Casting the net wide …

A  month or so ago I wrote a couple of posts about the green algae that were thriving in the River Wear this summer (see “Keeping the cogs turning …” and “More green algae from the River Wear”).  In one of those, I promised to write a post about a related genus, Hydrodictyon.   I did try to find some recent populations but ran out of time so have fallen back on some old pictures along with more of Chris Carter’s spectacular photography.

Hydrodictyon reticulatum is commonly called the “water net” and can form extensive, and sometimes nuisance, growths, either floating in a lake or a slow-flowing river, or as a mat at the edge (see photos below).   The cylindrical cells are arranged in pentagons or hexagons which can be visible with the naked eye (hence the net-like appearance).   These have a mode of asexual reproduction that results in tiny zoospores being formed inside each cell.  Each of these develops into a small daughter cell whilst still inside the mother cell, and the ends of these daughters then join together to form mini-nets.  You can see this happening in Chris’ image at the top of the post: there are some young cells in the foreground with a mature “mother” cell full of “daughter” cells forming their own nets inside.  Eventually, the wall of the mother-cell disintegrates and the daughter net is released.

A mat of Hydrodictyon reticulatum from the lower River Tweed; b. macro, and, c. microscopic views of coenobia of H. reticulatum from Thrapston Lake, Northampton (b. and c. by Chris Carter).

This is an extremely effective way of enabling Hydrodictyon reticulatum to spread quickly when conditions favour its growth and the image below shows just how extensive these mats can be.   It is a species that is more common in the warmer parts of the world but it does occur in the UK as far north as Scotland and Brian Whitton has predicted that it is a species that is likely to be favoured in some climate warning scenarios.   Some authors have suggested that Hydrodictyon favours nutrient-rich water, but some of the locations where I have found it (the River Tweed in Scotland, Sunbiggin Tarn in Cumbria) do not meet this criterion.  Rather, I suggest that well-lit, relatively undisturbed summer conditions are the key factor and that this is more likely to be the case in lowland areas that are, in many cases, also rich in nutrients.  It is more likely to be a correlation than a cause, in other words.   Whatever the cause, there is a huge dichotomy between the beauty of the organism under the microscope and the nuisance that it can cause.

A huge growth of Hydrodictyon reticulatum at Manor Farm Weir on the Jubilee River (a flood alleviation channel of the River Thames near Maidenhead).  Photo: Environment Agency.

This mode of asexual reproduction – in which the zoospores aggregate inside the parent – is also a feature of Pediastrum.  Even though the shapes and dimensions of the organisms are very different, they share some fundamental properties.   Molecular phylogenetic studies have also shown that there is a close affinity between Hydrodictyon, Pediastrum and the other genera I mentioned in “Keeping the cogs turning …”.   However, their habits and ecology are very different and that raises some interesting questions about a different matter entirely … the subject of my next post.

Reference

John, D.M., Pentecost, A. & Whitton B.A. (2001).  Terrestrial and freshwater eukaryotic algae.   pp. 148-149.  In: The Changing Wildlife of Great Britain and Ireland (edited by D.L. Hawksworth).   Taylor & Francis, London.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

 

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Algae from the Alto Duoro …

From the highlands of Serra da Estrela w headed north-west towards the vineyards of the Duoro Valley from which the grapes that make port are picked.  I’m supposed to be on holiday but, as the narrow road twists and turns down a steep hillside, with vineyards on both sides, I see a case study in how humans alter rivers and their catchments to suit their needs.  I wonder if the passengers on the cruise ships that move sedately through this beautiful landscape have any idea of just how difficult this same journey would have been just fifty years ago.   Now there are 51 large dams within the watershed, regulating the flow and, at the same time, generating much-needed hydroelectricity.   Before these were in place, the only way to get the port from the quintas in the Alto Duoro to Porto was to load the barrels onto a “barco rabelo”, and then to plot a perilous path through the rapids before using a combination of sail, oars and oxen to make the slow journey back upstream (you can see videos of these journeys on YouTube).

