How green is my river?


I wrote recently about the problems of knowing whether changes we observe in the biology of streams and rivers are signs of long-term shifts caused by human activities or just the result of short-term variation (see “’signal’ or ‘noise’?”). An interesting paper has just been published that allows us to see our observations on the River Ehen into a broader perspective.   The paper was written by my friend Susi Schneider, of the Norwegian Institute for Water Research, and is based on long-term observations by herself and colleagues on the Atna River in Norway (illustrated above; see “A brief excursion to Norway” and subsequent posts).

First of all, here is a graph summarising our observations of biomass in the River Ehen over three years. You can see a fairly regular pattern emerging of low biomass in the summer (when grazing activity is most intense) and high biomass in the winter. But you can also see strong differences between years. There were much greater quantities of algae in winter 2013/14, for example, compared with winter 2014/15 and we are puzzling over why this may have happened.


Trends in algal biomasss in the River Ehen, Cumbria between October 2012 and November 2015.   Values are the means of measurements made at four separate sites in a 5 km stretch of the river below the outflow of Ennerdale Water. Arrows indicate the approximate time of year when surveys of the Atma River were performed.

Though the Atna is about 1000 km to the north-east of the Ehen, there are similarities: both rivers have soft water and low levels of human impact and, furthermore, one of the two sites Susi writes about is immediately below a lake, just as our sites on the Ehen are downstream of Ennerdale Water.   The difference is that we visit the Ehen monthly, whereas Susi only visits the Atna once a year, although she takes care to visit at the same time each year.   I’ve indicated the time of year of her visits on the graph of the River Ehen, to aid comparisons between the two datasets.

One of the problems we have in the Ehen is that there is anecdotal evidence of lower quantities of biomass in the past.   The large quantities of algae was the trigger for our study; as is often the case, we generally do not start monitoring until a problem is perceived, which means that we then don’t have the baseline data that we need to understand the causes.   One of the interesting points that arise from Susi’s study is that there have been recent increases in algal cover at both the sites she studied.   Note that the pattern was different at the two sites (one just below a lake, one not). To put the two studies in perspective, the green box on the graphs from the Atma shows the length of time of our study covers, compared to Susi’s.

The reasons for the high algal cover in the Atna include cool, wet summers, driven by the North Atlantic Oscillation, and high discharges in August (i.e. the month before Susi’s regular visits). The former is a natural cyclical phenomenon; high August discharges are, in turn, a consequence of the cool, wet summers, and probably exert their effect by removing the grazers that would naturally control the biomass.   High discharges in the spring (i.e. 10 times the average discharge) also seem to have a major effect on the quantities of algae recorded later in the year.


Long term trends in algal cover at two locations in the upper catchment of the Atna River, Norway. The green box indicates the length of time covered by our observations on the River Ehen (2012-2015).  Graph from Schneider (2015).

What should we learn from this comparison?

  • Lesson 1: start monitoring at least 10 years before the problem arises, so that you have a strong baseline.   There is a serious point here, as environmental monitoring is likely to be one major casualty of the cuts in current spending. When problems do arise, the availability of historic data from the site is inevitably very useful but the quid pro quo is that you may need to invest in data collection even when there is no obvious short-term justification for that monitoring.
  • Lesson 2: following on from this, regard environmental monitoring is an insurance policy, insofar as you may not need to make a “claim” on every single site where you monitor.   In effect, this means accepting that some monitoring data that you collect may be redundant. The problem is that you don’t know which data will be redundant until at least a decade or so after you have collected it.   However, the complex nature of many of the problems that we face, particularly where there are interactions with climate (as in the Atna), you will not be able to make evidence-based decisions without long runs of reliable data.
  • Lesson 3: when dealing with algal growth in rivers (which reflects interactions between the physical, chemical and biological environment), do not try to draw any conclusions until you have measurements from years when the North Atlantic Oscillation is in each of its positive and negative phases.   Susi’s paper shows the problems of unravelling the complexities of biological interactions with climate. We need to think in “decades”, not “years” if we are to truly understand environmental change.
  • Lesson 4: simple measurements of criteria that can be easily understood by non-technical stakeholders aid communication. In both the Atna and the Ehen, measurements relate directly to public perceptions of healthy versus unhealthy rivers.   We have all the nerdy details of what algae are found at each site in both the Ehen and Atna, but the take-home message can be put across in terms of “how green is my river?”
  • Lesson 5: if you have to sample at widely-spaced intervals (i.e. yearly, as in Susi’s study), make sure that you sample at the same time every year.
  • Lesson 6: all of these lessons can be ignored if you are a politician with ambitions to create a leaner public sector. The sad truth is that the consequence of failing to invest in monitoring networks is not likely to be apparent for several years (well beyond the next General Election, to be blunt). Almost any aspect of public spending can be hacked away by a skilled political operator, so long as the effects of these decisions are chronic and slow to manifest themselves …


