Puzzling puddles on the Pennine Way …

We were in Upper Teesdale last weekend for one of our regular walks through the reserve,  hoping to catch an early glimpse of the spring gentians (Gentiana verna) for which Teesdale is famous (see “Blue skies and blue flowers in Upper Teesdale …”)  but, before we got to the sugar limestone grassland where these live, we had to walk along a length of the Pennine Way from Langdon Beck towards Cauldron Snout.  After a cold, wet spring we have had a period of warm, dry weather and we were walking under blue skies in shirtsleeves.   Puddles, unusually for this part of the world, were a rarity and, stepping across one of the few that persisted, I noticed a cloud of tadpoles.

Now frog spawn and tadpoles are quite common in Upper Teesdale at this time of year and barely deserve a second look.   However, the water in this particular puddle had a reddish-brown tinge of a slightly different hue to the mud around the margins.  This suggested to me that it was probably some kind of algae and that it might be worth taking a small sample in order to see what the tadpoles were feasting upon.

A puddle straddling the Pennine Way in Upper Teesdale with the cloud of tadpoles at the right hand side.  The reddish-brown colour is caused by Trachelomonas.   The lower picture and inset show close ups of the tadpoles (about a centimetre long) feeding on algae.  

Once I had a drop of this suspension on a slide and peered through my microscope, the nature of the coloration revealed itself to be a mass of tiny almost-circular cells moving rapidly across the field of view.   Taking the magnification up yet further revealed these cells to be Trachelomonas, a relative of Euglena (see “More from Loughrigg Fell …”).   Cells of Trachelomonas are enclosed in a rigid case called a “lorica”, which means that the cells are not constantly changing shape, as is the case for Euglena.   This lorica, in turn, is often impregnated with iron and manganese salts, which impart a reddish-brown tinge seen both in the micrographs of individual cells and in the colour of the water in the puddle.  Like most species of Euglena, Trachelomonas has a single flagellum, which emerges from the lorica through an apical pore surrounded by a “collar”.   I’ve included a link to a YouTube video which shows the movement of individual cells.  The lorica can be ornamented with warts, granules or short spines and these can be used to differentiate the species.

Trachelomonas from a puddle straddling the Pennine Way in Upper Teesdale.  The upper photograph shows the reddish-brown tint that the cells impart to the water in the puddle and the lower photographs show individual cells.  The flagellum is particularly clear on the cell second from left whilst the ornamentation on the lorica is obvious on the right hand cell (possibly a different species).  Scale bar: 20 micrometres (= 1/50th of a millimetre).

https://www.youtube.com/watch?v=cFUzjFGIGq8&feature=youtu.behttps://youtu.be/cFUzjFGIGq8

A short video of Trachelomonas illustrating the mode of movement, powered by the single flagellum.  This video was taken the day after the sample was collected, by which time many of the cells had ceased movement.

Fifty-five species of Trachelomonas have been recorded from Britain and Ireland, along with 15 forms and varieties.   Curiously, County Durham is a “hot spot” for this genus, with ten species recorded from Cassop Vale, just a few kilometres from where I live, and six from Croft Kettle (both locations have been the subject of posts on this blog).  The reason for this is, however, rather prosaic: when the Freshwater Algal Flora of the British Isles was being written, there were no UK-based experts on several groups of algae, including the Euglenophyta (see “An inordinate fondness for … algae”).   Konrad Wołowski, a Polish specialist, was invited to write the chapter for this group and was flown over for a trip around the country to get to know the British Euglenophyta flora.  One of the stops on this tour was Durham, where Brian Whitton introduced him to local sites.   As ever, when examining the distribution of the less conspicuous members of plant and animal kingdoms, one has to ask whether a “hot spot” represents a genuine biological phenomenon or is simply the result of intensive activity by one of the few people who know what they are looking for.

By the way, if you are interested in the natural history of this area, it is worth digging out the latest version of The Natural History of Upper Teesdale, which has just been published by Durham Wildlife Trust.   I contributed to the chapter on freshwater life but there is much here about all aspects, from geology and geomorphology through to the “Teesdale rarities” and the people who live in the dale.   You can find more details here .

References

Gater, S. (2018) (editor).  The Natural History of Upper Teesdale.   Mosaic, Middleton-in-Teesdale / Durham Wildlife Trust.

John, D.M., Whitton, B.A. & Brock, A.J. (2011). The Freshwater Algal Flora of the British Isles. 2nd Edition. Cambridge University Press, Cambridge.

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Eutrophic or euphytic?

A paper has just been published that should be required reading for anyone interested in the management of nutrients in in ecology.   It is a follow-up of a 2006 paper with the catchy title “How green is my river” that set out to provide a conceptual framework for how rivers responded to enrichment by nutrients.   That original paper contained several good ideas but, crucially, not all of them were underpinned by evidence.  A decade on, several of the predictions and statements made in that original paper have been tested, and the time has come to re-examine and modify that original conceptual model.

My reaction to the 2006 paper was that it was very interesting but not fully reflective of the rivers in my part of Britain, whose rougher topography produced quite different responses to nutrient enrichment than that proposed in their original model.   That criticism has been addressed in the revised version, which places greater emphasis on the physical habitat template, which means that it is more broadly applicable than the original version.   But that, in turn, got me wondering about the continued relevance of a term such as “eutrophication” to rivers.