A replica of a barco rabelo moored in the Rio Duoro at Porto, September 2018.

The Rio Douro is a type of river that is rare in the UK but very common throughout the rest of Europe in that it crosses (and, for part of its course, forms) national boundaries.  There are a few rivers in Ireland which straddle borders (the Foyle is one, and some of the headwaters of the Shannon can be found in County Fermanagh) but, mostly, this is a complication that our river managers do not have to face.  By contrast, eighty per cent of the Rio Douro’s catchment lies in Spain (where it is called the Duero) and it is actually the largest watershed on the Iberian Peninsula.   The whole European project, and its environmental policy in particular, makes so much more sense when you are looking at a well-travelled river.

Our immediate objective was the Quinta do Bomfin at Pinhão, which produces grapes for Cockburns’, Dow’s and Taylor’s ports.  However, after a morning walking through the vineyards and following a tour of the winery (the robot that has replaced human grape treaders has, we learned, been carefully calibrated to match the pressure that a human foot exerts, lest the grape seeds are crushed, imparting bitterness to the resulting wine) plus some port tasting, the lure of the river was too strong.

A view across the Douro Valley from Quinta do Bomfin at Pinhão.   This, and the previous two photographs, were taken by Heather Kelly.

The river bank at Pinhão is lined with rip rap (loose stones) enclosed in mesh cages to protect it from erosion from the waves created by the many cruise ships that make their way up the river with tourists.   This, along with the floating jetties at which they embark and disembark, meant that it was not easy to get access to the river; however, I eventually found a small slipway close to the point where a small tributary joins.  There were a few loose stones with a green film in shallow water that I could just reach, plus some algal mats coating the concrete of the slipway at water level.   I managed to get small samples of each to bring back for closer examination, attracting the usual curious stares from passers-by in the process.

The mats on the slipway were composed of an alga (technically, a cyanobacterium) that has featured in this blog on several occasions in the past: Phormidium autumnale (see “In which the spirit of Jeremy Clarkson is evoked”).   This is the time of year when the Douro is at its lowest so living at this point on the slipway means that it spends a small part of the year exposed to the air, but most of it submerged.

Phormidium cf autumnale on a slipway beside the Rio Douro at Pinhão, September 2018.  The left hand image shows the mats on the lower part of the slipway; the right hand image shows individual filaments.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The stones beside the slipway had a thick greenish film which, when I looked at it under a microscope, turned out to consist largely of bundles of thin cyanobacterial filaments belonging to a relative of Phormidium: Homoeothrix janthina (kindly identified for me by Brian Whitton).   Homoeothrix differs from Phormidium in that the filament are often slightly tapered, rather than straight-sided and usually aggregated into colonies, often growing vertically towards the light rather than intertwined to form mats.   It is a genus that I see in the UK (including, sometimes, in the River Wear) but which I have not previously written about on this blog.   The photos below show tufts of filaments but it would be quite easy to imagine several of these clumps joined together to form a hemispherical colony, before I disrupted them with my vigorous sampling technique.

Left: the rip rap at the edge of the Douro at Pinhão from which I sampled algae in September 2018; right: the stone after vigorous brushing with a toothbrush.

Bundles of filaments of Homoethrix janthina from the River Douro at Pinhão. Scale bar: 20 micrometres (= 1/50th of a millimetre).

Many of my posts try to make the link between the algae that I find in lakes and rivers and physical and human factors in those water bodies and their surroundings.  That is not an easy task in a large river basin such as that of the Douro as there is so much more of a hinterland including large towns in Spain such as Valladolid.   The river, to some extent, integrates all of these influences and, whereas the vines around Pinhão have their roots in nutrient-poor granite and schist soils, the river’s journey to this point has covered a range of different rock types, including chalky clay soils in the Spanish part of the catchment and the water reflect this.   This cocktail of physical alteration and pollution, shaken up with a dash of international relations, recurs in the largest rivers throughout Europe and is either a fascinating challenge for an ecologist or a complete pain in the backside, depending on your point of view.

