1. Urea accounts for half of global agricultural fertiliser applications, yet little is known of its role in eutrophication of freshwater ecosystems, nor how it interacts with phosphorus (P) in regulating phytoplankton composition, especially during spring and autumn.
2. To identify when and how urea and P inputs interact across the ice‐free period, we conducted seven monthly fertilisation experiments in 3,240‐L mesocosms from ice‐off to ice‐formation in a hypereutrophic lake. In addition, we ran bioassays with ammonium (NH₄⁺) to compare the effects of urea with those of NH₄⁺, the immediate product of chemical decomposition of urea.
3. Analysis of water‐column chlorophyll a and biomarker pigments by high‐performance liquid chromatography revealed that addition of inorganic P alone (100 µg P L^–1 week^–1) had no significant impact on either algal abundance or community composition in hypereutrophic Wascana Lake. Instead, fertilisation with urea (4 mg N L⁻¹ week^–1) alone, or in concert with P, significantly (p < 0.05) increased algal abundance in spring and much of summer, but not prior to ice formation in October. In particular, urea amendment enhanced abundance of cryptophytes, chlorophytes, and non‐diazotrophic cyanobacteria during April and May, while fertilisation in summer and early autumn (September) increased only chlorophytes and non‐diazotrophic cyanobacteria.
4. Comparison of urea mesocosms with NH₄⁺ bioassays demonstrated that urea lacked the inherent toxicity of NH₄⁺ in cool waters, but that both compounds stimulated production during summer experiments.
5. This study showed that urea pollution can degrade water quality in P‐rich lakes across a variety of seasonal conditions, including spring, and underscores the importance of quantifying the timing and form of N inputs when managing P‐rich freshwaters.

中文翻译：

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富营养化硬水湖中尿素和磷对浮游植物丰度和群落组成的季节性变化

1. 尿素占全球农业肥料施用量的一半，但鲜为人知的是它在淡水生态系统富营养化中的作用，也不清楚如何与磷（P）相互作用来调节浮游植物的组成，特别是在春季和秋季。
2. 为了确定在无冰期尿素和磷的输入何时以及如何相互作用，我们在富营养化湖泊中从冰期到冰层形成的3,240-L中等范围进行了七个月度施肥实验。此外，我们使用铵（NH ₄ ⁺）进行生物测定，以比较尿素和尿素化学分解的直接产物NH ₄ ⁺的影响。
3. 通过高效液相色谱分析水柱叶绿素a和生物标志物色素，发现单独添加无机磷（100 µg P L ^–1 周^–1）对富营养化的沃斯卡纳湖的藻类丰度或群落组成均无显着影响。取而代之的是，单独施用尿素（4 mg N L ^-1 周^–1）或与P一起施肥（p <0.05）在春季和夏季的大部分时间增加了藻类的丰度，但在十月份的冰层形成之前并未增加。尤其是，尿素改良剂在4月和5月提高了隐藻类，绿藻类和非营养型蓝细菌的含量，而夏季和秋季初（9月）的施肥仅增加了绿藻类和非营养型蓝细菌。
4. 尿素中膜和NH ₄ ⁺生物测定的比较表明，尿素缺乏凉水中NH ₄ ⁺的内在毒性，但是在夏季实验中这两种化合物都能刺激生产。
5. 这项研究表明，尿素污染会在包括春季在内的各种季节条件下降低富磷湖泊的水质，并强调在管理富磷淡水时量化氮输入的时间和形式的重要性。