DISCO: A low-cost device-instrumented Secchi disk for water clarity observations

Water clarity regulates irradiance penetration in aquatic environments, influencing physical and biological dynamics: irradiance penetration affects heat transfer in the water column and provides energy through photosynthetically active radiation (PAR) in the euphotic zone, which is vital for light-dependent organisms. The ability to accurately assess water clarity is therefore important in several aquatic science contexts, from data analysis and process interpretation to modeling. Common metrics used to quantify water clarity include the vertical irradiance attenuation coefficient $K$, a measure of irradiance penetration, and the Secchi disk depth ($z_{\mathrm{SD}}$), a measure of water visibility. The enduring simplicity and low cost of the Secchi disk has made it a global standard for measuring water clarity for almost two centuries. In contrast, $K$ is typically determined using expensive instruments that measure underwater irradiance profiles. This highlights the need for innovative, cost-effective methods that integrate both types of measurements. Here we present DISCO, a low-cost, easy-to-build instrument that retains the traditional appearance of a Secchi disk, and is equipped with photoresistors (also known as light-dependent resistors, LDRs) both looking upwards and downwards for planar irradiance measurements. DISCO is also equipped with low-cost temperature and pressure sensors, all connected to an ArduinoUNO board. DISCO was tested in two mountain lakes together with high resolution PAR, temperature and pressure sensors to calibrate the LDRs and validate its performance. The results show that the proposed instrument is able to measure the irradiance attenuation coefficients with an error of less than 10$\%$ compared to the reference PAR sensor.