Biodiversity affects ecosystem function, and how this relationship will change in a warming world is a major and well-examined question in ecology. Yet, it remains understudied for pico-phytoplankton communities, which contribute to carbon cycles and aquatic food webs year-round. Observational studies show a link between phytoplankton community diversity and ecosystem stability, but there is only scarce causal or empirical evidence. Here, we sampled phytoplankton communities from two geographically related regions with distinct thermal and biological properties in the Southern Baltic Sea and carried out a series of dilution/regrowth experiments across three assay temperatures. This allowed us to investigate the effects of loss of rare taxa and establish causal links in natural communities between species richness and several ecologically relevant traits (e.g. size, biomass production, and oxygen production), depending on sampling location and assay temperature. We found that the samples' biogeographical origin determined whether and how functional redundancy changed as a function of temperature for all traits under investigation. Samples obtained from the slightly warmer and more thermally variable regions showed overall high functional redundancy. Samples from the slightly cooler, less variable, stations showed little functional redundancy, i.e. function decreased when species were lost from the community. The differences between regions were more pronounced at elevated assay temperatures. Our results imply that the importance of rare species and the amount of species required to maintain ecosystem function even under short-term warming may differ drastically even within geographically closely related regions of the same ecosystem.

生物多样性影响生态系统的功能，这种关系将如何在变暖的世界中改变，这是生态学中一个主要且经过深思熟虑的问题。然而，对于微浮游植物群落而言，它仍然未被充分研究，它们全年都为碳循环和水生食物网做出了贡献。观测研究表明，浮游植物群落多样性与生态系统稳定性之间存在联系，但仅有很少的因果关系或经验证据。在这里，我们从波罗的海南部两个地理相关的区域（具有不同的热学和生物学特性）中抽取了浮游植物群落，并在三个测定温度范围内进行了一系列稀释/再生实验。这使我们能够研究稀有类群丧失的影响，并根据采样位置和测定温度，在自然界中物种丰富度与若干与生态相关的特征（例如大小，生物量产生和氧气产生）之间建立因果关系。我们发现样品的生物地理起源决定了所调查的所有性状是否以及如何随温度变化而改变功能冗余。从稍暖和热变的区域获得的样品显示出总体高的功能冗余。来自稍微凉爽，变化较小的站点的样本显示出很少的功能冗余，即当物种从群落中消失时，功能降低。在升高的测定温度下，区域之间的差异更加明显。