Diatoms of North AmericaThe modus operandi of this blog from the start has been to write in a “natural history” style about aspects of the natural world that are otherwise only discussed using the opaque language of hard science. My rationale is that the state of our water is now highly politicised, but most of the organisms that inhabit our rivers and lakes – and whose presence and abundance determine their condition - cannot be seen with the naked eye. We feel an emotional attachment to our rivers and lakes, but have little comprehension of the biodiversity that they contain. There is, as a result, a disconnect, which recalls Baba Dioum’s quotation, “In the end we will only conserve what we love, we will love only what we understand, and we will understand only what we are taught”. My posts during 2026 therefore will try to correct this, with overviews of some of the most common types of microscopic diatoms, one of the most abundant groups of microscopic organisms.
In the middle of the 17th century, a Dutch draper called Anton van Leeuwenhoek walked a few kilometres from his home in Delft to a pond, took a sample of water from this pond and looked at it under one of the primitive microscopes that he had constructed. The Dutch at this time were at the forefront of exploration, and he may well have encountered sailors would have been arriving at Delft with tales of the exotic lands that they had visited. Yet what van Leuwenhoek saw under his microscope was stranger by some orders of magnitude than any of these tales. His gift to humankind was awareness of a hitherto hidden aspect of nature: exotica right on our own doorsteps.
You can reconstruct van Leuwenhoek’s journey by visiting a stream or pond close to your own home. Pick up a submerged stone and run your finger across the surface. The likelihood is that it is slimy to the touch. Now take a toothbrush (not available in van Leuwenhoek’s day!) and brush some of this slimy layer into a bottle or jar. Add some stream water and take it home. You have, in your hands, a veritable zoo of exotic organisms, the only limitation being that most are too small to be seen with the naked eye. The next step, then, is to put a drop of this suspension of slime and stream water under a microscope and have a look at it when magnified at least 100 times.
Navicula antonii from the River Teme at Powick Bridge, July 2025. Scale bar: 10 µm (photos: Lydia King).
My guess is that you will see at least a few boat-shaped cells containing yellow-brown structures gliding around. These are diatoms from the genus Navicula. To see them properly you will need to magnify them to 1000x but lower magnifications will let you see enough to be intrigued. My rationale for telling you the name of an organism in a sample that I have not examined is simply that Navicula is one of the most common genera of diatoms in streams and ponds and that their boat-shaped outline is one of their characteristic features, along with their capacity to move around.
The genus Navicula was formally described in 1822, as befits a genus with some relatively large* members that would have been conspicuous with the relatively primitive microscopes available at the time. It was, for a long time, used as a catch-all for all diatoms that were approximately boat-shaped but 19th century microscopists were quick to see differences amongst these and to separate these as species in their own right. Navicula quickly grew intio a large, sprawling genus found across both freshwater and marine environments.
Navicula capitatoradiata from the River Teme at Powick Bridge, July 2025. Scale bar: 10 µm (photos: Lydia King).
The rise of the Navicula supergenus was aided by the practice of identifying diatoms using just the characteristics of empty silica cases (“frustules”). This meant that some important features such as the number and shape of chloroplasts was lost as samples were oxidised to remove organic matter before they were examined. Identification was also limited by the capacity of optical microscopy, which meant that some of the very fine features of the diatom frustule were not revealed until scientists turned to electron microscopy in the 1960s. By 1986, the genus sprawled across 152 pages and 57 plates of the Süsswasserflora von Mitteleuropa but the publication of this book also coincided with the implications of studies with electron microscopes bearing fruit. In the four decades that followed, many of the species recognised as “Navicula” in this book were hived off into separate genera (sometimes resurrected genera created in the 19th century but subsequently subsumed into Navicula). The slimmed down descriptions of “true” Navicula occupies just 38 pages and 14 plates of the Süsswasserflora.
Navicula cryptotenella from the River Teme at Powick Bridge, July 2025. Scale bar: 10 µm (photos: Lydia King).
Even after this balkanization, Navicula remains a large and widespread genus, so there is a strong possibility of finding at least a few specimens on a slide (especially if the sample comes from a lowland area). Under the right conditions, they can be the most abundant diatom in a sample and some species form patches that are visible with the naked eye (see “The ecology of cold days …”). These patches are most common in late winter and early spring, but other species thrive in summer and autumn, and there can be a succession of species even at a single site.
A summary of the ecology of Navicula might be that it is a genus of lowland, well-buffered moderately-enriched streams and rivers, and littoral zones of ponds and lakes. However, Navicula is such a large genus that, whilst generalisations are possible, there are always exceptions. A few species have a distinct preference for low nutrients (e.g. Navicula angusta, Navicula notha) and for very soft water (e.g. Navicula leptostriata) and there are several species that are associated with brackish conditions (e.g. Navicula bottnica: see: An excuse for a crab sandwich, really …”). Moreover, “true” Navicula are not abundant in water where there is very heavy organic pollution. However, between these extremes, this still leaves a wide range of freshwater habitats where Navicula can thrive, often with several species occurring simultaneously.
Navicua radiosa from a shallow calcareous pond near York. Scale bar: 10 µm
Though the traditional approach to diatom ecology has been to define species and genera in terms of their preferences for a chemical environment, the genus as a whole is, I suspect, defined by the ability to adjust its position within the slime layer that coats stones. A film that is a millimetre thick is equivalent, from the perspective of a typical Navicula, to a 25-story building crowded with other microorganisms all competing for a limited oxygen supply whilst pumping out their own waste products. Being able to move upwards allows Navicula to access the sunlight it needs for photosynthesis as well as to avoid the fetid conditions lower down.
Classic ecological theory (the “competitive exclusion principle”) would suggest that two closely-related species should not thrive in the same habitat. The reason why we often find several diatoms from the same genus in a sample is most likely not that they are, strictly, “sharing” the habitat, but that they have preferences for microhabitats but that our routine sampling methods are too crude to enable us to separate these. This separation may be in time as well as in space, as the turbulent world of a streambed will mean that adjacent stones are turned over and algae attached to them are scoured or grazed off in different ways, such that each has a slightly different “history”, allowing different diatoms to thrive on each.
There is, in short, a lot more that we don’t know about Navicula than that we currently do. It offers a fertile field for informed research into functional ecology rather than just matching forms to simplistic measures of water chemistry.
* “large”, in this context, means at least a 20th of a millimetre long.
