As 2019 draws to a close, I have looked back at all the data I have collected from Castle Eden Burn over the past twelve months. I chose this location precisely because it was different to my usual haunts and, despite having visited this Dene and others along the Durham coast for over thirty years, I realised that I had never had a look at the algae. Dry river beds are not the most obvious hunting grounds for aquatic biologists, after all. This year, I put that right over the course of a number of visits between January and November and in this post I am summarising what I found.
I found a total of 77 different diatoms in the six samples that I collected, not to mention green and yellow-green algae (see “When the going gets tough …”) and mosses (see “A thousand little mosses …”). Of these diatoms, 48 were rare and infrequent, only found in one or two samples, and never forming more than one percent of the total number of diatoms present. Of the remainder, only two were found in every sample (Humidophila contenta-type and Achnanthidium minutissimum) whilst another eight formed at least ten percent of the total on one occasion. Numbers of each species waxed and waned over the year: Humidophila contenta-type was abundant in the sample from my first visit in January 2019 but relatively scarce thereafter. In comparison, Luticola frequentissima was very abundant on two occasions (more than 80% of individuals), quite abundant on three other occasions but absent from the sample from my final visit in November.
Some of these differences are due to the variable flow regime: the stream was dry on three occasions, ponded on one and flowing on just two occasions. Those occasions when there was no running water were those when the proportions of diatoms that are tolerant to desiccation (see “Life out of water …”) were most abundant, forming from 20 to 97 percent of all individuals. When there was running water, it was motile Nitzschia species that dominated. In fact, there was a strong negative correlation between proportions of desiccation-tolerant and motile taxa in the samples, indicating that the diatoms responded rapidly to the changing pressures experienced in the stream. There was also a relationship between the proportions of desiccation-tolerant diatoms and the number of taxa recorded – the latter is a good measure of the level of physiological stress experienced in a stream.
What of the diatoms themselves? Humidophila contenta-type was one of the two ever-presents. It is, however, very small (few of those in our samples were more than a 100th of a millimetre long), making it difficult to photograph and, indeed, to discern many of the features of the valve. This species sometimes forms short chains though I did not see any in the Castle Eden Burn samples. It is strange to think that, when I first started to identify diatoms, this was considered to be part of the genus Navicula. Since then, it has moved into the genus Diadesmis before finally being transferred to the new genus Humidophila by Rex Lowe and colleagues in 2014. Some recently-described Humidophila species cannot be differentiated from H. contenta without a scanning electron microscope, so I have referred to this as “Humidophila contenta-type”.
Humidophila contenta ag. from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres. Photograph: Lydia King.
The most abundant diatom in samples collected during the dry periods was Luticola frequentissima. I started the year referring to this as “Luticola mutica” but was gently corrected by colleagues more au fait with recent literature than me. Luticola mutica is larger (length: 11-28 µm; breadth: 6-9.5 µm) and has more widely-spaced striae (16-18 / 10 µm) than L. frequentissima (length: 7 – 13.8 µm breadth: 4.8 – 6.8 µm; striae: 20 -24 / 10 µm). The specimens in the plate below all fit the description for L. frequentissima. Some of the large specimens have size ranges that overlap with L. mutica (though even the largest specimen as a striae density consistent with L. frequentissima). L. mutica is associated with more brackish habitats whilst L. frequentissima prefers freshwaters.
Luticola frequentissima from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100th of a. millimetre). Photographs: Lydia King.
Simonsenia delognei is another characteristic species of habitats that dry out periodically. This species, which is in the same family as Nitzschia, is quite small and only lightly silicified so easily overlooked. It was common early in the year, but rare thereafter. Whether this is a real characteristic of the species or an artefact of the conditions in Castle Eden Burn this year is difficult to tell as it is not a particularly common species so there are few other records against which this trend can be compared.
Simonsenia delognei from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.
Two other species of Nitzschia were common: I illustrated N. clausii in “Out of my depth …” and have included photographs of N. sigma here. I’m intrigued that two of the most conspicuous Nitzschia in this sample are sigmoid in outline. I’ve visited the question of sigmoid diatoms before, and still don’t have any good explanation why a few diatoms have this outline (see “Nitzschia and a friend …”). Note, too, that Nitzschia species can be sigmoid in valve view (i.e. looking down from above) or girdle view (i.e. looking from the side), although the great majority of species are straight in both planes.
Nitzschia sigma from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100th of a millimetre). Photographs: Lydia King.
Finally, one more relative of Nitzschia that was found in a couple of samples, but never in large numbers, was Tryblionella debilis. The genus Tryblionella was treated as part of Nitzschia for much of the 20th century. As it appears to form a natural group with some distinctive characteristics, it is now generally treated as a distinct genus, although the molecular evidence indicates a complicated evolutionary history. The principle characteristic of the genus is a longitudinal undulation on the valve face that is most clearly manifest on those species in the genus which have visible striae. T. debilis is a small species with striae that are not resolvable with the light microscope; however, the undulations are just apparent as faint longitudinal lines running along the valve face.
Tryblionella debilis from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100th of a. millimetre). Photographs: Lydia King.
That’s a lot of diatoms from a stream that is not always a stream. I am sure that someone with interests in other groups of algae could probably make similarly long lists for some of those, and a more thorough exploration of habitats within the stream could add to the number of diatoms. That’s before suggesting a molecular study, which might well reveal cryptic diversity (i.e. significant taxonomic variation that is impossible to discern with a light microscope) within the species I have already described. The greater our capacity to unravel the mysteries of the microscopic world, the more, it seems, we discover we don’t know.
Lowe, R.L., Kociolek, P., Johansen, J.R., Van de Vijver, B., Lange-Bertalot, H. & Kopalová, K. (2014). Humidophilagen. nov., a new genus for a group of diatoms (Bacillariophyta) formerly within the genus Diadesmis: species from Hawai’i, including one new species. Diatom Research 29: 351-360.