How a Spanish secondary education professor laid the foundations for aquatic ecology in early 20th century Spain
In the early 20th century, as Spain navigated between tradition and modernization, a secondary education professor ventured into the rivers, lakes, and wetlands of the Spanish geography with a curious gaze and a sampling net. Celso Arévalo was not a conventional laboratory scientist but a pioneer of aquatic ecology when this discipline didn't even have a consolidated name in the Spanish scientific landscape1 . His work would lay the foundation for future generations to understand that beneath the surface of aquatic ecosystems lay complex biological relationships vital to environmental health.
"A river is not just flowing water, it is a universe to be discovered"
At a time when ecology was an emerging science and limnology (the study of continental waters) was just beginning to define itself as a formal discipline in Europe, Arévalo introduced in Spain a novel way of studying aquatic ecosystems that were then grouped under the label of hydrobiology1 . His legacy directly connects with today's science that seeks to understand and protect our valuable water resources against pollution, invasive species, and climate change.
When Celso Arévalo began his research in the second decade of the 20th century, Spain practically lacked systematic studies on its aquatic ecosystems. Ecology as the integrative science we know today was then a fragmented discipline in different specialties that received various names and lacked coordination1 .
Scientists like August Thienemann in Germany and François-Alphonse Forel in Switzerland had laid the foundations of modern limnology, but these advances were slow to cross Spanish borders.
Arévalo became the bridge that brought these ideas to Spain, adapting and developing them to study the Peninsula's aquatic ecosystems.
Arévalo preferred the term "hydrobiology" to describe his biological and ecological research in aquatic systems1 . This terminological choice was not trivial: it reflected his integrative approach that combined elements of biology, water chemistry, and physical geography to understand the overall functioning of rivers, lakes, and wetlands.
Arévalo's methodology was characterized by meticulous fieldwork and the systematization of observations. In an era lacking sophisticated technology, his main tool was sharp observational ability and meticulous sample collection. His research generally followed a rigorous protocol:
Arévalo's research faced significant technical challenges. Without today's electronic measurement equipment, parameters had to be determined through manual chemical tests and visual comparisons. Microscopes of the time had limited capabilities, and preserving samples for later study required creative solutions with basic chemical products.
Despite these limitations, Arévalo and his contemporaries laid the methodological foundations that would allow the development of aquatic ecology in Spain in the following decades, creating the first standardized protocols for studying freshwater ecosystems.
Among Celso Arévalo's many investigations, an integral study of a natural lagoon conducted between 1923 and 1925 stands out for its methodological relevance. This work represented the first documented attempt in Spain to completely characterize an aquatic ecosystem in all its dimensions. The procedure, reconstructed from his publications, followed these steps:
Choice of representative sampling points covering different areas of the lagoon (littoral, benthic, pelagic)
Sample collection in the four seasons of the year to capture seasonal variations
Measurement of parameters such as temperature at different depths, pH, dissolved oxygen and basic ionic composition
Sampling of phytoplankton, zooplankton, benthic macroinvertebrates, aquatic vegetation and fish
Identification and counting of species under microscope, establishment of trophic relationships
Integration of all data to understand the functioning of the ecosystem as a whole
The data obtained by Arévalo revealed for the first time in Spain the complexity and seasonality of continental aquatic ecosystems. His meticulously handwritten data tables showed how biological communities varied not only spatially within the lagoon but also throughout the year, responding to changes in environmental factors.
