I’ve written many posts over the years about algae that grow in lakes, mostly based on samples I’ve collected whilst paddling around in the shallow littoral zone. One feature that I have not previously mentioned is that the attached algal flora changes quite dramatically as you descend into the deeper parts of the lake. These habitats have only been thoroughly explored since SCUBA diving became possible in the middle of the twentieth century. Before then, what was known of underwater algae came from chunks of rock dredged up from depths in nets.
This was the case in Lake Constance, which straddles the borders of Germany, Austria and Switzerland and, in particular, in the northern arm, the Überlinger See, where there are steep underwater cliffs composed of Molasse (a sedimentary sandstone/shale association). Studies by Lauterborn and Zimmerman revealed crusts and mats smothering these cliffs and extending to great depths. As you descended from the shallower, lighter water to the depths the composition shifted from predominately green algae to brown and red algae. Zimmerman captured these changes in a sketch of the algae on a ledge on the cliff wall at about fifteen metres depth, showing the red alga Hildenbrandia rivularis tucked away in the shady recesses whilst Cladophora profunda (now regarded as a synonym of Aegagropila linnaei, which we met in “A load of balls …”) was more abundant on the lighter, more exposed faces. The big interest, though, were the extensive growths of a freshwater brown alga, Bodanella lauterbornii.
The Phaeophyta – brown algae – are much more common in marine environments than in freshwaters. A few species can be found but these form small, inconspicuous growths that are frequently overlooked. Lauterborn and Zimmerman’s record of a rare brown algae in the depths of Lake Constance (it has only been recorded in three other lakes in Austria) came to light during the preparation of a Red List of algae in Baden-Württemberg and led to a search to see if it was still present.
The study proved fruitless, with all the submerged cliffs covered not by algae but by extensive growths of invasive mussels, mostly zebra mussels, Dreissena polymorpha. My illustration tries to convey the extent to which zebra mussels smother the surfaces but is still an underestimate, compared to the photographs taken by divers in this study, which show mussels filling all available spaces and often growing over each other. Zebra mussels were first recorded in Lake Constance in the 1960s and it is possible that the algal assemblages described by Zimmerman were already declining at this point. In the period since the original surveys, Lake Constance received considerable nutrient enrichment which would have boosted phytoplankton growth, in turn reducing the limited light available to the deep water algae yet further. So we can envisage a two-stage decline: first gradual darkening of an already very shaded habitat followed by a coup de grace of mussels covering all available surfaces.
The absence of the deep water algal assemblages from present day Lake Constance means that my painting of the 1920s-era Molasse cliffs is very much “imagined”, in the tradition of Henry de la Beche and his followers, as described in my previous post. Wolfgang Schūtz, Lydia King and Marco Cantonati, who had collaborated on the recent resurvey thought that an artist’s impression of the Molasse cliffs then and now might be a useful addition to the English-language report of their work currently in preparation. We had Zimmerman’s original diagram of the arrangement of algae on a rock crevice to work from but that gave us no idea of colour or of the lateral extent of the patches. Did the Bodenella, for example, form isolated colonies or continuous mats? I put together some sketches based on my own interpretations then the others critiqued these and I used their comments to produce new versions. After two or three iterations, we had an arrangement with which we all agreed. Whether this is an accurate portrayal is another question: all we needed to agree was that was a plausible arrangement, given what we know of these organisms from our own experience.
Our last encounter with Aegagropila linnaei in this blog was as balls of filaments in a lake in Ireland but it can alos be found forming mats or turfs of interwoven filaments on stones in streams, so I went with this growth form in the illustration. Zimmerman’s diagram suggests it had a fringe of filaments trailing in the water but this is not my experience of A. linnaei in rivers, so I left them out. You can decide: it’s a case of his artistic license versus mine.
But what do we know of Bodenella lauterbornii, a rare, deep water brown alga? None of us had seen it living so my colonies are based on another brown alga, Heribaudiella fluviatilis. Microscopically, these look quite different, with Heribaudiella having tightly packed erect filaments composed of just a few cells whereas Bodenella seems, from illustrations, to have longer, more loosely-packed filaments. However, one study has suggested that these species may be genetically identical so it is possible that the difference in growth form and habit is due to the habitat. I’ve depicted it as being quite dark brown, suggesting more accessory pigments than you would find in algae closer to the surface, in order to harvest as much of the meagre light as possible. That raises another point: both pictures are painted much brighter than would really be the case at fifteen metres depth. At best, both would depict a spot of Caravaggio-esque brightness (the diver’s torch, in this case, rather than a candle) amid the stygian gloom of Lake Constance’s depths.
It is not exactly ichthyosaurs chasing plesiosaurs but the essence of this picture is the same as in Duria Antiquior: lost worlds are as likely to be a hundred years old as 100 million years old and the evidence we have rarely tells us the whole story. A leap of imagination, seasoned with knowledge and experience, is always necessary to bring lost worlds to life.
Eloranta, P., Kwandrans, J. & Kusel-Fetzmann, E. (2011). Die Sūsswasserflora von Mitteleuropa 7: Rhodophyceae and Phaeophyceae. Springer Spektrum, Heidelberg & Berlin.
Lauterborn, R. (1922): Die Kalksinterbildungen an den unterseeischen Felswänden des Bodensees und ihre Biologie.– Mitteilungen des badischen Landesvereins für Naturkunde und Naturschutz 8: 209-215.
McCauley, L.A.R., Wehr, J.D., 2007. Taxonomic reappraisal of the freshwater brown algae Bodanella, Ectocarpus, Heribaudiella, and Pleurocladia (Phaeophyceae) on the basis of rbcL sequences and morphological characters. Phycologia 46: 429–439.
Schütz W., King L., Cantonati M. & Leist N. 2020. Algenbestände an den Molasse-Steilwänden des Überlinger Sees (Bodensee) – früher und heute. Carolinea 78: 5-18.
Zimmermann, W. (1927): Über Algenbestände aus der Tiefenzone des Bodensees. Zur Ökologie und Soziologie des Tiefseepflanzen. – Zeitschrift für Botanik 20: 1–28.
Wrote this whilst listening to: Ancient Heart by Tanita Tikaram, taking me back to the early 1990s.
Cultural highlights: More nostalgia: watched Shakespeare in Love on the iPlayer
Currently reading: Washington Black by Esi Edugyan
Culinary highlight: Chocolate, lime and Sichuan pepper cake.