25.03.2025New Publication: How Untangling Quenching Mechanisms Can Improve Fluorescence-Based Monitoring of Phytoplankton

With a microscopical size smaller than a human hair, most phytoplankton species are invisible to the human eye. But despite their size, these organisms play a vital role in aquatic ecosystems, as they form the foundation of the aquatic food web. Any changes in their population size can disrupt the balance of an entire aquatic ecosystem. Phytoplankton also contributes significantly to the regulation of the global carbon cycle. Like green plants on land, they take up carbon dioxide and release oxygen, thereby helping to regulate the Earth's atmosphere. The size and health of phytoplankton populations indicate how well aquatic ecosystems are functioning but also point to broader environmental changes linked to climate change. To study changes in phytoplankton populations on a global scale, researchers use remote sensing technologies, such as satellite data, to cover large extents of aquatic environments. But how can something as tiny as phytoplankton be detected from space?

One way scientists assess phytoplankton in water bodies is by detecting chlorophyll-a, a green pigment found in phytoplankton that allows them to capture sunlight for photosynthesis, the process by which light energy is converted into biomass. The concentration of chlorophyll-a provides important insights into the amount of phytoplankton populations. However, rather than measuring chlorophyll-a directly, researchers often estimate it by using a faint light emitted by phytoplankton. This phenomenon, known as sun-induced chlorophyll fluorescence (SIF), occurs when chlorophyll-a re-emits a small part of the absorbed sunlight as a subtle glow. This glow adds to the reflected light by water and can be detected by sensitive satellites from space. This method has been used in the past to study vast, open waters, such as oceans.

However, it has rarely been applied to inland water bodies like lakes. One reason is that these waters are considered "optically complex". In other words, many other factors, like suspended sediments or colored dissolved organic matter found in lakes, can interfere with the light signals measured by satellites and complicate their interpretation. To address this gap, a team of researchers led by Remika Gupana, a former member of two inter-institutional remote sensing research groups at UZH and Eawag, has explored how to improve the use of fluorescence as a chlorophyll-a estimate, even in complex waters. To do this, they took a closer look at a mechanism called quenching, which affects the relationship between chlorophyll-a and fluorescence estimates.

Just like a lamp that is dimmed to save energy or prevent overheating, phytoplankton can regulate how they use sunlight. When phytoplankton absorbs light for photosynthesis, not all that energy is used to help the organism grow. Some is released as heat, and some is emitted as light (fluorescence). However, the intensity of this emitted light can vary, depending on whether the energy is being used for its original purpose - photosynthesis (called photochemical quenching or PQ) or safely released as heat to protect the cells (called non-photochemical quenching, or NPQ). Understanding which type of quenching is occurring is important because it affects how much of that faint fluorescent glow can be observed. Without making this distinction, satellite signals can be misinterpreted, leading scientists to underestimate or overestimate the amount of chlorophyll-a, and therefore phytoplankton, present in the water.

To better understand how these quenching mechanisms affect the fluorescent signals detected from space, the researchers used Lake Geneva as a test site. The researchers collected data over a four-year period from the floating research platform LéXPLORE. This platform is equipped with instruments that monitor changes in water conditions and related water optical properties. By combining in situ measurements of chlorophyll-a with spectroscopy data, the team could test two different methods for estimating fluorescence from satellite measurements and compare how well they relate to actual chlorophyll-a levels under different quenching conditions. One commonly used fluorescence retrieval method is called FLH (Fluorescence Line Height) and estimates fluorescence by measuring a small increase in the measured light signal at a specific wavelength associated with the fluorescence emission. Another method is the FPH (Fluorescence Peak Height) approach which uses a more detailed model to separate fluorescence from light reflected by water, making it especially useful in complex water environments. The investigation of retrieved fluorescence together with chlorophyll-a measurements allowed the team to observe how shifts between PQ and NPQ affect fluorescence detected by satellites.

The study showed that the relationship between fluorescence and chlorophyll-a concentration becomes much clearer when the two quenching mechanisms are differentiated. By separating cases where absorbed light was mainly used for photosynthesis (PQ) from those where it was mostly released as heat (NPQ), the researchers could demonstrate that the accuracy of SIF-based chlorophyll-a estimates largely improves. The study also found that phytoplankton tend to switch from PQ to NPQ at higher light levels. The researchers identified a transition zone of light intensity, where this shift happens. Recognizing this shift ultimately helps explain why fluorescence signals can vary, even when chlorophyll-a levels stay the same.

While these results are promising for the in situ measurements, applying them to satellite data proved more difficult. The team tested various data processing methods for Europe’s Sentinel-3 satellite, but none could fully match the precision of in situ measurements. That is largely because the fluorescence signal from phytoplankton is extremely faint, and current approaches to analyze satellite data face methodological challenges in compensating atmospheric effects in measured data. The authors suggest that this challenge can be overcome in the future: with advances in sensor technology providing more detailed spectral data, and optimized methodology, researchers will be better equipped to observe and understand changes in phytoplankton populations from space.

This study is the result of a close collaboration between the two remote sensing research groups at UZH and Eawag. By sharing their expertise and pooling resources, the teams were able to realize an innovative project that builds on a long-standing partnership.