Spatially and temporally detailed measurement of ocean, river and lake chemistry is key to fully understanding
the biogeochemical processes at work within them. To obtain these valuable data, miniaturised
in situ chemical analysers have recently become an attractive alternative to traditional manual sampling,
with microfluidic technology at the forefront of recent advances. In this short critical review we
discuss the role, operation and application of in situ microfluidic analysers to measure biogeochemical
parameters in natural waters. We describe recent technical developments, most notably how pumping
technology has evolved to allow long-term deployments, and describe how they have been deployed in
real-world situations to yield detailed, scientifically useful data. Finally, we discuss the technical challenges
that still remain and the key obstacles that must be negotiated if these promising systems are to
be widely adopted and used, for example, in large environmental sensor networks and on low-power
underwater vehicles.