Spring in Ennerdale …

My latest trip to Ennerdale Water, in the Lake District, has yielded its usual crop of spectacular views and intriguing questions (see “Reflections from Ennerdale’s far side”).   This time, my curiosity was piqued by lush growths of green algae at several locations around the lake shore.  The knee-jerk reaction to such growths is that they indicate nutrient enrichment but I am always sceptical of this explanation, as lush green growth are a common sight in spring (see “The intricate ecology of green slime …”) and these often disappear within a month or two of appearing.

Two points of interest: first, the lake seems to be lagging behind the River Ehen, which flows out of Ennerdale Water.   We often see these lush growths of algae on the river bed in winter but by this time of year the mass of algae there is lower than we saw in the lake littoral.   Second, the lake bed looks far worse (see photograph below, from the north-west corner of the lake) than the actual biomass suggests.

Filamentous algae (Ulothrix aequalis) smothering cobble-sized stones in the littoral zone of Ennerdale Water, April 2017.

Under the microscope, this revealed itself to be unbranched filaments of a green algae, whose cells each contained a single band-shaped chloroplast lapping around most of the perimeter.   This is Ulothrix aequalis, a relative of Ulothrix zonata, which I wrote about a few times last year (see link above).   Like U. zonata, this species is very slimy to the touch and, I suspect, the payload of mucilage adds to the buoyancy of the organism and means that we look down on a fine mesh of filaments which trap light and add to the unsightly appearance of the lake bed at this point.   That this part of the lake shore is close to a tributary stream draining some improved pasture triggers some suspicions of agricultural run-off fuelling the algal growths but, looking back at my notebook, I see that the lake bed was almost clear of green algae when we visited this location in July last year.  I suspect that a return visit this summer would also show a clean river bed.  Appearances can often be misleading (see “The camera never lies?”).

Ulothrix aequalis from the littoral zone of Ennerdale Water, April 2017.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

This was not the only site that we visited that had conspicuous growths of green algae, though the mass of algae was greatest here.   All of the sites at the western end had these growths (see “A lake of two halves” for an explanation of geological differences within the lake) but, curiously, the genus of alga that we found differed from site to site.   In addition to Ulothrix aequalis in this corner of the lake, we found Mougeotia on the south side and Spirogyra close to the outfall.  This diversity of forms is, itself, intriguing, and I have never read a convincing explanation of what environmental conditions favours each of these genera.   I see both spatial and temporal patterns of green algae in the River Ehen too and, again, there is no satisfactory explanation for why the species I find can differ along short distances of the river and between monthly visits.

The Mougeotia and Spirogyra both have another story to tell, but that will have to wait for the next post …

Bollihope Bhavacakra*


My explorations of the biology of Ulothrix zonata have taken me from Bollihope Burn in Weardale (see “Bollihope Burn in close-up”) to upper Teesdale (see “The intricate ecology of green slime”) and one of the outcomes is this representation based on the diatom-smothered filaments that I observed in Bollihope Burn, close to the sink hole.   The picture illustrates the suggestion that I made in the post about Bollihope Burn – that the switch from “vegetative” to “reproductive” mode leads to less energy being available for the alga to manufacture the slime that it needs to stop epiphytes gaining a foothold.   By intercepting the limited light that penetrates into the water, these algae can shade the host plant to such an extent that it cannot gain the energy it needs to grow.   The mucilage is the equivalent of the “anti-fouling paint” that mariners use to stop barnacles encrusting their hulls.

My image shows a single healthy filament of Ulothrix zonata at the bottom right of the image and, on the left, two filaments of cells that are producing flagellated gametes that will eventually be released.  I write “gametes” with mild trepidation, as they may also be nascent zoospores associated with the asexual phase (see below).   A third filament, in the background, is composed mostly of empty cells that have already released their gametes.   There are no “male” or “female” gametes; any two can combine to form a zygote, so long as they come from different filaments.   This zygote then attaches to the substratum and does little more over the summer.

