Microplastic concentrations in the South-East Bay of Biscay (SE BoB): From observations and estimations to distribution of floating microplastic particles (a Lagrangian approach).

Supervisors: Oihane C. Basurko, Anna Rubio (AZTI).
The South East Bay of Biscay was recently described as a cul de sac for marine floating litter. LIFE LEMA project was launched in 2016 to help localities of the Basque Country tackle the problem of plastic accumulation in coastal waters and beaches. Macro-plastic distribution was therefore studied to improve the collection at sea by local fishermen. However, this report represents the first attempt to estimate microplastic distribution and relation to coastal ocean dynamics in the coastal waters of the Basque Country. Microplastic samples were collected from September to November 2017 (“winter characteristics”) in the Gipuzkoa coast and from June to September 2018 (“summer characteristics”) in the Iparralde coast. In average, microplastic concentrations were observed three times higher in summer 2018 (1,682,020 items per km2) than winter 2017 (314,574 items per km2). Those concentrations were in the same order of magnitude than the ones measured in the Mediterranean Sea, characterised as highly contaminated. However, important temporal and spatial variations were observed during sampling. Using the Lagrangian COSMO model forced with IBI surface current fields, complementary analyses were performed to characterise the dynamic of the floating microplastics. The seasonality aspect of this pollution was highlighted by the simulations and critical accumulation of particles was observed during summer, in the innermost side of the Bay, mainly along the French coast. Metocean conditions favourable to the accumulation of floating debris were proposed to explain such accumulative patterns. Other factors such as river input, geophysical characteristics, anthropogenic activities and distance to the coast were also suggested to explain such temporal and spatial variability in concentrations. According to the trajectories simulated backward in time, microplastics mainly originated locally from the coast. In the simulations, an important number of particles were backtracked to the two main estuaries (Adour and Nervion) suggesting they could play a key role in the introduction of microplastics in the coastal waters of the Basque Country. These observations were matching the ones mentioned by other authors in the same area of study. However, sampling effort and complementary analyses are still needed to explain microplastic dynamic in the South East Bay of Biscay. Beaches and estuaries are strategic (and low coast/effort) sampling sites to target in future microplastic studies. Specific treatment and collection effort could therefore be proposed to efficiently manage the coastline suffering of microplastic accumulation; and the role played by estuaries in microplastic distribution could be verified.