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Phytoplankton blooms appear in oceans, seas, bays, canals, and increasingly in lakes.
The Operational Earth Imager (OLI) aboard the Landsat 8 satellite captured this image of an algae bloom in Lake Villarrica in Chile on May 2, 2023. Ground observations and additional satellite image analysis indicate that cyanobacteria form the light blue-green swirls seen in the image Of natural colour.
Lago Villarrica, located on the slopes of the volcano of the same name, attracts visitors with its stunning beaches and recreational opportunities. The lake is surrounded by small towns and tourist areas on its southern shore, with agricultural areas further afield. Runoff from agriculture and urban development carries nutrients such as nitrogen and phosphorus to the lake. When the numbers of microscopic cyanobacteria increase in response, blooms often appear on satellite images.
“Freshwater lakes frequently bloom,” said Lian Rodríguez Lopez, an environmental science researcher at the University of San Sebastian who studies Chilean lakes using remote sensing. A combination of warm surface water and nutrient-laden runoff is likely responsible for the more regular flowering.
In a recent study using Landsat imagery, Rodriguez-López estimated the amount of chlorophyll a, an indicator of algal blooms, in Lake Villarrica from 2014 to 2021. He found that despite generally good lake water quality, chlorophyll a values approach the coast and near my hometown. Villarrica and Pucon. He concluded that the most important source of nutrients for these blooms came from urban pollution, although agriculture may have played a role as well. In fact, the pollution is significant enough that the lake goes from an oligotrophic state, with low nutrient levels and high clarity, to a mesotrophic state, with intermediate nutrient levels and biological productivity, he added.
Warm water also provides a more hospitable environment for algae. Like many bodies of water around the world, Lake Villarrica gets warmer along with the weather. In another study, Rodríguez López used Thermal Moderate Resolution Imaging Spectroradiometers (MODIS) to analyze the surface temperatures of Chilean mountain lakes between 2000 and 2016. He reported statistically significant warming trends in 12 of the 14 lakes studied, including Lake Villarrica. He wrote that the trend was “consistent with observations from the site and with an increased frequency of toxic cyanobacteria in Lake Villarrica.”
With the help of monitoring efforts by the community group Vigilantes del Lago, blooms in the lake have been documented almost every Australian summer (January or February) since 2008. Occasionally, occurring in the fall (April or May), such an event is more recent. The most common cyanobacteria that make up these blooms belong to the genus dolichospermumwhich can be toxic.
The frequency of potentially dangerous flowering has become a concern in local communities. Joined by citizen scientists’ efforts to monitor and protect these changing Chilean lakes, remote sensing could provide an effective way to complement the labor-intensive work on land. According to Rodriguez-Lopez, satellite imagery can be used to create an early warning system to notify the public of algal blooms and establish policies to prevent them.
NASA Earth Observatory images by Wanmei Liang, using Landsat data from the USGS. Reporting by Lindsey Dorman.