I’ll come back to the Douro in a few weeks, once I’ve had a chance to have a closer look at the diatoms.  Meanwhile, I have one more stop on my travels along the Rio Douro, at the port lodges of Vila Nova de Gaia to try some vintage port …

Reference

Bordalo, A.A., Teixeira, R. & Wiebe, W.J. (2006).  A water quality index applied to an international shared river basin: the case of the Douro River.  Environmental Management 38: 910-920.

The end of the journey: port maturing in barrels at Cockburn’s lodge in Vila Nova de Gaia.

 

Notes from the Serra de Estrela

At the end of my last post I suggested that the next time I wrote it may be from Portugal.   In reality, tiredness and, to be frank, a steady consumption of Vino Verde intervened and this post may be about Portugal but is not, alas, written from that country.   Our travels took us from Lisbon northwards to Covilhã, a town on the edge of the Serra da Estrela mountain range, then onwards to the Duoro valley and Porto, and finally back to Lisbon.   The lower part of the Duoro is the home to many of the Vino Verde vineyards, although our focus was mostly on the vineyards further upstream from which the grapes for port are grown.  I’ll write more about the Duoro in a later post but, first, I want to take you on a journey to the Serra da Estrela.

These are the highest mountains in mainland Portugal (there is a higher point in the Azores) but summit with a summit at 1993 metres at Torre.  Unusually, for the highest peak in a mountain range, there is a road all the way to the top, along with a couple of shops and a small bar/restaurant.   On the day we visited, a couple of hardy cyclists had toiled their way up from the plains but most of the visitors had driven up.   We had stopped on our route up from Covilhã to explore the granite landscape and botanise so felt that we had earned our bica and Pastéis de Nata by the time we got to the very top.

Much as I appreciate a summit that satisfies a caffeine addiction, the real interest lies elsewhere, with the road up from Covilhã passing through some dramatically-eroded granite outcrops, composed of huge boulders apparently perched precariously on top of each other.  These resemble the granite “tors” we find in Dartmoor in south-west England, and have a similar origin.   The area around the tors had distinctive vegetation that will, no doubt, be described in greater length in a post on Heather’s blog before too long.   The free-draining sandy soils that the granite landscape creates mean that there was not a lot of surface water for me to indulge my own passions, so I will have to take you to another part of the Serra da Estrela for the remainder of this post.

Granite landscapes near Torre in the Serra da Estrela Natural Park in northern Portugal, September 2018.  

We found an inviting stream as we were walking near Unhais de Serra, at the southern end of the Natural Park.  The first plants to catch our eye were a submerged Ranunculus species with finely-divided leaves and five-petelled white flowers sitting at the water surface.   As well as these, we could see shoots of patches of water dropwort (Oenanthe sp.) and, looking more closely, several of these appeared to be growing out of dark coloured patches which turned out to be a submerged moss overgrown with algae (more about which a little later).   I am guessing that, once the rains come, much of these mini-ecosystems will be washed downstream leaving just a few moss stems to be colonised again next year.

Submerged vegetation in the stream at Unhais de Serra in September 2018 (40°15’44” N 7°37’21” W).  The top photograph shows a Ranunculus species and the lower photograph shows mosses overgrown with algae (a mixture of Cyanobacterial filaments, diatoms and coccoid green algae), within which young plants of Oenanthe sp. have taken root (top photograph: Heather Kelly).

Somewhat to my surprise there were also some patches of Lemanea.   This is a red algal genus that I usually associate with late winter and spring in my own part of the world, so I had not expected to find such prolific growths at this time of year at lower latitudes.   Maybe Iberian species of Lemanea behave differently to those with which I am familiar?

