Table_1_Tropical Coastal Wetlands Ameliorate Nitrogen Export During Floods.XLSX

Wetlands can increase resilience to extreme climatic events and have a key role in protection and water quality improvement in coastal ecosystems. Studies in tropical coastal wetlands at a catchment scale are scarce, and most work has been undertaken on small, temperate wetlands. In this study, we tested whether natural coastal wetlands in a tropical catchment (Tully-Murray, Queensland, Australia) could ameliorate nitrogen (N) exported to the Great Barrier Reef during a flood event. We measured denitrification rates in different types of coastal wetlands (mangroves, saltmarshes, waterbodies with macrophytes, and floodplain wetlands dominated by Melaleuca spp.) to assess their potential contribution to N losses during the 6-day duration of a flood in March 2018. Denitrification potential was variable across the landscape, and we identified “hotspots” in sub-catchments with high NO3--N concentrations (0.4–0.6 mg L−1) and large areas of wetlands (>800 ha, >40% of the sub-catchment). These hotspots can denitrify up to 10 t of NO3--N per day during a flood. We used our measured denitrification rates to provide input parameters for a model that includes the main biogeochemical processes affecting N transformations within wetlands (nitrification, denitrification, plant uptake, sedimentation, anammox, and mineralization), and accounts for transport via the duration, depth, and flow of water. Model simulations of a sub-catchment of the Tully-Murray indicate that flood inundation of large areas of natural wetlands (>40% of the sub-catchment area) could potentially remove 70% of the incoming NO3--N load in the first 24 h of the flood. The management and restoration of coastal tropical wetlands could play a critical role in sustaining the health of coastal ecosystems through water quality improvement.

滨海湿地能够提升生态系统应对极端气候事件的韧性，在海岸生态系统的保护与水质改善中发挥关键作用。目前针对流域尺度热带滨海湿地的研究较为匮乏，现有工作多聚焦于小型温带湿地。本研究以澳大利亚昆士兰州塔利-墨里（Tully-Murray）流域内的天然滨海湿地为对象，探究其能否缓解洪水事件期间向大堡礁（Great Barrier Reef）输出的氮负荷。我们针对不同类型的滨海湿地——红树林、盐沼、大型水生植物水体以及以白千层属（Melaleuca spp.）为主的泛滥平原湿地——测定了反硝化速率，以评估其在2018年3月那场为期6天的洪水期间对氮素流失的潜在贡献。研究发现，反硝化潜力在流域内存在空间异质性，我们在硝态氮（NO3--N）浓度为0.4~0.6 mg·L⁻¹、湿地面积占比超40%且总面积大于800公顷的子流域中识别出了反硝化“热点区域”。此类热点区域在洪水期间每日可通过反硝化作用移除多达10吨硝态氮。我们将实测得到的反硝化速率作为输入参数，用于构建包含湿地内主要氮素生物地球化学转化过程（硝化作用、反硝化作用、植物吸收、沉积作用、厌氧氨氧化（anammox）以及矿化作用）的模型，并纳入了洪水持续时长、水深与水流流速等水文输运过程。对塔利-墨里流域某一子流域的模型模拟结果显示，当大面积天然湿地（占子流域面积40%以上）被洪水淹没时，可在洪水发生后的前24小时内移除最多70%的输入硝态氮负荷。由此可见，热带滨海湿地的管理与修复工作，可通过改善水质在维系海岸生态系统健康中发挥关键作用。