Urban streams: What are diatoms trying to tell us?
How often, as we walk through cities, do we come across small streams and channels crossing the urban landscape? At first glance, many of these environments may seem like little more than altered channels, characterized by the removal of riparian vegetation, channel straightening, bank sealing, and alterations to their natural characteristics. But these environments have a story to tell.
Urbanization involves not only the physical transformation of habitats. Urban areas are also important sources of multiple environmental stressors, including nutrients, organic matter, metals, and other contaminants associated with surface runoff. Yet, even in these environments, which may appear inhospitable, life persists. Organisms continue to inhabit these streams, although they are exposed to conditions that may represent a constant challenge to their survival and persistence. Among them are diatoms!
In 2023, when I started my master’s degree, this became my challenge! To study diatoms along an urbanization gradient in Aveiro, Portugal. The question was simple, but it had not yet been addressed in the region. What happens to diatom communities as the city grows?
Figure 1. Pictures of selected sampling sites representing streams along the urbanization gradient. Most sites show reduced or absent riparian vegetation and concrete structures along the stream margins.
After a couple of months of examining diatom slides, we identified a community composed of 222 species. Quite diverse considering the environmental conditions, I would say. We also observed that, as urbanization increased, sensitive species became less abundant, while more tolerant species increasingly dominated the community under urban conditions.
However, one result particularly caught our attention. Alongside changes in community composition, we observed an increase in the occurrence of diatoms displaying teratological forms in the more urbanized areas. These abnormalities included irregular cell outlines and changes in striation patterns, indicating that, in urban areas, the effects of environmental conditions were not restricted to shifts toward more tolerant taxa, but could also manifest at the individual level through the occurrence of teratologies. Interestingly, we found that environmental factors associated with the occurrence of teratological forms varied seasonally. In winter, nitrate stood out as the main environmental factor associated with teratological occurrence, whereas in summer, metal concentrations in sediments emerged as the most relevant predictors.
Figure 2. Light micrographs of diatom valve views (normal and teratological). Scale bar = 10 µm. (a–d) Planothidium frequentissimum: (a) normal valve; (b–d) teratological valves. (e–h) Platessa oblongella: (e) normal valve; (f–h) teratological valves.
For more details, read the full study published in Inland Waters.
Schmitt, R., Feio, M. J., & Almeida, S. F. P. (2026). Seasonal effects of urbanization on diatom communities: a taxonomic and functional approach to stream ecological quality assessment. Inland Waters, 16(1). https://doi.org/10.1080/20442041.2025.2549543
Hello everyone! My name is Rafael Schmitt, and I am currently a research fellow at the University of Aveiro, Portugal, working under the supervision of Professor Salomé Almeida. My passion for freshwater diatoms began during my master’s degree, and since then, my research has focused on understanding how environmental changes shape diatom communities. I combine morphology-based assessments with, more recently, metabarcoding approaches to investigate how land-use and land-cover changes and environmental filters influence diatom diversity, community composition, and ecological responses.