In situ sensors are needed to further our understanding of phosphate flux dynamics in marine environments during short term events such as tidal cycles, algae blooms and runoff periods. Here, we present a fully automated in situ phosphate analyzer based on an inlaid microfluidic absorbance cell technology. The microfluidic device employs colorimetric absorbance spectrophotometry, using the phosphomolybdenum blue (PMB) assay modified by the addition of polyvinylpyrrolidone (PVP), to measure phosphate concentrations in seawater. Bench top calibrations were performed with both copper(II) sulfate dye and the PMB assay, as well as temperature sensitivity studies to characterize the sensor's performance in a range of conditions. It achieves a limit of detection of 15.2 nM, a limit of quantification of 50.8 nM, and a high in situ precision with a relative standard deviation of less than 1.5% across three consecutive measurements. Two consecutive field deployments are conducted as assessments for its intended in situ applications. The sensor is first deployed from a pier at a depth of 6 m, with simultaneous bottle samples taken to perform cross-validation. It is next deployed on the Stella Maris testbed, a multi-sensor seabed platform (MSSP), 100 m offshore and 9 m deep in the inlet to the Bedford Basin in Nova Scotia, Canada. Over 300 successful phosphate measurements were acquired, showing the influence of the tidal cycle, and confirming the sensor's viability in observing nutrient flux dynamics with nanomolar variations.