Duckweed delights …

I’m hopping around in both time and space in recent posts.  The previous one was based on a visit to the Lake District in June whilst this one crosses the Pennines to report on a visit to Cassop Pond in May.   There is always a lag when studying diatoms, because of the preparation steps involved, but it has been longer than normal this time due to the pressure of other work.   In The Diatoms of Cassop Pond, I wrote that I had found 98 different species in samples collected in January, February and March.  Having looked at samples from April and May, this list has now been extended to 134 species but this is about more than just “twitching” because I’m also intrigued by how habitats around the lake margin differ in the diatoms they host.  

One of the most distinctive features of Cassop Pond are the extensive flocs of the liverwort Riccia natans, and I’ve noted the predilection of nitrogen-fixing diatoms (principally Epithemia adnata) for this substratum in previous posts (see “Working their passage …”).   One part of the margin, however, is dominated by duckweed (Lemna minor).   There is always some duckweed mixed in with the Riccia flocs, but it is usually a minor component.  How different, I wondered, were the diatoms when I looked at an almost pure growth of duckweed?  

Monoraphid diatoms from Cassop Pond, April and May 2021.   a., b. Cocconeis euglypta; c.,d. C. lineata; e. C. pseudolineata; f.g. Cocconeis euglypta/lineata/pseudolineata raphe valves; h.-l. Lemnicola hungarica (two raphe and three rapheless valves).   Scale bar: 10 micrometres (= 1/100th of a millimetre).

I had a hunch that I knew the answer, because I wrote about the epiphytes of duckweed in a previous post (“The green mantle of the standing pond …”), describing work in a Norfolk pond that had identified two species as being particularly characteristic epiphytes: Lemnicola hungarica and Sellaphora saugeressii (formerly S. seminulum).   I had found both of these in other samples from the pond, but they were more abundant on the Lemna than elsewhere.  L. hungarica accounted for five percent of the total number of valves, whilst S. saugeressii accounted for three percent.   By contrast, the Lemna samples had much less Epithemia (a single valve, compared to ten percent in the sample from Riccia) whilst the assemblages on the two plants were both dominated by Cocconeis spp.   The difference in representation of Epithemia is intriguing, based on what we know about its ecology, as it suggests that the Lemna assemblages are less nitrogen-limited than the assemblages on Riccia.   My hunch is that the most Lemna-rich area of the pond is a heavily-shaded inlet and, with the light needed for photosynthesis in such short supply, the plants grow more slowly and, as a result, the demand for nitrogen is lower.

One other species that I found for the first time in the May sample was Adlafia bryophila, a small diatom whose habitat is described as “locally frequent on intermittently wet bryophytes, aerophilous”.  In Cassop Pond it seems to show an opposite tendency: avoiding the bryophyte (Riccia fluitans) and inhabiting a fully aquatic habitat.   A good lesson in the need to treat ecological notes in the taxonomic literature with a healthy dose of scepticism.   

Biraphid bisymmetrical diatoms from Cassop Pond, April and May 2021.  a. Brachysira neoexilis; b. Navicula cryptotenella; c. N. cf. reichardtiana; d. Cavinula cocconeiformis; e. Adlafia bryophila; f.-h. Sellaphora saugeressii.   Scale bar: 10 micrometres (= 1/100th of a millimetre).
Heteropolar and dorsiventral taxa from Cassop Pond, April and May 2021.  a. Gomphonema capitatum; b. G. exilissimum; c. G. calcifugum; d.,e. Rhoicosphenia abbreviata (raphe and girdle views); f. Reimeria sinuata; g. Encyonema neogracile.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

There’s a lot more going on in Cassop Vale than just the changes I’ve written about in the pond itself.  You can follow changes in the higher plants in Heather’s blog [] but our two approaches still miss out great swathes of the biodiversity.  There is a lot more that could be written about the bryophytes, for example, though that is way outside both of our comfort zones.  And we haven’t even started thinking about the animal life.   Nonetheless, we can both agree that the little we have discovered only serves to remind us both about how much more there is to learn, and how difficult it is to make generalisations even about a habitat that we know reasonably well.

Bacillariales and Surriellales from Cassop Pond, April and May 2021.  a. Nitzschia acicularis; b. N. dravillensis; c. N. paleacea; d. N. inconspicua; e. Tryblionella debilis; f. Surirella angusta.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Some other highlights from this week:

Wrote this whilst listening to:    As The Love Continues, new album by Mogwai, headliners at Green Man next weekend.  Getting back into the festival vibe …

Cultural highlights:  Shiva Baby, a comedy set in a Jewish community in New York

Currently reading:  Culture Warloads: My Journey into the Dark Web of White Supremacy by Talia Lavin which, by macabre coincidence, includes a chapter on the world of incels.

Culinary highlight: Sunday lunch at a local restaurant.  Good food but, more importantly, the first time the whole family had eaten a meal together since April 2019.

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