Seasonal Variations Alter the Impact of Functional Traits on Plankton Dynamics

Gaining understanding of food-web processes often requires a simplified representation of natural diversity. One such simplification can be based on functional traits, as functionally similar species may provide a similar contribution to ecosystem level-processes. However, understanding how similarity in functional traits actually translates into similar contributions to ecosystem-level properties remains a challenge due to the complex ways in which traits can influence species' dynamics. Moreover, in many communities, seasonality alters the abiotic and biotic forcing regime, causing ongoing changes to patterns of species' dominance; groups of species do not stay intact but are rather continuously subjected to changes throughout the year. Using long-term high frequency measurements of phytoplankton in Lake Constance, we investigated the effect of seasonal changes on the relationship between functional similarity and temporal dynamics similarity of 36 morphotypes, and the relative contribution of different functional traits during the different parts of the year. Our results revealed seasonal differences in the overall degree of synchronization of morphotypes' temporal dynamics and how combinations of functional traits influence the relationship between functional trait similarity and temporal dynamics similarity, showing that different forcing regimes change how species cope with their environment based on their functional traits. Moreover, we show that the individual functional traits matter at different periods of the year indicating that species which are dynamically similar at certain parts of the year may not be at others. The differential strength of the overall and individual impact of functional traits on species' temporal dynamics makes the cohesion of a pair of functionally similar species dependent on the different forcing. Hence, simplifying food webs based solely on functional traits may not provide consistent estimates of functional groups over all seasons.

要阐明食物网的运作机制，往往需要对自然生物多样性进行简化表征。此类简化可基于功能性状（functional traits）开展，因为功能相似的物种对生态系统级过程的贡献往往较为相近。然而，由于功能性状影响物种种群动态的机制极为复杂，如何将功能性状相似性准确对应至生态系统属性贡献的相似性，仍是一项亟待解决的科学难题。此外，在多数群落中，季节性变化会改变非生物与生物驱动体系，导致物种优势格局持续发生动态调整；物种类群并非一成不变，而是在全年中不断经历组成与结构的变化。本研究依托康斯坦茨湖浮游植物的长期高频监测数据，针对36个浮游植物形态型，探究了季节性变化对功能相似性与时间动态相似性之间关联的影响，以及不同功能性状在一年中不同时段的相对贡献程度。研究结果显示，浮游植物形态型时间动态的整体同步程度存在显著季节差异，同时功能性状组合对功能相似性与时间动态相似性之间关联的调控模式也随季节变化，这表明不同驱动体系会基于物种的功能性状，改变其适应环境的策略。此外，本研究还发现，单一功能性状的作用强度随季节时段发生变化，这意味着在一年中某些时段表现出时间动态相似性的物种类群，在其他时段未必如此。功能性状对物种种群动态的整体与单一影响强度存在显著差异，使得一对功能相似物种的类群聚合程度随驱动体系的不同而变化。因此，仅基于功能性状对食物网进行简化表征，无法在所有季节中都准确估算功能类群的组成与特征。