Data from: In situ warming strengthens trophic cascades in a coastal food web

Global warming may affect most organisms and their interactions. Theory and simple mesocosm experiments suggest that consumer top–down control over primary producer biomass should strengthen with warming, since consumer respiration increases faster with warming than plant photosynthesis. However, these predictions have so far not been tested on natural communities that have experienced warming over many generations. Natural systems display a higher diversity, heterogeneity and complexity than mesocosms, which could alter predicted effects of warming. Here we used an artificially heated part of the northern Baltic Sea (the Forsmark Biotest basin) to test how warming influences trophic interactions in a shallow coastal food web with four trophic levels: omnivorous fish, invertivorous fish, herbivorous invertebrates, and filamentous macroalgae. Monitoring of fish assemblages over six years showed that small invertivorous fish (gobiids, sticklebacks and minnows) were much less abundant in the heated basin than in unheated references areas. Stomach content analyses of the dominating omnivorous fish – Eurasian perch Perca fluviatilis – revealed a strikingly different diet within and outside the Biotest basin; gammarid crustaceans were the dominating prey at heated sites, whereas invertivorous fish (e.g. gobiids) dominated at unheated sites. A 45-day cage experiment showed that fish exclusion did not affect the biomass of algal herbivores (gastropods and gammarids), but reduced algal biomass in heated sites (but not unheated). This suggests that warming induced a trophic cascade from fish to algae, and that this effect was mediated by predator-induced changes in herbivore behavior, rather than number. Overall, our study suggests that warming has effectively shrunk the food chain from four to three trophic levels (algae, gammarids and perch), which have benefitted the primary producers by reducing grazing pressure. Consequently, warming appears to have restructured this coastal food web through a combination of direct (physiological) and indirect (species interactions) effects.全球变暖可能会影响绝大多数生物及其种间相互作用。理论研究与简易中型实验生态系统（mesocosm）实验表明，消费者对初级生产者生物量的下行控制作用会随变暖增强，因为消费者的呼吸速率随温度升高的增幅快于植物的光合速率。然而，截至目前，这些预测尚未在经历过多代变暖的自然群落中得到验证。自然生态系统相较于中型实验生态系统，具有更高的物种多样性、环境异质性与复杂性，这可能会改变变暖的预期效应。本研究借助波罗的海北部一处人工加热海域——福什马克生物试验盆地（Forsmark Biotest basin），在包含四个营养级的浅海海岸食物网中检验变暖对营养相互作用的影响：该食物网涵盖丝状大型藻类、植食性无脊椎动物、食虫性鱼类以及杂食性鱼类四个营养级。为期六年的鱼类群落监测结果显示，相较于未加热的对照区域，加热盆地内的小型食虫性鱼类（虾虎鱼、棘鱼与雅罗鱼）丰度显著降低。对优势杂食性鱼类——欧亚河鲈（Perca fluviatilis）的胃容物分析显示，试验盆地内外的该鱼类食谱存在显著差异：加热位点的优势猎物为钩虾（gammarid）类甲壳动物，而未加热位点的优势猎物则为食虫性鱼类（如虾虎鱼）。一项为期45天的围栏实验表明，鱼类排除处理并未改变植食性动物（腹足类与钩虾）的生物量，但却降低了加热位点的藻类生物量（未加热位点无此效应）。这一结果表明，变暖催生了从鱼类到藻类的营养级联效应，且该效应是由捕食者诱导的植食动物行为变化而非种群数量变化所介导。综上，本研究表明变暖已使该食物链从四个营养级有效缩减为三个营养级（藻类、钩虾与欧亚河鲈），通过降低植食压力使初级生产者获益。因此，变暖似乎通过生理直接效应与物种相互作用间接效应的共同作用，重构了该海岸食物网。