Response of planktonic cladocerans (Class: Branchiopoda) to short-term changes in environmental variables in the surface waters of the Bay of Biscay

From January 2001 to December 2008, 73 surface plankton samples and 45 vertical profiles of sea temperature, salinity, dissolved oxygen and pH were collected on a monthly basis from a single sampling station located in the Bay of Biscay (43°37N; 1°43W) (North-East Atlantic). Two types of North Atlantic Oscillation (NAO) indexes were included in the data set and submitted to a Canonical Correspondence Analysis and Spearman non-parametric test. Significant breaks and levels in time series were tested using a data segmentation method. The temperature range varies from 11°C to 25°C. It begins to rise from April until August and then decline. Low salinity values occur in mid-spring (36PSU) in autumn. Dissolved oxygen mean values were around 8mg/l. In summer, when temperature and salinity are high, surface water layer is always accompanied with a significant deoxygenation, and the process reverses in winter. pH mean values range was 7.78–8.33. Seasonal and inter-annual variations of the two NAO indexes are strongly correlated to one another, but do not correlate with any hydrological or biological variable. Five of the seven cladocerans species which are present in the Bay of Biscay were found in this study. There is a strong pattern in species succession throughout the year: Evadne nordmanni is a vernal species, while Penilia avirostris and Pseudevadne tergestina occur mainly in summer and autumn. Evadne spinifera has a maximum abundance in spring, Podon intermedius in autumn, but they both occur throughout the year. However, for some thirty years, the presence of species has tended to become significantly extended throughout the year. During the 2001–2008 period, there was a noticeable decline and even a disappearance of the categories involved in sexual reproduction as well as those involved in parthenogenesis, in favor of non-breeding individuals.