e"> Harmful cyanobacteria are a globally growing concern. They produce a large variety oftoxic compounds,

including saxitoxin and its many structural variants, a group of potent neurotoxins collectively called

paralytic shellfish toxins or PST. Nucleic acid based detection methods, such as qPCR, have been

proposed as potential screening and monitoring tools for toxic cyanobacteria, but it is not clear how well

the presence and quantity of saxitoxin biosynthesis (sxt) genes can be used to predict the production of

PST in the environment. In this study, the prevalence of three sxt genes and their co-occurrence with

paralytic shellfish toxins in the environment was investigated. The sxtA, sxtG and sxtB genes were present

on average in 31% of the samples collected from lakes and brackish coastal waters on A˚ land Islands,

Finland, during the three-year monitoring period. PST detection frequency varied from 13% to 59% from

year to year, and concentrations were generally low. On average higher sxtB copy numbers were

associated with PST detection, and although a positive correlation between gene copy numbers and toxin

concentrations was observed (Spearman rank correlation, r = 0.53, P = 0.012), sxt gene presence or

quantity didn’t reliably predict PST production. Sequencing of sxtA fragments and identification of main

cyanobacteria indicated that the likely candidate responsible for PST production in the samples belonged

to the genus Anabaena.