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Knowledge and predictability of turbid river plumes is of great importance because they modulate the properties of the seawater adjacent to river mouths. The Loire and Gironde Rivers form the most important plumes in the Bay of Biscay, as they provide > 75% of total runoff. The development of the turbid plume under the influence of its main drivers was analyzed using Moderate Resolution Imaging Spectroradiometer satellite data from the period 2003–2015. River discharge was found to be the main driver, followed by wind, which also had an important effect in modulating the turbid plume during periods of high river discharge. Seaward and upwelling favorable winds enhanced the dispersion of plumes on seawater, whereas landward and downwelling favorable winds limited mixing with the adjacent ocean water. The maximum extension of the turbid plume was reached under landward winds. In addition, the spatio-temporal evolution of the East Atlantic pattern and the North Atlantic Oscillation was observed to affect the dynamics of plumes: positive values of both indices favored a greater extension of the plume. Thermohaline properties differed inside and outside the area affected by both rivers. In particular, these rivers maintain winter stratification inside the turbid plume, which results in a different warming ratio when compared with the adjacent ocean.

This contribution summarizes the scope and results of a selection of the studies presented to the XV International Symposium of Oceanography of the Bay of Biscay (ISOBAY 15), that took place in Bilbao (Basque Country, Spain) from 22nd to 24th June 2016. Firstly, a brief introduction to the status of the Bay of Biscay, its presence in scientific literature over the last two decades, and the aims of the present edition of ISOBAY are presented. A second part shows the main findings of the works submitted for publication in this special issue. They represent new steps in the knowledge of oceanographic, geological and biogeochemical processes, biodiversity, ecosystems´ structure and functioning, and human impact in the Bay of Biscay. On the other hand, they also contribute to the overall knowledge of marine systems.

European eel can be found at the Bay of Biscay after a long migration across the Atlantic. The duration of migration, which takes place at larval stage, is of primary importance to understand eel ecology and, hence, its survival. This duration is still a controversial matter since it can range from 7 months to > 4 years depending on the method to estimate duration. The minimum migration duration estimated from our lagrangian model is similar to the duration obtained from the microstructure of eel otoliths, which is typically on the order of 7–9 months. The lagrangian model showed to be sensitive to different conditions like spatial and time resolution, release depth, release area and initial distribution. In general, migration showed to be faster when decreasing the depth and increasing the resolution of the model. In average, the fastest migration was obtained when only advective horizontal movement was considered. However, faster migration was even obtained in some cases when locally oriented random migration was taken into account.

Turbid plumes formed by the main Iberian rivers were analyzed and compared in order to determine similarities and differences among them. Five Atlantic rivers (Minho, Douro, Tagus, Guadiana and Guadalquivir) and one Mediterranean river (Ebro) were considered. Plume extension and turbidity were evaluated through synoptic patterns obtained by means of MODIS imagery over the period 2003–2014. River discharge showed to be the main forcing. In fact, the dependence of plume extension on runoff is moderate or high for all rivers, except for Ebro. In addition, most of river plumes adjust immediately to runoff fluctuations. Only the extension of Tagus and Guadalquivir plumes is lagged with respect to river runoff, due to the high residence time generated by their large estuaries. Wind is a secondary forcing, being only noticeable under high discharges. Actually, the dependence of plume extension on wind is moderate or high for all rivers, except Guadalquivir and Ebro. All the Atlantic rivers show the maximum (minimum) near- field plume extension under landward (oceanward) cross-shore winds. The opposite situation was observed for Ebro River. Tide is also a secondary forcing although less important than wind. Actually, the dependence of plume extension on tide is only high for Guadiana River. Nevertheless, all Atlantic river plumes still have some dependence on semidiurnal tidal cycle, they increase under low tides and decrease under high tides. In addition, Tagus River plume also depends on the fortnightly tidal cycle being larger during spring tides than during neap tides. This is due to particular shape of the estuary, where the river debouches into a semi-enclosed embayment connected to the Atlantic Ocean through a strait. Ebro River constitutes a particular case since it has a low dependence on runoff and wind and a negligible dependence on tide. In fact, its plume is mainly driven by the Liguro-Provencal coastal current. Guadalquivir River also shows some unique features due to its high sediment load. It generates the largest Iberian plume in terms of turbid signal and extension even being the second smallest river in terms of discharge.

The near-surface transport of exogenous plastics that enter the Bay of Biscay near Cape Ortegal (NW Spain), the main coastal entrance of marine litter into this semi-enclosed sea, were analyzed using a particle tracking model over the period 2009–2018. Four wind drag coefficients were considered to account for the different windages due to the diverse sizes of marine litter. The combined effect of winds and currents results in a higher concentration of floating plastics nearshore favoring beaching. On the other hand, the movement of the smallest plastics (

The northwestern coast of the Iberian Peninsula is characterized by a high primary production mainly supported by coastal upwelling, creating an extraordinary commercial interest for fisheries and aquaculture. Considering chlorophyll- a (Chl- a ) as an indicator of primary production, its spatio-temporal variability was researched in this study in the surface water of this upwelling region from 1998 to 2007. Satellite derived Chl- a , Sea Surface Temperature (SST) and Ekman transport data as well as the inflow of the main rivers discharging into the study area were used to investigate the origin of the Chl- a concentration. Empirical Orthogonal Function (EOF) analysis of weekly Chl- a images was performed, as well as correlation analysis between Chl- a concentration, Ekman transport and river discharge. EOF results suggest that the highest Chl- a concentration occurs near the coast up to 60 km offshore. The interannual variability of Chl- a , SST and Ekman transport was also studied considering summer and winter months. Generally, 2005, 2006 and 2007 were the most productive years during the summer months with high Chl- a concentrations along the coast associated to the strong upwelling conditions observed. Otherwise, 1998 seemed to be the most productive year during winter. The absence of upwelling favorable conditions together with localized low SST and considerable discharges, suggests that the high Chl- a concentrations observed during this period are mainly due to the entrance of nutrients through river runoff. However, in winter, high concentrations of colored dissolved organic matter (CDOM), associated with river runoff, are present in the ocean surface, leading to an erroneous strong signal of the satellite. During winter correlations of 0.58 and 0.49 were found between Chl- a concentration and Douro and Minho discharges, respectively, evidencing that high Chl- a concentration was related with river runoff. Otherwise, during summer, Chl- a and Ekman transport exhibited a correlation of − 0.38 indicating that high Chl- a is associated to upwelling events. In summary, it was found that the spatio-temporal variability of Chl- a along the northwestern coast of Iberian Peninsula exhibited the clear influence of upwelling events during summer. Conversely, the variability during winter was mainly due to entrance of nutrients through the rivers discharge which flow into the area.