A ballast water monitoring device and a method for detecting live phytoplankton are disclosed. The device comprises a chamber for receiving a sample, at least one light source to emit light towards the sample, a light detector to receive light from the sample and generate a light signal, and a controller. The controller is configured to control the at least one light source to emit a single pulse of light, calculate the variable fluorescence [Fv] of the sample in response to the pulse of light, at time intervals less than the duration of the pulse of light, compare the calculated variable fluorescence to a predetermined reference limit, and perform an action if the calculated variable fluorescence is greater than the predetermined reference limit.