Both sides now …

Ennerdale_looking_NW_July16

I diverted from my usual haunts in the upper River Ehen in Cumbria recently in order to explore Ennerdale Water in greater detail.   I am used to see it from the western end as we do our fieldwork, but the length of the journey to and from the River Ehen means that we rarely have time to linger.  Finally, however, we found a July day when we could circumnavigate the lake.  “July day”,”Lake District” and “fieldwork” sounds like an intoxicating combination.  However, the photograph above shows it was not quite as idyllic as it might have been (or, even, as it was on the day before).  Hence the title of this post, borrowed from a beautiful Joni Mitchell song which includes the line “But clouds got in my way”.

In the far past, the lakes of the Lake District were thought to have “evolved” at different speeds following their formation at the end of the last Ice Age.  Ennerdale Water and Wastwater, surrounded by hard volcanic rocks which erode very slowly, were regarded as the two most “primitive” lakes, whilst Windermere and Esthwaite Water were thought to be the two most “evolved”.   That is now known not to be the case: the geology is very important in determining the type of lake, not just because erosion is the source of the inorganic salts that give the water a particular chemical character, but also because this influences how man uses the lake.   In the case of Ennerdale Water, only about five per cent of the catchment is cultivatable, and this, in turn, influences the amount of inorganic fertiliser that is added to the meagre supply of salts provided by the underlying rocks.   Ennerdale is, as a result, one of the least chemically-disturbed of all English lakes.

At the far south east end of the lake, close to where the River Liza enters the lake, I was intrigued to see some very dark spots on the rocks.  They looked like they might be cyanobacterial colonies, so I picked a few off with my forceps and put them into a bottle for later investigation.  When I was able to look at them, the following day, I saw clumps with brown filaments radiating out, and each gradually narrowing towards the tip.   Closer examination showed that the cells that made up each filament had a blue-green colour, but were each enclosed in a brown pigmented sheath.   The filaments showed a characteristic form of “false” branching, in which the daughter filament breaks off from the mother, but is contained within the same sheath.   At the base of many of the filaments, I could see a modified cell (slightly lighter and less granular than the others) called a “heterocyst”, which was responsible for nitrogen-fixation.   These are all characteristics of the genus Rivularia, which is a good indicator of very high status water.

Ennerdale_cobble_with_Rivul

A cobble (about 15 cm long) from the littoral zone of Ennerdale Water, SW end.  The dark cyanobacterial colonies are about 3-4 mm across.

Rivularia_Ennerdale_July16

Rivularia biasolettiana from the littoral zone at the south east end of Ennerdale Water, Cumbria, UK.  a. low power (x100) image showing radiating filaments gradually narrowing in width; b. filaments showing false branching (a heterocyst is present, but hard to see); c. false branching in a filament of Rivularia with heterocyst arrowed.   Scale bar (b. and c.): 10 micrometres (= 1/100th of a millimetre).

The presence of Rivularia here was interesting to me for several reasons.   First, it continues a series of observations that suggest that this genus is not confined to hard water habitats in Britain and Ireland, as once thought (see “more about Rivularia” and links) although earlier posts have also referred to its presence in soft water habitats in Norway.   I’m also fairly sure that the organism that I collected from this trip to Ennerdale is different to the one that I find in the River Ehen and, indeed, at other locations around the Ennerdale perimeter, but that is a subject for another day.

The second comment to make is that the presence of cyanobacteria (blue-green algae) is usually a sign of an unhealthy, enriched habitat, not the very high quality habitat that Ennerdale, in fact, represents.  Blue-green algae in the phytoplankton is, very often, a bad sign, suggesting enrichment by inorganic nutrients.  Several species combine the ability to fix nitrogen with positive buoyancy, which means that they have two distinct advantages over other algae as they compete to exploit limited light and nutrients.  Some of these blue-green algae are also toxic, which has implications for how the lake and its water is used.

Those blue-green algae that live attached to surfaces in streams and in the littoral zone of lakes play by a different set of rules, however.  As they cannot use positive buoyancy to compete for light, they are more likely to be overgrown by faster-growing algae in the scrabble to capture available light.   This means that investment in expensive nitrogen fixation machinery is only an advantage when other algae, too, are very short of nutrients.  And a shortage of nutrients is the natural state for most freshwater ecosystems.

I chose the title of this piece, originally, because I felt that I had looked at Ennerdale Water from both sides now.  However, the same lyric could just as well apply to blue-green algae.   The water manager sees them as a problem; in some situations, however, they can be a positive sign.   So we can, in fact, look at blue-green algae from both sides too … from give and take and still somehow … I really don’t know life (microscopic life especially) at all ….

