osmotic pressure (49.04 ± 0.03%) and the lowest cost in stomata (11.08 ± 3.00%) during the rainy season; and (4) although mangroves showed differences in trait values between dry and rainy seasons, their responses to drought stress remained consistent. These findings provided new theoretical insights into how mangroves maintain high carbon gain and water-use efficiency under extreme environmental conditions, which is important to improve mangrove conservation efforts and contribute to climate mitigation policies.

Mangrove sediments in the South China Sea, particularly in the Hainan Island region, play a crucial role in regulating heavy metal migration and sequestration. However, the impact of converting mangrove areas to fish and shrimp culture ponds on heavy metal pollution in the Bamen Bay Mangrove Reserve is unclear. This study evaluates the pollution levels and ecological risks of Cr, Zn, Pb, Cu, and As in sediments from three land-use types using pollution indices (CF, PLI, RI) and the geo-accumulation index (I_geo). Multivariate analysis explores the relationships between metals and their potential sources. The results show significant differences in pollution levels (p < 0.05), with culture ponds having the highest pollution and ecological risk (RI = 73). As is the primary ecological risk factor (Er = 129). Zn and Cr are positively correlated with organic matter, while As and Pb show negative correlations with pH and salinity. Culture ponds increase heavy metal load and ecological risk, adversely impacting the mangrove ecosystem. These findings provide scientific support for land-use management and pollution control in mangrove wetlands.