I made an journey via Paris to Orleans last weekend to wave off my wife and daughter as they walk part of the Camino de Santiago. The part of this austere Medieval pilgrimage route that they chose to follow happens to be one that meanders along the Loire Valley, passing chateaux and wineries en route. Apparently, blisters are providing an element of self-mortification to keep the spirit of pious ascetism alive.
Orleans has a beautiful Medieval old town, dominated by a cathedral, along with plenty of reminders that it was here that Joan of Arc whupped the English. More importantly, for a freshwater ecologist, a river runs through it. The mighty Loire – the longest river in France – rises in the Masif Central and then flows north until it reaches Orleans, then it swings round to flow west to join the Atlantic Ocean at Nantes, a total length of just over 1000 km. The river at Orleans is broad and shallow, divided into two channels by a wooded island; the left-hand chnnel is braided, with many small gravel islands, some with grass and herbaceous vegetation, and the water is generally shallow. However, the bed of the river itself was a dark green in colour. The Loire Valley is known as the “Garden of France” and this gives a clue to the scale of nutrient enrichment that we might expect in the river. In the backwaters, this green backdrop was enlivened by patches of red due to the aquatic fern Azolla (see “Escape to Southwold”).
Patches of Azolla floating over algae-smothered substrata in a backwater of the River Loire at Orleans. The picture at the top of the post shows a view looking downstream from the left side of the Pont George V.
I brought a sample of the algae from some stones that were just within reach of the shore home on the Eurostar in a Perrier Water bottle for a closer inspection and was surprised by the diversity. In particular, I noticed several clumps of a narrow cyanobacterium that proved hard to photograph (possibly Homoeothrix janthina: see “Algae from the Alto Duoro”) along with many green algae. I also saw Cladophoraglomerata, which is one of the classic indicators of nutrient rich conditions, along with Stigeoclonium, two forms of Spirogyra, two forms of Oedogonium and myriad green unicells and coenobia. Stigeocloniumis another good indicator of nutrient enrichment,as the filaments narrow to long colourless “hairs” when key nutrients are scarce – these cells are physiological adaptions to scavenge phosphorus and their absence at Orleans shows that this nutrient is not in short supply (see “A day out in Weardale”).
Stigeoclonium cf. tenue (based on properties of erect filaments) from the River Loire at Orleans, September 2019. Scale bar: 10 micrometres (1/100thof a millimetre).
As well as green algae and cyanobacteria, there were also a lot of diatoms. A few of these are illustrated below, and I’ll describe the diatoms in more detail in a future post. As well as those I have photographed, I also saw long chains of a species of Fragilaria and another small araphid (possibly Staurosira) as well as Ulnaria ulna and some cells of Gomphonema and Navicula. Note, in particular, the tube of Encyonema (possibly E. prostratum). I’ve talked about tube-forming diatoms before (see “An excuse for a crab sandwich, really …”) but still can’t really explain what advantage this structure confers on a diatom. What we can, perhaps, say, is that tube-dwelling is one of a several habits represented in the diatoms from Orleans – chains, erect, free-living motile, epiphytes and more – and the mass of other algae create a rich diversity of microhabitats that the diatoms can exploit.
Some diatoms from the River Loire at Orleans: a. – d. Diatoma vulgare; e. Cocconeis pediculus; f. Encyonemasp. Scale bar: 10 micrometres (= 1/100thof a millimetre). The identity of the tube-dwelling form will have to wait until a cleaned sample is available.
This abundance and diversity of green algae reminded me of some of the lush growths I had seen in UK rivers during the 2018 heatwave (see “Talking about the weather …”). In a healthy river in the UK, I would expect to find less conspicuous growths than this, as invertebrate grazers would keep the algal biofilm shorn to a short stubble. If, for any reason, the invertebrates cannot keep up with the algal growth, then a feedback loop is set up: the algae suck the valuable oxygen the invertebrates need from the water, the grazing reduces in intensity yet further, leading to a situation such as that I saw in the River Loire is the result.
Warm weather exacerbates the situation because water holds less oxygen at high temperatures. In my posts about the River Wear last summer I commented that the plants in the river suggested that the river was more enriched with nutrients than was actually the case and I suspect that this was the result of these interactions. The more southerly, more continental climate of the Loire Valley will experience these effects more often and it is possible that what I am looking at in Orleans may become the norm for UK rivers, as global warming intensifies. Over the past decade I’ve worked on how to estimate the threshold concentrations of nutrients that a healthy river can endure. However, nutrients rarely exert a direct effect on the plants and animals in a river but, instead, work through their effect on other factors such as oxygen. That will mean that global warming may wipe out any benefits of expensive nutrient reduction programs based on current estimates of the relationship between nutrients and river ecosystem health. That’s a rather depressing prospect.
So I’ll end on a more cheerful note: the reason Heather and Rosie chose to start their Camino in Orleans was that they wanted to avoid a flight if possible. At least that is how they sold it to me. That they also chose to start their pilgrimage in a town close to the Sancerre vineyards may be pure coincidence. Or maybe not ….
Surt, M.M., Jansen, M.A.K., Harrison, S.S.C. (2011). Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river. Freshwater Biology 56: :2580-2593
Wilco, C.E., Verbrak, P., Durance, I., Vaughn, I.P. & Ormerod, S.J. (2016). Field and laboratory studies reveal interacting effects of stream oxygenation and warming on aquatic ectotherms. Global Change Biology 22: 1769-1778.
Orleans cathedral, August 2019.