Aquatic ecosystems provide essential services to society; from acting as a sink for atmospheric CO2 to food production. However, aquatic environmental health and services are under risk of irreversible damage, requiring continuous fit-for-purpose monitoring. The aim of the PolTrans project is to build a low-power, compact, submersible sensor for the autonomous measurement of birefringent particles in water, namely 1) calcium carbonate minerals (CaCO3) produced by planktonic organisms, which play an important role in ocean CO2 storage and are at risk from ocean acidification, and 2) microplastic particles in aquatic environments, which constitute a major pollution problem. This breakthrough technology will fill crucial observational gaps in the global effort to monitor the ocean’s biological CO2 storage and microplastics pollution in aquatic environments. We will build and validate a prototype and substantiate the claims of our filed patent.