Vertical structure of plankton communities in areas of European eel larvae distribution in the Sargasso Sea

The European and American eels spawn in the subtropical convergence zone (STCZ) in the Sargasso Sea, a dynamic and relatively productive area that is strongly influenced by front and eddy formations and subducted high-saline water masses. To understand how the physical and biological environments may affect the early life history of eels, we conducted a detailed bio-physical investigation of the water column at a site of high eel larvae abundance. Diel measurements and sampling in the upper 300 m revealed strong variations in hydrographic conditions and mean depths of different taxonomic groups; however, characteristics patterns of distribution were apparent. Most species showed diel vertical migrations, ascending about 20-30 m at night, whereas examples of night-time downward migration were also seen. European eel larvae were among the species showing more extensive diel vertical migration: their population mean depth changed from 160 m at day to 100 m at night where abundance peaked at 45 m depth. Distribution and migration of eel larvae corresponded to patterns observed for small hydrozoans, supporting a proposed predator-prey linkage. The study demonstrates the diverse and vertically strongly structured plankton community of STCZ where larvae of eel and other fish find a wide range of potential niches