This post is still mostly about the ecology of Lough Down, though it draws heavily upon photographs taken at Lough Cullin, in County Mayo, Ireland. Lough Down shares some characteristics with Lough Cullin, but more with Lake Wobegon. I suspect that the summers in Lough Down are not quite as long or as hot as those experienced around Lake Wobegon but, also, that a walker passing around the edge of Lake Wobegon would not see the round balls of algae that are a common sight along the shoreline of Lough Down. Maybe I’m wrong: if anyone in Minnesota knows differently, please do let me know.
From a distance, these look like an unsightly mass of algae cast up on the foreshore; it is only when you get close that you see that this mass is, in fact, composed of a large number of discrete spherical growths. You can see, in the photograph below, where Bryan Kennedy, my erstwhile Lough Down correspondent, has built a few of these into an Andy Goldsworthy-esque sculpture. These are “Cladophora balls”, a phenomenon encountered in lakes around the northern hemisphere. In Japan, they’ve even made their way onto postage stamps.
Aegagropila linnaei washed up on the shore of Lough Cullin, Co. Mayo, Ireland. The photograph at the top of the post also shows Lough Cullin (photographs: Bryan Kennedy).
Until relatively recently, as their name suggests, the alga from which these balls are formed, belonged to the genus Cladophora, a frequent subject on this blog (see “The pros and cons of cell walls” for a recent example). Like Cladophora, these are branched filaments composed of relatively large, multinucleate cells with a reticulate chloroplast. This was, however, recognised as a different species to Cladophora glomerata, the common species of enriched lowland rivers: Cladophora aegagropila. However, recent molecular studies have shown that it is not so closely related to Cladophora glomerata as its outward appearance suggests, leading to the resurrection of a very old name, Aegagropila linnaei.
In rivers, Aegagropila linnaei forms carpet-like growths of short filaments, not growing into the long wefts that we associate with Cladophora glomerata. However, C. glomerata can sometimes be profusely branched as well, so telling these two species apart both in the field and under the microscope can be tricky. One of the most useful characteristics is that the branches of Aegagropila are sub-terminal, meaning that they arise just below the end of the parent cell, rather than at the end, as is the case in Cladophora (see diagram below). It is strange that two such similar species in appearance are, in fact, not particularly closely-related. This is, however, an important distinction asA. linnaei prefers, as far as we can tell, less enriched conditions than C. glomerata.
Left: a section through a ‘Cladophora’ ball from Lough Cullin and, right, profusely-branched filaments of Aegagropila linnaei. Photographs: Bryan Kennedy.
Why does it form these distinctive spherical growths in lakes? I have not managed to find a paper that gives an authoritative explanation so here are a few possibilities, none mutually exclusive. First, filamentous algae that display apical growth and copious branching tend to form hemispherical growths if attached and spherical ones if not. We’ve seen that for Cyanobacteria such as Rivularia (see “More about Rivularia”) and Gloeotrichia (see “Rewriting history at Talkin Tarn”). Second, the constant ebb and flow in the lake littoral zone will create a physical stress on attached carpets of Aegagropila leading, eventually, to parts becoming detached. Third, the profuse branching that is characteristic of Aegagropila will mean that adjacent filaments will become entangled around another, creating a Velcro-type effect. Finally, the apices of the filaments will continue to grow towards the light, meaning that the free-floating balls gradually expand in size.
Aegagropila’s dislike of nutrient-rich conditions mean that the number of places where it is found has been decreasing over recent decades. It was, for example, recorded from several locations in the Netherlands in the past but not since 1967. There are records from in the UK, but mostly from the more remote regions. There are also a number of records from loughs in Ireland, as is the case here The river form is, in my opinion, hard to differentiate unequivocally from Cladophora glomerata without very careful examination and this raises the spectre of “identification by association”, particularly when it is recorded by macrophyte surveyors who often do not have time to check material under the microscope. Christian Boedeker, who has done much of the recent work on Aegagropila, thinks that a limited dispersal capability will mean that it will be slow to re-colonise habitats once it has been lost.
So that’s another day over here at Lough Down, a quiet lake that no-one has visited but everyone has got to know very well. It’s one of those places, I like to think, where naturalists notice all of nature, not just the pretty, cuddly and exciting things. Everyone leaves a little wiser, even if only because they have noticed that something everyone else overlooks is, actually, a thing of great intrinsic beauty. As Garrison Keillor himself once said: “Thank you, God, for this good life and forgive us if we do not love it enough”.
Diagrams of branching patterns in Aegagropila linnaei (a.) and Cladophora glomerata (b.). Note how the branches of A. linnaei arise just below the end of the cell (“sub-terminal”, indicated by arrows) whereas the branches of C. glomerata arise at the ends.
Boedeker, C., & Immers, A. (2009). No more lake balls (Aegagropila linnaei Kützing, Cladophorophyceae, Chlorophyta) in The Netherlands? Aquatic Ecology. https://doi.org/10.1007/s10452-009-9231-1
Boedeker, C., Eggert, A., Immers, A., & Wakana, I. (2010). Biogeography of Aegagropila linnaei (Cladophorophyceae, Chlorophyta): A widespread freshwater alga with low effective dispersal potential shows a glacial imprint in its distribution. Journal of Biogeography. https://doi.org/10.1111/j.1365-2699.2010.02309.x
Boedeker, C., Kelly, C. J., Star, W., & Leliaert, F. (2012). Molecular phylogeny and taxonomy of the Aegagropila clade (Cladophorales, Ulvophyceae), including the description of Aegagropilopsis gen. nov. and Pseudocladophora gen. nov. Journal of Phycology. https://doi.org/10.1111/j.1529-8817.2012.01145.x
Some other highlights from this week:
Wrote this whilst listening to: This post has been a long time in gestation, so ’ve listened to a lot. These included Bob Dylan’s Shot of Love, Infidels and Real Live, as well as Courtney Barnett’s A Sea of Split Peas and Arvo Pärt’s Tabula Rasa.
Cultural highlights: The National Theatre At Home’s Streetcar Named Desire, starring Gillian Anderson.
Currently reading: JK Rowling’s Harry Potter and the Philosopher’s Stone. Comfort reading.
Culinary highlight: I have to admit that fish, chips and mushy peas from Bells in Gilesgate was hard to beat.