The Lemanea species found in the stream at Unhais de Serra in September 2018.  The top photograph shows it growing in situ and the lower photograph is a close-up.  The filaments are about a millimetre wide.

The dark film itself contained a variety of algae, some of which I have put in a plate below.   There were some cyanobacterial filaments which looked like Oscillatoria to me but which were not moving (their life between collection and examination was less than ideal).  There were also a large number of diatoms, mostly Navicula and Surirella.  Again, both would have been moving around in a healthy sample but were static when I got around to examining them; the chloroplasts in the Surirella, in particular, were not in very good condition).  I also saw some chains of Fragilaria species and several small green algae (especially Monoraphidium, discussed in the previous post).  I’ll return to the diatoms in a future post, once I have been able to get permanent slides prepared and examined but first impression is that I am looking at a community from a low nutrient, circumneutral environment.

Some of the algae living in the dark films overgrowing mosses in the stream at Unhais de Serra in September 2018.   a. – c.: Navicula angusta; d. –g. Surirella cf. roba; h. – i. two different chain-forming Fragilaria sp.; j. – k.: Navicula cf cryptocephala; l. – m.: Oscillatoria sp.    Scale bar: 20 micrometres (= 1/50th of a millimetre). 

The diatoms, in particular, reiterate the important point that notwithstanding the huge number of new species that have been described in recent years, it is possible to peer through a microscope at a sample from anywhere in Europe and see a familiar set of outlines that, for the most part, give a consistent interpretation of environmental conditions wherever you are (see, for example, “Lago di Maggiore under the microscope”).   That same rationale applies, to some extent to other organism groups too: we have recently shown this for macrophytes in shallow lakes for example.   Likewise, the geology here was shaped by the same broad forces that created the landscape of south-west England even if local climate means that the flora surrounding the tors in the Serra da Estrela is adapted to more arid conditions than that on Dartmoor.    It is important that, when we travel, we see the differences but, perhaps even more important in this fractured age, that we see the similarities too.

References

Chapuis, I.S., Sánchez-Castillo, P.M. & Aboal Sanchero, M. (2014).  Checklist of freshwater red algae in the Iberian Penisula and the Balearic Islands.   Nova Hedwigia 98: 213-232.

Poikane, S., Portielje, R., Deny, L., Elferts, D., Kelly, M., Kolada, A., Mäemets, H., Phillips, G., Søndergaard, M., Willby, N. & van den Berg, M. (2018).   Macrophyte assessment in European lakes: Diverse approaches but convergent views of ‘good’ ecological status.  Ecological Indicators 94: 185-197.

More green algae from the River Wear

Having discussed some of the recent name changes in green algae in the previous post, I thought that I would continue this theme using some of the other taxa that I found in the samples I collected from the River Wear a couple of weeks ago.   The plate below shows some specimens that, 20 years ago, I would not have hesitated to call Scenedesmus, characterised by coenobia of either four cells or a multiple of four cells arranged in a row.   Over 200 species, and 1200 varieties and forms have been recognised although there were also concerns that many of these so-called “species” were, in fact, variants induced by environmental conditions.  A further problem is that Scenedesmus and relatives do not have any means of sexual reproduction.  This means that any mutation that occurs and which does not have strong negative effects on the organism will be propagated rather than lost through genetic processes.  Working out what differences are really meaningful is always a challenge, especially when dealing with such tiny organisms.

Scenedesmus and Desmodemus species from the River Wear, Wolsingham, September 2018.  a. and b. Scenedesmus cf ellipticus; c. Desmodesmus communis.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

The onset of the molecular era shed some new light onto these problems but, in the process, recognised differences within the genus itself that necessitated it being split into three, two of which are on the plate below.  Scenedesmus, in this modern sense, has cells with obtuse (rounded) apices and mucilage surrounding the cells whilst Desmodesmus has distinct spines at the apices of marginal cells and, sometimes, shorter ones elsewhere too.   In addition to these there is Acutodesmus, which is similar to Scenedesmus (i.e. without spines) but whose cells have more pointed (“acute”) ends and which does not have any surrounding mucilage.   A further genus, Pectinodesmus, has been split away from Acutodesmus on the basis of molecular studies, although there do not seem to be any features obvious under the light microscope which can differentiate these.

