Encystment of dinoflagellate in tidal coasts of Gabes Gulf (Tunisia) : analyse of long term observations

Abstract

In phytoplankton monitoring survey in Gulf of Gabes, detection and study of cysts were averred to be as essential as
those of toxic species. The cysts can’t only provoke blooms, but some toxic responses related for presence of these
forms are detected in our coasts. In order to understand the bloom dynamics and occurrences of encystment, we
analyzed 20 years of harmful algae data acquired a monitoring program of toxic phytoplankton in Gabes gulf.
Encystement phenomenon is regular and frequent in more stations in Gabes gulf, but it affected low variety of
species. For 20 years of monitoring we detect only 8 different group of cysts in this area. Also, cysts distribution
showed a synergy with blooms. Spring seems to be the season were diversity of cysts is recorded and higher
densities are observed in North of Gabes gulf. Encystment may be induced with abrupt variation in temperature and
salinity. Presence of an important hydrodynamic mouvement (tide) can be responsible of rehappening particules.
Key Words: cysts of dinoflagellate, spatio-temporal variation, Gabes gulf.