Transitory phenomena …

Fieldwork in the River Ehen has been an unusually pleasurable experience over the past few months, even to the extent of abandoning waders altogether and wearing just a thin pair of neoprene beach shoes and shorts as I worked.   Curiously, there were few obvious signs of the prolonged period of low flow here, but that is partly due to the pumps installed by United Utilities to keep the river running whilst the lake was drawn down (see “Life in the deep zone …”).   I did, however, find some intriguing green patches on fine sediments at the margins.

Most of the bed in this part of the river consists of much coarser sediments than these which are, I suspect, silt and sand deposited on the occasions when Ben Gill (which joins the Ehen immediately below Ennerdale Water) is flowing.   Current velocity is lower at the edges of the river, allowing fine sediments to settle out and create temporary sandbanks.   One decent spate will be all that is needed, I suspect, to wash much of this downstream.  However, there has not been a period of prolonged high flow for several months and there is, as a result, a thin green mat of algae growing on the upper surface of this sediment.

Mats of Oscillatoria on fine sediments beside the River Ehen just downstream from Ennerdale Water, August 2018.   The total length of the mats in the left hand photograph is about one metre. 

I scraped up a small sample to examine under my microscope.  I was expecting to see the broad filaments of the cyanobacterium Phormidium autumnale which I often find at a site about five kilometres downstream (see “’Signal’ or ‘noise’?”) but what I saw was much narrower filaments, some of which were slowly gliding forwards and backwards.   These belong to a species of Oscillatoria, a relative of Phormidium that is common in the plankton.  A few species, however, do live on surfaces and can, as I could see in the Ehen, form mats.  I have, in fact, described a different mat-forming species of Oscillatoria (O. limosa) from the River Wear close to my home (see “More from the River Wear”) and this, too, had been favoured by a long period of warm weather and low flow.   The filaments in the River Ehen were much narrower – just a couple of micrometres wide – and had relatively long cells (two or three times longer than wide) but, in other respects, they clearly belonged to the same genus.

Microscopic views of Oscillatoria filaments from the River Ehen, August 2018.   The upper photograph was taken at medium magnification (400x) and the lower image was taken at 1000x.  The constant motion of the filaments means that it is not possible to use stacking software to obtain a crisp image.  Scale bar: 10 micrometres (= 1/100th of a millimetre). 

The motion that I could see is thought to be due to a layer of tiny fibres (“microfibrils”) which wind around the inner layer of the cell wall in tight spirals.   Movement is caused by waves that are propagated along these fibres, meaning that the filament actually rotates as it moves (though this is almost impossible to see with a light microscope).   The filaments can move either towards or away from light, depending on the intensity, at a speed of up to 11 micrometres per second (that’s about a millimetre a day or, for any petrolheads who are reading, 0.00004 kilometres per hour).  This allows the filaments can adjust their position so that they are neither in the dark nor exposed to so much light that they are likely to do damage to their photosynthetic apparatus (see “Good vibrations under the Suffolk sun” for more about this).   The result is that filaments will tend to converge, Goldilocks-style, at the point where light conditions are “just right”.  You can see some sediment particles settling on the top of the mat in one of the images and we can expect the filaments to gradually adjust their positions, incorporating these particles, over time.

Last year, I wrote about Microcoleus, a relative of Oscillatoria, which formed mats on saltmarshes and explained how this could be the first stage of colonisation of damp habitats by plants (see “How to make an ecosystem”).   We are seeing the same processes happening here, but the life expectancy of these mats is much lower.  They may well be gone next time I visit, depending on how the Cumbrian climate behaves over the next couple of weeks.   They are transitory phenomena, here today and gone tomorrow but, like the subjects of some of my other recent posts, particularly favoured by the long period of settled weather that we have enjoyed over recent weeks.

Reference

Halfen, L.F. & Castenholz, R.W. (1971).  Gliding motility in the blue-green alga Oscillatoria princeps.  Journal of Phycology 7: 133-145.

Note: you can read more about how the heatwave has affected fresh water in the Lake District in Ellie’s McKay’s recent post on Freshwaterblog

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Two-faced diatoms …

Back in March I reflected on the challenges involved in discriminating species of Gomphonema (see “Baffling biodiversity …”).   That there were several species in the sample which prompted the article was indisputable; that some of those species were, individually, quite variable was also clear.  The former issue I resolved, to some extent, by reference back to Hutchinson’s “Paradox of the Plankton” but the latter was harder to explain.

