Exploring light transmission through ice-snow cover and water in lakes

Solar radiation is a fundamental source of energy for lakes. It regulates the physical and bio-chemical balance of the lake by influencing thermal stratification, biological productivity, and biochemical processes. Its importance becomes even more critical during ice-covered periods when external forces, like wind, are inhibited by the ice-snow cover. Despite this, a fraction of solar radiation can still penetrate the ice and snow, becoming the main external source of energy. This radiative heating destabilizes the upper part of the water column, which is inversely stratified in ice-covered lakes, leading to radiatively driven convection. While the importance of irradiance (solar radiant flux per unit area, Wm−2) in lakes is well-established, having in-situ measurements or reliable estimates of irradiance penetration remains a challenge. A key parameter used to estimate water clarity and irradiance attenuation in the water column is the attenuation coefficient, which quantifies the exponential decrease of irradiance with depth. Although the attenuation coefficient can be determined from irradiance profiles measured with radiometers, their high cost limits their use. A more affordable alternative is the Secchi disc, which has historically been used to measure Secchi depth and estimate water visibility. To effectively estimate water clarity, we developed a low-cost DIY (Do-It-Yourself) instrument using low-cost photoresistors. The instrument preserves the historical Secchi depth measurement while incorporating the more informative data from irradiance attenuation coefficient, which is obtained more affordably than with a proper radiometer. In ice-covered lakes, obtaining in-situ measurements becomes even more difficult due to logistical challenges. Both ice and snow exhibit exponential attenuation of irradiance, and the effect of surface albedo becomes crucial. Accurate estimates of albedo and attenuation coefficients of ice and snow are essential for understanding irradiance conditions beneath the ice. By conducting laboratory spectrometer measurements on lake ice cores, we assessed the internal variability of light transmittance, evaluating the impact of non-homogeneities, such as bubbles, within the ice. This intrinsic variability, along with the variability that can be in the field due to meteorological conditions, results in a wide range of values for albedo and attenuation coefficients in the literature. Consequently, selecting appropriate reference values for these parameters is difficult. To address this, we analyzed a large dataset of in-situ measurements of both irradiance above and below the ice-snow cover, and ice and snow thickness, using a Bayesian model. This model generated probability distributions for the albedo and attenuation coefficients of both ice and snow, providing reliable reference values for further research. These estimates are crucial not only for understanding present conditions but also for predicting future changes under different climate scenarios. As climate change is expected to alter ice and snow cover thickness and duration, significant changes in under-ice irradiance conditions are likely, with profound effects on winter limnological processes. To quantify future trends in Swedish lakes, we combined these distributions with ice and snow thickness data simulated from the air2water model under ISIMIP3b future climate scenarios. The model predicts a decline in ice thickness and duration in Swedish lakes, especially in the southern region, with ice cover likely to disappear or become extremely short in duration by 2100 under the worst-case climate scenario. This reduction in ice duration is expected to lead to an increase in irradiance penetration into the lakes, with potentially significant ecological consequences. In some southern Swedish lakes, ice cover is already intermittent, and differences in bio-chemical properties between ice-covered and ice-free years have been observed. This research underscores the importance of irradiance observations and modelling in lakes. It significantly improves our ability to model and quantify light transmission through both the water column and the ice-snow cover, providing valuable insights for future ecological studies.