Rapid Ecological Shift in a Sierran Lake due to Climate Change

Lakes can accumulate sediments and protect them from disturbance for thousands of years, making them excellent repositories of environmental information. Samples of lake sediment retrieved during coring expeditions can be analyzed to reconstruct critical characteristics of past environments, such as local vegetation communities, fire frequency, and temperature.

Illustration of a coring rig installed on a raft. The casing (essentially an empty tube) is pushed into the sediment. Credit: http://www.museum.state.il.us/muslink/behind/htmls/cr_bot_pal3.html

Coring lake sediment.

This is an example of a sediment core. Different layers, often associated with seasonal changes, can be seen in the sediment. Photo credit: Jamie Howarth

To investigate whether changes associated with global warming could be detected in sediment that has accumulated in lakes in the Sierra Nevada, Laura Streib and her colleagues collected sediment from June Lake, which is in the eastern Sierras and about 20 km south of Mono Lake.

Location map of June Lake (red square).

Google Earth image of June Lake. The tracks above the lake are ski runs in the June Mountain ski area.

In this study, published by Global Change Biology, the authors looked at how mixing of the lake water has changed over time. As anyone who has jumped into a lake knows, the water is warm near the surface, but it gets progressively colder the deeper you go; this effect is known as 'water column stratification,' and if the water is vertically mixed, for example by the wind, the water column becomes less stratified. The authors determined that, for June Lake, the strength of the stratification depends primarily on the duration of ice and snow cover, with lake level, water clarity, and wind speed playing secondary roles. As a way of tracking the strength of the water column stratification, the authors used fossil diatoms recovered in the sediment from the core. Diatoms, a type of algae, are unicellular organisms that live in water. 

Images of diatoms. Because their cell walls are composed of silicon, they are well-preserved in sediment.

Diatoms are useful for this type of study because different species of diatoms prefer different types of environments. The authors measured the abundance of two different types of diatoms, one that thrives in lakes that are strongly stratified and one that does better in waters that are more mixed, and calculated the ratio of the two. In addition, they used carbon-14 and lead-210 dating to assign ages to different layers in the sediment and provide a detailed chronology of the changes in the lake.

They found that, beginning in the early 20th century, the lake transformed from being poorly stratified to strongly stratified. Moreover, this transition was abrupt (in a geological sense), occurring in just a few decades. Because this change was coincident with a rapid increase in surface air temperature, the authors attribute it to global warming. In particular, the increase in air temperature appears to have reduced the amount of time that the lake is covered by ice and snow and increased the temperature of the surface water, both potential causes for enhanced thermal stratification.

Figure 5 (cropped) from Streib et al. (2021). The L-S index is a ratio of the abundance of two types of diatoms. Negative numbers indicate poor thermal stratification, positive numbers indicate strong stratification. The index changes abruptly in the first part of the 20th century (grey bar), a period of time that also saw an increase in global surface air temperature (SAT anomaly) and temperatures in North America (NA anomolies).

The striking result from this interesting study is that, even though global warming appears gradual to us, once certain thresholds are reached, the structure of an ecosystem can flip like a switch. Another example of this type of behavior may be seen in Sierran forests. Mature trees are able to survive droughts because their long roots can access water stored deep within the soil and bedrock. However, if large fires are followed by drought, tree seedlings won't be able to establish themselves and, within a few decades, forests will be replaced by shrubs that can tolerate drier conditions. Finally, studies like the one featured here are important because they give us critical information about these environmental thresholds and serve as a warning about how ecosystems can suddenly change from one state to another due to changes in climate.