In order to allow detection of toxic algae in the field a portable semi-automated nucleic acid biosensor was developed in the ALGADEC project (www.algadec.net). This device enables the electrochemical detection of microalgae from watersamples in less than two hours, without the need of expensive equipment. The detection of the toxic algae is based on a sandwich hybridisation that is carried out on a multiprobe sensorchip. It allows the simultaneous detection of 14 different target species. The main steps of the probe to target hybridisation and analysis process are executed automatically, whereas a manual filtering and a lysis procedure has to be done before the automatic measurement. Probe sets for the relevant toxic algae have been developed for three different areas in Europe: Skagerrak in Norway, the Galician coast in Spain and the area of the Orkney Islands in Scotland. The probe sets have been tested with laboratory strains for specificity and the applicability of the device was evaluated in the field. The scope of our current work is the automation of the sampling and sample preparation in order to obtain a fully automated nucleic acid biosensor system, that can be used in combination with other autonomous marine sensor systems. Furthermore, the biosensor is going to be adapted and validated for the monitoring of key phytoplankton species in the North Sea.