1. Filter‐feeding zooplankton in lakes feed on a mixture of phytoplankton, bacteria, and terrestrial particles and the proportions and nutritional value of these components can be highly variable. However, the extent to which food quality interacts with food quantity in affecting overall zooplankton performance is not yet fully resolved.
2. Here we performed laboratory feeding experiments to test how the performance of the unselective filter feeder Daphnia galeata was affected if various quantities of high‐quality food (the phytoplankton Rhodomonas) were diluted with low‐quality food such as heterotrophic bacteria (Pseudomonas) or terrestrial detritus particles (t‐POM) from the riparian zone of a boreal forest stream. We hypothesised: that increased proportions of bacteria and t‐POM in the diet will lead to decreased survival, somatic growth; and reproduction of Daphnia despite the presence of phytoplankton; that these effects are more pronounced for t‐POM than for heterotrophic bacteria; and that this response is stronger when phytoplankton availability is low.
3. Increasing the concentrations of Pseudomonas affected Daphnia survival, growth, and reproduction negatively when Rhodomonas was available at intermediate (0.37 mgC/L) and high (0.55 mgC/L) quantities. When Rhodomonas quantity was low (0.22 mgC/L), the addition of Pseudomonas generally resulted in better Daphnia performance except at very high concentrations of the bacterium relative to Rhodomonas. In contrast, the addition of t‐POM was detrimental for overall Daphnia performance at all Rhodomonas concentrations.
4. Daphnia performance was best described by a model including the interaction between food quality and quantity, with stronger negative effects on Daphnia when high‐quality food was supplemented with t‐POM than with Pseudomonas.
5. The results indicate that the ability of zooplankton to use low‐quality food is affected by the concurrent availability of high‐quality food. Furthermore, food sources that can be used but do not fulfil dietary requirements of grazers (e.g. bacteria), may still provide nutritional benefits as long as other complementary food components are available in sufficient quantities to compensate for biochemical deficiencies.
6. Therefore, we conclude that heterotrophic bacteria, but not peat layer t‐POM, can be an important component of zooplankton diets in boreal lakes, especially if the concentration of phytoplankton is low.

中文翻译：

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水蚤对高品质藻类，异养细菌和异源颗粒有机物的不同组合的饮食表现

1. 湖泊中以浮游动物为食的浮游生物以浮游植物，细菌和陆地颗粒的混合物为食，这些成分的比例和营养价值可能存在很大差异。但是，在影响浮游动物整体性能方面，食品质量与食品数量相互作用的程度尚未完全解决。
2. 在这里，我们进行了实验室饲养实验，以测试如果用异养细菌（Pseudomonas）或陆生碎屑等低质量食物稀释各种数量的优质食物（浮游植物红藻），会对非选择性滤食性食嘴蚤（Daphnia galeata）的性能产生影响来自北方森林溪流河岸带的颗粒（t-POM）。我们假设：饮食中细菌和t-POM的比例增加将导致存活率下降，体细胞生长；浮游植物的存在和水蚤的繁殖；这些影响对于t-POM而言比对异养细菌更明显; 当浮游植物的利用率低时，这种反应会更强。
3. 增加的浓度假单胞菌影响水蚤存活，生长和繁殖负时Rhodomonas可用在中间（0.37 MGC / L）和高（0.55 MGC / L）量。当Rhodomonas量为低（0.22 MGC / L），的添加假单胞菌通常导致更好的水蚤除了在非常高的浓度相对于细菌的性能Rhodomonas。相反，在所有红景天浓度下，添加t-POM对整体水蚤的表现都是有害的。
4. 用包含食物质量和数量之间相互作用的模型来最好地描述水蚤的性能，当优质食品中添加t-POM而不是假单胞菌时，水蚤对水蚤的负面影响更强。
5. 结果表明，浮游动物使用劣质食物的能力受到高质量食物同时供应的影响。此外，只要其他补充食品成分的量足以弥补生化缺陷，可以使用但不能满足放牧者饮食要求的食物（例如细菌）仍可提供营养益处。
6. 因此，我们得出的结论是，在北方湖泊中，异养细菌而不是泥炭层t-POM可能是浮游动物饮食的重要组成部分，尤其是在浮游植物浓度较低的情况下。