A diverse influence of dissolved natural organic matter on light irradiance measurements in lakes

Light penetration plays a vital role in lakes and drinking water reservoirs, influencing fundamental processes such as primary production and thermal budgets. The Secchi depth (Z(SD)) and the compensation depth (Z(CD)) are commonly used measurements in this context. Z(SD) is determined through visual inspection using a Secchi disc, while Z(CD) represents the depth at which photosynthetic activity balances respiration and can be measured using a quantum irradiance sensor. Through in situ water-core samples from 23 lakes within a Lake District in Southeastern Norway, we observed that DNOM exerts a diverse influence on these light irradiance measurements. If DNOM concentrations are reduced to half or a quarter of the current concentration, similar to what was measured during the 1980s, the median Z(CD):Z(SD) ratios are estimated to have decreased by approximately 30 and 60% since then, respectively. Conversely, a plausible future climate-driven doubling or quadrupling of the present DNOM concentrations are estimated to further decrease the median Z(CD):Z(SD) ratios in the Lake District with approximately 10 and 25%, respectively. From this, the Z(CD):Z(SD) ratios seem to have experienced a considerable long-term decline attributed to both climate change and the recovery from acid rain, and a further climate-driven decrease is expected.