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

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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.