In my image, the Ulothrix filaments have been colonised by needle-like cells of Fragilaria gracilis, Achnanthidium minutissimum cells on short stalks, and a couple of cells of Gomphonema pumilum.   There are also a couple of cells of Ulnaria ulna and some zig-zag colonies of Diatoma tenuis.   The effect of these cells on the appearance of the Ulothrix zonata is marked, smothering the filaments entirely so that, with the naked eye, the assemblage appears brown rather than green.

The diagram below summarises the life cycle of Ulothrix zonata and emphasises the point that the green filaments that most people associate with this alga are only a small part of the story.  The cell contents divide in one of two ways.  The first produces zoospores, each with four flagellae, which are released, settle and grow directly into a new vegetative filament.  The second, however, produces a number of cells which are smaller but otherwise look similar to the zoospores except that each has two rather than four flagellae.  These gametes then fuse with gametes produced by another filament to produce a “zygote” which, in turn, germinates to produce several (typically eight) zoospores, each with four flagellae, from which new vegetative filaments grow (see illustration of putative “germlings” in “The intricate ecology of green slime”).


The life-cycle of Ulothrix zonata, following Lokhorst and Vroman (1974).   “2n” refers to diploid stages; “n” refers to haploid phases (note that the vegetative filament is also haploid).

The asexual phase can be produced at any time, but is stimulated by high temperatures; the sexual phase, however, is more strictly regulated.   The formation of gametes only occurs under “long day” conditions, which means that it will happen as daylight hours extend in the spring.   By contrast, the division of the zygote requires short day conditions and low temperature, meaning that the zygote is relatively inactive over the summer months, only dividing to produce zoospores, and ultimately, new filaments, in late autumn and winter.   This creates a useful niche for the organism during a period of the year when nutrients are relatively plentiful in upland rivers (as they are washed off the land following rainfall) and grazers are relatively inactive.   It also means that this apparently simple green filament actually has some sophisticated controls that regulates how and when it divides.

I’ve talked about algal life cycles in the past, commenting that the concepts behind these are not always easy to grasp (see “Reflections from the trailing edge of science …”).   The problem is that undergraduates of my generation were taught this as part of a broader overview of plant evolution and the variations between patterns in different groups tended to befuddle rather than enlighten students already struggling to grasp the big picture.   The interesting twist to my explorations of Ulothrix zonata is that it has shown how the idiosyncrasies of an organism’s life-cycle can have a practical significance that helps the organism survive in a particular habitat.   Knowing about the life cycle can, in turn, inform our understanding of processes occurring within a stream or river.  The problem is that these topics have largely fallen off the agenda both for teaching and research, so we are generally limited to interpreting descriptions from old journals, and often forget completely the role that these factors may play in creating the mosaic of algae in a stream.


Lokhorst, G.M. & Vroman, M. (1974).  Taxonomic studies on the genus Ulothrix (Ulotrichales, Chlorophyceae) III.  Acta Botanica Neerlandica 23: 561-602.

* “Bhavacakra” is a symbolic representation of the cyclical nature of existence used in Tibetan Buddhism.  The title of this post is also an affectionate tribute to Brian Moss, who died a few days ago.

The intricate ecology of green slime …


The first really warm weather of the year was a perfect excuse to visit Upper Teesdale and remind the world that spring gentians are not the only botanical wonders in this part of the world.   Our route to the gentians follows the course of the Tees up from Langdon Beck, alongside Falcon Clints (pictured above) and finally a scramble up beside Cauldron Snout to Widdybank Fell.  Along the way we saw birds eye primrose and butterworts growing alongside the path and, up on Widdybank Fell the spring gentians were approaching their best, and we saw some blue moor grass (Sesleria caerulea) too.  But my eyes were focussed beyond the terrestrial vegetation, and soon homed in on some green films in the river as we were sitting down to eat our lunch.   I had not come prepared for fieldwork today, so had to lean out precariously from the bank to grab a sample which I then had to store in an empty Twiglets bag for the journey home.


Sampling algae in the River Tees near Falcon Clints, May 2016

I had a hunch that I had found some more Ulothrix zonata (see “Bollihope Burn in close up …”) but could not be sure in the field.  The film of algae on the rock surface was slimy, and, as far as I could tell with the naked eye, unbranched, but there are other algae that share these properties and I could not know for sure until I had a specimen under my microscope.   This means that algal natural history lacks the immediacy that is associated with larger organisms and one of my campaigns this year is to see just how much can be achieved with field microscopes.  Today, however, was supposed to be a day off.  Hence the need to requisition a Twiglets packet.