The genera Ankistrodesmus and Monoraphidium present a similar situation.  In the past, these long needle- or spindle-shaped cells would all have been considered to be Ankistrodesmus.   Some formed small bundles whilst others grew singly and this, along with a difference in their reproductive behaviour, was regarded as reason enough for splitting them into two separate genera.   Both were present in the Wear this summer, but only Monoraphidium presented itself to me in a manner that could be photographed.  Two species are shown in the plate below.   Recent molecular studies seem to not just support this division but also suggest that each of these could, potentially, be divided into two new genera, so we’ll have to watch out for yet more changes to come.

Monoraphidium species from the River Wear, Wolsingham, September 2018.  a. and b.: M. griffthii; c. M. arcuatum.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The final illustration that I managed to obtain is of another common coenobium-forming alga, Coelastrum microporum.   Though the three-dimensional form makes it a little harder to see, cell numbers, as for Pediastrum, Scenedesmus and Desmodesmus, are multiples of four.  I apologise if the picture is slightly out of focus, but it is a struggle to use high magnification optics on samples such as these.  The oil that sits between the lens and the coverslip conveys the slight pressure from fine focus adjustments directly to the sample, meaning that the cells move every time I try to get a crisper view.  That means it is impossible to use my usual “stacking” software.   The answer is to use an inverted microscope so that the lens was beneath the sample.  However, I do this type of work so rarely that the investment would not be worthwhile.

That’s enough for now.   I’m off on holiday for a couple of weeks, so the next post may be from Portugal and perhaps I will also find time to sample the River Duoro as well as the products of the vineyards in it’s catchment…

Coelastrum microporum from the River Wear,Wolsingam, Septmber 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

References

An, S.S., Friedl, T. & Hegewald, E. (2008).  Phylogenetic relationships of Scenedesmus and Scenedesmus-like coccoid green algae as inferred from ITS-2 rDNA sequence comparisons.   Plant Biology 1: 418-428.

Hegewald, E., Wolf, M., Keller, A., Friedl, T. & Krienitz, T. (2010).  ITS2 sequence-structure phylogeny in the Scenedesmaceae with special reference to Coelastrum (Chlorophyta, Chlorophyceae), including the new genera Comasiella and Pectinodesmus.   Phycologia 49: 325-355.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

Krienitz, L., Bock, C., Nozaki, H. & Wolf, M. (2011).   SSU rRNA gene phylogeny of morphospecies affiliated to the bioassay alga “Selanastrum capricornutum” recovered the polyphyletic origin of crescent-shaped Chlorophyta.  Journal of Phycology 47: 880-893.

Trainor, F.R. & Egan, P.F. (1991).  Discovering the various ecomorphs of Scenedesmus: the end of a taxonomic era.   Archiv für Protistenkunde 139: 125-132.

Keeping the cogs turning …

A few algae lift my soul when I see them under the microscope through their beauty.   To see such intricate yet symmetrical organisation in something too small to be visible with the naked eye never ceases to amaze and delight me.   One of the genera that has that effect is the green alga Pediastrum, which forms cog-like colonies: flat plates of cells whose outer members bear horn-like projections.   One of its representatives, Pediastrum boryanum, was common in the River Wear when I visited recently (see previous post).   You can see, from the illustration above (the scale bar is 20 micrometres – 1/50th of a millimetre – long), the characteristic disc-like arrangement of cells, always in multiples of four (there are 16 in the colony above).   There are many species of Pediastrum, differing in the shape of both the inner and marginal cells, and the number and length of the horns.

