A transboundary case of investigating longitudinal water and sediment connectivity using microplastics in the Thayatal/Podyji National Park

Microplastics are increasingly recognized as potential tracers of anthropogenic activities and sediment transport. Due to their irregular shapes and low densities, these particles exhibit transport behaviours that differ significantly from naturally occurring sediments. However, analysing their distribution and characteristics can help identify microplastic sources and sinks, offering insights into their transport dynamics within the environment. This study focuses on the Thayatal/Podyjí National Park, situated along the border of Lower Austria (Austria, AT) and Southern Moravia (Czech Republic, CZ), where plastic pollution has not previously been monitored. Water samples (approximately 5 m³ each) were collected in triplicate from five locations along and across a hydrologically disconnected section of the Thaya/Podyjí River, spanning from Vranov (CZ) to Hnanice (CZ). Sampling was conducted using a 30 cm in diameter plankton net with a mesh size of 150 μm in early November 2024. Flow velocity was measured across the width of the channel using MF Pro flowmeter. Between each sampling phase, the net was cleaned three times using pressurized filtered (40 µm) water at 3-4 bars. To assess the influence of tributaries and their connectivity to the main river channel, samples were also taken from three tributaries: Klaperův Creek (CZ), Kaja Creek (AT), and the Fugnitz River (AT). Additional samples were collected along the Fugnitz River, a medium-sized agricultural stream (catchment area 131 km²) that has been the focus of connectivity research over the past decade. This study is conducted alongside ongoing research on microplastic transport at the catchment scale, with a particular emphasis on lateral connectivity in the Fugnitz catchment. The overarching aim of this research is to evaluate the potential of microplastics as tracers of geomorphological connectivity and to improve understanding of their behaviour in comparison to natural sediments. This analysis is supported by erosion modelling and grain size analysis. Geomorphological features of the Thaya/Podyjí River, along with anthropogenic factors influencing connectivity, are being analysed using remote sensing and GIS tools. In the sedimentology laboratory, water samples from the Thaya/Podyjí section undergo organic matter reduction using Fenton’s reagent (H₂O₂ with an Fe²⁺ catalyst) and density separation with potassium formate (HCO₂K; density 1.45 g/cm³). Microplastic particles are then analysed with an ATR-FTIR Lumos II microscope, with manual identification performed on 25% of the filter area. Ongoing protocol validation includes blank tests and recovery rate analyses to ensure methodological reliability. As of now, results from 3 locations along the main channel were produced. From upstream to downstream, location 1 had 6.4±1.31 particles per m³, location 2 had 10.2±0.8 particles per m³ and location 3 had 10±3.39 particles per m³. In the upcoming spring, additional fieldwork may be performed to evaluate seasonality of discharge as well as other sampling parameters affecting the discharge i.e. depth and time of sampling.