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Rivers and streams are a primary transport vector for microplastics (MPs), connecting terrestrial sources to marine environments. While previous studies indicated that pore-scale MPs can accumulate in streambed sediments, the specific MPs transport and retention mechanisms in fluvial systems remain poorly understood. As part of this technical note, we present a novel method for a quantitative analysis of the spatiotemporal transport and retention of pore-scale MPs in an experimental flume. A continuous mass balance for MPs in surface water was achieved using two online fluorometers, while a laser-induced Fluorescence-Imaging-System was developed to track and quantify the spatial migration of MPs through the streambed sediments. The detection limit was < 1 µg/L for 1 µm polystyrene microbeads with the fluorometers and 3 µg/L for the Fluorescence-Imaging-System. The system was able to quantitatively track the advective transfer of MPs into the streambed sediments: a process that has yet not been observed experimentally. Results showed that MPs infiltrated into the streambed sediments up to a depth twice the bedform amplitude. This work provides a novel experimental method to quantitatively monitor MP transport through porous media and advective exchange of MP across the streambed interface.

Biogeochemical hot spots are defined as areas where biogeochemical processes occur with anomalously high reaction rates relative to their surroundings. Due to their importance in carbon and nutrient cycling, characterization of hot spots is critical to accurately predict carbon budgets in the context of climate change. However, biogeochemical hot spots are difficult to identify in the environment, as sampling resolutions are often too coarse to find these areas in the subsurface. Here, we present imaging results of a geophysical survey using the non-invasive induced polarization (IP) method to identify biogeochemical hot spots of carbon turnover in a minerotrophic wetland. To interpret the field-scale IP signatures, geochemical analyses were performed on freeze-core samples obtained in areas characterized by anomalously high and low IP responses. Our results reveal large variations in the electrical response, with the highest IP phase values (> 20 mrad) corresponding with high concentrations of phosphates (> 4000 μM), an indicator of carbon turnover. Moreover, analysis of the freeze core reveal negligible concentrations of iron sulfides. The extensive geochemical and geophysical data presented in our study demonstrates that IP images can assess changes in the biogeochemical activity in peat, and identify hot spots.

Although a major part of marine microplastic (MP) pollution originates from rivers and streams, the mechanistic behavior of MP in fluvial systems is only poorly understood. MP enter fluvial systems from e.g. waste water treatment plant (WWTP) effluents, sewer overflows during heavy rain events, agricultural runoff, aerial input/atmospheric fallout, road runoff or via fragmentation of plastic litter. As part of this project we want to investigate the hydrodynamic transport mechanisms that control the behavior and re-distribution of MP in open channel flow and the streambed sediments. Hydrodynamic conditions in open channel flow are represented in an experimental flume environment. Different porous media materials (e.g. aqua beads, glass beads and sand) are used in the flume experiments to shape typical bed form structures such as riffle-pool sequences, ripples and dunes. The aim of this experimental setup is to create hydrodynamic flow conditions such as hydraulic jumps, low and high flow velocity environments for which the transport and sedimentation behavior of MP can be investigated under realistic conditions. Hydrodynamic flow conditions in the flume are characterized using a Laser-Doppler-Anemometry (LDA) and Particle Image Velocimetry (PIV). Detection and tracking of fluorescent MP-particles in open channel flow and in porous media will be achieved with a fluorescence-camera-system.

Mercury (Hg) is a toxic trace element known to highly accumulate in aquatic biota and uptake by algae is the main entry-point into the aquatic food chain. However, the effect of changes in solar irradiance and climatic conditions on Hg uptake by algae is largely unknown. We analyzed the link between sediment Hg accumulation, cyclic changes in total solar irradiance (TSI), and related changes in lake productivity as well as climate during the past 4.5 thousand years (kyr) in sediments of a small highly productive lake in Southern Patagonia (53°S). The analyses encompass proxies for TSI (based on 10Be), sediment geochemical composition, and lake productivity (FTIR spectra, and hydrogen-index). The sediment record shows high concentrations of organic matter (median 70%) and strong variations in Hg accumulation rates (14–53 μg m2 yr−1) which correspond to changes in aquatic productivity and TSI. Accumulation of Hg was highest during dryer periods when irradiance and lake productivity was high. During these periods, accumulation was up to fourfold higher compared to those of lower TSI, lower productivity, and wetter climatic conditions. Higher amounts of algae biomass can increase Hg scavenging by organic particles and Hg export to the sediment. To obtain mass balance, we assume that the increase in sediment Hg accumulation rates during periods of lower Hg fluxes from the catchment (drier) caused a decrease in water phase Hg and in rates of Hg evasion from the lake.

Radon (R86222n) as a hydrological tracer offers a method for studying short to medium term groundwater - surface water interactions. These high frequency processes play an important role in wetland hydrology and biogeochemistry and may influence their contribution to the global carbon cycle. Therefore, there is a definite need for robust methods to measure high resolution

Cyclic changes in total solar irradiance (TSI) during the Holocene are known to affect global climatic conditions and cause cyclic climatic oscillations, e.g., Bond events and related changes of environmental conditions. However, the processes how changes in TSI affect climate and environment of the Southern Hemisphere, especially in southernmost South America, a key area for the global climate, are still poorly resolved. Here we show that highly sensitive proxies for aquatic productivity derived from sediments of a lake near the Chilean South Atlantic coast (53 °S) strongly match the cyclic changes in TSI throughout the Holocene. Intra-lake productivity variations show a periodicity of ~200–240 years coherent with the time series of TSI-controlled cosmogenic nuclide 10Be production. In addition TSI dependent periodicity of Bond events (~1500 years) appear to control wetness at the LH site indicated by mineral matter erosion from the catchment to the lake assumingly through shifts of the position of the southern westerly wind belt. Thus, both intra-lake productivity and wetness at the southernmost South America are directly or indirectly controlled by TSI.