Multivariate substrate characterization: The case of shellfish harvesting areas in the Rias Altas (north-west Iberian Peninsula)

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. Grain-size 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 (Rias 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 Fe2O3) or by the imprint of the Ollo de Sapo and Manto de Mondonedo (high levels of SiO2, Rb, K2O and Ba). The multivariate analysis also separated the sandbanks with higher terrestrial influence in the inner part of the inlets (high levels of Al2O3, Zn, Ba and TiO2) from those with higher marine influence, which were located in the outer part (high levels of Sr, CaO and CaCO3). 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.