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

内陆水域，如湖泊和水库，在陆地和原生有机碳的运输和转化中发挥着重要作用。水生碳动态的关键内部途径，即颗粒有机物(POM)的下沉，在很大程度上依赖于水柱的垂直密度分层，而由于大气变暖，水柱的垂直密度分层在最近几年在全球范围内有所增加。然而，水柱分层对POM转化的整体影响到目前为止还没有定量化。现有的对大尺度湖泊碳循环进行建模的努力忽视了湖泊间的异质性。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，例如湖泊雪，从而成为水生碳循环的组成部分。我们专注于下沉POM，特别是浮游动物身体--而不是浮游植物聚集体。首先，下沉的浮游动物身体数量丰富，代表着微生物定植的热点和加速微生物有机质再矿化的活动。其次，浮游动物身体将活性有机物(LOM)输送到更深的水层，并可能增加异养活性，刺激顽固性有机物(ROM)的降解。尽管浮游动物身体在水生系统碳循环中的重要作用的暗示越来越频繁，但在水生科学中，死亡浮游动物仍然在很大程度上被忽视，主要是由于方法学的限制。垂直剖面，特别是浮游动物身体相对于其他POM的丰度以及它们的比下沉速度，构成了死后释放LOM的关键参数，并伴随着在更大水深时刺激ROM的降解，实际上仍不清楚。这项建议旨在阐明水柱密度梯度对POM下沉行为的变化作用，特别是浮游植物聚集体和浮游动物身体，以及它们对内陆水域微生物碳循环的影响。将在选定的德国和俄罗斯的湖泊中检验以下假说：1)尖锐的密度界面--温跃层和/或化跃层--影响浮游植物和浮游动物聚集体的命运，并通过作为大的下沉颗粒的陷阱来控制水体中有机物的降解强度，例如浮游植物和浮游动物聚集体，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身体等不稳定的POM的垂直剖面和动态显著影响水体中有机污染物的垂直分布模式。不稳定的POM的存在和随后LOM在跃层中的释放刺激了更深水层中的ROM降解。

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.