Trophic interactions in the pelagic area of lakes and the opposing effects of fish feeding (top-down) and phytoplankton biomass (bottom-up) on zooplankton communities are central topics in limnology. We hypothesized that zooplankton size distributions should be a more sensitive approach to disentangle top-down and bottom-up effects than the commonly measured zooplankton biomass. We examined zooplankton size distributions from 148 samples collected during summer months in the upper and lower pelagic layers of a deep mesotrophic lake among 13 years of sampling. Top-down effects, namely fish size and biomass, and bottom-up effects, including water temperature and total phosphorus and chlorophyll a concentrations, were considered. To add robustness to our analyses, we expressed the zooplankton size distributions as size spectra based on log-binning, as continuous size spectra and by the size diversity, a measure that has been developed to mimic taxonomic diversity indices. Among numerous regressions tested, significant top-down or bottom-up effects could rarely been detected. Our results indicate that the overall zooplankton size distribution was not significantly affected by fish predation and lake productivity measured as total phosphorus or chlorophyll a concentration. However, we found negative correlations between fish biomass and the preferred zooplankton prey, including Bosmina longirostris, Daphnia cucullata and nauplii in the upper depth layer. However, due to their small body size, low biomass and therefore relative small contribution to the zooplankton size distribution, predation on preferred zooplankton species did not translate into a statistically significant modification of the entire size spectrum. Consequently, the size spectrum seems to be relatively robust against predation effects, but might reflect the lake-wide energy availability and transfer efficiency in the food web.

湖泊中上层地区的营养相互作用以及鱼类摄食（自上而下）和浮游植物生物量（自下而上）对浮游生物群落的相反影响是湖泊学的中心议题。我们假设浮游生物的大小分布应该是一种比通常测量的浮游生物量更灵敏的方法，可以消除自上而下和自下而上的影响。我们在13年的采样中检查了夏季中深营养湖中上层和下层上层的148个样本中浮游动物的大小分布。考虑了自上而下的影响，即鱼的大小和生物量，以及自下而上的影响，包括水温以及总磷和叶绿素a的浓度。为了增加分析的稳健性，我们基于对数分箱将浮游动物的大小分布表示为大小谱，作为连续的尺寸谱并通过尺寸多样性，已开发出一种可模拟分类学多样性指数的方法。在测试的众多回归中，很少会检测到明显的自上而下或自下而上的影响。我们的结果表明，以总磷或叶绿素a的浓度衡量，鱼类捕食和湖泊生产力对浮游动物的总体分布没有显着影响。但是，我们发现鱼类生物量与首选浮游生物猎物之间存在负相关关系，包括 我们的结果表明，以总磷或叶绿素a的浓度衡量，鱼类捕食和湖泊生产力对浮游动物的总体分布没有显着影响。但是，我们发现鱼类生物量与首选浮游生物猎物之间存在负相关关系，包括 我们的结果表明，以总磷或叶绿素a浓度衡量，鱼类捕食和湖泊生产力对浮游动物的总体分布没有显着影响。但是，我们发现鱼类生物量与首选浮游生物猎物之间存在负相关关系，包括上深层的Bosmina longirostris，Daphnia cucullata和无节幼体。但是，由于它们的小体型，低生物量以及因此对浮游动物尺寸分布的贡献相对较小，因此对首选浮游动物物种的捕食并不能转化为整个尺寸谱的统计学显着变化。因此，规模谱似乎对捕食效应具有较强的抵抗力，但可能反映了整个湖泊的能源供应和食物网中的转移效率。