An inexpensive method for the measurement of photosynthetically active radiation profiles in waterbodies

We developed an inexpensive light instrument (ILI) and protocol for the measurement of photosynthetically active radiation (PAR) at discrete depths underwater. We tested the accuracy of the ILI against a submersible radiometer (RAD) and a Secchi disk method. The ILI consists of a PAR sensor (HOBO MX2202; Onset) attached to a flat disk that ensures that the sensor faces upwards when lowered through the water column. The flat disk is attached to a rope marked at 1-m increments (to 20 m) that allows for the measurement of PAR at discrete depths. A weight is attached to the bottom of the flat disk to prevent it from drifting. The ILI is much cheaper (US $130), less bulky, and lighter (1.1 kg) than the RAD (US $40,000, weight 9 kg). We tested our method in a mesotrophic lake and in an oligotrophic lake in summer and fall 2023. The correlations between the RAD and ILI measurements of irradiance at depth (as a percentage of surface irradiance), and the depth to which 1% of the PAR at the surface penetrated the water (Z_1%PAR), were high and did not significantly differ. There was a difference of 8% in the Z_1%PAR determined by the ILI compared with that determined by the RAD. However, the estimated Z_1%PAR based on the Secchi disk data was 37% deeper than that calculated with the RAD data. Our method should enable the study of underwater PAR in remote regions where transporting bulky and expensive equipment is impossible due to logistic and/or financial constraints. Moreover, our method can be used for the direct measurement of PAR at discrete depths.