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This study describes the phytoplankton community in sheltered euhaline waters of the Basque coast (southeastern Bay of Biscay). Phytoplankton composition, cell size, abundance, biomass and the presence of potentially toxic taxa, together with chlorophyll a, nutrients and hydrographic and optical conditions were measured, from August 2016 to August 2017, in the Mutriku port, with the main aim of assessing the suitability of the phytoplankton community as a food resource for bivalves. The water column in Mutriku showed the typical environmental conditions of Basque marine waters, with no significant nutrient enrichment caused by anthropogenic pressures. Haptophytes represented the greatest contribution to cell abundance (31-47%), and diatoms were the dominant group in terms of biomass (52-79%),which could favour mussel growth due to their high fatty acid content. In addition, the size structure of the phytoplankton community was suitable for mussel ingestion, since the predominant cell size was 2-20 mu m. Regarding toxic phytoplankton, the genera that pose a risk for human health and those that affect negatively mussel physiology and survival were considered. Altogether, ten toxic phytoplankton taxa were identified, contributing in less than 5% to the total cell abundance of Mutriku. However, median chlorophyll a concentration was low (0.5 mu g L-1), reflecting the oligotrophic conditions of the area. Therefore, even if the composition of the phytoplankton community could be favourable for bivalve aquaculture, biomass values are low compared to other zones of bivalve production

Phytoplankton communities have long been used as water quality indicators within environmental policies. This has fostered the development of national and international phytoplankton monitoring programs, but these networks are subject to sources of uncertainty due to laboratory issues. Nevertheless, studies regarding the interference associated with these aspects are not well-documented. Hence, a long time series (2003-2015) from the Basque continental shelf (southeastern Bay of Biscay) was analyzed to evaluate the uncertainty given by laboratory strategies when studying phytoplankton variability. Variability in phytoplankton communities was explained not only by environmental conditions but also by changes in fixatives (glutaraldehyde and acidic Lugol's solution) and laboratory staff. Based on Bray-Curtis distances, phytoplankton assemblages were found to be significantly dissimilar according to the effect of changes in the specialist handling the sample and the employed fixative. The pair-wise permutational multivariate analysis of variance (PERMANOVA) showed significant differences between the two fixatives utilized and also between the three taxonomists involved. Thus, laboratory-related effects should be considered in the study of phytoplankton time series.

237 p., El fitoplancton, como base de las redes tróficas marinas, es la principal fuente de energía para los bivalvos filtradores, entre otros. En el País Vasco, el reciente interés en el cultivo de bivalvos en aguas costeras llevó a la instalación de una planta piloto frente a la costa de Mendexa (Bizkaia). En este contexto, surge la necesidad de estudiar y conocer la dinámica y composición de la comunidad fitoplanctónica en la costa vasca, dada su implicación en el crecimiento de bivalvos. Para ello, se han utilizado dos fuentes de información. Por una parte, se ha analizado una serie temporal de más de diez años a lo largo de toda la costa vasca; y, por otra parte, se ha llevado a cabo un estudio específico en la planta piloto. En general, en términos de disponibilidad alimentaria para los mejillones, se han encontrado atributos fitoplanctónicos favorables para el crecimiento de mejillones, como son la presencia de blooms, la contribución de las diferentes fracciones de tamaño de célula y la dominancia de las diatomeas y los dinoflagelados, principalmente. En cuanto al riesgo de toxicidad en humanos asociado a especies tóxicas de fitoplancton mediante la ingesta de mejillones, la presencia de toxina en mejillón se detectó en el 60% de los casos, aunque sólo un 15% hubiera implicado un riesgo para la salud humana. Todos estos casos se dieron en primavera y se asociaron al ácido ocadaico, siendo Dinophysis acuminata la especie causante.