Abstract
The Baltic Sea observing system combines ship-based, autonomous, and remote-sensing components, yet major observational gaps persist in the shallow and dynamic coastal zone. Here, we present the first evaluation of a micro-autonomous underwater vehicle (micro-AUV) for high-resolution mapping of the Baltic Sea water column and assess its potential integration into regional coastal observing systems. Eleven missions were conducted in 2025 across three contrasting environments—the shallow estuarine Bay of Matsalu, the Suur Strait, and the coastal slope of the Baltic Proper—covering depths from 1 to over 100 m. The micro-AUV operated reliably under diverse hydrographic conditions, capturing fine-scale frontal, submesoscale, and benthic-layer structures, as well as a dual upwelling cell. Comparisons with reference profiles confirmed the high accuracy of temperature and salinity measurements, and strong correlations for chlorophyll-a fluorescence and turbidity. The vehicle’s endurance (up to 40 km or ~7 h at 1.5 m s −1 ) enables efficient mesoscale surveys, while its maneuverability allows operation in very shallow or complex coastal areas where gliders and floats cannot be used. The results demonstrate that micro-AUVs can fill a critical observational gap between ship-based surveys, gliders, FerryBoxes, and coastal stations by extending coastal observations offshore and resolving vertical structures inaccessible to remote sensing. Despite limitations in endurance and communication range, micro-AUVs represent a promising, cost-efficient addition to integrated coastal observing systems, supporting targeted process studies and rapid-response missions in dynamic environments such as the Baltic Sea.