Continuous monitoring of estuarine systems, such as Narragansett Bay, remains in its infancy. Living biosensors capable of sensitive and specific detection of low levels of bioavailable nitrogen could provide a new and innovative approach to monitoring nitrate fluctuations in estuarine systems in real time. The design and evaluation of a fully modular, scalable sensor system based on an optical sensing element and a nitrate sensing bio-reporter will be described. Preliminary characterizations of the sensor robustness and sensitivity will also be reported for nitrate standards and estuarine water samples.