Lake pycnoclines trap organic particles forming hot spots of accelerated carbon cycling in the water column

湖泊和水库等内陆水域在陆地和原生有机碳的运输和转化中起着重要作用。水生碳动态的关键内部途径，即颗粒物有机物（POM）的下沉，强烈依赖于水柱的垂直密度分层，这是由于最近全球大气变暖而增加的。然而，水柱分层对POM转化的整体影响至今仍未量化。现有的大尺度湖泊碳循环模型忽略了湖泊间的异质性。我们的简单分解模型显示了湖泊大小和水文对陆地碳分解的强烈影响，从而为基于过程的湖泊碳循环尺度化提供了机会。直到最近，死亡的浮游动物才被确定为下沉POM的重要组成部分，例如湖雪，因此是水生碳循环的组成部分。我们专注于下沉的POM，特别是浮游动物尸体-除了浮游植物聚集体。首先，下沉的浮游动物尸体可能丰富，代表微生物定植和活动的热点，加速微生物有机质再矿化。其次，浮游动物的尸体将不稳定有机物（LOM）运输到更深的水层，并可能增加异养活动，刺激顽固性有机物（ROM）的降解。尽管关于浮游动物尸体在水生系统碳循环中的重要作用的提示越来越频繁，但由于方法的限制，死亡的浮游动物在水生科学中仍然很大程度上被忽视。垂直剖面，特别是浮游动物尸体丰度与其他POM的关系，以及它们的特定下沉速度，构成了死后LOM释放的关键参数，以及在更深的水深下伴随的ROM降解的刺激，实际上仍然未知。本研究旨在揭示水体密度梯度对POM沉降行为的影响，特别是浮游植物聚集体和浮游动物尸体，以及它们对内陆水域微生物碳循环的影响。以下假设将在选定的对比德国和俄罗斯湖泊中进行测试：1)尖锐的密度界面-温跃线和/或趋化线-影响植物和浮游动物聚集体的命运，并通过充当大型下沉颗粒（例如植物和浮游动物聚集体）的陷阱来控制水柱中OM降解的强度，刺激密度梯度薄层中的微生物活动。2)浮游植物聚集体和浮游动物尸体等不稳定POM的垂直剖面和动态显著影响水体中ROM浓度的垂直分布格局。在较深水层中，由于存在不稳定的POM和随后在斜斜层中释放LOM， ROM的降解受到刺激。

Inland waters such as lakes and reservoirs play a substantial role in the transport and transformation of both terrestrial and autochthonous organic carbon. The key internal pathway for aquatic carbon dynamics, i.e. sinking of particulate organic matter (POM), strongly depends on vertical density stratification of the water column, which due to atmospheric warming has increased globally during the recent past. Yet, the integral effect of water column stratification on POM transformation remains non-quantified to date. Existing efforts to model broad-scale lake carbon cycling ignore inter-lake heterogeneity. Our simple decomposition model demonstrates strong effects of lake size and hydrology for terrestrial carbon decomposition and thus provides an opportunity for process-based scaling of lake carbon cycling.Only recently, dead zooplankton has been identified as an important component of sinking POM, e.g. lake snow, and thus as integral part of the aquatic carbon cycle. We focus on sinking POM, in particular zooplankton carcasses - aside of phytoplankton aggregates. Firstly, sinking zooplankton carcasses can be abundant and represent hot spots of microbial colonization and activity accelerating microbial organic matter remineralization. Secondly, zooplankton carcasses transport labile organic matter (LOM) to deeper water layers and may increase heterotrophic activities, stimulating the degradation of recalcitrant organic matter (ROM). Although hints on an important role of zooplankton carcasses for carbon cycling in aquatic systems become more frequent, dead zooplankton is still largely ignored in aquatic science mainly due to methodological limitations. Vertical profiles, especially of zooplankton carcasses abundance in relation to other POM as well as their specific sinking velocities, which constitute key parameters for the post-mortal release of LOM and concomitant stimulation of ROM degradation at greater water depth, remain practically unknown. This proposal aims to shed light on the changing role of water column density gradients for sinking behavior of POM, in particular phytoplankton aggregates and zooplankton carcasses, and their impact on microbial carbon cycling in inland waters. Following hypotheses will be tested in selected contrasting German and Russian lakes: 1) Sharp density interfaces - thermoclines and/or chemoclines - affect the fate of phyto- and zooplankton aggregates and control the intensity of OM degradation in the water column by acting as traps for large sinking particles, e.g. phyto- and zooplankton aggregates, stimulating microbial activities in thin layers of the density gradients.2) Vertical profiles and dynamics of labile POM such as phytoplankton aggregates and zooplankton carcasses significantly affect vertical distribution patterns of ROM concentration in the water column. ROM degradation in deeper water layers is stimulated in the presence of labile POM and subsequent release of LOM in the pycnocline.