People have been using the term “eutrophic” to describe lakes with high concentrations of nutrients since early in the 20th century.   As the century progressed, evidence of a causal relationship between inorganic nutrients and algal biomass, and the consequences for other components of lake ecosystems grew.   With this foundation, it has then become possible to predict the benefits of reducing nutrients and there are plenty of case studies, particularly from deep lakes, that demonstrate real improvements as nutrient concentrations have declined.

Attempts to apply the same rationale to rivers have, however, met with far less success.   Legislation to reduce nutrients in rivers has been in force in Europe since 1991 (the Urban Wastewater Treatment Directive, followed by the Water Framework Directive) and whilst this has led to reductions in concentrations of phosphorus in rivers (see  “The state of things, part 2”), there has, in most cases, not been a corresponding improvement in ecology.   There are a number of reasons for this but, at the heart, there was a failure to understand that the tight coupling between nutrients and biology that was the case in deep lakes did not also occur in running waters.   What was needed was recognition of fundamental differences between lakes and rivers, and “How green is my river?” and, now, this new paper have both contributed to this.

However, one consequence of recognising the importance of the physical habitat template alongside nutrients is to challenge the relevance of the term “eutrophic” when describing rivers.   “Eutrophic” literally means “well-nourished” so is appropriate in situations where high nutrients cause high plant or algal biomass.   This high biomass (strictly speaking, the primary production arising from this biomass) then creates problems for the rest of the ecosystem (night-time anoxia caused by plants consuming oxygen being a good example).   If high biomass can arise due to, let’s say, removal of bankside shade or alteration to the flow regime, perhaps (but not always) in combination with nutrients, then perhaps we need a term that does not imply a naïve cause-effect relationship with a single pressure?

My suggestion is to shift the focus from nutrients to plant growth by using the term “euphytic” (“too many plants”) as this would shift the emphasis from simply driving down nutrient concentrations (expensive and not always successful) towards reducing secondary effects.  It is possible that strategies such as planting more bankside trees, for example, or altering the flow regime or channel morphology (see “An embarrassment of riches …”) may be just as beneficial, in some cases, as reducing nutrient concentrations.   That said, we also have to bear in mind that nutrients may have an effect well downstream, so focus on amelioration of effects within a particular stream segment will never be a complete solution.

I should emphasise that a lot of work has been done in recent years to understand the concentrations of nutrients that should be expected in undisturbed conditions, and also to understand the nutrient concentrations that lead to changes in community structure in both macrophytes and algae.   These show that many rivers around Europe do have elevated concentrations of nutrients and I am not trying to side-step these issues.  I do, however, think it is important that regulators can prioritise those rivers in greatest need of remediation and, in most cases, they do this without considering the risk of secondary effects.

It is, largely, a matter of semantics.   I have been involved in many conversations over the past couple of decades about how to improve the state of our rivers.  Many of those have centred on the importance of reducing nutrient concentrations (which would be, indisputably, a major step towards healthier rivers).  But there is more to it than that.  And Mattie O’Hare and colleagues are helping to open up some new vistas in this paper.

Note: the photograph at the top of this post shows the River Wear at Wolsingham.  This stretch of the river captures many of the challenges facing river ecologists: nutrient concentrations are relatively low and there is good bankside shade.  However, the flow of the river is highly altered due to impoundments upstream and a major water transfer scheme.  How do all these factors interact to create the often prolific algal growths that can be seen here, particularly in winter and spring?

References

Hilton, J., O’Hare, M., Bowes, M.J. & Jones, J.I. (2006).  How green is my river?  A new paradigm of eutrophication in rivers.   Science of the Total Environment 365: 66-83.

O’Hare, M.T., Baattrup-Pedersen, A., Baumgarte, I., Freeman, A., Gunn, I.D.M., Lázár, A.N., Wade, A.J. & Bowes, M.J. (2018).  Responses of aquatic plants to eutrophication in rivers: a revised conceptual model.   Frontiers in Plant Science.   9: 451

Grazing on algae …

I comment on the role that grazers play in controlling algal biomass in rivers in these posts and this is the time of year when I, myself, take a more participatory role.   As it is spring, Lemanea fluviatilis is thriving in our rivers (the cleaner ones, at least) and I could not resist grabbing a couple of handfuls whilst out in the field recently for culinary purposes.

This time, I followed the routine I described in “More from the Lemanea cookbook … ” and washed, air-dried and then cut-up some Lemanea filaments into short lengths (they need to be about a centimetre long, otherwise they can form clumps).   My experience is that the fishy taste of Lemanea is a fine complement to freshwater fish, so decided to use it in a warm potato salad which I then served underneath a salmon fillet seasoned and sprinkled with dill and then wrapped in foil and baked with a couple of knobs of butter.

The warm potato salad needs a mayonnaise made from one egg yolk and about 150 ml of olive oil into which a couple of tablespoons of lemon juice are stirred, along with salt and pepper.   Add a generous handful of dried Lemanea to this and leave to soften for about 20 minutes, and also add a teaspoon of capers and a small handful of land (or water) cress.  Cook and drain enough new potatoes for two, then cut these into small chunks and stir the mayonnaise and algae mixture into these.   Divide between two warmed bowls and place half the salmon fillet on top of each.  Finally, add a few fresh pea shoots as a garnish, along with a wedge of lemon, and serve.

Definitely worth repeating.

Warm potato salad with lemon and Lemanea, served with salmon fillets.