Winter diatom blooms and their effect on lake ecosystems (Acronym: DIABLO - DIAtom BLOoms)

全球变化正在迅速改变湖泊的冬季条件。然而，后果很难预测，因为湖沼学家历来关注的是从春季到秋季的“植被期”，而对冬季生态学知之甚少。直到最近，冬季通常被认为是生态休眠的，因为在春季开始分层之前，没有足够的光照来生长浮游植物。与这种观点相反的是，在许多湖泊中，浮游植物--特别是大细胞硅藻--可以在冬末分层之前形成密集的水华。这种现象鲜为人知，但在温带湖泊中并不少见。此外，有证据表明，这些水华可以通过隔离营养物质和减少浮游植物生物量，在随后的季节强烈改变湖泊生态系统。我们迫切需要更好地了解这些水华，因为冰盖正在减少，冬季混合在温带发生变化，我们预计这些水华将变得更加频繁。在这个项目中，我们想要描述晚冬硅藻水华的原因和后果。我们的主要假设是，如果1)混合期有足够的持续时间，2)湖深和水的清晰度允许一定的平均光照，大细胞硅藻物种可以在冬末发展出高生物量。反过来，冬末的硅藻通过沉积作用剥离了地表水中的营养物质，从而改变了春季和夏季的湖泊生物地球化学和生态。我们的方法结合了1)对来自德国四个监测最严格的湖泊和水库的超过100年的高质量数据的分析，所有这些湖泊和水库都具有所需的深度和水透明度范围，并支持大型冬季硅藻生物量；2)对我们研究的一个湖泊冬季和春季的关键过程进行了实地测量，以及3)对三个研究湖泊进行了耦合的一维水动力-生态模型。我们计划揭示冬季条件与季节性浮游植物和营养动态之间的关系，并确定是否存在促进冬末硅藻并可能导致替代稳定状态的反馈。我们将量化碳和营养物质的垂直通量、冬季浮游植物接种量和内部营养物质储存。最后，建模工作将把所获得的知识整合到数学过程描述中。预期成果是以概念框架和预测模型的形式对晚冬硅藻水华的发生及其对生态的影响进行全面的系统了解。这将有助于关闭季节循环，并更好地了解控制湖泊水质的因素。

Global change is rapidly altering winter conditions in lakes. However it is difficult to predict the consequences because limnologists have historically focused on the “vegetation period” from spring through autumn and little is known about winter ecology. Until recently winter was generally considered to be ecologically dormant because there is insufficient light for phytoplankton to grow until stratification begins in spring. Contrary to this view, there are many lakes in which phytoplankton – specifically large-celled diatoms – can form dense blooms in late winter before stratification. This phenomenon is little known yet not uncommon in temperate lakes. Moreover, there is evidence that these blooms can strongly alter lake ecosystems in the subsequent seasons by sequestering nutrients and decreasing phytoplankton biomass. We urgently need to better understand these blooms because ice cover is decreasing and winter mixing is changing in temperate zones and we expect these blooms to become more frequent as a result. In this project we want to characterize the causes and consequences of late-winter diatom blooms. Our main hypothesis is that large-celled diatom species can develop high biomasses in late winter if 1) the mixing period is of sufficient duration and 2) lake depth and water clarity permit a certain amount of average light. In turn, late-winter diatom blooms strip nutrients from surface water through sedimentation, which alters lake biogeochemistry and ecology in spring and summer. Our approach combines 1) an analysis of over 100 collective years of high quality data from four of Germany’s best-monitored lakes and reservoirs, all of which have the required depth and water clarity range and support large winter biomasses of diatoms, 2) field measurements of key processes during winter and spring in one of our study lakes, and 3) coupled 1-D hydrodynamic-ecological modelling of three of the study lakes. We plan to expose the relationships between the winter conditions and the seasonal phytoplankton and nutrient dynamics, and determine the presence of feedbacks that promote late-winter diatoms and can lead to alternative stable states. We will quantify vertical fluxes of carbon and nutrients, winter phytoplankton inocula, and internal nutrient storage. Finally the modelling effort will integrate the knowledge gained into mathematical process descriptions. The expected outcome is a comprehensive systems understanding in the form of a conceptual framework and predictive models of the occurrence of late-winter diatom blooms and their effect ecology in the rest of the year. This will help to close the seasonal loop and better understand the factors that control water quality in lakes.