Clearing up the “green lake” phenomenon

This summer, the water in Mono Lake appeared noticeably clearer than it has during most summers over the last decade. Historically, the clarity of Mono Lake’s water fluctuates seasonally, appearing green and opaque in the winter as algae grows and then clearing up in the summer as brine shrimp hatch and eat the algae. However, starting in 2014, researchers and recreationalists alike noticed the lake stayed green with algae during the summer, greatly reducing water clarity and visibility.

This “green lake” phenomenon of summers with significantly decreased water clarity persisted until 2017, another exceptionally wet year akin to this year. 2018 and 2019 were somewhat in-between years with regards to water clarity, and the remaining years until 2023 have had exceptionally opaque, green water. Anecdotally, 2023 appears to have even greater clarity than 2017, but data are still being collected and processed.

“Why is this happening?” is the question the Mono Lake Committee, researchers, and visitors have been asking.

In the big picture, for decades Mono Lake has been below the level the California State Water Resources Control Board set to ensure a healthy salinity level for the lake’s overall ecosystem function. When the lake stayed green in the summer of 2014, Mono Lake had dropped to a low level, and high salinity, not seen since the mid-1990s. Since then the lake has been fluctuating around that low level, including the summers when the water remained green and opaque. How the low lake level and higher salinities influence the many different limnological factors tied to the green lake condition is not yet fully understood.

Other important factors to consider when investigating the green lake phenomenon include: brine shrimp abundance, size, grazing patterns, and life cycle changes; algae composition and abundance; water temperature; stratification within the lake; and, as in this year, large inputs of fresh water to the lake.

Excitingly, the Committee has secured grant funds from the California Department of Fish & Wildlife to fund UC Santa Barbara limnologists to shed some light on the green lake phenomenon. This project will seek to solve this Mono Lake mystery as a supplement to the ongoing limnology research at Mono Lake required by DWP’s water license.

This post was also published as an article in the Fall 2023 Mono Lake Newsletter. Top photo by Elin Ljung.