I have found Pediastrum on many occasions in the Wear in the past, but never quite as abundant as it was in my most recent samples.  Pediastrum boryanum is the species I find most often, here and elsewhere, but other species occur too.  I have also found Pediastrum in some unusual places, including deep in lake sediments when I was searching for fossil pollen grains and there is evidence that the cell walls of Pediastrum contain both silica and a sporopollenin-like material (sporopollenin is the extremely tough material found in the outer walls of pollen grains (which probably explains why it had survived the fierce mix of chemicals that we used to prepare the lake sediments for pollen analysis).   I am guessing that the sporopollenin and silica both add some structural integrity to the cells.   There are references in the literature to Pediastrum being planktonic but I often find it in samples from submerged surfaces and associated with submerged macrophytes, so I suspect that it is one of those species that moves between different types of habitat.  It should not really be a surprise that a relatively large colonial alga with a payload of silica and sporopollenin in addition to the usual cellulose cell wall, is going to be common in benthic films in a river towards the end of a long, dry summer.

Pediastrum is another genus that has been shaken up in recent years as a result of molecular studies.  According to these, Pediastrum boryanum should now be called Pseudopediastrum boryanum although the Freshwater Algal Flora of the British Isles continues to use the old name.   Not everyone agrees with this split (see McManus and Lewis’ paper in the list below) but the divisions suggested by molecular data do also seem to match differences in morphological characteristics of the group (see Table below).

Pediastrum is part of the family Hydrodictyaceae and, as I was writing this, it occurred to me that I have never written about another interesting member of this family, Hydrodictyon reticulatum.  As I like to accompany my posts with my own photographs, I spent part of yesterday afternoon tramping around a location where I have found Hydrodictyon in the past.   All I got for my troubles, however, was two damp feet, so that post will have to wait for another day.

 

Differentiating characteristics of Pediastrum and similar genera (after Krienitz & Bock, 2011).

Genus Features
Pediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes
Lacunastrum Flat coenobia with large intercellular spaces, marginal cells with two lobes
Monactinus Flat coenobia with large intercellular spaces, marginal cells with one lobes
Parapediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes, each divided into two projections
Pseudopediastrum Flat coenobia without intercellular spaces, marginal cells with two lobes, each with a single projection
Sorastrum Three-dimensional coenobia, each cell with two or four projections.
Stauridium Flat coenobia without intercellular spaces, marginal cells “trapezoid”* with deep incision to create two lobes, each with a concave surface, though the lobes are not really extended into “projections”

* not all of the illustrations show marginal cells that are strictly “trapezoid” (e.g. with at least one pair of parallel sides).

References

Buchheim, M., Buchheim, J., Carlson, T., Braband, A., Hepperle, D., Krienitz, L., Wolf, M. & Hegewald, E. (2005).  Phylogeny of the Hydrodictyaceae (Chlorophyceae): inferences from rDNA data.  Journal of Phycology 41: 1039-104.

Good, B.H. & Chapman, R.L. (1978).  The ultrastructure of Phycopeltis (Chroolepidaceae: Chlorophyta). I. Sporopollenin in the cell walls.  American Journal of Botany 65: 27-33.

Jena, M., Bock, C., Behera, C., Adhikary, S.P. & Krienitz, L. (2014).  Strain survey on three continents confirms the polyphyly of the genus Pediastrum (Hydrodictyaceae, Chlorophyceae).  Fottea, Olomouc 14: 63-76.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

McManus, H.A. & Lewis, L.A. (2011).  Molecular phylogenetic relationships in the freshwater family Hydrodictyaceae (Sphaaeropleales, Chlorophyceae), with an emphasis on Pediastrum duplex.   Journal of Phycology 47: 152-163.

Millington, W.F. & Gawlik, S.R. (1967).  Silica in the wall of PediastrumNature (London) 216: 68.

