Seasonality of Pseudo-nitzschia spp. (Bacillariophyceae) in western Scottish waters

Some diatoms belonging to the genus Pseudo-nitzschia produce the neurotoxin domoic acid (DA), which causes amnesic shellfish poisoning (ASP). In Europe, accumulation of DA in shellfish has led to shellfish harvesting closures in western Scottish waters since 1999. However, little is yet known of the temporal appearance and succession of Pseudo-nitzschia species and how this may relate to environmental forcing. A phytoplankton monitoring programme was established for almost 3 yr at a coastal station in western Scottish waters to study the diversity and seasonal variation of Pseudo-nitzschia species in relation to physical and chemical parameters of the water column. Samples were collected with a high temporal resolution. Annually repeatable seasonal patterns of Pseudo-nitzschia abundance were evident. Pseudo-nitzschia cells were categorised by shape and size into 2 groups, the 'delicatissima-group' and the 'seriata-group'. Cluster analysis demonstrated the different dynamics of the 2 Pseudo-nitzschia groups. The delicatissima-group was dominated in spring by non-toxic P. delicatissima, while the seriata-group occurred mainly during the summer months. Summer to autumn blooms of both groups were composed of several species, including toxic P. australis and P. seriata and potentially toxic P. pseudo-delicatissima. Redundancy analysis indicated that 31% of the variance in Pseudo-nitzschia spp. abundance could be explained by the measured environmental variables. The repeatable annual nature of the blooms suggests that deterministic forecasting of the timing of future toxic Pseudo-nitzschia spp. events may be an achievable goal.