Study on ecological water demand in mangrove ecosystem utilizing salinity as a key habitat indicator

Mangroves are one of the vital ecological environments along tropical coasts, serving not only as natural barriers against tides and protecting shorelines but also as ideal habitats for the reproduction and habitation of aquatic life. Additionally, mangroves themselves hold substantial economic and medicinal value. However, the construction and operation of upstream hydraulic engineering projects at river estuaries have altered the hydrological characteristics downstream of the dam. These changes impact the freshwater area, salinity, distance of salty tides upstream, and sediment distribution at the estuary, significantly affecting the growth and reproduction of regional mangrove forests. This study focuses on the Xin’ying Bay estuary in the Beimen River of China, utilizing a three-dimensional hydrodynamic and salinity diffusion mathematical model to investigate the effects of varying discharge rates on distance of salty tides upstream, freshwater area, and the maximum salinity of the cross section. The study selects Rhizophora stylosa Griff and Avicennia marina (Forssk.) Vierh as key species within the estuarine mangrove ecosystem, using salinity as a critical ecological factor to establish the relationship between salinity and flow in typical sections, thereby constructing a research system for optimal ecological water requirements for mangrove ecosystems. The results show that there is a negative correlation between the distance of salty tides upstream, the maximum salinity of the section and the discharge flow, while there is a positive correlation between the area of the freshwater area and the discharge flow. A discharge rate of 1.86 m³/s (20% of the multi-year average flow at the dam site) in July and 3.25 m³/s (35% of the multi-year average flow) in August and September can meet the salinity requirements necessary for the maturation of embryos and growth of seedlings in Rhizophora stylosa Griff and Avicennia marina (Forssk.) Vierh. This study establishes a comprehensive system for studying mangrove ecosystems and their ecological water requirements, achieving the goals of ecological protection and quantifiable, manageable environmental water needs. The findings also provide new perspectives and significant references for understanding and protecting mangrove ecosystems.