Scientists from Crater Lake National Park, University of Trento, and the USGS, highlight how climate change will alter the deep-water mixing patterns in Crater Lake, OR, published in the Journal of Great Lakes Research. The findings show Crater Lake will mix to the bottom less frequently in the latter half of the 21st century due to a warming climate. The frequency that Crater Lake mixes all the way to the bottom is significant to the health of the lake because it cycles nutrients within the water column, adds dissolved oxygen to the deep waters, and allows heat from its hydrothermal vents to escape to the atmosphere.  

Most lakes routinely mix to the bottom by convection when cold dense water sinks to the bottom and warmer water rises to the top. During early winter and spring, Crater Lake has periods of limited convective mixing to 300-400 meters deep, but less frequent mixing to its deepest water at 594 meters. Water mixing all the way to the bottom of extremely deep lakes, like Crater Lake, requires special conditions. Including that water temperature at the surface be slightly colder than the temperature of maximum density that is nearly four degrees Celsius at atmospheric pressure. In a warming climate, the lake surface temperature will dip below four degrees Celsius less often. The research modeled future climate scenarios ranging in extremity, but all showing a clear trend of warming lake temperatures and decreasing deep-water mixing.  

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“In order to have confidence in the future simulations of deep mixing in Crater Lake, we ran the deep-mixing model using 24 combinations of future climate models, which had accurate historical performance in the Pacific Northwest,” said Susan Wherry, Hydrologist at USGS Oregon Water Science Center.  

The famous clarity and color of Crater Lake exists because algae (phytoplankton) in the water are sparse. Since algae need the nitrogen mixed up from the lake bottom to grow, it’s likely that with less deep-mixing events the lake would initially be even more clear. However, it’s not understood what happens when mixing fails to occur for long periods. Does nitrate continue to build up at the bottom and cause extreme algae conditions when deep-water mixing does happen? Under that possibility the clarity of Crater Lake would decrease. 

“Vertical mixing of water in winter is one of the most important drivers of lake ecology because it impacts growth of algae and water clarity, as well as prevents oxygen dead zones on the lake bottom by mixing new oxygen down from the surface,” said Scott Girdner, Lake Biologist at Crater Lake National Park. 

For any lake, a major concern with a loss of deep-water mixing is how quickly oxygen at the bottom gets used up by the decay of algae. Many lakes form oxygen “dead-zones” at the lake bottom. Dead zones are uninhabitable for aquatic life. This new modeling project addresses that concern and shows that Crater Lake’s oxygen levels do not decline very fast, at least until late in the century. Even in the warmest climate scenarios with little deep-water mixing, dissolved oxygen dead zones do not form at the deepest points. This is because Crater Lake has relatively few algae growing in the water column, thus, less decay and oxygen depletion.   

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Crater Lake receives heat and salts from hydrothermal vents at its base, which sets it apart from other very deep lakes in temperate climates. In warmer climate scenarios, deep-water mixing conditions are rare and the heat from the hydrothermal vents cannot circulate to the top where they are vented to the atmosphere. Combined with warming from the air above, water temperatures would increase rapidly given these conditions.  

The results in this journal article are possible simulated scenarios. The change in the mixing patterns could be moderate if climate warming is reduced or less than predicted.  

"Lake models provide valuable insights into lake dynamics and can be used to support the development of management policies for preserving aquatic ecosystems. Here we used a simple yet powerful model, that allowed us to explore future climate change scenarios in Crater Lake," said Sebastiano Piccolroaz, Assistant Professor at the University of Trento. 

Overall, Crater Lakes happens to be in a particular setting and situation where warming weather conditions clearly impact important processes in the lake. In addition to deep-water mixing, many aspects of lake ecology are impacted by warming weather. Visit here to learn more. 

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