We evaluated trends in dissolved organic carbon (DOC) and associated changes in water transparency and epilimnion thickness to better understand the implications of regional increases in DOC concentration in lakes. Long‐term monitoring of a suite of physical, chemical, and biological data from six to 12 lakes in Acadia National Park in Maine was paired with high‐frequency sensor monitoring of one lake as a model system. Water transparency declined across study sites since 1995 as DOC increased and chlorophyll remained stable, suggesting that this was not a signal of increased eutrophication. As clarity declined, some lakes experienced reduced epilimnion thickness. The degree to which transparency changed across the lakes was dependent on DOC concentration, with a larger decline in transparency occurring in clear water lakes (−0.3 m yr−1) than brown water lakes (−0.1 m yr−1). DOC concentration was an important explanatory variable for reduced epilimnion thickness in short‐term sensor measurements. A regional decline in water transparency across all lakes and reduction in epilimnion thickness in a limited number of systems appeared to be acting as a sentinel for changes in atmospheric deposition and regional weather that modified the delivery of DOC from the watershed.

The transparency regulator hypothesis (TRH) proposes that water transparency determines the relative importance of visual predation vs. ultraviolet radiation (UVR) in driving zooplankton out of the surface waters during the day. To test this hypothesis, we used a combination of observational and experimental approaches to examine the effects of UVR, photosynthetically active radiation (PAR), fish, temperature, and food resources on the daytime vertical distribution and diel vertical migration (DVM) of the copepod Hesperodiaptomus arcticus in a set of lakes in the Canadian Rocky Mountains. Across lakes, H. arcticus daytime vertical distribution increased with both UVR transparency and depth of food resources, and was not related to PAR, thermal structure, or the presence of fish. We also observed a strong positive relationship between UVR and H. arcticus daytime depth distribution in a single lake that varied in transparency over time. The presence of substantial DVM in H. arcticus in two transparent lakes coupled with the lack of DVM in a less transparent lake indicated a weakening effect of UVR in lower transparency conditions. Finally, we deployed open‐bottomed vertical columns constructed of UVR‐blocking and UVR‐transmitting material overnight in two lakes of contrasting transparency. The following day, we observed significantly more copepods in UVR‐blocking columns in the more transparent lake but no UVR effect in the less transparent lake. Collectively, our results provide multiple lines of evidence supporting the TRH by highlighting the changing role of UVR as a driver of DVM across lakes of varying transparency.

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We examined factors regulating water transparency in a set of 33 lakes located in the Canadian Rocky Mountains. Eighteen lakes had catchments that included glaciers and 15 did not. In each lake, we quantified midsummer attenuation rates for three ultraviolet wavelengths and photosynthetically active radiation and measured chromophoric dissolved organic matter (CDOM) absorbance, turbidity, and chlorophyll a fluorescence. We also used GIS to quantify characteristics of lake catchments. Across lakes, turbidity and CDOM absorbance were arrayed on orthogonal gradients that intersected in a region of low turbidity and low CDOM absorbance. Nonglacially fed lakes had low turbidity and attenuation rates were regulated by CDOM absorbance, which increased with the percentage of the catchment covered by vegetation. Glacially fed lakes had low CDOM absorbance and attenuation rates increased with turbidity, which increased with the percentage of the catchment covered by permanent ice. Glaciers are retreating rapidly in the Canadian Rockies. As catchments become deglaciated, turbidity will likely decrease and the regulator of lake transparency is likely to switch to CDOM, which will have broad implications for lake ecosystems.