Long term observations on Chattonella subsalsa (Rhaphidophyceae) blooms and related environmental conditions in a Mediterranean lagoon (Santa Giusta Lagoon, Sardinia, Italy)

Chattonella Biecheler (Raphidophyceae) exhibits a worldwide distribution, and includes deleterious species causing fish kills in natural environments and aquaculture systems, with very important ecological damage and economic losses in different areas of the planet. Since the mid- 1990s, Santa Giusta Lagoon (Sardinia Island, Italy, Western Mediterranean Sea), one of the Italian Long Term Ecological Research network stations has suffered Chattonella blooms, associated with fish kills. During the blooms, difficulties in the taxonomic determination of Chattonella species with traditional light microscopy methods had not permiIted definitive classification of Chattonella at the species level due to the species pleomorphology, and loss of morphological characteristics in fixed samples. Recently, the application of molecular methods on fixed samples taken during the blooms allowed us the identification of C. subsalsa Biecheler. This species exhibits relatively recent history as a harmful species. Data on C. subsalsa is scarce, in particular many knowledge on its ecology, life cycle and fish-killing mechanisms have yet to be obtained. Pluriannual ecological data (1990-2012) from Santa Giusta Lagoon were analysed to evaluate environmental conditions favouring the blooms. Moreover, the presence of resting cysts in the sediments was investigated C. subsalsa blooms (density > 1.5 x 106 cells l-1) were observed in 1994, 1998, 1999 and 2010. These events always coincided with massive fish deaths C. subsalsa cell densities showed a significant positive correlations with temperature and salinity. Correlations with nutrients were less clear, probably for the high availability of nutrients in the lagoon. However the increase in ammonia seems to be an important factor in influencing C. subsalsa dynamic. The presence of resting cysts in the sediments was confirmed by their germination, implying an inner source of inoculum for future blooms. The study contributes to the knowledge on C. subsalsa, providing valuable ecological data from the field.