Data from: Spatial, temporal, and experimental: three study design cornerstones for establishing defensible numeric criteria in freshwater ecosystems

1.Nutrient over‐enrichment increasingly threatens global water resources. Stressor‐response studies specifically designed to identify levels of nutrients strongly associated with undesirable ecological conditions are needed to inform numeric nutrient criteria that protect inland waters.

2.Diatoms are important components of aquatic life, which support higher trophic levels and are sensitive to nutrient enrichment. We tested a framework that relies on stressor‐response modelling of phosphorus (P) enrichment and stream diatom assemblages across many field locations, multiple years and seasons within years, and under controlled experimental conditions to inform nutrient criteria development.

3.Diatom species composition was nonlinearly correlated with total phosphorus (TP) throughout the two‐year field study. This occurred despite temporal shifts in species composition between two hydrologically distinct years and over eight seasons.

4.Species assemblages on rocks transplanted from a low P stream to mesocosms representing a P enrichment gradient (8, 20 and 100 μg L−1) shifted into two groups over time. Species composition on rocks in low (20 μg L−1) and high (100 μg L−1) P mesocosms was consistent with assemblages at P‐enriched field sites, whereas rocks in control (8 μg L−1) mesocosms had significantly different species composition, consistent with low P field sites. Species composition on rocks transplanted from high P streams did not shift as dramatically and were not significantly different after exposure to different P treatments in mesocosms.

5.Threshold Indicator Taxa Analysis identified synchronous declines in several diatom species that culminated in assemblage thresholds associated with TP concentrations > 20 and 25 μg L−1 for 2006 and 2007 respectively.

6.Synthesis and applications. Diatom assemblages show consistent responses to nutrient enrichment despite temporal shifts associated with confounding factors common in stream ecosystems. Regulators should include diatom assemblage responses when developing numeric nutrient criteria. We present a framework that includes spatial, temporal and experimental components, and has broad applicability for use in different ecological settings to evaluate ecological endpoints and set limits for a variety of contaminants threatening freshwater ecosystems throughout the world.

1. 营养物过度富集正日益威胁全球水资源。为制定可保护内陆水体的数值化营养物标准，亟需开展针对性的胁迫-响应研究，以明确与不良生态状况显著相关的营养物浓度阈值。
2. 硅藻（Diatoms）是水生生物群落的重要组成部分，可为更高营养级的生物提供支撑，且对营养物富集反应敏感。本研究构建并验证了一套研究框架，该框架依托磷（P）富集与溪流硅藻群落的胁迫响应模型，整合了多野外采样点位、跨年度及年内多季节的野外观测数据，同时纳入受控实验条件下的数据集，用于支撑营养物标准的制定工作。
3. 为期两年的野外监测全程表明，硅藻物种组成与总磷（TP）呈非线性相关关系。尽管在两个水文特征迥异的年份间，以及年内八个监测季节中，物种组成均发生了时序性变化，但这一非线性关联始终保持稳定。
4. 研究将采自低磷溪流的岩石基质，移植至模拟磷富集梯度（浓度分别为8、20和100 μg·L⁻¹）的中型实验生态系统（mesocosms）中，结果显示岩石表面的硅藻群落组成随时间推移分为两个类群。在20 μg·L⁻¹（低浓度）与100 μg·L⁻¹（高浓度）磷浓度的中型实验生态系统中，岩石表面的物种组成与磷富集野外点位的硅藻群落特征一致；而对照组（8 μg·L⁻¹）的岩石物种组成则存在显著差异，其群落特征与低磷野外点位相符。与之相对，采自高磷溪流的岩石基质，在经不同磷浓度处理的中型实验生态系统中，其物种组成并未发生显著变化，且不同处理组间无显著差异。
5. 阈值指示类群分析（Threshold Indicator Taxa Analysis）识别出多种硅藻物种的同步衰减现象，其对应的群落阈值分别对应2006年总磷浓度>20 μg·L⁻¹，以及2007年总磷浓度>25 μg·L⁻¹。
6. 综合与应用。尽管溪流生态系统中常见的混淆因子会引发物种组成的时序性变化，但硅藻群落对营养物富集的响应仍保持一致。监管机构在制定数值化营养物标准时，应将硅藻群落的响应特征纳入考量。本研究提出的研究框架整合了空间、时序与实验三类数据集，具备广泛的适用性，可用于全球范围内不同生态场景下的生态终点评估，以及为各类威胁淡水生态系统的污染物设定限值。