Notes from Windermere

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Just before the trip to the Shetland Islands I wrote about in the previous post, I spent two days in the Lake District teaching a course on identifying macroalgae for the Freshwater Biological Association.  It coincided with a period of gorgeous weather, showing Windermere at its absolute best (as the photo at top of the post shows).  Only a month ago my wheels were spinning in the snow on Whinlatter Pass (see “How to make an ecosystem (2)”).

Looking up Windermere towards the high peaks of the Lake District’s volcanic centre, I find myself reflecting on how geology creates the diversity in landscapes and aquatic features that, in turn, creates variety in the microscopic flora and fauna (see “The Power of Rock”).   A nuanced understanding of the aquatic world requires one to view the grand panorama at the same time as focussing on organisms that are scarcely visible with the naked eye.

One of the locations that we visited during the course was Cunsey Beck, which flows out from Esthwaite Water and, a few kilometres later, into Windermere.   Esthwaite is one of the more productive of the lakes in this region and we usually find a healthy crop of algae in the beck.   This year was no exception and, amongst the different forms we collected were some long straggly growths that had a slighty gelatinous feel.  Back at the laboratory we put part of one of these growths under the microscope and saw a large number of individual cells set in a jelly matrix.   This identified the alga as Tetraspora gelatinosa, a green alga that I have written about before (see “More from the Atma River …”) although not for some time.

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Tetraspora gelatinosafrom Cunsey Beck, Cumbria, May 2019.   The picture frame is about five centimetres wide.

The genus Tetraspora gets its name from a mode of division that leaves many of the daughter cells in groups of four (visible in the lower illustration).  These, in turn, are embedded in mucilage, and repeated divisions can lead to growths becoming visible with the naked eye.   Three species have been recorded from Britain and Ireland, of which the Cunsey Beck population is most likely to belong to T. gelatinosa.   In the past, it might have been called Tetraspoa lubrica, which has a more tubular thallus; however, this is now thought to just be a growth form of T. gelatinosa that is associated particularly with fast-flowing rivers.  As far as I can tell, no-one has performed any detailed molecular genetic studies on this genus to better understand the relationships between these different growth forms so we will have to go with current convention for now.

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Tetraspora gelatinosaunder the microscope.   Cells in the foreground are about ten micrometres in diameter.   Photograph by Hannah Kemp.

I’ve seen Tetraspora in a wide range of habitats – on stones in fast-flowing, relatively soft water rivers in Norway and growing on plant stems in the littoral zone of hard water ponds in Ireland.   Most of my records are from the spring, though I should add that spotting some of the smaller gelatinous colonies (barely more than near-transparent dots on the stone surface) does take some practice and I suspect that I have missed it on a few occasions too.

The microscopic image of Tetrasporawas taken during the course using a Carson Hookupz, a neat device which allows a smartphone to be attached to a microscope (or any other optical device).   It takes a little fiddling to get the set-up right but, once this has been achieved, the quality of pictures we obtained was excellent.   My microscope engineer tells me that he is selling large numbers of these to schools and colleges as it means that students can capture images during practical classes that they can subsequently use in reports or just (as was the case during our course) as an aide mémoire.

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The Carson Hookupz 2.0 as it comes out of the box (left) and (right) in action during the Identifying Macroalgae course at the Freshwater Biological Association.

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Looking north from Miller Ground towards the central Lake District peaks as the sun sets.  The photograph at the top of the post was taken from nearby but shows the view in early morning.  

 

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More from the Atma River …

As we worked our way down the Atma River, the diversity of algae increased, although the river did not yield up its secrets easily.   At each site, Susi had to make a careful scrutiny of the stones on the river bed using an Aquascope to find a series of spots, blobs and tufts which, experience had told her, were likely to consist of algae.  The Hydrurus, which we met in the previous post, was conspicuous but many of the others were very easily overlooked.

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Susi using an Aquascope to search for algae in the Atma River, Norway, July 2013.

The small jelly-like growths on the top surface of several of the submerged stones are a case in point.  It takes a practised eye to spot these on the apparently smooth rock surfaces but, under the microscope, they resolve into distinct colonies of small green cells, each with a tiny cup-shaped chloroplast.   This is Tetraspora gelatinosa, a green alga which I often find in spring in the UK, often attached to vegetation at the edges of lakes.   The colonies grow by simple division of the cells, with the “daughters” often remaining in close proximity, which is why the genus is called “Tetraspora”.

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Tetraspora gelatinosa: the left hand image shows the gelatinous growths on the upper surface of a stone from the river bed; the right hand image shows the cells in their mucilaginous matrix (scale bar: 20 micrometres = 1/50th millimetre); inset: a group of four Tetraspora cells from within the matrix.

Elsewhere in the same stretch of river we found dark olive-green patches at and around water level, so that they spent part of the time submerged and part exposed to air, but never so high on the boulders that they dried out entirely.  These were formed by a blue-green alga (Cyanobacterium) Stigonema mamillosum.   Most blue-green algae live either as isolated cells or simple filaments but Stigonema have a relatively advanced morphology, with filaments that are several cells wide and branched.  The individual cells have the characteristic blue-green colouration that gives the group its name, but the sheath within which they live has a brownish hue.  This is common in blue-green algae that live in areas subject to bright light and is due to a compound called scytonemin which acts like a natural sunscreen, protecting the cells from the damaging effects of ultra violet radiation.

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Stigonema mamillosum: the left hand image shows the Stigonema colonies (arrowed) growing in the “splash zone” just above water level on a boulder in the Atma River in Norway.  The scale bar is one centimetre long. The central image is a low magnification view of the colonies, showing the side branches arising from the central filament whilst the right-hand image shows a higher magnification view of the filament (scale bar: 50 micrometres = 1/20th millimetre).

The dense network of Stigonema filaments acts like a sponge, trapping water so that the colony did not dry out and, at the same time, creating a habitat within which other algae could survive.  I saw some thinner blue-green algal filaments growing on the Stigonema as well as several diatoms here.

The public’s perception of blue-green algae is usually negative because they often proliferate in lowland lakes and reservoirs where they can produce toxins, which limits recreational use of the water.  However, my experience is that many types of blue-green algae are extremely sensitive to pollution and, as a consequence, are good indicators of high quality habitats.   One of our challenges for the next few years is learning how to build this information into our assessments.