How François-Alphonse Forel Founded Limnology
The revolutionary vision of a Swiss scientist who wove together biology, chemistry, physics, and geology to create the study of inland waters.
Explore the StoryImagine a single body of water—a lake—seen through the lens of every scientific discipline at once. This was the revolutionary vision of François-Alphonse Forel, the Swiss scientist who, in the late 19th century, wove together threads of biology, chemistry, physics, and geology to create an entirely new field: limnology, the study of inland waters 7 .
Born in 1841 in Morges on the shores of Lake Geneva, Forel was a medical doctor by training. However, his true passion was the vast lake at his doorstep 7 .
"He realized that to understand a lake, one could not study these aspects in isolation; they were all interconnected parts of a single, complex system."
Among Forel's most ingenious and enduring contributions is the Forel-Ule scale, developed in collaboration with German limnologist Wilhelm Ule 5 . In an era before digital sensors and satellite imagery, Forel needed a simple, reliable method to quantify the apparent colour of lake and ocean waters.
The Forel-Ule scale is a set of 21 coloured liquid vials, created by mixing stable inorganic salts like copper sulfate, potassium chromate, and cobalt sulfate in precise recipes 5 6 .
To use the scale, an observer lowers a white Secchi disk into the water and, at half the depth at which the disk disappears from view, compares the colour of the water above the disk to the set of vials 5 . The matching number becomes the water's Forel-Ule Index (FUI).
The 21 colors of the Forel-Ule scale from blue (oligotrophic) to brown (eutrophic)
| Forel-Ule Colour Group | Typical FUI Range | Common Water Constituents | Indicated Water Type |
|---|---|---|---|
| Blue | 1 - 3 | Low concentrations of all substances | Oligotrophic (Nutrient-poor) |
| Green | 4 - 10 | Higher levels of phytoplankton | Productive, potentially eutrophic |
| Yellow/Brown | 11 - 21 | High CDOM or mineral turbidity | Humic (organic-rich) or sediment-laden |
This colour is not merely aesthetic; it is a visual integrator of the water's contents. The specific hue depends on the concentration and type of optically active substances in the water, primarily phytoplankton, coloured dissolved organic matter (CDOM), and inorganic suspended particulate material 3 .
Forel's scientific curiosity was not confined to water. He was also a renowned myrmecologist—a scientist who studies ants. In the 1880s, he conducted a fascinating series of experiments to understand how ants communicate, particularly how they distinguish friends from foes 2 .
He gathered a large number of ants from different species and colonies.
He carefully removed the antennae from a significant number of these ants. Antennae are the primary sensory organs for ants, crucial for touch and smell.
He placed the antennae-less ants in a box with other ants and observed their behaviour, comparing it to the normal, aggressive interactions typically seen between ants from different colonies.
The results were striking. Forel found that ants without antennae were no longer aggressive towards ants from other colonies 2 .
This was a critical discovery. It confirmed his hypothesis that ants use their antennae to detect chemical signals (pheromones) or other cues that allow them to recognize nestmates and identify intruders. This "contact theory" of communication was a fundamental contribution to insect psychology and sensory biology 2 .
Hypothetical visualization of ant aggression levels in Forel's experiment
"While this experiment was on ants, it exemplifies the meticulous, observational approach that Forel applied to all his research, including his limnological work."
Forel's work is far from a historical relic. Modern technology has breathed new life into his methods, particularly the Forel-Ule scale.
Today, scientists use satellites like MODIS and Sentinel-2 to derive the FUI over vast areas of the global ocean and countless inland lakes 1 . By analyzing the "water-leaving radiance" detected by these satellites, researchers can calculate the tri-stimulus values that correspond to the 21 colours of the Forel-Ule scale 3 .
This allows for high-frequency, large-scale monitoring of water bodies that would be impossible with field observations alone. For instance, a 2024 study used MODIS satellite data to reveal the water colour evolution in China's large lakes over the past two decades, tracking changes driven by climate and human activity 1 .
Hypothetical satellite data showing FUI changes over time in a large lake system
The simplicity of the FUI has also made it a perfect tool for citizen science. Projects like the "Citizen's observatory for Coast and Ocean Optical Monitoring (Citclops)" and the "Eye on Water" app empower the public to contribute valuable environmental data 3 5 .
By using a handheld scale or even a smartphone app to report water colour, citizens can help scientists build a dense network of observations, improving our understanding of local and global water quality changes 3 .
| Tool | Description | Key Advantage |
|---|---|---|
| Handheld FU Scale | Traditional set of 21 coloured vials or a modern plastic scale with lighting filters 5 | Low-cost, easy to use, no power required |
| Satellite Remote Sensing | Algorithms convert satellite-measured reflectance into the FUI 1 | Global coverage, long-term data records |
| Smartphone Apps | Mobile applications that allow users to estimate water colour 3 5 | Democratizes data collection, high spatial coverage via crowdsourcing |
Forel's research, spanning limnology and myrmecology, relied on both simple tools and innovative creations.
| Tool/Material | Function in Research |
|---|---|
| Secchi Disk | A white disk lowered into the water to measure transparency (Secchi depth) and provide a background for determining water colour with the FU scale 5 . |
| Forel-Ule Kit | The set of 21 pre-prepared coloured solutions used as a reference for classifying the apparent colour of a water body 5 6 . |
| Chemical Salts (Copper Sulfate, Potassium Chromate, Cobalt Sulfate) | Used in precise recipes to create the stable, coloured solutions for the Forel-Ule scale 5 . |
| Microscope | Essential for examining the tiny plankton and microorganisms he discovered in lake water, forming the base of the lacustrine food web 8 . |
| Ant Colonies | The subject of his myrmecological studies; he observed their social structures and communication methods 2 . |
Forel used precise chemical mixtures to create his color scale
Systematic observation of physical, chemical and biological parameters
Detailed study of ant behavior and communication
François-Alphonse Forel was a true pioneer. He looked at Lake Geneva and saw not just a body of water, but a dynamic, integrated microcosm. His creation of limnology provided a new framework for understanding the lakes, reservoirs, and ponds that are vital to human societies and global ecosystems.
The Forel-Ule scale, a tool born of 19th-century ingenuity, continues to be relevant in the 21st century, bridging the gap between historic oceanographic data and modern satellite technology and citizen science 3 6 .
"Forel's legacy is a powerful reminder that sometimes, the most profound scientific insights come from seeing the connections between all things—from the chemical cues detected by an ant's antenna to the subtle shift in a lake's colour that signals a changing planet."
His work laid the groundwork for us to better monitor, understand, and protect our precious freshwater resources for generations to come.