by Hilary Dugan – Why are lakes blue? Or green? Or brown? The color of a lake reflects what’s in the water. Pure water appears blue, lakes with a lot of algae appear green, and lakes with high carbon concentrations appear brown. Water color is not synonymous with water quality, but it does provide insight into what might be happening beneath the surface.
Much like the surrounding landscape, lake color can change seasonally. Often, lakes are ‘greener’ in the summer when algae growth is at its highest. Sometimes lakes are exceptionally clear in the spring, due to huge populations of ravenous zooplankton feasting on algae. And, once in a while, lake water might appear surprisingly brown due to blooms of single-celled algae called diatoms.
For a single lake, observing the annual pattern of lake color can tell us about the local ecosystem. But imagine we could track the color of all lakes! At such a large scale, landscape patterns in lake color might inform us about the impacts of climate or land-use change, and how lakes might be storing or emitting carbon.
To systematically monitor the color of thousands of lakes, eyes on the ground quickly become impractical, and it makes sense to turn to eyes in the sky – satellites.
In a new paper, Dr. Simon Topp and colleagues harness the powerful US Landsat satellites to analyze summer lake color from 1984 to 2020 across more than 26,000 lakes. “Satellite imagery contains a ton of information, far more than we capture with our eyes. If we process satellite imagery in the right way, we can use it to replicate how we see the world,” says Dr. Topp. (Click here for a cool GIF of these seasonal changes in action).
In this paper, scientists convert how satellites see lakes (in individual reflectance bands) into how humans perceive color. They could then look at how lakes change seasonally and over time. For example, Figure 1 shows seasonal lake color for three well-known US lakes: 1) Oregon’s Crater Lake is consistently blue, and shows no seasonal pattern, 2) Wisconsin’s Lake Mendota shows a seasonal pattern of ‘greening’ in the summer, and 3) Florida’s Lake Okeechobee has browner water and, in the last 12 years (2008-2020), has shown less seasonal change than it historically did. 
Topp’s results show that lakes in the US could be generalized into five distinct seasonal patterns that mapped onto well-known patterns in highly studied lakes. Lakes either 1) got greener in the spring, 2) became greener in the summer, 3) had spring and fall color changes, 4) stayed blue all year, or 5) stayed green all year.
Remotely mapping 26,000 lakes is impressive, but can we also use this dataset to investigate our favorite Wisconsin lakes? Of course we can. Figure 2 shows the overall color distribution of Lake Mendota and Trout Lake. Maybe no surprise here, Lake Mendota is often much greener than Trout Lake.
Topp’s research has a knack for taking complicated remote sensing data and producing intuitive results that provide a wealth of information that can be used by limnologists and non-limnologists alike. Topp noted “As the lake remote sensing community and satellites improve, large-scale analyses such as this one will move from simple metrics like color towards quantitative qualities like chlorophyll-a and dissolved carbon.” Satellites also provide equity in limnological mapping.
Although we hold our Wisconsin lakes close to our heart, global remote sensing provides much needed data to parts of the world that are vastly understudied. In the future, remote sensing might be the key to global limnology.
To learn more about lake water properties and light (color): https://youtu.be/TWuhKmT9dac
Research article: Topp et al. 2021 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020WR029123