Schneider S. (2015). Greener rivers in a changing climate? – Effects of climate and hydrological regime on benthic algal assemblages in pristine streams. Limnologica 55: 21-32.


Subsidiarity in action

Back in June, I collected a sample from the edge of Lago di Maggiore in Italy and performed an impromptu analysis to see if the outcome, based on my experience of British lakes, was in any way comparable with that of my Italian colleagues (see post of 17 June 2013).  This is necessary if the EU’s environmental legislation is to provide a level playing field for all Member States and has occupied much of my professional life over the past eight years or so.   Standing beside the Atma River, I decided to conduct a similar experiment, once again scraping some of the slippery film from the submerged rocks into a small bottle that I could slip past airport security in my hand luggage.


A chain of three cells of Tabellaria flocculosa from the Atma River in Norway, July 2013.  The scale bar is 10 micrometres (1/100th of a millimetre) long.

The most abundant diatom in the sample, by far, was Tabellaria flocculosa, comprising well over 90% of all the diatoms that I saw.   This forms long chains, each cell attached to the next by a pad of mucilage at one corner, which tangle in and around the other algae (in this case, mostly green alga such as Mougeotia).   There were a few diatoms that I could not identify in this sample, because of the ad hoc method of analysis, but there were enough that were identifiable for me to be able to run the results through the calculations which we use in the UK to evaluate the status of rivers.

Had I performed this analysis on a UK river, I would have concluded that it was at “high status” (i.e. very close to its natural state), exhibiting no signs of enrichment by nutrients or of acidification.   The good news is that this is what Susi concludes, based on her own analyses, which use the algae other than diatoms.  So this informal exercise gives me double confidence:  that standards in Norway and the UK are similar, and that we can reach the same conclusions using two different groups of algae.

The most surprising aspect, for me, is that I still had a good mobile signal whilst sampling this site.  I usually associate “high status” with remote locations where the mobile signal is non-existent.

Ecological yin and yang …

One of the sites we visited had a lot of fine, silty material at the margins, washed into the river following floods a few weeks before our visit.   There were a few light green patches on this silt which were dotted with oxygen bubbles as the algae made the most of the bright sunlight.  Under the microscope the green patches resolved into filaments of the blue-green alga Phormidium (probably P. autumnale or a relative).  You can see from the picture that this is a much simpler organism than the Stigonema that we met in the previous post, consisting just of straight, unbranched filaments.  However, it is effective at growing around the silt particles, creating a “mat” of algal filaments. The Phormidium filaments are capable of limited gliding motion which means that they can adjust their position to get the maximum benefit from the light.


A patch of Phormidium autumnale (or a close relative) growing on silt at the side of the Atma River, July 2013.   The air bubbles are about two millimetres across.

The next time there is a flood in the Atma, these banks of silt will probably be washed away, along with the Phormidium colonies.   However, we have seen very similar colonies form more substantial growths in the River Ehen (see post of 24 April 2013), perhaps reflecting a more stable habitat though these, too, could be washed away by the larger floods.

