This boulder is to me what madeleines were to Marcel Proust: it stirs memories. You’ve already read about it in Hunger Games and in some older posts. When I looked at it on my most recent visit in early December, I not only remembered, but I noticed that it was different. It is time to take a closer look.
The first thing to notice is that more of it is exposed than back in October. The lake must be about twenty centimetres lower. Second, the zonation of algae is back. When I visited in August, I commented on the clear distinction between bands of cyanobacteria and green algae (see “More from the splash zone …”). In October, the green algae had disappeared. Now, in December, they are back again. Microscopic examination showed these to be mostly Spirogyra, as in August, along with smaller quantities of Mougeotia, Oedgonium and Klebsormidium.
Third, the growths of Tolypothrix on the side of the boulder are less conspicuous. I’m guessing that it has been outcompeted by the green algae. I’m also guessing that it is present, and possibly even serving as a “rooting medium” for the green algae. It is another example of the “patchiness” in space and time of aquatic algal assemblages (see previous post for more about this).
Algae below the waterline on the littoral boulder in Wastwater in December 2022. The photo captures about 20 centimetres depth and cyanobacteria and green algae are both visible. The photograph at the top of the post shows the boulder with Great Gable in the background.
This drop in lake levels meant that a flat “plateau” of rock on the right-hand side which had been submerged was now largely exposed. In my earlier post, I had commented on how this had been dotted with Tolypothrix colonies. Tolypothrix was, on this trip, still conspicuous in depressions on this surface – miniature “rock pools”, as it were, that were rewetted at regular intervals by waves breaking against the boulder.
I’ll keep this post brief. It is just an update on a ever-changing situation. Having made a comparison with Marcel Proust, I need to be mindful of length. À la Recherche du Temps Perdu spills over into seven volumes and over 3000 pages. Most of us have heard about it, but almost none of us have read it. I don’t want that to be my fate too.
The “rock pool” on the right-hand end of the boulder. It is about 30 centimetres across, and growths of Tolypothrix (up to about a centimetre in diameter) are just visible.
Wrote this whilst listening to: Phoebe Bridger’s EP of Christmas songs, including her exquisite cover of Merle Haggard’s If We Make It Through December.
Currently reading: How to Feed a Dictator by Witild Szabłowski. Interviews with the personal chefs of Idi Amin, Saddam Hussein and others.
Cultural highlight:Standing on the Sky’s Edge, new musical set in a brutalist high-rise development in Sheffield. Features songs by Richard Hawley.
Culinary highlight: Christmas dinner, cooked by my daughter at her home in Sheffield.
Clouds were low enough to obscure the views that I used to illustrate my last post about Wastwater and Wasdale (see “Hunger games …”), so I have started this post with a photograph of the underwater landscape of the River Irt, rather than the distant mountains. You should be able to see three distinct algal lineages here: diatoms forming the yellow-brown patches in the foreground and left-centre, cyanobacteria along with mosses forming black patches on the centre-right boulder and green algae in the right foreground and growing amidst the black cyanobacteria/moss patches. The underwater landscape is verdant at the same time as the surrounding fields and fells are at their most depleted.
Is ”verdant” even the right word? It comes from the French word for “green” yet green is just one of a number of hues on display. All these, however, are the outcome of mixtures of green chlorophyll and other pigments, so we may have strayed from a literal definition of verdant whilst still being within the popular meaning of lush vegetation. But, before we get lost in etymology, let’s think about why some rivers in the Lake District present this counterintuitive burst of colour in the depths of winter.
We do not yet have a definitive answer but suspect that the temperature of the water plays an important role. The lushest algal growths are found in rivers immediately downstream of lakes which act, as it were, as huge “water-source heat pumps” – warming up gradually during the spring then cooling slowly throughout the rest of the year (see graph below). The difference between the River Irt and nearby River Calder (which does not have a lake upstream) is small when measured as absolute temperature but nonetheless means that there is a third more heat energy in the River Irt every day at this time of year compared with the Calder. It all adds up.
Water temperature (2021 & 2022) and benthic chlorophyll (2019-2022) trends in the River Irt downstream of Wastwater.
That’s not the whole story. The flow regime in rivers downstream of lakes is not as harsh as it is in rivers without lakes upstream, and there must be differences in the intensity of grazing by invertebrates. But small differences in temperature have effects on a wide range of processes in rivers, and there is no reason why it should not be an important factor in determining algal dynamics in Lake District streams either.
