Reflections from Ennerdale’s Far Side …

Ennerdale Water is, as I have described in earlier posts, is a lake of two halves, with a south eastern end influenced by granite and the north western end by softer mudstones and sandstones.  That has a big effect on the algae that we find in the littoral zone, with Cyanobacteria (blue-green algae) abundant in the south-east end and Chlorophyta (green algae) more conspicuous at the other end.   Diatoms are conspicuous in the littoral zone all around the lake, although there are some differences in the types of species encountered.  That is a story for another day, but I did find one species in some of the samples I collected from the south-eastern end that point to one other influence on the ecology of Ennerdale’s littoral zone.

Look at the photograph at the start of this post.  It was taken as I walked up to the south-eastern end (circa NY 127 140) and shows the view up the lake, with Angler’s Crag visible on the left hand shore in the distance.   The River Liza enters the lake on the right hand side (just out of the frame) and the low lying area between the River Liza and the raised ground where I was standing is an area of wet heath with a range of Sphagnum species and several boggy pools.   The shoreline of the lake itself is formed by a shingle spit which acts as a barrier between the wet heath and the lake itself.

The shingle spit separating the wet heath at the south-east end of Ennerdale from the lake itself.   Photographed in January 2017.

Several of the diatoms that I found at this end of the lake were species that I associate with acid conditions although, curiously, the limited chemical data that we have does not show a lower pH here than elsewhere in the lake.   I suspect that the proximity to the acid Sphagnum heath may lead to occasional pulses of acid water entering this area and exerting a subtle effect on the attached algae before being diluted by the water of the lake as a whole.   Of the species that I found, the most intriguing was Stenopterobia sigmatella, a long, sigmoid diatom with a single plate-like chloroplast.

The genus Stenopterobia fulfils most of my criteria for a genuinely rare diatom (see “A “red list” of endangered British diatoms”).   I only have 11 records in my dataset of 6500 samples, and in only one case did Stenopterobia constitute more than one percent of the diatoms in the sample.   These samples are all from acid habitats (mean pH: 6.1), with low nutrient concentrations (never more than 2 mg L^-1 reactive phosphorus).  Those for which we have location information are plotted below.   The record in East Anglia needs further investigation (meaning: “I don’t believe it … but I haven’t had a chance to track down the slide for a closer look”). If we ignore this, the distribution is confined to mountainous regions of western Britain, and these Ennerdale samples also fit this trend, although the lake has soft water and is circumneutral rather than acid.

Stenopterobia sigmatella is another diatom with a sigmoid outline, and this brings me back to a question that I have posed before (see “Nitzschia and a friend …”): what advantages does a sigmoid outline confer on a diatom?  I cannot think of any other genera of algae that has species with a sigmoid outline, which only adds to the mystery. All of the diatoms that are sigmoid are motile, so I guess that the explanation may be linked to movement, but I don’t know for sure what the reason may be.   For all of the rich diversity that we see in diatoms, there is still, to pick up on a phrase from my biography of Humboldt, a “poverty of meaning” …

Stenopterobia cf sigmatella from Ennerdale Water, October 2016 and January 2017.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).

A distribution map of records of Stenopterobia in Great Britain.   S. curvula is a synonym for S. sigmatella (see taxonomic note below).  Map prepared by Susannah Collings (see “Why do you look for the living among the dead?” for more details of how this was done)

A valve of Stenopterobia densestriata.  Photograph from the ADIAC database (photographer: Micha Bayer).  Scale bar: 10 micrometres (= 1/100^th of a millimetre).

Taxonomic note

I have used the name “Stenopterobia sigmatella” in this post, but this still needs confirmation as there is a closely-related species, S. densestriata (Hustedt) Krammer 1987 (see image above).  S. sigmatella has < 24 striae in 10 micrometres whilst S. densestriata has > 26 striae in 10 micrometres.  S. densestriata also has slightly smaller overall dimensions.

David Mann made the following comment about Stenopterobia sigmatella on the website Common Freshwater Diatoms of Britain and Ireland (predecessor to the new Diatom Flora of Britain and Ireland: “A nomenclatural mess. For most of the 20th century, this species was referred to (wrongly) as S. intermedia. Ross (in Hartley, 1986) stated that there is an earlier name, sigmatella, that could be applied to this species and made a new combination S. sigmatella. Unfortunately, this was wholly ignored by Krammer (in Lange-Bertalot & Krammer, 1987; and see Krammer & Lange-Bertalot, 1988) who made the new combination S. curvula. However, Nitzschia curvula of W. Smith is preceded by N. sigmatella of Gregory (1856, 1854, respectively).”   The references can all be found on the Common Freshwater Diatoms website.