Intertidal shellfish banks in estuarine areas are influenced by a wide variety of environmental conditions, including the physical and chemical characteristic of the water that floods the grounds. In this study we carry out a cross-comparison of the hydrographic characteristics and nutrient fertilization patterns of the waters that laps the shellfish grounds of a group of five drowned valleys collectively known as "Rías Altas", located in the western Cantabrian coast (NW Iberian Peninsula). This region is affected by coastal upwelling from June to August resulting in a water exchange between the embayments and the shelf varying between 31.5 and 220.5 m 3 s −1. The fresh water discharge followed a seasonal cycle too, with maxima in winter and minima in summer, and longterm average flow rates ranging from 5.9 m 3 s −1 to 22.2 m 3 s −1. Significant differences were observed among the five embayments with regard to temperature and salinity and also with inorganic nutrients, chlorophyll a and particulate organic matter concentrations in the continental waters. Highlights: • Continental waters dictate the different fertilisation patterns among the Rías Altas • Extension and geomorphology of the drainage basins explains continental nutrient inputs • Continental nutrient inputs control organic matter concentrations in the inner rías • Production in the inner rías is extremely P-limited

The productivity of intertidal shellfish banks is affected by a wide variety of environmental parameters. In this study, a battery of multivariate analyses including generalized linear mixed models, hierarchical cluster analysis and principal component analysis were performed to define the spatial organization of sandbanks and to identify the variables driving the grouping. Grainsize distribution and calcium carbonate, organic matter, Si and Al oxides, and trace metals content were the variables used to characterize the intertidal sediments. Field data were collected through the upper 50 cm in 57 sites located in shellfish sandbanks from five coastal inlets of the western Cantabrian coast (Rı ´as Altas, north-west Iberian Peninsula). Generalized linear mixed models showed that hardly any variable differed with core depth, and the hierarchical cluster analysis and principal component analysis revealed that the banks organized around four clusters. This grouping was dictated by the influence of the imprint of the Cabo Ortegal complex material (with high levels of MgO, Mn, Cr, Ni, V and Fe 2 O 3 ) or by the imprint of the Ollo de Sapo and Manto de Mondo ñedo (high levels of SiO 2 , Rb, K 2 O and Ba). The multivariate analysis also separated the sandbanks with higher terrestrial influence in the inner part of the inlets (high levels of Al 2 O 3 , Zn, Ba and TiO 2 ) from those with higher marine influence, which were located in the outer part (high levels of Sr, CaO and CaCO 3 ). Furthermore, it was observed that both axes of main ordination were related to the annual average concentration of chlorophyll-a and inorganic nitrogen in the water column above those sediments, highlighting the interdependence between chemical composition of the overlying water and sediments' characteristics. This approach, combining parametric models and multivariate analyses of textural and geochemical sediment composition data, proved to be useful for characterizing intertidal substrates where shellfish species live.