Microbial response on organic matter in river-retention zones

Although harboring only a small fraction of the worlds water masses, lotic (running water) systems are well known as critical links in global organic matter cycles. Recent estimates illustrate that fluvial net heterotrophy is significant on a global scale, and organic matter in transit (including also aged, recalcitrant material) is likely metabolized more or less continuously throughout fluvial networks. The transformation and microbial utilization of organic carbon in rivers seems to depend on the existence of opportunities (increased residence time based on channel morphology, floodplains, retention zones etc.), which potentially enhance the carbon exchange with the atmosphere. Surprisingly, the role of floodplains and retention zones, which are extending the residence time of organic molecules in transport, is poorly studied regarding the processing of this material by bacteria (prokaryotes). Natural floodplains and riparian zones are known to increase the heterogeneity of a river system. Here, the driving force for lateral exchange processes is hydrological connectivity, which may profoundly influence biological and biogeochemical processes. Almost all large rivers in Europe are greatly reduced in spatial heterogeneity due to severe alterations in the catchment, damming and regulation, some large rivers even being among the world`s most degraded ecosystems. Given the renewed global interest in fluvial networks, it is timely to study the role of river heterogeneity on microbial activity and carbon cycling by tackling the following objectives: impact of variable hydrology, hydrologic storage and retention on organic matter diversity and variability; impact of variable hydrology, hydrologic storage and retention on microbial activity, diversity and organic matter turnover; significance of retention zones for abundance of suspended, metabolically highly active aggregates as sites of organic matter utilization; significance of retention zones and varying hydrology on CO 2 - remineralization, thus augmenting the metabolic performance of fluvial ecosystems. The temporal and integrative character of this study combines important issues of microbial and river ecology with biogeochemical aspects, and will significantly contribute to the field of aquatic ecology in a changing world.

This study, conducted in a semi-natural, urban-influenced river-floodplain system was able to demonstrate that typically variable hydrological conditions and the existence of retention zones have significant impact on diversity and microbial utilization of organic matter. This also influences the related emission of carbon-gasses (CH4 u. CO2) to the atmosphere.The study demonstrates that floods are important for delivering organic matter, which, despite its terrestrial origin, can be effectively utilized by aquatic bacteria in such river- floodplain systems, depending on the hydrological conditions. The presence of active floodplains, characterized by hydrological connectivity with the main channel, creates the opportunity to process organic material, which is typically resistant to degradation. The often rapidly changing environmental conditions in such water bodies appear to create specific spatiotemporal patterns thus influencing the community structure and enzymatic activity of degrading bacteria. This indicates functional redundancy and metabolic plasticity of the involved microorganisms.Particularly during post-flood periods, the carbon sources that support bacterial activity are variable in the different types of backwaters. In disconnected floodplain sections with high terrestrial input, bacterial activity can also be driven by locally produced aquatic carbon sources (exudates of phytoplankton origin). In semi-isolated backwaters, the presence of fresh labile material from phytoplankton even enhances activity of key enzymes for degradation also of terrestrial material. In frequently flooded river-floodplain sections, bacterial activity was mainly driven by enzymatic degradation of particulate phytoplanktonmaterial. As an added value, using data from former projects conducted in this system, it can be concluded that lysis of bacterial cells due to viral attack contributes significantly to a pool of rapidly cycling carbon in this environment with typical high proportion of terrestrial carbon of low quality.Data on gas emission of important greenhouse gases (CH4 u. CO2) from the river- floodplains emphasize the importance of floods differing in magnitude; for example 34% more methane was emitted from the entire system during a year with a 100-yr flood compared to a hydrologically normal year.All these findings imply that the predicted increased frequency of extreme flooding events will have significant consequences for microbial processes and related carbon gas emission. This apparently will impact the contribution of such systems to overall carbon flux.