Part of the problem stems, I suspect, from the reliance on morphology to characterise species.  We assume that, because a group of organisms share a set of visible characteristics, then they must also share genes which determine those characteristics and that, in turn, implies a common ancestry.   Turning that assumption on its head, we assume that groups of microscopic algae that appear different to each other belong to different species.   However, a dog lover might point out that Chihuahuas and Great Danes look very different but are, in fact, the same species.   One of the challenges of those of us who study algae is deciding just how much variation in form is typical within a species, and at what point differences are such that they represent more than one species.

Gomphonema sarcophagus from Pitsford Water, Northamptonshire, showing Janus cells.  Photographs by Ingrid Jüttner.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

So what should we make of the diatom valves in the image above?   The valve outlines and breadths are similar but the striae densities are so different that we might think that they belong to two separate species.   However, I recently stumbled, by chance, on a 1998 paper by Stacy McBride and Robert Edgar which discussed the topic of “Janus cells”.  Janus, you may remember, is the Roman god of time and is depicted with two faces, one looking back to the past and the looking to the future. His name has been appropriated, in this context, to describe diatoms that have frustules comprising two valves with different characteristics.   A few genera show consistent differences between the two valves – in Cocconeis and Planothidium, for example, one valve has a raphe whilst the other does not – and there are also differences in striae densities between the raphe and rapheless valves.   The term “Janus cell” is applied to diatoms where there are marked differences between the two valves but this is not a fundamental characteristic of the species or genus.   So, in the example above, we see some forms with much denser striae (11-13 in 10 mm) than others (7-8 in 10 mm).

We don’t know, from just looking at variability in populations, that this is not polymorphism within the species, in much the same way that some humans have attached ear lobes and others do not.   But, as diatom populations grow in number by repeated divisions of single cells, we can assume that most are clones of a small number of genotypes and, therefore, that the differences are due to ontogenetic variation.   What is interesting here is that this variation seems to create two distinct outcomes – coarsely or finely striated valves.  Some have suggested that such variation may be determined by differences in environmental conditions; however, the co-existence in a single population argues against this.

Gomphonema, as I have mentioned in earlier posts, is a genus that challenges taxonomists.  And, because ecologists depend upon taxonomists to give them a means of sorting diatom valves and frustules into meaningful categories, the environmental signals we get from Gomphonema species are often quite confused too.   The possibility of encountering Janus cells just throws one more curve ball into the mix.

Reference

McBride, S.A. & Edgar, B.K. (1998).   Janus cells unveiled: frustular morphometric variability in Gomphonema angustatum.   Diatom Research 13:293-310.

Round, F.E., Crawford, R.M. & Mann, D.G. (1990).  The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

Life in the colonies …

Another outcome of my visit to Ennerdale Water a couple of weeks ago in July (see “Life in the Deep Zone”) was some tiny green spheres in the sample I collected from one of the small streams flowing into the lake’s north-west corner.   The stream was very short, little more than a seepage arising from a wet rush-dominated area of a field just twenty metres or so from the lake margin and, at the point which I sampled, there was a tangle of filamentous algae (Stigeoclonium, Mirsrospora and Mougeotia) as well as a distinct diatom-dominated film on exposed stones.    The colonies looked like tiny peas in my sample tray but I suspect that they were attached to rocks or aquatic vegetation before I disrupted them. Under the microscope, these turned out to be colonies of the green alga Chaetophora pisiformis, a relative of Draparnaldia and Stigeoclonium, both of which I have written about before (see “The exception that proves the rule …” and “A day out in Weardale …”.  Like those, Chaetophora has branched filaments but they differ in forming well-defined colonies that are visible to the naked eye.

The pictures below show the form of colonies very clearly.  Chaetophora colonies are firm to the touch and cannot easily be squashed under a coverslip.   I overcame this by using a cavity slide, and taking one of the smallest colonies that I could find in order to photograph it with as little damage as possible.  Note how there is a very clear edge to the colony, whereas Draparnaldia and Stigeoclonium have a mass of filaments and mucilage but no obvious border between the “colony” and the surrounding environment.  Draparnaldia sometimes forms discrete colonies (see “The exception that proves the rule …”) but these are much softer and more easily squashed onto a slide.

Top: colonies of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, with a one cent coin for scale; bottom left: lower power (x40) view of a colony.  The picture frame is about two millimetres across; bottom right: medium power (x100) view of the same colony.

