Data from: A novel approach to determining dynamic nitrogen thresholds for seagrass conservation

1. Seagrass decline is often related to eutrophication, with sudden and drastic losses attributed to tipping points in nutrient loads. The identification of these threshold loads is an important step in the sustainable management of nutrient discharges. 2. In this study, a novel methodological approach is presented to simulate the spatial and temporal dynamics of coastal nitrogen loads, and its relationship to seagrass loss. The analysis allows the identification of nitrogen thresholds associated with the loss of Posidonia and Amphibolis spp. in Adelaide, South Australia. 3. The identified thresholds varied between 0.8 and 1.1 t N km-2 over 6 months, and explained up to 95% of all seagrass losses in the region. 4. Posidonia spp. was predominantly lost nearshore, indicating a threshold for this species in the higher end of the values reported here. In contrast, Amphibolis spp. was selectively lost further offshore, suggesting a lower threshold for the onset of fragmentation and decline. 5. Nitrogen pressure was concentrated over the wetter months of the year between late autumn and early spring. Pulsed pressure peaking in August likely affected thresholds by reducing resilience in the system. 6. Synthesis and applications. This study derives, for the first time, nitrogen load thresholds for the loss of the seagrasses Posidonia and Amphibolis spp., highlighting the low tolerance of these keystone temperate species to nitrogen pressure. The methodology developed as part of the work provides a tool to inform planning and improved management of nutrient discharges from land to facilitate seagrass conservation.1. 海草衰退通常与富营养化（eutrophication）相关，突发性的大规模海草损失可归因于营养盐负荷达到临界点。识别此类临界负荷是实现营养盐排放可持续管理的重要环节。 2. 本研究提出一种全新的方法学框架，用于模拟海岸氮负荷的时空动态及其与海草损失的关联。通过本分析可识别澳大利亚南澳州阿德莱德地区波喜荡属（Posidonia）与海菖蒲属（Amphibolis）物种衰退相关的氮负荷阈值。 3. 所识别的阈值在6个月周期内介于0.8至1.1 t N km⁻²之间，可解释该区域高达95%的海草损失事件。 4. 波喜荡属物种的衰退主要集中于近岸海域，表明该物种的临界负荷处于本研究报告数值区间的较高端；与之相反，海菖蒲属物种的衰退多发生于远岸区域，提示其破碎化与衰退起始的临界阈值更低。 5. 氮负荷压力集中于每年秋末至初春的湿季，8月达到峰值的脉冲式氮输入可能通过降低生态系统恢复力，进而影响临界阈值的判定。 6. 综合与应用。本研究首次推导了波喜荡属与海菖蒲属海草衰退所需的氮负荷阈值，凸显了此类温带关键物种对氮负荷压力的低耐受性。本研究开发的方法学框架可为陆地营养盐排放的规划与优化管理提供决策支撑，助力海草资源保护。