We report on a 10-year study of the dynamics of toxic dinoflagellate Karenia selliformis at 20 stations along the nearshore of the Gulf of Gabes (Tunisia) in relation to environmental variables. Dinoflagellates and K. selliformis have dominated the phytoplankton community and K. selliformis blooms have taken place since 2001. Phytoplankton groups showed significant variability among sampling sites, but did not vary significantly from year to year. Redundancy analysis revealed that salinity, nitrate and phosphorus significantly explained the variability of the main phytoplanktonic groups namely, diatoms, dinoflagellates, cyanobacteria and euglenoids, while K. selliformis exhibited a specific pattern. Because of the wide variability in K. selliformis abundance, we tested the influence of environmental variables on its presence/absence using a generalized linear mixed-effect model (GLMM). K. selliformis occurrence had a positive relationship with nitrate and a negative one with total phosphorus. The different spatial gradients of these two chemical variables led to spatial differences in K. selliformis development prevalent near touristic areas. Temperature was also retained in the GLMM and since it rose over the 10 years of the study, it likely supports the expansion of K. selliformis. The discrepancies between model predictions and observed occurrences suggest that consideration of other sources of environmental forcing may improve our understanding of the determinism of K. selliformis dynamics. Our study may be useful in the management of this ecosystem so as to plan for the best disposal options in the treatment of urban and industrial wastes in the gulf's coastal waters.