Viewed at higher magnifications, the branches of the filaments are clear. They tend to be clustered towards the tops of the filaments and, in this case at least, end abruptly, rather than tapering to fine hairs.  I explained in the posts mentioned above how these fine hairs are used by the algae as means of capturing the nutrients that they need.  Chaetophora can form these hairs, but it does so less often, in my experience, than Draparnaldia and Stigeoclonium.   There will be dead and decaying vegetation in the rush-dominated swamp from which the stream originates, and the enzymes that these algae produce will be able to harvest any phosphorus from organic particles that result from this decay.  That’s the theory for Stigeoclonium at least, but I suspect that the colonies of Chaetophora are also highly efficient recycling units: the filaments are embedded in a firm mucilage that is far more than an inert polysaccharide gunk.   Any phosphorus that is released from a filament will be far more likely to be hoovered up by another filament than to drift downstream whilst the phosphatase enzymes will also be on hand at the colony surface to savenge any stray nutrients from the seepage.  These tight colonial forms are, in other words, fortresses of plenty in an otherwise inhospitable landscape: well adapted to nutrient-stressed situations and, as a paucity of nutrients is the natural condition of streams, the presence of these colonies is a good sign that this stream is in good condition.

Filaments of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, July 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre). 

Reference

Whitton, B.A. (1988).  Hairs in eukaryotic algae.   pp. 446-480.  In: Algae and the Aquatic Environment (edited by F.E. Round).  Biopress, Bristol.

Getting started with microscopy

I talked about algae last week at an event organised by Durham Wildlife Trust (part of the publicity for The Natural History of Upper Teesdale) and I promised them a post about how to get started in microscopy and, more specifically, to start discovering more about algae.  I have illustrated the post with some of Chris Carter’s spectacular images of algae to whet your appetites for exploring the world of freshwater algae …

Broadly speaking, the natural historian wants a microscope for one of two tasks: to make small things bigger or to make invisible things visible.   There is not really a sharp dividing line between these two categories, as the illustrations of Cladophora filaments in “Summertime blues …” show.   You might start out looking at a handful of green slime pulled from your garden pond, but then you might see smaller algae growing on those filaments that you want to examine too.   The good news is that you should be able to get hold of a reasonable microscope with the capacity to magnify up to 400 times for about the same outlay as a digital SLR camera.   That should let you see all but the smallest algae.   If you are sure that your interests lie mainly in “making small things bigger” then you should consider a low power dissecting microscope (these are probably the best way of introducing children to microscopy, as there is a smooth transition between the tangible but small object that has piqued their interest and the larger, more detailed image that they see when they peer through the microscope’s eyepieces).

However, even though a basic microscope need not cost a fortune, good microscopes are expensive so my advice to a beginner is to search out a rerfurbished second hand microscope.  In north east England, I can recommend JB Microscopes but Google should help you find dealers elsewhere in the country.   A reasonably local supplier is necessary because you should really try out a microscope before you buy.   There are reputable mail order suppliers (e.g. Brunel Microscopes) but I would not want to spend a large sum on a piece of equipment that I had not had a chance to use first.

A colony of the diatom Meridion circulare.  The image at the top of the post shows desmids from the genus Micrasterias.   Both photographs by Chris Carter. 

If you are on a limited budget, I suggest you go for a good basic microscope with the option to fit a camera at a later stage.  It is possible to take a reasonable photograph by pointing a digital camera (or even a smartphone) down the microscope’s eyepiece and it is better to put up with the shortcomings of these images than to sacrifice the quality of the microscope itself.

Once you have your microscope, you will also need slides, coverslips, forceps, some plastic Pasteur pipettes, a couple of needles, a scalpel and some collecting tubes.  You can buy all of these from Brunel Microscopes and NHBS, both of whom cater for both the amateur and professional markets.   They also sell boxes of prepared slides, which are a good way to get some experience at using a microscope.

The microscopic world generally lacks the type of user-friendly well-illustrated identification guides that help us identify wild flowers, birds, butterflies and so on.   Most books are aimed at the academic market and are, consequently, expensive.   If you want to get started with freshwater algae, one useful resource is this guide to the larger algae found in rivers: RAPPER_manual_version1.7_May2016.  It was produced to accompany a method for rapid assessment of streams and rivers and, as the journey towards formal publication has stalled, I am happy to make it available here.

Hydrodictyon reticulatum, the water-net, photographed by Chris Carter.  500 mm (micrometres) is half a millimetre.

