Paralytic shellfish poisoning (PSP) occurs sporadically on the NE UK c
oast. The degree of toxicity shows considerable interannual variabilit
y, but particularly severe events occurred in 1968 and 1990. The time
sequence of PSP toxin production in 1990 is described and compared wit
h 1989 when no significant PSP toxin occurred. In 1990, PSP toxin was
widespread in shellfish samples taken on 300 km of coastline, from Ber
wick to Whitby, and toxin was present at high concentrations for >1 mo
nth. The distribution of Alexandrium tamarense cysts in the sediments
is described. High concentrations were found in the Firth of Forth and
also in a number of regions offshore of the Scottish and English coas
ts. A water transport model has been used to estimate back trajectorie
s, with the aim of determining the source of the A. tamarense bloom. T
he Firth of Forth has previously been suggested as the seed bed for A.
tamarense outbreaks in the area, but the transport model clearly show
s that A. tamarense moved inshore over a wide area in 1990; there was
no single source of the bloom. Sea surface temperatures, estimated fro
m satellite imagery, show that water temperatures were much higher at
the end of April 1990, when the bloom occurred, than in 1989 when PSP
toxin incidence was very low. These conditions would have resulted in
early seasonal stratification and would have favoured phytoplankton gr
owth in the water column.