Mesozooplankton composition and distribution in relation to oceanographic conditions in the Gulf of Cádiz, Spain

Two surveys were conducted during July 1994 and July 1995 in the Gulf of Cadiz with the aim of assessing temporal and spatial patterns in biomass (ZDV, zooplankton displacement volume) and mesozooplankton composition and their relationships with the oceanographic conditions. Differences between two successive summer seasons were found in this study. The upper water column was warmer and more saline in 1994. Mesozooplankton abundance and biomass were higher in 1994 than in 1995. However, the occurrence of several plankton species was remarkably regular. In the two summers, cladocerans (Penilia avirostris, Evadne spinifera, Evadne tergestina, Evadne nordmanni, and Podon spp) were the most abundant group followed by copepods and appendicularians. The dominance of cladocerans in the two summers was basically due to the high abundance of Penilia avirostris. The study of trends showed that the relative abundance of copepods increased throughout the summers, although this increase was significant only in the north inshore sites, where the influence of Atlantic water is higher. The mesozooplankton abundance and, specifically, Cladocera density showed a positive correlation with temperature and ZDV but showed a negative correlation with salinity and depth. ZDV, Copepoda and Appendicularia density did not show a significant relationship with oceanographic variables.