Useful websites include AlgaeVision and the Diatom Flora of Britain and Ireland.  As most freshwater algal genera are found throughout the world, Diatoms of North America is also a useful resource.

The Freshwater Biological Association have affordable booklets on the identification of desmids and diatoms and there is an AIDGAP key, too, for freshwater diatoms.   The latter is badly in need of updating but, people assure me, is still useful for beginners.

There are plenty of other online resources, but l would recommend visiting the website of the Quekett Microscopical Club, a long-established group of enthusiasts whose interests span the whole realm of natural history and optics.   www.microscopy-uk.org.uk is also worth a visit.   Both websites will help you as you start your explorations of the hidden worlds of nature.

Summertime blues …

My reflections on the effects of the heatwave on freshwater algae continued with the latest of my regular visits to the River Wear at Wolsingham.  A comparison of the picture above with that at the head of “Spring comes slowly up this way …” says it all: the sun was shining and the gravel berms that I usually use to enter the river were occupied by families with barbeques whilst their children splashed around in the water.   At times such as this, a grown man picking up stones and then vigorously brushing their tops with a toothbrush would have invited too many questions, so I slunk off 100 metres or so downstream and found a quieter spot to explore.

The biofilm in the main channel of the River Wear at Wolsingham, July 2018. 

The first thing I noticed was that the biofilm coating the submerged stones at the bottom of the river had a greenish tinge, rather than its usual chocolate brown appearance.  It also was more crusty and less slimy to the touch than I usually see in this river.  When I got a specimen under the microscope, I could see that the composition was completely different to that which I had observed in previous months.   Most samples from this location that I’ve looked at in the past have been dominated by diatoms, with occasional spring flourishes of filamentous green algae.  Today, however, the sample was dominated by small green algae – a group that I am not very confident at identifying.   My rough estimate is that these formed about three quarters of all the algae that I could see, with diatoms and cyanobacteria each accounting for about half of the remainder.   The most abundant greens were a tiny single-celled alga that I tentatively identified as Keratococcus bicaudatus, along with a species of Scenedesmus and Desmococcus communis.   There were also a number of cells of Monoraphidium arcuatum and some of Ankistrodesmus sp.

Two views of biofilms from the River Wear, Wolsingham in January 2018.   Left: from the main channel; right: from pools at the edge of the channel.

Green algae from the River Wear at Wolsingham, July 2018: a. Desmococcus communis; b. Monoraphidium arcuatum; c. Scenedesmus sp.; d. unidentified, possibly Keratococcus bicaudatus.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

However, there were also pools at the side of the channel, away from the main current but not so cut off that they were isolated from the river itself.   These were dominated by dense, brown filamentous growths, very similar in appearance to the Melosira varians flocs I described in “Some like it hot …”.  The filaments, however, felt coarser to the touch and, in close-up, could be seen to be branched, even without recourse to a microscope.   Once I got these under the microscope, I could see that they were filaments of Cladophora glomerata, another green alga, but so smothered with epiphytic diatoms (mostly Cocconeis pediculus) that they appeared brown in colour.

This combination of Cladophora glomerata and Cocconeis pediculus in the backwaters were as much of a surprise as the green-algae-dominated biofilms in the main channel.   These are species usually associated with enriched rivers (see “Cladophora and friends”) and, whilst I have seen Cladophora in the upper Wear before, it is an unusual occurrence.   Just as for the prolific growths of Melosira varians described in “Some like it hot …” it is tempting to leap to the conclusion that this must be a sign that the river is nutrient-rich.  However, the same conditions will apply here as there: “nutrients” are not the only resource that can limit plant growth and a steady trickle of phosphorus combined with warm, sunny conditions is just as likely to lead to prolific growths as a more conventionally “polluted” river.

Cladophora filaments smothered by the diatom Cocconeis pediculus in a pool beside the River Wear at Wolsingham, July 2017.   The frame width of the upper image is about 1 cm; the scale bar on the lower images is 20 micrometres (= 1/50th of a millimetre).

Another way to think of these situations is that, just as even healthy people are occasionally ill, so healthy streams can go through short periods when, based on a quick examination of plants and animals present, they exhibit symptoms associated with polluted conditions or simply (as for the first sample I described) different to what we usually expect to find.   A pulse of pollution might have passed downstream or, as seems to be happening at the moment, an unusual set of conditions lad to different organisms thriving.   Just as the ability to fight off infection forms part of a doctor’s understanding of “health”, so I expect that the River Wear will, in a few weeks time, be back to its usual state.   Healthy ecosystems, just like healthy humans, show “resilience”.   The irony is that, in this case, the “symptoms” are most obvious at a time when we are enjoying a summer better than any we’ve had in recent years.