It is often hard to convince people of the importance of algae in lakes and rivers.   My work can seem abstract and esoteric but these oxygen bubbles help us put it all into perspective.   Put simply, the algae are the engines of rivers, particularly fast-flowing rivers such as the Atma where higher plants cannot get established.  They use the sunlight to create simple sugars out of carbon dioxide and water and this, in turn, is the food for the midge larvae and other bugs which are, ultimately, eaten by fish.  The oxygen is a by-product of this process but also plays a role in keeping the river healthy. All of the other organisms in the river need oxygen if they are to survive, so there needs to be a source that can constantly replenish the supply.   Algae contribute to the yin and the yang of freshwater ecology: capturing the sun’s energy and then balancing this by producing the oxygen that other organisms need to release this energy again for their own needs. Those of us who study algae tend to get bogged down with putting names on all the microscopic shapes we find and too easily forget to explain the role that they play.


A network of filaments of Phormidium autumnale (or a close relative) growing amongst silt particles in the Atma River.  The inset shows a single filament (scale bar: 10 micrometres = 1/100th of a millimetre).

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.


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”.


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.


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.

A brief excursion to Norway

There is a heat wave in the UK as I write, but I am wearing a fleece, cagoule and waterproof over-trousers and wishing I had brought a wooly hat and gloves too. I am standing beside a stream 350 m north of Oslo, at an altitude of about 1000 metres, in the Rondane National Park in Norway.  We are at the tree line here, and there are still patches of  snow on the hillsides around us.  At my feet I can see low shrubby growths of the dwarf birch (Betula nana), a classic feature of “tundra” vegetation, interspersed amidst expanses of light-coloured lichens.


The mountains of the Rondane National Park, Norway, seen from the Nedre Dørålseter Turisthytte, July 2013.

I am here to help a colleague, Susi Schneider with some fieldwork and, in the process, to learn the Norwegian approach to ecological assessment.  However, my travels around Europe have already taught me that differences in scientific approach have to be set into broader contexts of environment and culture, and the patches of moose droppings scattered amongst the lichen are enough to remind me of the many differences that exist.

The streams in the upper part of the Atna River, which drains this part of the national park have extensive covers of a slippery, brown growth.  If you remove a stone and run your fingers through it, it has a slimy, viscous feel.   The overall visual effect is, frankly, off-putting but this is an entirely natural phenomenon: an alga called Hydrurus foetidus.   Under the microscope, the yellow-brown cells can be seen to be arranged in rows within this mucilage, branching at intervals to give a feathery appearance.   Hydrurus belongs to a group of algae called Chrysophytes, which are related to the diatoms, yet also distinct in many ways.  It can be found in the UK but only in the depths of winter in remote places.  It is much more common in Norway, even in July, partly because it is further north and partly because there are so many near-pristine streams here.


Hydrurus foetidus in the Atna River, Rondane National Park, Norway.  Left hand image shows Hydrurus smothering a submerged cobble; right hand image shows the mucilaginous growths on a stone removed from the water.

Susi’s conductivity meter gives us an extremely low reading, just 4 microSiemens /cm, meaning that this stream water is about as pure as distilled water and we both wonder out loud how any organism can find the sustenance to grow here.  There must, we presume, be occasional flushes of nitrogen, phosphorus and the other building blocks of life, perhaps following rain showers, one of which had soaked us a couple of hours earlier.  In any case, most of the biomass that we can see is the slimy mass around the cells, composed of carbohydrate, the most basic product of photosynthesis.  The recipe is simple: shake stream water with the carbon dioxide that is found naturally in the air (a turbulent stream is ideal for this purpose), then pour the mixture through the Hydrurus cells.  The result, judging by the number of midge larvae feeding on it, is delicious.


Hydrurus foetidus at two different magnifications under the microscope.  250 micrometres = a quarter of a millimetre.   Photographs by Chris Carter.

This still leaves us with a conundrum: that the goal of the EU legislation to which both Norway and the UK are signatories is natural or near natural ecosystems yet here we have just such an ecosystem yet one distinctly lacking in aesthetic appeal.  Nature is not only red in tooth and claw: it can also be brown, slimy and somewhat unappealing to the naked eye.  Quite how we convince the lay public of this is something I still haven’t fully solved.