| Season | Phytoplankton (ind/ml) | Zooplankton (ind/l) | Dominant Species |
|---|---|---|---|
| Winter | 840 | 32 | Diatoms, Rotifers |
| Spring | 1,520 | 118 | Diatoms, Cladocerans |
| Summer | 2,850 | 95 | Cyanobacteria, Copepods |
| Autumn | 1,210 | 67 | Diatoms, Rotifers |
| Parameter | Winter | Spring | Summer | Autumn |
|---|---|---|---|---|
| Surface temp. (°C) | 8.2 | 15.7 | 24.3 | 16.8 |
| Dissolved oxygen (mg/l) | 10.8 | 9.2 | 7.5 | 8.9 |
| Transparency (m) | 1.8 | 1.2 | 0.7 | 1.4 |
| pH | 7.2 | 7.8 | 8.4 | 7.5 |
These results demonstrated the seasonal dynamics of planktonic communities, with abundance peaks in spring and summer, and how these fluctuations affected the entire ecosystem. Arévalo was able to correlate these biological changes with variations in temperature, nutrient availability, and sunlight.
Research in aquatic ecology, both in Arévalo's time and today, requires specific tools to decipher the secrets of aquatic ecosystems. Here are some of the fundamental solutions and reagents in this field:
Winkler method reagents allow quantification of available oxygen, a crucial parameter for aquatic life.
Formol and Lugol's solution are essential for preserving plankton samples for later identification.
Used for studying bacteria and algae in bioremediation projects2 .
Key for evaluating the trophic status of water (nitrates, phosphates).
Modern technology that allows species identification through genetics2 .
For detecting pesticides, antibiotics and biocides in aquatic environments2 .
The pioneering work of Celso Arévalo finds its continuation today in research such as that of the Aquatic Ecology and Biotechnology group at IMDEA Water, which addresses current problems like contaminant risk assessment, microbial ecology of aquatic ecosystems, and the impact of emerging pollutants on microorganisms2 . This line of research, although technologically more advanced, maintains the integrative spirit that Arévalo defended.
European projects like AQUACROSS (2015-2018) have developed methodologies for the integral management of aquatic ecosystems that reflect that holistic vision that Arévalo intuited decades ago6 . This project sought precisely to overcome the fragmented approach that still persists in water management, developing an assessment framework that considers all interactions between human activities and aquatic ecosystems.
| Era | Main Focus | Characteristic Methods | Key Figures |
|---|---|---|---|
| 1920s | Description and inventory | Direct observation, basic sampling | Celso Arévalo |
| 1960s-1980s | Ecosystem structure and function | Field and laboratory experimentation | Ramón Margalef |
| 21st Century | Integral management and conservation | Satellite technology, modeling, genomics | Carlos Duarte, Sebastian Villasante |
The integrative vision that Celso Arévalo brought to Spanish aquatic ecology finds its maximum expression today in interdisciplinary approaches that combine ecology, biotechnology, economics, and management policies. Contemporary researchers like Sebastian Villasante today develop work on transformations of marine socio-ecological systems and ocean equity that reflect this evolution5 .
The connection between Arévalo's early research and today's science is particularly evident in initiatives like that developed by the working group "Water Quality and Guideline Levels for the Protection of Aquatic Biodiversity" of the Aquatic Ecosystem Assessment and Monitoring Network (REM.AQUA-CONICET), which has developed methodologies to derive guideline levels for the protection of aquatic biodiversity3 . These modern protocols share with Arévalo's work the fundamental objective of understanding and protecting aquatic ecosystems, although they now have much more sophisticated tools.
Celso Arévalo represents that archetype of the pioneering scientist who works at the frontier of knowledge, opening paths where others only see familiar landscapes. His legacy reminds us that science advances both through technological sophistication and through the quality of ideas and insatiable curiosity.
The challenges facing our aquatic ecosystems today—from chemical pollution to climate change, including invasive species—require more than ever that integrative vision that Arévalo defended2 6 . His example inspires new generations of scientists who, armed with powerful tools, do not forget that to protect an aquatic ecosystem, one must understand it in all its complexity.
As Arévalo demonstrated with his work, every river, every lake, every wetland is a world to discover, a natural laboratory where physical, chemical, and biological processes intertwine in a delicate dance that we must learn to decipher to protect our natural heritage and, ultimately, our own well-being as a society.