The eutrophication related to excess phosphorus within our freshwater systems has been a continuous area of concern for environmental scientists and agencies since the implementation of the Water Framework Directive in 2000. While discharge of phosphorus has decreased significantly in Europe over the past twenty-five years the issue of phosphorus pollution persists and has started to worsen. Because of trends of increasing phosphorus being reported across Europe, the need for portable, rapid, and reliable phosphate detection is required to determine the sources of excess phosphate and the effects our rapidly changing climate have on phosphorus diffusion. This research was designed with the goal of developing a low cost, portable, rapid, and complete analytical system for the detection of phosphate in freshwater systems. This new system utilises colorimetric detection of phosphate and the molybdenum blue method dried onto a custom microfluidic disc. A micro-spectrometer and single wavelength LED enables the system to be compact and easily portable with a comparable performance to its benchtop counterparts. The analytical range of the system is 5-400 μg/L, which encompasses the threshold value of 35 μg/L P for Irish rivers and groundwaters. The system’s compact size and weight of less than 2 kg makes it highly portable. With a run time of 15 minutes per ten samples, it enables the in-situ detection of phosphate for rapid on-site monitoring. To demonstrate the sensor, three rivers in the northwest of Ireland were identified which have historical orthophosphate readings in the range of 5 - 47 μg/L. With this microfluidic phosphate detection system, rapid in-situ detection and reliable, real-time monitoring of phosphorus in freshwater systems was demonstrated.