Assessment of the phytoplankton ecological status in the Basque coast (northern Spain) according to the European Water Framework Directive

This contribution focuses upon the use of phytoplankton as an indicator of ecological quality in the coastal waters of the Basque Country (Bay of Biscay, northern Spain). The Water Framework Directive (WFD) establishes a common water policy in the European Union. The phytoplankton, owing to its relationship with the eutrophication processes, is one of the biological elements considered within the WFD. Phytoplankton biomass, composition and abundance, together with frequency and intensity of blooms, are the metrics to be assessed according to the WFD. In this work, data on phytoplankton biomass and frequency and intensity of blooms along the Basque coast were analysed. Phytoplankton biomass was evaluated using the 90th percentile of the chlorophyll-a concentrations (Chl-a) over a 6-year period (2001–2006). For the evaluation of the frequency of blooms, data on composition and abundance of the phytoplankton communities were obtained, then analysed using three different approaches. The first approach involved the utilisation of only some of the taxonomic groups (diatoms and dinoflagellates). The second approach divided the phytoplankton community into two size categories (> 20 µm and 2–20 µm, cell diameter). The third approach used all of the phytoplankton taxa recorded in the sample. Evidence for the oligotrophy of the coastal waters of the Basque Country was provided by the low values obtained for the 90th percentile of the Chl-a (< 3 µg L− 1). Low intensity of upwelling activity, relatively small river loads and a narrow shelf characterise this coast, which can explain the low level of phytoplankton biomass. However, phytoplankton counts were relatively high compared with other neighbouring coastal waters a priori of higher trophic richness. These differences are discussed in relation to the methodologies for the analysis of cell counts which different monitoring programmes utilise in the European Union.