Life in the deep zone …

The view above – looking along Ennerdale Water from the western end – is one that I’ve used before in this blog.  The difference today is that there is about twenty metres of foreshore exposed.  Normally, water covers all the area in the foreground.   Not today: Ennerdale Water is one more victim of our present drought conditions.  During the winter, we often see water splashing over the weir at the outfall; today, the weir head is a metre above the lake level and flow in the River Ehen is maintained only by pumps installed by United Utilities.

When W.H. Pearsall visited Ennerdale Water in the 1920s, he considered it to be one of the most primitive of the Lake District’s lakes (see “The power of rock …”).   However, this supposedly wild lake had been tamed by a weir since the middle of the 19th century in order to provide drinking water for Whitehaven and Workington and surrounding areas.   That, in turn, has consequences for the river downstream, especially at times such as this when, unless augmented by pumps, there would be no water in the River Ehen below the outfall.   At some point in the next decade, a new water infrastructure project will pipe water to west Cumbria from Thirlmere, after which the weir can be removed and fluctuations in water level in both lake and river will be more natural.

The weir at the outfall of Ennerdale Water, with the fish pass at the far end. 

Meanwhile, however, I was able to explore areas of the lake littoral zone that would normally be hidden from me.  My notebook, for example, records my observations that this part of the lake shore has a stony bottom yet, as can be seen from the picture above, these form a belt about 20 metres wide, after which there is firm sand.   Normally, this would be close to the limit of safe wading but, today, I could walk out with just a pair of thin neoprene shoes.   Looking down, I could see a number of tufts of the alga Nitella flexilis growing in this sand.   I’ve written about this species before (see “Finding the missing link in plant evolution …”) and have seen it in the lake before, but not in this particular location. Standing with the lake water lapping against my shins I could bend down and take some photographs of these with my underwater camera that give this usually chilly location a semi-tropical feel.

It is a useful reminder to those of us who dabble in lake littoral zones and think that we understand their ecology that a lot happens beyond the depth in which we can safely wade.   Marco Cantonati and colleagues, for example, have found big changes in the composition of the algal flora of Alpine lakes when they used Scuba diving to explore the depths of their littoral zones.  No doubt, we would see similar changes if we were to try the same in the Lake District.

References

Cantonati, M., Scola, S., Angeli, N., Guella, G. & Frassanito, R. (2009).  Environmental controls of epilithic diatom depth-distribution in an oligotrophic lake characterized by marked water-level fluctuations.   European Journal of Phycology 44: 15-29.

Cantonati, M. & Lowe, R.L. (2014).  Lake benthic algae: toward an understanding of their ecology.  Freshwater Science 33: 475-486.

 

Some like it hot …

My reflections on algae that thrive in hot weather continued recently when I visited a river in another part of the country.  As this is the subject of an ongoing investigation, I’ll have to be rather vague about where in the country this river flows; suffice it to say it is in one of those parts of the country where the sun was shining and your correspondent returned from a day in the field with browner (okay, redder) arms than when he started.   Does that narrow it down?

A feature of some of the tributaries, in particular, was brown, filamentous growths which, in close up, could be seen to be speckled with bubbles of oxygen: a sure sign that they were busy photosynthesising.  These were most abundant in well-lit situations at the edges of streams, away from the main flow.   Under the microscope, I could see that these were dominated by the diatom Melosira varians, but there were also several filaments of the cyanobacterium Oscillatoria limosa, chains of the diatom Fragilaria cf capucina and several other green algae and diatoms present.

Melosira varians is relatively unusual as it is a diatom that can be recognised with the naked eye – the fragile filaments are very characteristic as is its habitat – well lit, low-flow conditions seem to suit it well.   It does seem to prefer nutrient-rich conditions (see “Fertile speculations …”) but it can crop up when nutrient concentrations are quite low, so long as the other habitat requirements are right for it.  The long chains of Melosira (and some other diatoms such as Fragilaria capucina and Diatoma vulgare) help the cells to become entangled with the other algae.   I could see this at some sites where the Melosira seemed to grow around a green alga that had been completely smothered by diatoms and was, I presume, withering and dying.  In other cases, the Melosira filaments are much finer and seem to attach directly to the rocks.   Neither arrangement is robust enough for Melosira to resist any more than a gentle current which is why it is often most obvious at the edges of streams and in backwaters.   As is the case for Ulva flexuosa, described in the previous post, I suspect that the first decent rainfall will flush most of this growth downstream.   Another parallel with Ulva is that, despite this apparent lack of adaptation to the harsh running water environment, Melosira varians is more common in rivers and streams than it is in lakes.

