Data from: Nitrogen loads influence trophic organization of estuarine fish assemblages

Nutrient (N and P) loading may affect functioning in aquatic ecosystems by restructuring producer assemblages with flow-on effects to consumers. Trophic niche occupancy and trophic organization of consumers are key components of ecosystem function that have been increasingly investigated using quantitative isotopic niche indices. These indices are based on the premise that the isotopic values of consumer tissues indicate their assimilated diet. Typically, isotopic niche indices are calculated using only consumer isotope data, which limit their application for spatial and temporal comparisons because consumer isotopic niches depend on isotopic variability of available autotrophs.
We used measures of isotopic variability of autotrophs to standardize isotopic niche indices, which enabled us to compare trophic organization of fish assemblages in nine estuaries spanning a broad range of nutrient loading. We related standardized isotopic niche indices of fish assemblages to nitrogen and phosphorous loads and hydrological flushing of the estuaries in autumn and spring.
The estuarine fish assemblages studied here showed greater trophic diversity and less redundancy given moderate to high inorganic nitrogen loading. Taxonomic richness partly influenced three isotopic niche indices measuring trophic diversity, but not measures of redundancy. Similar patterns may occur in other systems in which nitrogen loads have increased but the diversity of primary producers has not been reduced to a single dominant source.
Our results demonstrate bottom-up controls of estuarine food webs. Effects of inorganic nitrogen loading were transmitted upwards through the food web to affect the trophic organization of higher trophic levels, demonstrating the crucial role of nitrogen for estuarine trophic dynamics.

养分（氮N、磷P）负荷可通过重塑水生生态系统的生产者群落（producer assemblages），并引发对消费者的级联效应，进而影响生态系统功能。消费者的营养生态位占据情况与营养组织形式，是生态系统功能的核心组成部分，目前学界已愈发多地通过定量同位素生态位指数（quantitative isotopic niche indices）对其开展研究。此类指数的构建前提是：消费者组织的同位素值能够反映其同化的食物来源。传统上，同位素生态位指数仅基于消费者的同位素数据计算所得，由于消费者的同位素生态位依赖于环境中可利用自养生物（autotrophs）的同位素变异性，这一局限制约了该指数在空间与时间维度上的对比应用。
本研究通过利用自养生物的同位素变异性数据，对同位素生态位指数进行标准化处理，借此得以对比9个覆盖不同养分负荷梯度的河口（estuaries）的鱼类群落（fish assemblages）营养组织形式。我们将鱼类群落的标准化同位素生态位指数，与秋、春两季河口的氮、磷负荷及水文冲刷状况进行关联分析。
本研究涉及的河口鱼类群落，在中等至高强度无机氮负荷条件下，表现出更高的营养多样性与更低的功能冗余性。分类丰富度（taxonomic richness）会对三项用于衡量营养多样性的同位素生态位指数产生部分影响，但并未作用于冗余性相关指标。在氮负荷升高但初级生产者（primary producers）多样性未被简化为单一优势类群的其他生态系统中，或可观察到类似的规律。
本研究结果证实了河口食物网的上行控制（bottom-up controls）效应。无机氮负荷的影响通过食物网向上传递，进而影响高营养级类群的营养组织形式，这凸显了氮在河口营养动态（trophic dynamics）中的关键作用。