Waterborne and shellfish-borne enteric viruses associated with wastewater-polluted coastal waters (e.g. Norovirus, Hepatitis A/E viruses, Adenovirus) represent a major threat to human health. Improved understanding of the locations and periods of heightened risks can help target mitigation measures and improve public health. We developed a river-estuary-coast model to simulate virus dispersal, driven by point source discharges and river flows in combination with tidal forcing. Viral inputs were based on measured wastewater adenovirus concentrations and the model was implemented with or without viral die-off. We applied the model to the Conwy river (North Wales, UK), through the estuary, to the Irish Sea coast where bathing waters and shellfisheries are known to be prone to viral contamination. Using a suite of scenarios, we showed that river flow was the primary control of viral export to the coast. Since the Conwy catchment is short and steep, and the estuary is small and river-dominated, short-duration high intensity 'flash floods' were shown to transport viruses through the estuary and out to sea, despite dilution or die-off effects. Duplicating flow events (i.e., storm clustering) did not double the virus export since the virus re-entered the estuary on the flood tide. The tidal magnitude and timing of high water relative to peak river flow were also important drivers regulating viral dispersal. A worst-case event simulation (i.e., combining high river flows with high viral loading and high spring tide) resulted in increased concentrations of virus at nearby coasts, although the spatial spread was similar to the previous scenarios. Our results suggest that impact models for predicting and mitigating episodes of poor microbiological water quality may require careful representation of the intensity and timings of river flow when evaluating pathogen exposure risk.

Estuaries are potentially exposed to compound flooding where weather-driven extreme sea levels can occur synchronously with extreme fluvial discharge to amplify the hazard. The likelihood of compound flooding is difficult to determine due to multiple interacting physical processes operating at sub-daily scales, and poor observation records within estuaries with which to determine potential future probabilistic scenarios. We hypothesize that fluvial extremes can occur within the peak of the surge in small/steep catchments because of rapid runoff times, whilst the length-scale in larger/flatter catchments will result in fluvial and marine extremes being out-of-phase. Data (15 min river flow and hourly sea level) spanning 40 years were analyzed to assesses the behaviour and timings of fluvial and sea level extremes in two contrasting estuaries: Humber and Dyfi (United Kingdom). Compound events were common in the Dyfi, a small/steep catchment on Britain’s west coast with fast fluvial response times. Almost half of the 937 skew-surge events (95^th-percentile) occurred within a few hours of an extreme fluvial peak, suggesting that flood risk is sensitive to the storm timing relative to high tide—especially since flows persisted above the 95^th-percentile typically for less than 12 h. Compound events were more frequent during autumn/winter than spring/summer. For the Humber, a larger/flatter catchment on the east coast with slower fluvial response times, extreme fluvial and skew-surge peaks were less frequent (half as many as the Dyfi) and compound events were less common (15% of events co-occurred). Although flows in the Humber persisted above the 95^th-percentile for typically between one and 4 days, hence overlapping several high tides and possibly other surges. Analysis of 56 flooding events across both estuaries revealed: 1) flooding is more common in the Dyfi than Humber; 2) Dyfi flooding is driven by 99^th-percentile flows lasting hours and co-occurring with a 95^th percentile skew-surge; 3) Humber flooding was driven by 95^th-percentile flows lasting days, or surge-driven—but rarely co-occurring. Our results suggest that compound flooding studies require at least hourly data (previous analyses have often used daily-means), especially for smaller systems and considering the potential intensification of rainfall patterns into the future.

Background: Larval connectivity between distinct benthic populations is essential for their persistence. Although connectivity is difficult to measure in situ, it can be predicted via models that simulate biophysical interactions between larval behaviour and ocean currents. The blue mussel (Mytilus Edulis L.) is widespread throughout the northern hemisphere and extensively commercialised worldwide. In the Irish Sea, this industry represents ~ 50% of Welsh shellfisheries, where cultivation is mainly based on wild spat. However, the main sources and amount of spat varied interannually (1100 tonnes harvest in 2014 against zero in 2018). The aim of this study is to characterise the structure and dynamics of the blue mussel metapopulation within the northern part of the Irish Sea. Methods: We develop a Lagrangian particle tracking model, driven by a high-resolution (from 30 to 5000 m) validated unstructured coastal hydrodynamic model of the Irish Sea, to simulate spatial and temporal variability of larval dispersal and connectivity between distinct mussel populations and potential settlement areas. Results: Our results showed that: (1) larvae positioned near the surface were strongly influenced by wind-driven currents suggesting that connectivity networks had the potential to span hundreds of kilometres; (2) in contrast, larvae positioned deeper in the water column were driven by tidal currents, producing intricate spatial patterns of connectivity between mussel beds over tens of kilometres that were consistent over time. Conclusions: Dispersal of mussel larvae in the tidally energetic Irish Sea during the April–May spawning season is potentially driven by wind-driven surface currents, as confirmed by fisherman observations of inter-annual variability in wild spat collection. These results have important implications for metapopulation dynamics within the context of climate change and sustainable shellfisheries management (i.e. gain and loss of populations and harvest areas according to wind conditions).

Hydrodynamic controls of the transport of suspended particulate matter (SPM), particulate nutrients and faecal indicator organisms (FIOs) in the river-estuary transition zone (RETZ) of a shallow macrotidal estuary were studied on tidal and seasonal timescales. The RETZ was found to be a conduit for SPM rather than a zone of particle accumulation during spring tides, with complex size fractionation and biogeochemical exchanges of particulate nutrient/FIO compositions. The downstream RETZ was characterised by flood-dominant currents, but with ebb-dominant turbulence due to the suppression of flood tide turbulence by salinity stratification created by lateral convergence; this produced a net seaward mass transport of SPM. Without lateral convergence in the narrower upstream RETZ, flood-dominant currents and flood-dominant turbulence were experienced. Hence the RETZ exported SPM landwards from its upper end and seawards from its lower end - a process observed throughout the year during spring tides and low-to-mean river flows. During neap tides when SPM concentrations were reduced, the RETZ became a zone of minor particle accumulation as its lower end imported SPM landwards from the estuary and its upper end imported SPM seawards from the river. During a river flood event, net SPM flux was significantly increased and was seawards throughout the RETZ. SPM mass concentration and carbon, nitrogen, phosphorous, and FIO concentrations peaked due to local resuspension and advection of an ephemeral estuarine turbidity maximum (ETM). The ETM formed on the advancing flood tide due to entrainment of material from intertidal flats. Flocculation and settling occurred at high slack water. The ETM was reconstituted by entrainment on the ebb and was composed of larger flocs than on the flood. Particulate nutrients and FIOs were associated with flocs in the 10-200 μm range but not with smaller or larger flocs. SPM concentrations in the resuspension component and ETM exceeded microbial water quality standards, emphasising the need for monitoring practices that consider tidal dynamics. The results from this study showing periodic SPM export from, rather than prolonged accumulation in, the RETZ and the influence of particle size fractionation on biogeochemical fluxes in the RETZ, are likely to be transferable to many other embayment-type estuaries on macrotidal coasts.