Annual cycle of Pseudo-nitzschia species in the outer Oslofjord, Norway

Summary

Monitoring of microalgae, including toxic species like Pseudo-nitzschia , is based on light microscopy; nevertheless precise identification of Pseudo-nitzschia at the species level requires verification by electron microscopy and/or molecular biological tools. Earlier investigations from Norwegian waters have shown a considerable variation in the species composition of the genus Pseudo-nitzschia, both geographically and seasonally. The genus contains more than 30 species and many of them are found in Norwegian waters (e.g. P. delicatissima, P. fraudulenta, P. granii, P. heimii, P. pungens, P. multiseries, P. calliantha. P. seriata and P. americana). Several species of Pseudo-nitzschia have been shown to be toxic, including P. seriata, P. callianta, P. delicatissima, P. multiseries and P. fraudulenta, which all may occur in Norwegian waters. Vertical net tows (0-20 m), quantitative water samples and hydrographical data have been collected monthly over one year, from June 2009 to June 2010, in the outer Oslofjord, southern Norway. The succession of Pseudo-nitzschia species have been recorded from acid cleaned samples viewed in the scanning electron microscope and transmission electron microscope. The abundance of Pseudo-nitzschia species has been determined and related to hydrographical conditions. The ability to produce domoic acid varies between species, thus a precise identification and knowledge about their geographical and seasonal occurrence at the species level is important to early warning systems serving the aquaculture industry.