Phosphorus (P) is an important nutrient to monitor in natural waters as it is a growth limiting nutrient. When levels of phosphorus are elevated, excessive growth of algae occurs. This can lead to hypoxic or anoxic waters, potential release of harmful toxins and it impacts negatively on the ecosystem. Phosphorus measurement in water involves the collection of grab samples, transport and storage of samples, and analysis using expensive analytical instrumentation. This is costly, time and labour intensive and is carried out infrequently as a result. In this work, P is measured as soluble reactive phosphate (SRP), to give an indication of P levels in fresh water. A device for in situ analysis of SRP was fabricated. This device consists of a centrifugal microfluidic lab-on-a-disc platform, with a housing unit. The disc operates by mixing the sample with ascorbic acid method reagents using centrifugal force. The coloured product is presented to an optical detection system consisting of a laser and photodiode. The limit of detection has been optimised by modification of the optical path length. The housing unit is a 3D printed portable box with a built in motor for disc rotation. This box also houses the optical detection system which consists of a laser and photodiode. The box facilitates the alignment of the detection system with the optical pathway of the disc for absorbance measurements in an environment free from ambient light. This device allows for rapid analysis times, compactness, ease of use, low cost analyses and low reagent consumption. Keywords: Phosphate, microfluidics, lab-on-a-disc, optical sensor