Ulothrix zonata on the surface of a submerged boulder in the splash pool below Cauldron Snout, Teesdale, May 2016.   In contrast to my usual close-up views of the algal world, this was taken with a 300 mm telephoto lens.

The pictures below reiterate the comments I made in my earlier post about the problems we have appreciating the three-dimensional structure of microscopic algae when removed from their natural habitat: the left hand image was taken underwater with my Olympus TG2 camera in macro mode and shows the filaments supported by the fast-flowing water of the Tees; the right hand view shows them collapsed into a gungy green slime on top of a cobble that I have removed from the edge of the river.   It is equivalent to looking at a mass of wracks and kelp smothering a rocky shore at low tide and trying to imagine these as the vertical fronds of a “kelp forest” at high tide.


Ulothrix zonata in the River Tees, near Falcon Clints, May 2016.  Left: close-up view of filaments in situ, taken underwater with an Olympus TG2 camera; right: a cobble removed from the margins of the river.

Looking down the microscope, there were definite signs that this was late season Ulothrix, even if it was not so overgrown as the population I had seen at Bollihope Burn.   First, the chloroplasts were not in a particularly healthy state – earlier in the year they would encircle most of the cell, whereas now they seem to be shrivelled and fill less than half of the cell length.  Second, there were several cells in “reproductive” mode, forming zoospores. And, third, there was an abundance of epiphytes, particularly on those that had already released their zoospores.   Had I come earlier in the year, I would have expected to see filaments virtually free of attached algae.  Now, they were abundant.   Many of the epiphytes were diatoms, similar to those I described from Bollihope Burn, but there were a few groups of a different alga, which I initially thought was either Characium or  Characiopsis but which is more likely to be a germling of Ulothrix zonata that has grown from one of the zoospores.

There are nuances to the natural history of this nondescript green slime.   Studies in the North American Great Lakes have shown that it thrives in high light conditions, in the shallow littoral (less than a metre deep in the littoral) and grows best when the temperature is less than five degrees.   This gives it a window of opportunity in our northern England streams, thriving in the late winter and spring before tree cover reduces the light available.  It is not the only alga that likes cool temperatures, and the quid pro quo of this habit is that it needs to produce copious mucilage in order to stop epiphytes growing on the cell wall and blocking out the light that it craves.   As days get longer and the water becomes warmer, so the organism shifts to a reproductive mode, producing zoospores and, from those, zygotes, that can hunker down and survive the inhospitable conditions that we call “summer”.

Oh yes, we saw a ringed plover too.  Almost forgot.


Ulothrix zonata from the River Tees, near Falcon Clints, May 2016.  Left: vegetative cells with degrading chloroplasts; right: “dead” cells with epiphytic cells that are probably germlings produced by U. zonata zoospores.   Scale bar: 25 micrometres (=1/40th of a millimetre).


Auer, M.T., Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1983).   Factors regulating the spatial and temporal distribution of Cladophora and Ulothrix in the Laurentian Great Lakes.  Pp. 135-145.   In: Wetzel, R.G. (editor) Periphyton of Freshwater Ecosystems.   Dr W. Junk, The Hague.

Graham, J.M., Kranzfelder, J.A. & Auer, M.T. (1985).  Light and temperature as factors regulating seasonal growth and distribution of Ulothrix zonata (Ulvophyceae).  Journal of Phycology 21: 228-234.


Bollihope Burn in close-up

Bollihope Burn does not disappear dramatically down a single swallow hole in the way that Gaping Gill swallows up Fell Beck on the slopes of Ingleborough.  Rather, there is a gradual diminishment of flow, as the river percolates through the joints in the limestone, before the remnants of the stream swirl down a final sinkhole (see “Co. Durham’s secret Karst landscape”).   I was intrigued to see how the organisms that inhabited Bollihope Burn reacted to these stresses so got down on my knees close to this final sinkhole to get a closer look.

