Relationships between meso- and macro-zooplankton communities and hydrology in the Senegal River Estuary

Abstract This project studied how meso- and macro-zooplankton communities (composition, abundance, and biomass) in the Senegal River Estuary varied in time and space depending on hydro-biological factors and human action. Samples were taken approximately every 45 days, from January to December 2003, at 3 sampling stations where the estuary was subject to different marine and continental influences. Temperature, salinity, fluorescence and oxygen were measured using a Seabird probe (SBE 19). Meso- and macro-zooplankton were collected using standard nets (200 μm and 500 μm mesh sizes respectively), towed horizontally at the surface and at a depth of 3 m–5 m. The study showed a marked seasonal variation which clearly influenced the structure and distribution of meso-zooplankton and macro-zooplankton communities. During the low water period (November–June) the marine influence prevailed, decreasing from the river mouth to the upstream part of the estuary near the Diama anti-salt dam. Zooplankton was abundant, containing mainly marine organisms and a few euryhaline species (i.e. Acartia clausi ) which colonized the estuary. During the high water period (July–October) the estuary was mainly under the continental influence and the impact of the annual flood. Zooplankton was less abundant but more diversified than during the low water season and was dominated by freshwater species which entered the estuary through the Diama Dam. By comparison with previous works, this study showed significant changes in the abundance of zooplankton and the types of community since the Diama Dam came into operation in 1985. The results also showed the initial effects on the pelagic ecosystem of a channel opened in October 2003 in the middle part of the estuary.