Eutrophication of shallow coastal ecosystems often manifests as dense mats of opportunistic macroalgae, degraded underlying sediments and displacement of ecologically important primary producers including seagrass beds. Ecological thresholds of drivers that cause these shifts in ecosystem state are needed to manage eutrophication symptoms before tipping points are crossed. This study quantifies total nitrogen (TN) load thresholds for abrupt changes in ecologically important seagrass habitat and the appearance of nuisance macroalgae as a function of estimated catchment TN load to shallow, intertidal estuaries in New Zealand. Results indicate that the extents of both macrophyte types are strongly governed by increasing TN load to the estuaries, while total suspended sediment load was not significant alone or as an interaction term. Seagrass extent was unpredictable below a critical TN load envelope (< 50 mg N m⁻² d⁻¹), likely because there are additional factors than TN that influence seagrass cover, but once surpassed, seagrass became restricted to less than 10% of intertidal area or absent. Conversely, nuisance macroalgae was predominantly absent unless TN load exceeded 41.15 ± 21.1 mg N m⁻² d⁻¹, expanding in extent by 2.7% of intertidal area for every 50 mg N m⁻² d⁻¹ increase above threshold levels. This research underscores the importance of managing diffuse nitrogen inputs to safeguard the structure and function of shallow estuarine ecosystems before tipping points in dominant macrophyte type are crossed.

中文翻译：

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潮间带浅海口从海草向讨厌的大型藻类转变的集水氮负荷阈值

浅层沿海生态系统的富营养化通常表现为机会主义大型藻类的致密垫层，底层沉积物的降解以及具有生态重要性的初级生产者（包括海草床）的迁移。在穿越临界点之前，需要引起生态系统状态发生这些变化的驱动器的生态阈值，以管理富营养化症状。这项研究量化了重要生态草海生境的突然变化和滋扰性巨藻的出现的总氮（TN）阈值，该阈值是新西兰浅潮间河口流域总TN负荷估算值的函数。结果表明，两种大型植物的范围受河口总氮负荷的增加强烈地控制，而总悬浮泥沙负荷单独或作为相互作用项并不显着。^-2 d ^-1），可能是因为除TN之外还有其他影响海草覆盖率的因素，但是一旦超过，海草就被限制在潮间带面积的10％以下或不存在。相反，除非TN负荷超过41.15±21.1 mg N m ^-2 d ^-1，否则基本上不存在讨厌的大型藻类，每增加50 mg N m ^-2 d ^-1超出阈值水平，潮间带面积就会扩大2.7％。这项研究强调了在穿越主要大型植物类型的临界点之前，管理扩散氮输入以保护浅河口生态系统的结构和功能的重要性。