Water flow plays a key role in determining chemical biodegradation in water-sediment systems.

Before agrochemicals can be registered and sold, the chemical industry is required to perform regulatory tests to assess their environmental persistence, using defined guidelines. Aquatic fate tests (e.g. OECD 308) lack environmental realism as they are conducted under dark conditions and in small-scale static systems, which can affect microbial diversity and functionality. In this study, water-sediment microflumes were used to investigate the impact of these deficiencies in environmental realism on the fate of the fungicide, isopyrazam. Although on a large-scale, these systems aimed to retain the key aspects of OECD 308 tests. Tests were carried out under both a non-UV light-dark cycle and continuous darkness and under both static and flowing water conditions, to investigate how light and water flow affect isopyrazam biodegradation pathways. In static systems, light treatment played a significant role, with faster dissipation in illuminated compared to dark microflumes (DT50s = 20.6 vs. 47.7 days). In flowing systems (DT50s = 16.8 and 15.3 days), light did not play a significant role in dissipation, which was comparable between the two light treatments, and faster than in dark static microflumes. Microbial phototroph biomass was significantly reduced by water flow in the illuminated systems, thereby reducing their contribution to dissipation. Comprehensive analysis of bacterial and eukaryotic community composition identified treatment specific changes following incubation, with light promoting relative abundance of Cyanobacteria and eukaryotic algae, and flow increasing relative abundance of fungi. We conclude that both water velocity and non-UV light increased isopyrazam dissipation, but the contribution of light depended on the flow conditions. These differences may have resulted from impacts on microbial communities and via mixing processes, particularly hyporheic exchange. Inclusion of both light and flow in studies could improve the extent they mimic natural environments and predict chemical environmental persistence, thus bridging the gap between laboratory and field studies.

Bedform characteristics and biofilm community development interact to modify hyporheic exchange.

The physical and biological attributes of riverine ecosystems interact in a complex manner which can affect the hydrodynamic behaviour of the system. This can alter the mixing characteristics of a river at the sediment-water interface. Research on hyporheic exchange has increased in recent years driven by a greater appreciation for the importance of this dynamic ecotone in connecting and regulating river systems. An understanding of process-based interactions driving hyporheic exchange is still limited, specifically the feedbacks between the physical and biological controlling factors. The interplay between bed morphology and sediment size on biofilm community development and the impact on hyporheic exchange mechanisms, was experimentally considered. Purpose built recirculating flume systems were constructed and three profiles of bedform investigated: i) flat, ii) undulating λ = 1 m, ii) undulating λ = 0.2 m, across two different sized sediments (0.5 mm and 5 mm). The influence of biofilm growth and bedform interaction on hyporheic exchange was explored, over time, using discrete repeat injections of fluorescent dye into the flumes. Hyporheic exchange rates were greatest in systems with larger sediment sizes (5 mm) and with more bedforms (undulating λ = 0.2). Sediment size was a dominant control in governing biofilm growth and hyporheic exchange in systems with limited bedform. In systems where bedform was prevalent, sediment size and biofilm appeared to no longer be a control on exchange due to the physical influence of advective pumping. Here, exchange rates within these environments were more consistent overtime, despite greater microbial growth. As such, bedform has the potential to overcome the rate limiting effects of biotic factors on hyporheic exchange and sediment size on microbial penetration. This has implications for pollutant and nutrient penetration; bedforms increase hydrological connectivity, generating the opportunity to support microbial communities at depth and as such, improve the self-purification ability of river systems.

Inclusion of seasonal variation in river system microbial communities and phototroph activity increases environmental relevance of laboratory chemical persistence tests.

Regulatory tests assess crop protection product environmental fate and toxicity before approval for commercial use. Although globally applied laboratory tests can assess biodegradation, they lack environmental complexity. Microbial communities are subject to temporal and spatial variation, but there is little consideration of these microbial dynamics in the laboratory. Here, we investigated seasonal variation in the microbial composition of water and sediment from a UK river across a two-year time course and determined its effect on the outcome of water-sediment (OECD 308) and water-only (OECD 309) biodegradation tests, using the fungicide isopyrazam. These OECD tests are performed under dark conditions, so test systems incubated under non-UV light:dark cycles were also included to determine the impact on both inoculum characteristics and biodegradation. Isopyrazam degradation was faster when incubated under non-UV light at all collection times in water-sediment microcosms, suggesting that phototrophic communities can metabolise isopyrazam throughout the year. Degradation rate varied seasonally between inoculum collection times only in microcosms incubated in the light, but isopyrazam mineralisation to 14CO2 varied seasonally under both light and dark conditions, suggesting that heterotrophic communities may also play a role in degradation. Bacterial and phototroph communities varied across time, but there was no clear link between water or sediment microbial composition and variation in degradation rate. During the test period, inoculum microbial community composition changed, particularly in non-UV light incubated microcosms. Overall, we show that regulatory test outcome is not influenced by temporal variation in microbial community structure; however, biodegradation rates from higher tier studies with improved environmental realism, e.g. through addition of non-UV light, may be more variable. These data suggest that standardised OECD tests can provide a conservative estimate of pesticide persistence end points and that additional tests including non-UV light could help bridge the gap between standard tests and field studies.

Longitudinal dispersion of microplastics in aquatic flows using fluorometric techniques.

Microplastics are an emerging environmental contaminant. Existing knowledge on the precise transport processes involved in the movement of microplastics in natural water bodies is limited. Microplastic fate-transport models rely on numerical simulations with limited empirical data to support and validate these models. We adopted fluorometric principles to track the movement of both fluorescent dye and florescent stained microplastics (polyethylene) in purpose-built laboratory flumes with standard fibre-optic fluorometers. Neutrally buoyant microplastics behaved in the same manner as a solute (Rhodamine) and more importantly displayed classical fundamental dispersion theory in uniform open channel flow. This suggests Rhodamine, a fluorescent tracer, can be released into the natural environment with the potential to mimic microplastic movement in the water column.