Different population dynamics of two PSP-toxinproducing species at NW Mediterranean Sea

There are evidences that HABs are increasing worldwide due to human causes, such as eutrophication and an increase in confined waters. In addition, it is also likely that climate variations can affect HABs dynamics. To understand future perspectives of coastal harmful events, we need to identify the underlying factors contributing to the current variations. Using a phytoplankton time series collected at 16 confined NW Mediterranean Sea locations, we investigated the long-term trends of two local PSP-toxinproducing species, Alexandrium minutum and A. pacificum, and their responses to different environmental drivers. These two species responded differently to the same environmental conditions along the Catalan coast (NE Spain). From 2000 to 2012, blooms of A. minutum in Catalan harbors doubled in their frequency whereas those of A. pacificum decreased slightly. In the 1990s, the first detection of the toxic bloom-forming A. pacificum in several localities of the NW Mediterranean coast suggested the expansion of this species. Our results show that, after the initial period of expansion, the abundance of A. pacificum declined such that by 2008 the species was no longer found in most sampling locations. The potential contributions of regional and local factors in this decrease in A. pacificum abundance are considered. For example, the colder water temperatures in 2004–2005 coincided with a reduction in A. pacificum abundances in most of the monitored stations. The possible link between these two events is assessed by analyzing data from a wider geographic area, including other NW Mediterranean locations in Sardinia and southern France.