From source to sink: part 1-characterization and Lagrangian tracking of riverine microplastics in the Mediterranean Basin.

The Mediterranean Sea is one of the most critically polluted areas due to its semi-enclosed structure and its highly anthropized shoreline. Rivers are significant vectors for pollutant transfers from the continental to the marine environment. In this context, a 3D Lagrangian simulation of the dispersion of riverine microplastics (MPs) was performed, which included the application of a recently developed model that reassessed the MP fluxes discharged by rivers. MP physical properties from river samples were further investigated to approximate vertical displacement in modeled ocean currents. The use of a high-resolution circulation model, integrating Stokes drift, turbulent diffusion, and MP sinking and rising velocities, enabled us to establish stock balances. Our simulation suggested that 65% of river inputs may be made of floating MPs drifting in the surface layer and 35% of dense MPs sinking to deeper layers. The Eastern Mediterranean tends to accumulate floating MPs, primarily originating from the Western Mediterranean Basin, where major river sources are concentrated. After 2 years of simulation, modeled stranding sequestered 90% of the MP inputs, indicating relatively short average residence times from a few days to months at most for particles at sea. Although spatial distribution patterns stabilized after this period and a steady state may have been approached, the surface concentrations we modeled generally remained below field observations. This suggested either an underestimation of sources (rivers and unaccounted sources), by a factor of 6 at most, or an overestimation of MP withdrawal through stranding, to be reduced from 90 to around 60% or less if unaccounted sinks were considered.

The missing ocean plastic sink: Gone with the rivers.

Plastic floating at the ocean surface, estimated at tens to hundreds of thousands of metric tons, represents only a small fraction of the estimated several million metric tons annually discharged by rivers. Such an imbalance promoted the search for a missing plastic sink that could explain the rapid removal of river-sourced plastics from the ocean surface. On the basis of an in-depth statistical reanalysis of updated data on microplastics-a size fraction for which both ocean and river sampling rely on equal techniques-we demonstrate that current river flux assessments are overestimated by two to three orders of magnitude. Accordingly, the average residence time of microplastics at the ocean surface rises from a few days to several years, strongly reducing the theoretical need for a missing sink.

Seafloor litter sorting in different domains of Cap de Creus continental shelf and submarine canyon (NW Mediterranean Sea).

We analyzed litter occurrence in 68 underwater video transects performed on the middle/outer continental shelf and submarine canyon off Cap de Creus (NW Mediterranean), an area recently declared Site of Community Importance (SCI). Low densities of urban litter were registered on the shelf (7.2 items ha-1), increasing in abundance towards the deepest part of the submarine canyon, with 188 items ha-1 below 1000 m depth. We hypothesize that the strong bottom currents that recurrently affect this area efficiently move litter objects from the shelf towards the deep. Of all litter items, approximately 50% had a fishing-related origin, mostly longlines entangled on rocks in the canyon head and discarded trawl nets in deeper areas. Over 10% of cold-water colonies observed had longlines entangled, indicating the harmful effects of such practices over benthic habitats. These results should be considered when designing mitigation measures to reduce litter pollution in Cap de Creus SCI.

Numerical modelling of 137Cs content in the pelagic species of the Japanese Pacific coast following the Fukushima Dai-ichi Nuclear Power Plant accident using a size-structured food-web model.

As result of the great east Japan earthquake on March 2011 and the damages of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), huge amount of radionuclides, especially 137Cs, were released to the Japanese Pacific coast. By consequence, several marine species have been contaminated by direct uptake of radionuclides from seawater or through feeding on contaminated preys. In the present study we propose a novel radioecological modelling approach aiming to simulate the radionuclides transfer to pelagic marine species by giving to the organism body-size a key role in the model. We applied the model to estimate the 137Cs content in 14 commercially important species of the North-Western Pacific Ocean after the FDNPP accident. Firstly, we validated the model and evaluated its performance using various observed field data, and we demonstrated the importance of using such modelling approach in radioecological studies. Afterwards, we estimated some radioecological metrics, such as the maximum activity concentration, its corresponding time and the ecological half-life, which are important in assessment of the previous, current and future contamination levels of the studied species. Finally, we estimated the time duration required for each species to reach the pre-accident 137Cs activity concentrations. The results showed that the contamination levels in the planktivorous species have generally reached the pre-accident levels since about 5 years after the accident (since 2016). While in the case of the higher trophic level species, although the activity concentrations are much lower than the regulatory limit for radiocesium in seafood in Japan (100 Bq kg-1), these species still require another 6-14 years (2018-2026) to reach the pre-accident levels.