Gomphonema exilissimum in the River Irt (Lund Bridge), December 2022. Scale bar: 20 micrometres (= 1/50th of a millimetre).
Do not misinterpret all this talk of the relative warmth of the River Irt in December. Plunging an arm into a northern English river in December is not for the faint-hearted. But you’ve got no other option if you want to know what organisms are responsible for those vivid yellow-brown patches on the stream bed. Back in the relative warmth of my study, I examined these with my microscope and saw that the bulk of the diatoms belonged to a single species – Gomphonema exilissimum. – growing on long branched stalks. Sometimes there were patches where other diatoms were prominent – Fragilaria tenera, Tabellaria flocculosa and Achnanthidium species. But these were growing on and around the matrix of Gomphonema stalks. Gomphonema, in this setting, is a “foundation species”, shaping the habitat in ways that let other organisms thrive. We think of the trees of a forest as foundation species but these “bushes” of Gomphonema play similar roles in streams, albeit on much smaller scales.
Gomphonema cf. exilissimum and Fragilaria cf. tenera in the River Irt (Lund Bridge), December 2022. Scale bar: 20 micrometres (= 1/50th of a millimetre).
We could, perhaps, argue that the mosses and cyanobacteria play similar roles. I’ve written about these before (see “As old as the hills …”) and also described how cyanobacteria can create conditions within which green algae can thrive in another stream in the western Lakes (see “Ever changing worlds …”). These relationships all help to create the patchworks visible with the naked eye (see also “The multiple dimensions of aquatic biofilms”). We need to think of the patterns not just in terms of variation across the stream bed, but also over time. One big spate could roll the stones on which the algae grow or rip the filaments from the surfaces, leaving spaces for new species to invade. It’s the ecological equivalent of Heisenberg’s Uncertainty Principle: we can predict that certain algae will live under certain conditions, but there is a limit to the accuracy of these predictions. It is not that our measurements are necessarily flawed, just that this patchiness – across space, through time and at multiple scales – is an inherent property of the system that needs to be respected.
I’ll end where I started: this is all happening in the midst of a northern English winter. The trees are bare and the water is cold. We watch the weather forecasts closely, and plan our fieldwork for the short gaps when water levels are low enough to permit safe wading. When we get to the rivers, though, we find richer growths – both in quantity and diversity – than we see at any other point in the year. We can only wonder …
Some other highlights from this week:
Wrote this whilst listening to: The Specials, following the untimely death of Terry Hall. In particular, their most recent album, Protest Songs:1924 – 2012.
Currently reading:Henning Mankell’s The Pyramid, short stories featuring the Swedish detective Kurt Wallander.
Cultural highlight: non-existent. A combination of winter lurgies and too much work to finish before the Christmas break
Back in October I wrote, somewhat apologetically, about my inability to switch off completely whilst on holiday (see “Reflections from Lago Trasimeno”). The flip side of that condition is that I can continue to revisit a holiday two months after I have packed my flip-flops and sunhat away for the year. On a damp, cold winter afternoon I can travel, virtually, back to warmer climes simply by peering through a microscope. Somehow, the presence of particular assemblages of diatoms synergises with the memory of being there to take me to a flow state, where time melts away and space ceases to have any relevance. For a couple of hours, at least, I can feel the Umbrian sun on my face again.
Looking at the diatoms of Trasimeno not only took me back to the summer, it also linked Trasimeno to some other places I have visited and written about here. It recalls Cassop Pond and Croft Kettle in my own neighbourhood (see “The diatoms of Cassop Pond” and “More about Croft Kettle”), but also turloughs in the west of Ireland (see “Famous for 15 minutes …”), small lochs in the Shetland Islands and shallow lakes in Greece that I have been studying. Genera such as Epithemia, Rhopalodia and Mastogloia all crop up together in these hard water habitats, irrespective of geography.
Diatoms from Lago Trasimeno. a., b. Diatoma moniliformis; c. Tabularia fasiculata; d. Cocconeis placentula. Scale bar: 10 micrometres (= 1/100th of a millimetre). The photograph at the top of the post shows Lago Trasimeno from Passignano sul Trasimeno, September 2022.
