Described herein is an underwater vehicle having a vehicle body with a buoyancy controller adapted to vary the buoyancy of the vehicle in order to control motion of the vehicle through an underwater environment. The vehicle further includes a sampling system and a sensor arrangement. The sampling system is adapted to sequentially sample fluid from the underwater environment at specified sampling times resulting in a sample sequence, each sample associated with a sample time and a fluid flow rate. The sensor arrangement includes a plurality of molecule sensors adapted to sense organic molecules in each respective sample of the sample sequence.