Talking about the weather …

September is here.  When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”).   As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year.   We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.

That does not mean, however, that there have been no changes in the algae on the submerged stones.  Some of these differences are apparent as soon as I pick up a stone.  Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments.   Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.

A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments.   Note, too, the many sand grains embedded in the biofilm.  The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become.  The cobble is about 20 centimetres across.

Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too.   There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous.   We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors.   And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions.  Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance.    In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.

A view of the biofilm from the River Wear at Wolsingham on 1 September 2019. 

There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September.   The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too.  The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum.   I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions.  That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations.   I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.

Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019.  a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum.   Scale bar: 50 micrometres (= 1/20th of a millimetre).  

It is not just the differences between months this year that I’m curious about.  I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition.   There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii.   My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath)..   As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm.   This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.

Other algae from the River Wear at Wolsingham, September 2018.    The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river.  This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”).   The British have a reputation for talking about the weather.   I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river.  Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.

A hitchhiker’s guide to algae …

One of the recurring themes of this blog is the hidden delights of natural history for anyone prepared to take a closer look at unprepossessing locations, so it is appropriate that we have found some quite rich habitats within walking distance of our home in County Durham.   I’ve written before about visits to Crowtrees, a local nature reserve (see “More pleasures in my own backyard” and “Natural lenses”) and Heather is also writing a series of posts about the ever-changing flora of this small vale at the foot of the Permian limestone escarpment (see “Crowtrees LNR July 2018 part 2: gentians to grasses” for the most recent and links back to previous ones).   I visited again last week, taking Brian Whitton along for company.

His interest was the red alga Chroothece ricteriana, which I described in one of my earlier posts about Crowtrees but we did not find it on this particular visit.   Instead, my eye was drawn to soft clouds of green filaments that floated just above the bed of the pond.   When I looked closely under my microscope, I saw that these were thin filaments of Oedogonium.  Typically, these had no reproductive organs, so cannot be named (see “Love and sex in a tufa-forming stream” for a rare exception), but all showed characteristic “cap cells” (see lower illustration).

Growths of Oedogonium in Crowtrees pond, August 2018.   The frame width is about 30 centimetres.   The photograph at the top of the post shows Brian Whitton searching for algae during our visit.

The diatom Achnanthidium minutissimum was growing on small stalks attached to the Oedogonium filaments, often alone but also in pairs and stacks of four, as the diatom cells divided and re-divided.  Oedogonium is a rougher alga to the touch than filamentous genera such as Draparnaldia, Stigeoclonium and Spirogyra, and often carries epiphytes, and I presume the lack of mucilage is a factor in this.   Achnanthidium minutissimum is a diatom that is very common on the upper surface of submerged stones in both lakes and rivers, but it is not fussy and I often see it as an epiphyte if conditions are right.  In this case, I suspect that the very hard water of Crowtrees Pond is a factor: calcium carbonate is constantly being precipitated from the water to create a thin layer of “marl” (see photo in “Pleasures in my own backyard”).   This makes life difficult for a tiny diatom that cannot move, so hitch-hiking a ride on the back of a filamentous alga that floats about the lake bottom makes a lot more sense.

Oedogonium filaments with epiphytic Achnanthidium minutissimum, from Crowtrees pond, August 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Oedogonium is an adaptable genus.  It is also common in the River Ehen (soft water, low nutrients) and I also find it in lowland polluted rivers too.  Being able to name the species would, I am sure, help us to better understand the ecology but this is, as I have already mentioned, problematic (see “The perplexing case of the celibate alga”).   However, in each of the cases I’ve mentioned, the epiphytes are different (Achnanthidium minutissimum here, Tabellaria flocculosa and Fragilaria species in the Ehen, Rhoicosphenia and Cocconeis placentula in enriched lowland rivers) and I suspect that these might offer an easier way to interpret the habitat than the filaments themselves, at least until someone finds a stress-free way of naming them.