Melosira varians-dominated filaments at the margins of a stream.  Top photograph shows the filaments smothering cobbles and pebbles in the stream margins (frame width: approximately one metre); bottom photograph shows a close-up (taken underwater) of filaments with oxygen bubbles (frame width: approximately one centimetre).

Algae from the filaments illustrated above: a. and b.: Melosira varians; c. Fragilaria cf capucina; d. Oscillatoria limosa.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The graphs below support my comments about Melosira varians preferring nutrient rich conditions to some extent.  Many of our records are from locations that have relatively high nutrient concentrations; however, there are also a number of samples where M. varians is abundant despite lower nutrient concentrations.   How do we explain this?   About twenty years ago, Barry Biggs, Jan Stevenson and Rex Lowe envisaged the niche of freshwater algae in terms of two primary factors: disturbance and resources.   “Resources” encompasses everything that the organism needs to grow, particularly nutrients and light, whilst “disturbance” covers the factors such as grazing and scour that can remove biomass.   They used this framework to describe successions of algae, from the first cells colonising a bare stone through to a thick biofilm.   As the biofilm gets thicker, so the cells on the stone get denser and, gradually, they start to compete with each other for light, leading to shifts in composition favouring species adapted to growing above their rivals (see “Change is the only constant …”).

The relationship between Melosira varians and nitrate-nitrogen (left: “NO3-N”) and dissolved phosphorus (right: “PO4-P”).   The vertical lines show the average positions of concentrations likely to support high (red), good (green), moderate (orange) and poor (red) ecological status (see note at end of post for a more detailed explanation).

They suggested that filamentous green algae were one group well adapted to the later stages of these successions but these, in turn, create additional opportunities for diatoms such as M. varians which can become entangled amongst these filaments and access more light whilst being less likely to being washed away.   If there is a period without disturbance then the Melosira can overwhelm these green algal filaments.   Nutrients, in this particular case, do play a role but, in this case, are probably secondary to other factors such as low disturbance and high light.  Using the terminology I set out in “What does it all mean?”, I would place M. varians in the very broad group “b”, with the caveat that the actual nutrient threshold below which Melosira cannot survive in streams is probably relatively low.   Remember that phosphorus, the nutrient that usually limits growth in freshwater, comprises well under one per cent of total biomass, so a milligram of phosphorus could easily be converted to 100 milligrams of biomass in a warm, stable, well-lit backwater.

Schematic diagram showing the approximate position of Melosira varians on Biggs et al.’s conceptual habitat matrix.

The final graph shows samples in my dataset where Melosira varians was particularly abundant and this broadly supports all that has gone before: Melosira is strongly associated with late summer and early autumn, when the weather provides warm, well-lit conditions with relatively few spates.

The case of Meloisra varians is probably a good example of the problem I outlined in “Eutrophic or euphytic?”  I have seen similar growths of diatoms in other rivers recently, due to the prolonged period of warm, dry conditions.  It is easy to jump to the conclusion that these rivers have a nutrient problem.  They might have, but we also need to consider other possibilities.   Like Ulva flexuosa in the previous post, Melosira varians is an alga that is enjoying the heatwave.

Distribution of Melosira varians by season.   The line represents sampling effort (percent of all samples in the dataset) and vertical bars represent samples where M. varians forms >7% of all diatoms (90th percentile of samples, ranked by relative abundance). 

Reference

Biggs, B.J.F., Stevenson, R.J. & Lowe, R.L. (1991). A habitat matrix conceptual model for stream periphyton. Archiv für Hydrobiologie 143: 21-56.

Notes on species-environment plots

These are based on interrogation of a database of 6500 river samples collected as part of DARES project.  Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive phosphorus.  This approximates to “soluble reactive phosphorus” or “orthophosphate-phosphorus” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards for rivers that are used here are site specific, using altitude and alkalinity as predictors.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.

There are no UK standards for nitrate-nitrogen in rivers; thresholds in this report are based on values derived using the same principles as those used to derive the phosphrus standards and give an indication of the tolerance of the species to elevated nitrogen concentrations.  However, they have no regulatory significance.