My waterproof Olympus TG2 (see “Getting close to pearl mussels with my underwater camera”) set to super-macro mode is equivalent to putting my head under the surface of the water and then peering at the rock through a magnifying glass … but gets fewer odd looks from passers-by.   Fortunately, this is an isolated corner of Weardale and passers-by were limited to a few rabbits, because sticking a camera into a stream to take a photograph of a stone is, itself, odd enough to attract stares from most people.

These close-up views of freshwater algae in their natural habitat continue to surprise me.  It is only in the last few years that waterproof digital cameras with macro facilities have fallen to an affordable price.  Before this, underwater photography required special kit that few freshwater biologists could afford.  Yet, removing a stone to photograph the algal growths meant that the algae were never photographed in their natural habitat, and were deprived of the buoyancy that the water afforded them.   I have plenty of photographs of green or brown gunk composed of different algae but, with the algae removed from their context, these photographs offer few insights into the biology of the stream bed.  The photograph below, however, shows a community with a distinct structure – a “turf” of near-vertical filaments waving in the gentle eddies of the stream as it swirls around before disappearing down the swallow hole.


A cobble in Bollihope Burn, close to the swallow hole, covered by a short “turf” of algae, April 2016.   Scale bar: approximately two centimetres.

Under the microscope, the structure of this “turf” starts to reveal itself.   The filaments appear to be aggregations of diatoms around dying filaments of the green alga Ulothrix zonata.   This is an alga that is common in Pennine streams in the winter and early Spring but which disappears as the weather starts to warm up. It often forms very conspicuous green patches on the river bed for a short period of time, as in the following picture, which I took a few kilometres away from my current location, in the River Wear at Wolsingham.   The difference in appearance between the alga in the two photographs is mostly due to the Bollihope population being smothered with diatoms whilst the Wolsingham population was virtually a pure growth of Ulothrix.   This may be partly due to the Bollihope picture being taken taken two months later than the Wolsingham image.   Ulothrix zonata produces copious quantities of mucilage and the Wolsingham population was slimy to the touch.  I rarely see epiphytes on this or any other slime-producing algae in their healthy state.   However, Ulothrix is a species that thrives in cold water.   Indeed, a study has shown that when the water starts to warm up and the day length increases, the Ulothrix filaments switch into their dispersal and reproductive modes and that is what may be happening here.   As the rate of photosynthesis declines, so there is less carbohydrate from which the slime molecules can be made and, as a result, less of a deterrence to any diatom looking for a perch.   From now until next winter, Ulothrix zonata will not be very obvious in the streams that I visit.  This is because the zygotes which are produced by sexual reproduction lie dormant until day length decreases and temperature drops.   At this point, they germinate and divide to produce zoospores which, in turn, grow into new Ulothrix zonata filaments.


Growths of Ulothrix zonata on cobbles in the River Wear at Wolsingham, February 2009. 

The photographs taken under the microscope illustrate this well.  On the left hand side there is one of the few healthy looking Ulothrix filaments that I found, with a chloroplast wrapped around the inside of the cell wall   On the right hand side you can see that the chloroplasts have gone, replaced by dark green blobs which are (I think) bundles of gametes awaiting release.   More significantly, you can also see several diatoms around the Ulothrix filament, taking advantage of it to lift themselves up above the rock surface.

The paradox is that these algae are entering their senescent phase just as most of the plant life in Weardale is flourishing.   This is probably not a coincidence: life in cold water means fewer grazing invertebrates and less shade to intercept the precious winter sunlight.   I suspect that algae, once masters of the planet, have gradually adapted and evolved to live a subordinate life, flourishing in those periods of the year when most of us are content to stay indoors.


Ulothrix zonata from Bollihope Burn, April 2016.  The left hand image shows a healthy vegetative filament; the right hand image shows zoospore production and colonisation by diatom epiphytes. 


Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1985).  Light, temperature and photoperiod as factors controlling reproduction in Ulothrix zonata (Ulvophyceae).  Journal of Phycology 21: 235-239.

van den Hoek, C., Mann, D.G. & Jahns, H.M. (1995).  Algae: an Introduction to Phycology.  Cambridge University Press, Cambridge.