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More about Rivularia

My post on Rivularia from softwater habitats (‘“Looking’ is not the same as seeing”’) prompted an email from Bryan Kennedy in Ireland with some pictures of Rivularia from a moorland stream in Co. Mayo in the west of Ireland, once again from a catchment completely lacking limestone.   Bryan estimates the calcium concentration in the water to be between 5 and 10 milligrams per litre, which means that the water here is very soft.  He also comments that it was recorded in the 1970s from the Caragh catchment in south-west Ireland (average calcium concentration: 2.15 milligrams per litre: see Heuff & Horkan, 1984).

Yellow_river_trib

A tributary of the Yellow River, Co. Mayo, Ireland (left) with dark brown / black colonies of Rivularia beccariana on a submerged stone (right).  Photos: Bryan Kennedy.

The photomicrographs show the colony structure very well with filaments radiating out from the centre.   The major difference between these and the Rivularia biasolettiana I photographed in Upper Teesdale (“Blue skies and blue flowers in Upper Teesdale”) is that colonies of the latter contain calcite crystals, though these were not visible in my images.  The right hand image shows the structure of Rivularia filaments very clearly; the tapering blue-green filament gradually narrowing to a colourless hair.   Note the colourless cell at the base of the filament.  This is the “heterocyst”, and is the location where nitrogen fixation takes place.  This is a very useful adaptation in the nutrient-poor habitats where Rivularia is found, as it means that, like peas and beans, it can capture nitrogen directly from the atmosphere, rather than relying upon dissolved minerals.

Nitrogen-fixation, however, needs a lot of energy and organisms do not fix nitrogen if there is a ready supply available from other sources.   Once nitrogen is abundant, species such as Riviularia are at a competitive disadvantage and it is no surprise that Rivularia and it’s close relatives are found only in remote parts of the country, given the extent to which nitrate fertiliser washes off the land and into streams and rivers.   Even in upland areas, there are often nitrogen compounds in rain water, much of it originating in the exhaust emissions from our cars.   One wonders if Rivularia might have been much more widespread a hundred years ago than is the case now.

Rivularia_beccariana_tribYe

A close-up of a Rivularia beccarina colony from the tributary of the Yellow River, Co. Mayo, Ireland.  Photos: Bryan Kennedy.  

Reference

Heuff, H. & Horkan, K. (1984).  Caragh.  Pp. 363-384.  In: Ecology of European Rivers (edited by B.A. Whitton).   Blackwell Scientific Publications, Oxford.

 

 

 

“Looking” is not the same as “seeing” …

Looking down my microscope at a sample from the upper Ehen a couple of months ago, I saw something that I was not expecting.   The geology of the catchment of the River Ehen means that the water in this area is very soft yet here was an alga I usually associated with strongly calcareous geology.   I double checked to make sure that I had not made a mistake and, once I was confident of my identification, I wondered if it was a freak occurrence (see “When is a record not a record?”).

The organism that I was looking at was a species of the cyanobacterium (blue-green alga) Rivularia, which we last saw in Upper Teesdale (“Blue skies and blue flowers in Upper Teesdale”).  In Upper Teesdale it formed distinct hemispherical colonies yet here I could just see bundles of filaments.  Maybe they were fragments of colonies washed in from elsewhere or which I had disrupted as I had collected my samples?   One other difference is that the Upper Teesdale samples all had calcite crystals within the colonies whereas my samples from the soft water of the River Ehen lacked these.

Rivularia_in_Ehen

The cyanobacterium Rivularia from the upper River Ehen, March 2013.   The main image shows a bundle of filaments in sheaths with a single filament in the inset.  The arrows indicate the heterocysts.  Scale bar: 10 micrometres ( = 100th of a millimetre).  

Brian Whitton and Alan Pentecost summarised their records of Rivularia in a short article in The Phcyologist and commented that all but one came from catchments where there was some limestone (the exception was Haweswater in the Lake District).   This sample from the River Ehen is, therefore, unusual for the UK although we do know that Rivularia is found in soft waters in Norway, including the River Atma, subject of several posts back in July 2013.

Since I first noticed it in November, I have seen it at the most upstream of my four sites on the River Ehen every time I have visited.  It is possible that “chance favours the prepared mind”: I didn’t “see” what I was not expecting but, having found it once, I was alert to its presence on every subsequent visit.   On the other hand, I have looked at many other samples from soft water habitats and am fairly confident that there was no Rivularia present in these.   And I am sure that Brian and Alan have also looked at enough soft water habitats for their generalisation about Rivularia’s preference for calcareous habitats to be sound.

And why just at this one site and not at the others, all within about four kilometres of the lake outfall?  What is so special about this particular location?   Part of my fascination with the lower plants is that we can still make discoveries, still turn preconceptions on their heads, still approach a visit even to a familiar site with anticipation …