I found 32 species in the sample I collected on the shore of Isola de Maggiore, and could have found more had I been inclined to spend more time. I always prefer to spread my effort over several samples from different locations or collected on different dates rather than to “mine” a single sample to exhaustion. 32 is a respectable haul for a single sample: not especially species rich but, at the same time, not depauperate either.
About ten percent of these belong to the genera Epithemia and Rhopalodia which are capable of nitrogen fixation. These are relatively large diatoms, so they will represent a greater proportion of the biomass and biovolume than a simple count of individuals suggests. No great surprise, perhaps, that Lago Trasimeno in September, is short of nitrogen as these are also ideal conditions for the bacteria which break down the nitrogen compounds that run off the surrounding farmland and release it back to the atmosphere. My earlier post on the algae of this lake pointed out the abundance of the cyanobacterium Gloeotrichia, another nitrogen-fixing organism, again emphasising the nitrogen-limited state of the lake at this time of year.
More diatoms from Lago Trasimeno. a. Navicula crptotenelloides; b. Navicula cryptotenella; c. Navicula cf. microdigitoradiata; d. Navicula duerrenbergiana; e. & f. Mastogloia baltica (two focal planes); g. Haslea spicula; h. Cymbella neocistula; i. Encyonema caespitosum; j. Seminavis strigosa; k. & l. Amphora sp.; m. & n. Amphora pediculus. Scale bar: 10 micrometres (= 1/100th of a millimetre).
About a third of the diatoms belonged to genera such as Navicula and Nitzschia that are capable of movement. Large numbers of these can often be signs of human disturbance, but not always. In this case, I suspect that the naturally hard water leads to the precipitation of calcium carbonate, which smothers surfaces and challenging any organism living there. Being able to constantly adjust position means that a motile diatom has an advantage over its sessile cousins. A further twist to the story is that 20% of the diatoms are tolerant of saline conditions, which ties in with some evidence that the concentration of ions in the lake is increasing (see references in earlier post). The two most likely reasons are abstraction of water from the lake for irrigation and climate change so this, too, is a potential sign of human disturbance. Greater evaporation also makes it more likely that calcium carbonate will precipitate at the same time as the lake water gets more salty. My interpretation of conditions, based on a single sample, is that the water is getting close to being “brackish” rather than strictly “fresh”. If trends for global warming continue, then shallow lakes in warm regions such as this will become more salty, with implications for the ecosystem services that they provide to the region.
Diploneis cf. elliptica, photographed at three focal planes. Scale bar: 10 micrometress (= 1/100th of a millimetre)
I’ve found at least one of the motile diatoms – a small Nitzschia – in brackish lakes in Greece too. It does not correspond to any Nitzschia I’ve seen described (“e.” in the plate of Nitzschia and Tryblionella below), so one challenge for the next few months is to try and find out some more about this, comparing Italian and Greek populations and digging a little deeper into the literature.
More diatoms from Lago Trasimeno. a. Epithemia sorex; b., c. Epithemia adnata; d., E. frickeri; e., E. turgida. Scale bar: 10 micrometress (= 1/100th of a millimetre).
This slide is my “souvenir” from Trasimeno. We associate the word souvenir with knickknacks and trinkets, but the word has its roots in the French word meaning “remember”. Peering through a microscope on a cold winter day in northern England is enough to transport me back to sunny days in Umbria. At the same time, the diatoms that I see bring me back to my own locale because I can find several of them in a pond that is walking distance from my house. At the same time, finding salt-tolerant diatoms so far from the sea is a reminder of the encroaching reality of global warming. All these pictures combine to tell a story … happy memories, certainly, but no guarantee of a happy ending.
More diatoms from Lago Trasimeno: a. Nitzschia acicularis; b. Nitzschia filiformis var. conferta; c. Nitzschia cf. palea; d. N. palea var. tenuirostris; e. Nitzschia sp.; f. Tryblionella sp. Scale bar: 10 micrometress (= 1/100th of a millimetre)
Some other highlights from this week:
Wrote this whilst listening to: Low’s Christmas and Sufjan Stevens’ Songs for Christmas. Two very leftfield Christmas albums.
Currently reading:more Nadime Gordimer. This time it is Livingstone’s Companions, a book of short stories.
Cultural highlight: A recreation of local artist Norman Cornish’s house and studio at Beamish Museum in County Durham.
Culinary highlight: “dirty broccoli” – broccoli roasted with a peanut and chilli marinade.
microscopesandmonsters 