Vegetation characteristics control local sediment and nutrient retention on but not underneath vegetation in floodplain meadows.

Sediment and nutrient retention are essential ecosystem functions that floodplains provide and that improve river water quality. During floods, the floodplain vegetation retains sediment, which settles on plant surfaces and the soil underneath plants. Both sedimentation processes require that flow velocity is reduced, which may be caused by the topographic features and the vegetation structure of the floodplain. However, the relative importance of these two drivers and their key components have rarely been both quantified. In addition to topographic factors, we expect vegetation height and density, mean leaf size and pubescence, as well as species diversity of the floodplain vegetation to increase the floodplain's capacity for sedimentation. To test this, we measured sediment and nutrients (carbon, nitrogen and phosphorus) both on the vegetation itself and on sediment traps underneath the vegetation after a flood at 24 sites along the River Mulde (Germany). Additionally, we measured biotic and topographic predictor variables. Sedimentation on the vegetation surface was positively driven by plant biomass and the height variation of the vegetation, and decreased with the hydrological distance (total R2 = 0.56). Sedimentation underneath the vegetation was not driven by any vegetation characteristics but decreased with hydrological distance (total R2 = 0.42). Carbon, nitrogen and phosphorus content in the sediment on the traps increased with the total amount of sediment (total R2 = 0.64, 0.62 and 0.84, respectively), while C, N and P on the vegetation additionally increased with hydrological distance (total R2 = 0.80, 0.79 and 0.92, respectively). This offers the potential to promote sediment and especially nutrient retention via vegetation management, such as adapted mowing. The pronounced signal of the hydrological distance to the river emphasises the importance of a laterally connected floodplain with abandoned meanders and morphological depressions. Our study improves our understanding of the locations where floodplain management has its most significant impact on sediment and nutrient retention to increase water purification processes.

Automated in Situ Oxygen Profiling at Aquatic-Terrestrial Interfaces.

Optical sensing technologies provide opportunities for in situ oxygen sensing capable of capturing the whole range of spatial and temporal variability. We developed a miniaturized Distributed Oxygen Sensor ("mDOS") specifically for long-term in situ application in soil and sediment. The mDOS sensor system enables the unattended, repeated acquisition of time series of in situ oxygen profiles at a subcentimeter resolution covering a depth of up to one meter. As compared to existing approaches, this provides the possibility to reveal highly variable and heterogeneous oxygen dynamics at a high, quasi-continuous resolution across both scales. The applicability of the mDOS to capture both intra- and interday fine-scale variability of spatiotemporal oxygen dynamics under varying hydrological conditions is exemplarily demonstrated. We specifically aim at estimating the dependency between oxygen dynamics and hydrologic conditions along the measured profiles. The mDOS system enables highly detailed insights into oxygen dynamics in various aquatic and terrestrial environments and in the inherent transition zones between them. It thus represents a valuable tool to capture oxygen dynamics to help disentangling the coupling between underlying hydrological and biogeochemical process dynamics.

Robust optode-based method for measuring in situ oxygen profiles in gravelly streambeds.

One of the key environmental conditions controlling biogeochemical reactions in aquatic sediments like streambeds is the distribution of dissolved oxygen. We present a novel approach for the in situ measurement of vertical oxygen profiles using a planar luminescence-based optical sensor. The instrument consists of a transparent acrylic tube with the oxygen-sensitive layer mounted on the outside. The luminescence is excited and detected by a moveable piston inside the acrylic tube. Since no moving parts are in contact with the streambed, the disturbance of the subsurface flow field is minimized. The precision of the distributed oxygen sensor (DOS) was assessed by a comparison with spot optodes. Although the precision of the DOS, expressed as standard deviation of calculated oxygen air saturation, is lower (0.2-6.2%) compared to spot optodes (<0.1-0.6%), variations of the oxygen content along the profile can be resolved. The uncertainty of the calculated oxygen is assessed with a Monte Carlo uncertainty assessment. The obtained vertical oxygen profiles of 40 cm in length reveal variations of the oxygen content reaching from 90% to 0% air saturation and are characterized by patches of low oxygen rather than a continuous decrease with depth.