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 limits 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 foodwebs. Effects of inorganic nitrogen loading were transmitted upwards through the foodweb to affect the trophic organization of higher trophic levels, demonstrating the crucial role of nitrogen for estuarine trophic dynamics.氮（N）、磷（P）养分负荷（nutrient loading）可通过重构水生生态系统的生产者群落（producer assemblages）并产生级联效应（flow-on effects）影响消费者，进而改变生态系统功能。消费者的营养生态位占据度（trophic niche occupancy）与营养结构（trophic organization）是生态系统功能的核心组成部分，近年来已有越来越多研究借助定量同位素生态位指数（isotopic niche indices）对其展开探究。该类指数的构建前提为：消费者组织的同位素值可反映其同化的食物来源。通常而言，同位素生态位指数仅基于消费者的同位素数据计算得到，但由于消费者的同位素生态位会受现有自养生物（autotrophs）的同位素变异影响，这一局限限制了该指数在空间与时间对比研究中的应用。 本研究通过引入自养生物的同位素变异特征对同位素生态位指数进行标准化（standardize）处理，得以在涵盖广泛养分负荷梯度的9个河口（estuaries）中开展鱼类群落（fish assemblages）的营养结构对比研究。我们将秋季与春季采集的河口鱼类群落标准化同位素生态位指数，与河口的氮、磷养分负荷及水文冲刷（hydrological flushing）状况进行关联分析。 本次研究涉及的河口鱼类群落表现出如下特征：当中等至高水平的无机氮负荷存在时，其营养多样性（trophic diversity）更高，营养冗余度（trophic redundancy）则更低。分类丰富度（taxonomic richness）会对3项表征营养多样性的同位素生态位指数产生部分影响，但未对营养冗余度的相关指标产生显著作用。类似的模式或可出现在其他氮负荷升高、但初级生产者（primary producers）多样性未被简化为单一优势种的生态系统中。 本研究结果证实了河口食物网（foodwebs）的上行控制（bottom-up controls）效应：无机氮负荷通过食物网逐级向上传递，最终影响高营养级（higher trophic levels）的营养结构，凸显了氮元素在河口营养动态（trophic dynamics）中的关键作用。