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Abstract Microplastic research has mainly concentrated on open seas, while riverine plumes remain largely unexplored despite their hypothesized importance as a microplastic source to coastal waters. This work aimed to model coastal accumulation of microplastic particles (1–5 mm) emitted by the Po River over 1.5 years. We posit that river-induced microplastic accumulation on adjacent coasts can be predicted using (1) hydrodynamic-based and (2) remote sensing-based modelling. Model accumulation maps were validated against sampling at nine beaches, with sediment microplastic concentrations up to 78 particles/kg (dry weight). Hydrodynamic modelling revealed that discharged particle amount is only semi-coupled to beaching rates, which are strongly mouth dependent and occur within the first ten days. Remote sensing modelling was found to better capture river mouth relative strength, and accumulation patterns were found consistent with hydrodynamic modelling. This methodology lays groundwork for developing an operational monitoring system to assess microplastic pollution emitted by a major river. Graphical abstract Unlabelled Image Highlights • Beach sediment microplastic concentrations up to 78.8 particles/kg dry weight • Water sampling (max 84.1 particles/m3) highest along outer plume and within river • Daily river discharged microplastic particles only semi-coupled to beaching rates • Microplastic beaching rate strongly river mouth dependent and occurs within 10 days • Remote sensing exposure modelling validated against hydrodynamic accumulation model [ABSTRACT FROM AUTHOR]

In February 2012, a severe cold spell in the European region triggered a massive production of very dense water on the northern Mediterranean Sea shelves. The spreading phase of the newly formed dense water was extensively studied in the Adriatic Sea by means of 2 ship surveys and 5 moorings fully equipped to monitor the flow of the bottom layer. For the Adriatic Sea, opposite to the Gulf of Lions, the area of cascading is far from the source area and this implies substantial modifications, adjustments and dilution of the source water mass along its path, with a spreading phase lasting several months. Indeed all the moorings detected events, although weaker than in the preceding months, until June 2012. The surveys detected 2 branches of NAdDW on the shelf, the first branch not denser than 29.7 kg/m 3 and the second branch not denser than 29.5 kg/m 3 . Despite the extremely dense water generated in the Northern Adriatic, during events of dense-water flow, moorings recorded temperatures generally between 12.5 and 13 °C, seldom less. Temperatures along the shelf break also did not fall below 13 °C at depths greater than 400 m. Turbulent mixing, therefore, heavily modified the cascading plumes, which left the shelf with thicknesses between 10 and 30 m. Mooring data in the lowermost 100 mab suggest that the thickness of the cascading layer increased by several tens of meters downslope, as a consequence of entrainment. Detraining frictional layers as well as locations of active cascading were identified mostly by isolated casts, highlighting the submesoscale domain of the downsloping plumes. The use of LADCP data allowed identification of very energetic bottom flow (40–50 cm/s in many locations), with otherwise little signature in tracers, not previously observed. The Bari Canyon System (BCS) was so far recognized as a hot spot for cascading in the Southern Adriatic. However, during the 2012 event, this is not the only preferred site for cascading. Significant dense flow was detected in other locations. The northernmost mooring site, closer to the inception of the cascading process, in particular showed active cascading and several dynamical differences from the BCS: denser water with thinner boundary layer, events organized in multiple pulses with sub-inertial periodicity and with very short duration (12 h to 1 day) that is generally not seen in other locations. [ABSTRACT FROM AUTHOR]

The northern Adriatic Sea is a basin where dense shelf waters form during winters if the following favorable conditions are attained: high salinities, mainly related to low river water discharge (Po river in primis) during fall season, and strong heat fluxes induced by cold and dry Bora wind events blowing from north-east. Extremely favorable conditions to Northern Adriatic Dense Waters (NAdDW) formation characterized winter 2012 when, following several months of low Po river runoff, a strong event of Cold Air Outbreak (CAO) occurred from the end of January until mid-February. Consequently, the mean temperature of northern Adriatic waters dropped to about 6°C and exceptional densities (potential density anomaly locally exceeding 30.0 kg m − 3 ) were reached. The production and spreading mechanisms of dense water in the Adriatic Sea have been modeled by means of the COAWST (Coupled-Ocean–Atmosphere-Wave-Sediment-Transport) modeling system. The model builds upon a high-resolution (1 km spaced horizontal grid), fully 3-D primitive equations hydrodynamic model coupled with a phase-averaged wave model and sediment routines, and is driven by simulated atmospheric forcings. The dataset used to assess model outputs relies on the measurements acquired during the dedicated field campaigns “Operation Dense Water”, a set of two rapid response cruises carried out in southern Adriatic during winter 2012, and by five mooring arrays deployed in the Southern Adriatic Margin (SAM) that allowed the continuous acquisition of temperature, salinity, currents and suspended matter samples. Results from the integrated data-model approach suggest that the NAdDW propagates along the shelf to the southern basin following both a shallower vein and a deeper stream, with a process characterized by a strong variability (mean value of 0.31 Sv with peaks rapidly growing in the first weeks after the CAO up to 2.19 Sv). Additionally, COAWST capability to couple different numerical models allowed to disentangle the relative importance of aspects on dense water generation, mixing and spreading, demonstrating how coupled runs can lead to volumes up to 50% larger with respect to uncoupled simulations. Additional light is shed on the transport pathways off the shelf and on possible sediment transport phenomena in the area, in this benefiting from an unprecedented spatial resolution and a new bathymetry reflecting very high resolution data acquired via multi-beam techniques. Although dense water propagation appears as a relatively large-scale process involving the whole western side of the SAM, topographic local discontinuities and seabed slopes appear crucial in triggering descent and governing flow patterns. The presence of suspended sediment along the water column, despite not significantly influencing the overall fluxes across sections, is responsible of pulling part of the veins towards deeper zones. [ABSTRACT FROM AUTHOR]

Cold Air Outbreaks (CAOs) over shallow seas may lead to dense water formation episodes, enhancing water, heat, nutrient and sediment exchanges across the continental margin, with associated seabed reshaping. During winter 2012, a CAO episode characterised by exceptional intensity stroke the northern Adriatic Sea, one of the most effective cool engines driving the Mediterranean circulation, providing a paramount opportunity for an integrated investigation of dense shelf water dynamics. In the present study, we describe this event using a fully coupled modeling approach exploring the effects of mutual interactions among atmosphere, ocean currents and sea surface waves, usually not completely accounted for, in the resulting dense water formation. Whilst atmospheric fields appear to be marginally affected by coupled dynamics in the present case, implications for sea surface elevation and circulation are far from negligible. Measurements collected in the northern Adriatic Sea showed that a physically consistent description of energy exchanges between ocean and atmosphere provides an improved estimate of heat fluxes and of air and sea temperatures. In addition, the explicit inclusion of wave action within the modeling system further enhances the modulation of air-sea exchanges and the propagation of its effect along the water column, resulting in a different intensity of northern Adriatic gyres and in different water fluxes flowing through the formation basin. Through these main controls on the water volume involved in the densification process and on the intensity of momentum input and cooling, a coupled modeling strategy accounting for atmosphere-waves-currents interactions can turn out to be crucial for improving the quantification of thermohaline properties and energy content, newly formed dense water mass, and provide a better description of its migration pathways and rates of off-shelf descent. [ABSTRACT FROM AUTHOR]