Nutrient Distribution and Nitrate Processing in a Mangrove Tidal Creek Affected by Submarine Groundwater Discharge (SGD)

Investigations of the spatial-temporal variations of nutrients within mangrove coastal zones are essential for assessing the environmental status of an aquatic ecosystems. However, major processes controlling nitrate cycle along the submarine groundwater discharge (SGD) pathway from the mangrove areas to adjacent tidal creek remain underexplored. A time series measurement over a 25 h tidal cycle was conducted in Qinglan Bay tidal creek (Hainan Island, China). Samples of the creek seawater, nearby terrestrial groundwater and sediment porewater were collected for analysis of nutrient concentrations and dual nitrate isotopic compositions (delta 15N- NO3- and delta 18O-NO3-). We observed high concentrations of nutrients in terrestrial groundwater, and their concentrations decreased with distance towards the coastal area. Using dual nitrate isotopes and a MixSIAR model, we concluded that ammonia nitrogen fertilizer (Amm-NF) is the primary nitrate sources for groundwater, while creek water nitrate originates from various sources, including manure and sewage, soil organic nitrogen, and chemical fertilizer. The isotopic ratios (delta 18O: delta 15N) suggest active nitrification occurs in low tide creek water, whereas assimilation and limited denitrification occurs in high tide creek water and groundwater. Based on previous report of SGD rate in the region, we calculated the flux of SGD-derived nutrient export to coastal water, and the result indicates low N: P ratio and N limitation in the tidal creek. This study highlights the complexity of different sources and biogeochemical processes that impact the nutrient distribution and nitrate dynamics in the nearshore mangrove ecosystem. Understanding these processes is essential for effectively protecting and managing of the mangrove aquatic ecosystem.