Lab on a Secchi disk: A prototype open-source profiling package for low-cost monitoring in aquatic environments

Owing to the high cost of commercial optical sensors, there is a need to develop low-cost optical sensing packages to expand monitoring of aquatic environments, particularly in under-resourced regions. Visual methods to monitor the optical properties of water, like the Secchi disk and Forel-Ule color scale, remain in use in the modern era owing to their simplicity, low-cost and long history of use. Yet, recent years have seen advances in low-cost, electronic-based optical sensing. Here, the designs of a miniaturized hand-held device (mini-Secchi disk) that measures the Secchi depth and Forel-Ule color are updated. We then extend the device by integrating a small electronic sensing package (Arduino-based) into the Secchi disk, for vertical profiling, combining historic and modern methods for monitoring the optical properties of water into a single, low-cost sensing device, that measures positioning (GPS), light spectra, temperature, and pressure. It is charged and transfers data wirelessly, is encased in epoxy resin, and can be used to derive vertical profiles of spectral light attenuation and temperature, in addition to Secchi depth and Forel-Ule color. We present data from a series of deployments of the package, compare its performance with commercially available instruments, and demonstrate its use for validation of satellite remotely sensed data. Our designs are made openly available to promote community-based development and have potential in communicating and teaching science, participatory science, and low-cost monitoring of aquatic environments.