On the pleasures of untidiness …

“Go, sit upon the lofty hill,
and turn your eyes around
where waving woods and waters wild,
do hymn an autumn sound.” Elizabeth Barrett Browning, The Autumn 1833

Naturalists are explorers, of a sort. They are not necessarily of a type that ranges far and wide in search of exotic phenomena.  That’s one sort of exploration, for sure.  Another type is to travel to the same place over and over again and watch how it changes.   That type of journey is through time rather than space, but it is no less of an adventure, so long as your senses are attuned to what is happening around you.   Indeed, watching how much one location can change both through seasons and between years offers a cautionary tale to anyone who thinks that isolated, concentrated bursts of activity at a distant location (what our archetypal “explorers” are doing) are likely to yield definitive insights.

With that thought in mind, I was back at the River Ehen last week.   I’ve been coming here for eight years now, so I think I know it quite well but one of the things I have learned is that there is always more to discover.   Sometimes, this is because the river really has changed, sometimes (quite often, I suspect) it is because I have noticed something that has been there for a while but which I have hitherto overlooked.  Sometimes, it is a mixture of the two.

That fallen tree on the right hand side of the image at the top of the post is a case in point.   When I first started coming here, eight years ago, there was a patch of Nitella flexilis tucked under the footbridge close to the right bank.  A couple of years ago it disappeared for no apparent reason. I presume that it had been dislodged in a spate and washed downstream.   Then, over the past few months, I’ve gradually seen patches of Nitella appear again, though not in the same place.  Some of the most prolific stands are in the lee of the tree, where fine sediments have gradually accumulated.   It grows there along with Myriophyllum alterniflorum (alternate water milfoil) and Callitriche brutia ssp. hamulata (intermediate water starwort), creating a hotspot of macrophyte diversity, at least until that tree is dislodged and this area is exposed to the full force of the Ehen’s current again.  

Nitella flexilis (left) and Callitriche brutia var. hamulata (right) in sediments downstream of a fallen tree in the River Ehen, November 2020.  The fallen tree can be seen just to the right of the footbridge in the photo at the top of the post.

Aquatic ecologists can get quite misty-eyed at the mention of large woody debris because of the important role it can play in supporting biodiversity.   This example in the River Ehen is a good example of how natural treefall creates patches within a stream that creates a set of conditions enabling a range of species to thrive.   I’ve focussed on one of our larger freshwater algae in this particular example, but there is evidence of similar benefits working on invertebrate and fish populations too.   One day, probably after an exceptional flood, I’ll come back and find that this fallen tree has been washed downstream and these patches of finer sediments, along with the algae and plants that they support, have been eroded away.   Patches exist in time as well as in space: you just need to think in another dimension.  

The problem is that this type of woody debris makes rivers look untidy and owners and bailiffs are prone to remove it.   Anything that holds back water can cause localised flooding, which may be a problem for the owner of adjacent land; however, slowing down the flow is, generally, beneficial for communities downstream.   So that’s another reason why we should not pull out fallen tree trunks.   Once upon a time, a fallen tree trunk was only ever seen as a nuisance, now it is a “public good” with at least two tangible benefits for streams and rivers.   “Tidiness”, however, is not one of those but, then, look at an ecologist’s study or even the boot of his or her car, and you will see that it is rarely a trait that we endorse either.   

Reference

Thompson, M.S.A., Brooks, S.J., Sayer, C.D., Woodward, G., Axmacher, J.C., Perkins, D.M. & Gray, C. (2019).  Large woody debris “rewilding” rapidly restores biodiversity in riverine food webs.  Journal of Applied Ecology 55: 895-504.  

Some other highlights from this week:

Wrote this whilst listening to:  Stevie Wonder.  Because, halfway through Lockdown 2.0, we neeed cheering up.  

Cultural highlights:  To the Ends of the Earth: a Japanese film about a team making a travel documentary in Uzbekistan.   If Lost in Translation is about westerners discombobulated by Japanese culture, then this is (Lost in Translation)².  Many of the scenes were set around locations in Tashkent and Samarkand that we recognised from our visit in 2017. 

Currently reading:   Why I No Longer Talk to White People About Race by Reni Addo-Lodge.

Culinary highlight:   A toss-up between Bouillabaisse and a water cress and horseradish risotto. 

Finding the missing link in plant evolution …

I noticed some patches of a small, bright-green plant fully submerged in the slack waters beside the River Ehen a kilometre or so below the outflow from Ennerdale Water.   My first look suggested a tangle of narrow, flaccid grass-like stems but a closer examination showed that each of the narrow stems had whorls of branches arising from them at intervals (similar to the pattern seen in horsetails).   Their size and structure suggested an affinity to the vascular plants such as Myriophyllum and Juncus that I also saw in the river that day but, actually, these plants belong to an algal genus called Nitella.   I could not see any reproductive organs on these plants, but other evidence points to them being Nitella flexilis.

Nitella belongs to a group of algae called the Charophytes which have been the subject of vigorous debate by phycologists and evoloutionary scientists for a long time.    The author of one recent paper went so far as to claim that “no group in the plant kingdom has presented greater difficulties in classification …”.   They are usually placed in the green algae (Chlorophyta) although some people regard them as a separate division entirely.  Charophytes, in fact, have several characteristics, as well as their size and pigmentation, that suggest that they may be the closest algal relatives of land plants.   These similarities extend to the reproductive organs (see Chris Carter’s image of Chara virgata in a post from July last year).

Nitella flexilis photographed in the River Ehen, 18 March 2014.  Scale bar: 1 centimetre.

Under the microscope, some oddities of Nitella and other charophytes become clear.  The branchlets , despite their size, are composed of single giant cells, each with many tiny chloroplasts around the margins and a large central vacuole.  If you watch closely, you can see the cytoplasm close to the vacuole within these cells streaming around (though this proved to be hard to capture on video).   These giant cells made the charophytes very popular with physiologists, as they were easy to manipulate in the laboratory.

A cell of a branchlet of Nitella flexilis from the River Ehen collected on 18 March 2014.  Scale bar: 1/10^th of a millimetre.

Their large size and distinctive appearance has meant that Charophytes were often recorded by mainstream botanists who usually ignore algae completely.   This means that we have a better understanding of their distribution than is the case for most freshwater algae.   These show records of Nitella flexilis and relatives to be particularly clustered around the Lake District and other areas associated with soft water, though more often in ponds and lakes than in rivers.   It is certainly not a common genus so this is one more feature that makes the River Ehen such a fascinating river to study.

References

Bennici, A. (2008).  Origin and early evolution of land plants.  Problems and considerations.   Communicative and Integrative Biology 1: 212-218.

Karol, K.G., McCourt, R.M., Cimino, M.T. & Delwiche, C.E. (2001).  The closest living relatives of land plants.  Science (New York) 294: 2351-2353.

Ruhfel, B.R., Gitzendanner, M.A., Soltis, P.S., Soltis, D.E. & Burleigh, J.G. (2014).  From algae to angiosperms – inferring the phylogeny of green plants (Viridiplantae) from 360 plastid genomes.  BMC Evolutionary Biology 14: 23.