Substantial progress has been made in mapping the distributions of metal micronutrients throughout the ocean over the last 30 years, but there remain information gaps, particularly during seasonal transitions and in remote regions. Trace metal micronutrients are integral to the functioning of marine ecosystems and the export of particulate carbon to the deep ocean. A remaining challenge is to develop in situ sensing technologies necessary to capture the spatial and temporal variabilities of micronutrients characterized with short residence times, variable sources, and nanomolar to sub-nanomolar concentrations in open ocean settings. Development of these sensors will allow investigation of the biogeochemical processes at the necessary resolution to constrain fluxes, residence times, and the biological and chemical responses to varying metal inputs in a changing ocean. To encourage a more widespread use of in situ sensors by academics and resource managers, the challenge is not only to develop devices that are robust, compact, easy to operate and amenable to long term deployments (>month) but also to produce sensors that can meet or come close to the same stringent accuracy and precision criteria that are achieved in the laboratory. Specifically, one unmet need is a sensor capable of determining Fe concentrations in real time on CTD rosettes, moorings, and autonomous vehicles. Significant progress can be made towards the development of an ocean Fe-sensor based the new generations of microfluidic, solid state, voltammetric, and other technologies. For example, advances in “Lab-on-a-Chip” technology combined with new chemical probes show promise in achieving limits of detection required to understand the oceanic Fe system. Some of these probes (such as for Zn) have been successfully adapted to seawater analysis at sub- nanomolar levels in the laboratory but none have been adapted for Fe, nor for in situ use. Similarly, immobilization of fluorescent probes onto fiber optic style sensors may hold promise as well. Electrochemical techniques exhibit high sensitivity in the lab, and their use in in situ sensor-systems is an area of possible development. The overall goal is the production of an in situ sensor capable of analyzing Fe at low to subnanomolar levels for periods of time exceeding 1 month. The chosen technology would preferably be applicable to other trace nutrients.