Today’s Image of the Day from NASA Earth Observatory features Clear Lake in northern California, which is one of the oldest freshwater lakes in the United States. 

In mid-May 2024, the waters of Clear Lake were transformed into a vivid tapestry of bright green swirls, as seen in this image captured on May 15, 2024 by the Operational Land Imager-2 (OLI-2) on the Landsat 9 satellite. 

Clear Lake algal bloom is visible from space 

The algal bloom, potentially comprised of blue-green algae (cyanobacteria) and other phytoplankton, was documented from space. Direct sampling is needed to precisely identify the algal organisms present in the lake.

Cyanobacteria, single-celled photosynthetic organisms, are notorious for some species’ ability to produce microcystin, a toxin harmful to the skin, liver, and kidneys. 

Large algal blooms in Clear Lake 

Clear Lake, about 60 miles north of San Francisco Bay, is a naturally nutrient-rich body of water that has historically supported large algal populations dating back to the end of the last ice age, around 10,000 years ago, based on sediment core studies.

Recent human activities have exacerbated nutrient levels in the lake, leading to a rise in harmful algal blooms. 

Out of over 130 algal species that have been identified in the lake, three types of blue-green algae pose health risks under certain conditions. These blooms typically appear in the spring and late summer, as reported by Lake County officials.

Nutrient influx from agriculture, vineyards, inadequate septic systems, gravel mining, and an abandoned mercury mine, along with natural factors such as lakebed sediment disturbances caused by waves and fish, have contributed to the lake’s water quality decline. 

Chlorophyll-a and cyanobacteria 

This year, data from a satellite-based ocean color instrument on Sentinel-3, processed further by the NOAA National Ocean Service, showed increased concentrations of chlorophyll-a and cyanobacteria leading up to May 15.

As of May 25, local monitors have not reported microcystin levels for the current bloom. Nonetheless, the high algal density poses risks to aquatic ecosystems, as decomposing phytoplankton deplete oxygen, potentially leading to hypoxic conditions and aquatic dead zones.

More about Clear Lake 

Clear Lake is the largest natural freshwater lake wholly within the state of California. Geological estimates suggest that it formed about 2.5 million years ago. 

The lake is a popular destination for fishing, particularly for largemouth bass, and its scenic beauty attracts many visitors for boating, bird watching, and other recreational activities. 

Clear Lake’s ecosystem supports a diverse range of wildlife, but it also faces environmental challenges such as water quality issues, partly due to agricultural runoff and other human activities.

Freshwater lakes and climate change 

Freshwater lakes are significantly impacted by climate change, which manifests in various ways, affecting both their ecological balance and their roles in local environments. 

Warming lake water

As global temperatures rise, lakes experience changes in water temperature and ice cover. Warmer water tends to hold less oxygen, which can harm fish and other aquatic life; it can also encourage the growth of harmful algal blooms that further deplete oxygen and release toxins into the water. 

Ice cover

Reduced ice cover during winter changes the seasonal rhythms of lakes, affecting species that rely on ice cover for part of their life cycle, like some cold-water fish and plants.

Water level fluctuations 

Additionally, changes in precipitation patterns and increased evaporation due to higher temperatures can alter water levels. These fluctuations not only impact the aquatic habitats but also affect water availability for human use, agriculture, and hydroelectric power generation. 

The increasing frequency of extreme weather events, like storms and droughts, leads to more pronounced cycles of flooding and low water levels, creating challenges for managing water resources and preserving lake ecosystems. 

As lakes are integral to their local climates, biodiversity, and economies, these changes can have widespread implications for the communities and ecosystems that depend on them.

Image Credit: NASA Earth Observatory 

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