The rippling effects of a warming climate and wildfire on lakes in northern Alaska

NPS/Danielle Knapp

We’ve been monitoring lakes in Alaska for twenty years and most of the time, we see small changes. But in this lake, and several others, we saw a dramatic change. As we investigated the cause of this change, we gained insight into how a warming climate and thawing permafrost may have a greater impact on lakes and the surrounding vegetation than we first thought.

Specific conductance continued to rise slowly each year after the fire until 2019, when the seep developed. Shortly afterward, measurements increased dramatically.

Over the past two decades, a group of scientists with the National Park Service’s Central Alaska Inventory & Monitoring Network have been tracking changes in a remote lake located in Yukon-Charley Rivers National Preserve. The lake has been undergoing changes since monitoring began in 2003; the year before, a large wildfire burned the vegetation surrounding the lake. The impacts of the fire were immediately observed in the lake, nutrient concentrations increased, and specific conductance nearly doubled. Specific conductance is a measure of ions such as calcium, sulfate, magnesium, and chloride available in the water. These compounds often enter freshwater lakes via groundwater in contact with mineral soil, bedrock, or thawing permafrost.

Most of the impacts appeared to be short-lived. By 2010, nutrient and carbon concentrations had largely returned to pre-fire concentrations. Specific conductance, however, continued to rise. In 2019, the warmest summer on record in Alaska, specific conductance rose exponentially. Dead vegetation was visible near the lake, and large beds of water lilies were dying. Specific conductance continued to rise in the lake through 2021. Remotely sensed imagery showed two new features on the landscape. Field visits in 2022 identified the features as groundwater seeps. The specific conductance of the water coming from the seep was over 5,987 µS/cm, more than 15 times the specific conductance in the lake. The water coming out of the seep—laden with calcium, magnesium, and sulphate—had killed the terrestrial vegetation adjacent to the seep, as well as the large beds of water lilies growing in the lake.

During the winter of 2021-2022, scientists used remote sensing to identify the cause of the rapid change in specific conductance. Two brown patches appeared in 2019, the features were only visible on extremely high-resolution imagery (Worldview-2, 0.5 m resolution). The features were most visible using infrared imagery, which highlighted the contrast between the living and dead vegetation. The upper row shows that the seep was absent in 2016, present in 2019, and persisting in 2021.

The lower row is Planet images (3 m resolution) from each year showing the sudden appearance of the groundwater seep in 2019.

Specific conductance in the seep was more than 15 times higher than measurements taken at the lake surface, ion concentrations were so high they were toxic to nearby vegetation.

The seep developed in response to ground warming that occurred after the fire destroyed the thick organic layer that protected the permafrost (ground that remains frozen for two or more years) from thawing. The change in soil temperatures allowed the permafrost to thaw and the active layer thickness to increase. This shift made it possible for less-cold-tolerant trees like birch and willow to grow. It also enabled subsurface flow paths, such as the seep, to develop. These new flow paths likely allowed groundwater to be in contact with solute-laden bedrock adjacent to the lake, changing the lake’s water chemistry and killing both the terrestrial and aquatic plants nearby. This long-term monitoring project provides a unique opportunity to track the impacts of fire and ongoing climate warming on lake ecosystems.

Lake vegetation shift

Left image
A lake before its water chemistry shift, July 2018.
Credit: NPS/Amy Larsen

Right image
The same lake after its water chemistry shift, July 2022. Notice the loss of water lilies in the lake and dead vegetation.
Credit: NPS/Amy Larsen

These two pictures (taken from different perspectives) show the difference in lake vegetation. In the left image, the lake before the chemical shift (July 2018), shows lots of water lilies in the lake and bright green vegetation around the lake. In the right image (July 2022), the water lilies are gone and vegetation is dying around the lake.

Cascading effects of climate change and wildfire on a subarctic lake: A 20-year case study of watershed change

Abstract

Mean annual air temperature has been increasing in Alaska since the 1970s and is expected to continue to increase through the current century, resulting in significant environmental changes (e.g., permafrost thaw, shifts in vegetation community composition and distribution, increased wildfire frequency and severity). Because there is little long-term monitoring data available on lake ecosystems in the subarctic it is difficult to predict how lakes will respond. Here we present data from an ~20-year long-term lake and climate monitoring program in Interior Alaska. A significant portion of the lake catchment was burned by wildfires in 1986 and again in 2004. As a result, much of the vegetation in the lake catchment was converted from spruce-dominated forest to deciduous forest, indicating likely permafrost degradation. At a nearby monitoring site absent of fire effects, mean annual ground temperatures at 50 and 90 cm warmed significantly from about −2°C to about −1°C over the 20-year monitoring period. Warming of similar or greater magnitude is inferred at the study site due to fire effects. Lake water analyses before and after the 2004 fire show a significant postfire increase in dissolved organic carbon, total nitrogen, and total phosphorous that persisted for a short period (~3 years) and was followed by small, but significant, increases in specific conductance and ion concentrations. Approximately 15 years postfire sulfate and cation concentrations in the lake increased exponentially due to the development of a groundwater seep near the lake. The seep likely formed as a result of permafrost thaw creating new subsurface flowpaths in response to long-term climate warming and fire effects. In 2021, specific conductance and sulfate concentrations reached 657 μS/cm and 71 mg/L respectively, exceeding tolerance thresholds of the water lily Nuphar lutea. In 2021, N. lutea beds that had occupied the lake since 1981 were no longer visible. Depending on local catchment characteristics, source waters and flow paths contributing to small subarctic lakes will continue to evolve uniquely in response to climate change and thawing permafrost. This case study shows the cascading effects of warming soil and wildfires on catchment characteristics as well as terrestrial and aquatic vegetation and lake water chemistry.

Larsen, A. S., D. L. Rupp, D. K. Swanson, and K. R. Hill. 2023. Cascading effects of climate change and wildfire on a subarctic lake: A 20-year case study of watershed change. Ecosphere 14(7): e4558.