Coastal ecosystems are important interface zones between the terrestrial and the marine realm. Due to the continuously increasing anthropogenic pressure on most coastlines worldwide, a significant increase in the effort to monitor and assess changes in the coastal systems has been proposed. In the framework of COSYNA (Coastal Observation System of the Northern and Artic Seas), a cable connected underwater observatory for long term exposure even under extreme environmental conditions has been developed. Two prototypes of the COSYNA underwater node system have been installed in 2012 in the southern North Sea and in an Artic Fjord System (Kongsfjord Svalbard) and are operated since then continuously. These systems provide the logistic underwater platforms to operate standard sensors like ADCP and CTD as well as complex sensors like a continuous plankton recorder or a stereo-optical fish detection device with a data transmission rate up to 1 GHz year in both ecosystems. The main scientific objective of the COSYNA underwater node technology is the continuous assessment and (near) real time analysis of environmental parameters in the COSYNA target environments the North Sea and the Arctic Sea. The continuous data stream of the main oceanographic, hydraulic and biological parameters sampled synchronously in the two ecosystems year round even under extreme conditions like severe storms in the North Sea or ice coverage in the Arctic are used to monitor, analyse and to model ecosystem behaviour with respect to abiotic environmental dynamics and environmental shifts.