Krill species play a pivotal role in energetic transfer from lower to upper trophic levels. However, functional feeding responses, which determine how food availability influences ingestion rates, are still not well defined for northern krill species. Here, we estimated and compared the functional feeding responses on natural communities of phytoplankton and mesozooplankton of two coexisting species, Meganyctiphanes norvegica and Thysanoessa raschii. We tested the influence of the presence of phytoplankton on the ingestion rate and the selectivity of both krill species when feeding on zooplankton prey. We performed a series of feeding experiments using increasing concentrations of natural phytoplankton (64 taxa; 2 to >50 μm) and mesozooplankton (28 taxa; ~100–2000 μm) assemblages and the latter in presence and absence of phytoplankton. Results revealed that both krill species exhibited a Holling type III feeding response on phytoplankton. However, T. raschii was able to exploit efficiently the highest phytoplankton concentrations. Our experiments highlighted that the presence of phytoplankton modified the functional feeding response on mesozooplankton preys of M. norvegica, but not that of T. raschii. Similarly, the presence of phytoplankton influenced the feeding selectivity on mesozooplankton preys, although both species showed contrasting selectivity patterns. In addition, we estimated the energy needs in relation to the daily rations. T. raschii satisfied its energy needs by feeding either on high phytoplankton concentrations or on low mesozooplankton densities, whereas M. norvegica did not cover its metabolic costs efficiently by feeding on phytoplankton only, even at high phytoplankton concentrations.

中文翻译：

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北极和北极磷虾对天然浮游植物和浮游动物的功能性摄食反应

磷虾种类在从低营养级到高营养级的能量转移中起关键作用。但是，对于北方磷虾种类来说，功能性喂养反应（决定食物可利用性如何影响摄入速率）仍然没有很好的定义。在这里，我们估计并比较了两种同时存在的物种Meganyctiphanes norvegica和Thysanoessa raschii对浮游植物和中层浮游动物自然群落的功能性摄食反应。。我们测试了浮游植物的存在对以浮游动物为食的两种磷虾种类的摄食率和选择性的影响。我们进行了一系列饲喂实验，使用浓度增加的天然浮游植物（64个分类单元； 2至> 50μm）和中浮游动物（28个分类单元；〜100–2000μm）组合，以及后者在存在和不存在浮游植物的情况下进行。结果表明，两种磷虾种类都对浮游植物表现出Holling III型摄食反应。但是，罗氏锥虫能够有效利用最高的浮游植物浓度。我们的实验强调，浮游植物的存在改性上的浮游动物捕食的功能摄食响应M. norvegica，但不是说的T. raschii。同样，浮游植物的存在影响了中游浮游动物捕食的选择性，尽管两种物种都表现出相反的选择性模式。此外，我们估计了与每日口粮有关的能源需求。T. raschii由高浓度的浮游植物或低浮游动物密度要么喂养满足其能源需求，而M. norvegica没有只，即使在高浓度的浮游植物浮游植物喂养有效地覆盖其代谢成本。