The algae that got away …

Unusually, for a blog that is mostly about algae, this post is about the absence of algae.  Not the complete absence of algae; that would be extremely unlikely but certainly the absence of visible algae from locations where, at this time of year, they are usually either conspicuous from the bankside or apparent as soon as you get close to the stones on which they usually thrive.  

We are now well into the season of unpredictable weather.  Most of the rainfall, and subsequent increases in flow are an inconvenience when trying to plan fieldwork (see “On fieldwork”), but have only limited effects on the river itself.   This is particularly the case for the River Ehen where the lake just upstream from our sampling sites acts as a huge buffer that damps down the impact of any changes.  A couple of weeks ago, however, there was heavy rain, leading to very high river levels and some local flooding.    I had watched this via the weather forecast and on the hydrographs, but this was my first opportunity to see what this had actually done to the river.

River levels in the River Ehen in October and November 2021 (from www.riverlevels.co.uk).  The horizontal red line shows the maximum river levels that are typically encountered and the two arrows indicate the dates of our two most recent visits.  The photo at the top of the post shows Croasdale Beck.  

November is usually the month when the algae in the river are at their most prolific, which may come as a surprise as this is also the time of year when the trees have lost most of their foliage.  The reasons are still not entirely clear, but it is likely that the invertebrates that would normally graze the algae have finished their lifecycles and hunkered down for the winter whilst the algae are benefiting from the lake-warmed water.   On this visit, however, the stones on the river bed had only a slight covering of algae, certainly not apparent with the naked eye. 

Memories can be deceptive, however.   When I scanned back through photographs of the river bed, the river bed rarely looked particularly algae-rich. That’s partly because a lot of the algae that are present are diatoms, whose yellow-brown colouration blends in with their rock substrata.   Three of the six years whose photos I checked had visible green growths, but 2021 had unusually low concentrations of all algae, not just greens.   However, that needed a measurement rather than an observation to confirm.   2019 was clearly an aberration in terms of the extreme quantities that we found, for as yet unknown reasons, but the photos show that algae are not always obvious here, possibly because this particular site has a lot of small, easily-rolled pebbles rather than cobbles and boulders, and the algae are easily scoured away.

Photos of the bed of the River Ehen just downstream from the outflow of Ennerdale Water, photographed in October or November over six years.

Compare the images in this plate with those I presented in “The Wrong Kind of Green?”   My point in that post was to show how the quantities of visible algae varied over the course of a year in a stream.  The photographs of the River Ehen’s riverbed in this post show how much variation there is in the same month between years.  Now try to rank these photos in terms of the quantity of algae and compare your findings with the graph below, which shows the quantities of algae that we measured.  Whilst 2019 clearly has the most algae by both reckonings, 2018 comes second with the measurements but not the photos, whereas 2016 probably comes second from photos.   So there is rough, but far from perfect, agreement between the two approaches.   We’ve also noticed, over the years, that green algae tend to be patchier than the other groups.  You can see this with the photos, but it also means that our measurements of green algae are more variable than those of other groups.   

Algal biomass (as chlorophyll concentration) at the same location in the River Ehen in October or November since 2016. 

Something else that we noticed was that some of the stones that presented a very “clean” surface had conspicuously darker and slimier undersurfaces.   These must have been stones that were rolled by the storm and we wonder if this creates miniature “compost heaps” under the stones as the light-deprived algae slowly wither and are decomposed by the microorganisms in the stream.   That, in turn, would suck some of the oxygen out of the surface sediments, perhaps imperilling the young mussels that need a steady supply of oxygen-rich water flowing through the sediments in order to survive. 

Two views of the same cobble from the River Ehen in November 2021: the upper surface on the left, almost devoid of algae, and the lower surface on the right which had more algae and which was, we presume, facing upwards until the stone was rolled during the recent high flows.  

A few kilometres away, Croasdale Beck, the River Ehen’s unruly sibling, also shows the effects of the rain.  Just above the site where we sample the stream has cut a wide meander into the bank, through which the channel flows through unconsolidated sediments.   At about this time of year in 2017 I wrote about how a storm just before I visited had changed the path of the water through this channel (see “What a difference a storm makes”) and today we noticed that the channel had shifted back to where it had been before the 2017 flood (photo at the top of the post).  

That was due to Storm Ophelia and, as I write, Storm Arwen is battering the countryside.   I will not know what effect this has on the River Ehen until I get there for my next round of field work but – here’s the rub – it might also mean that fieldwork plans have to be postponed.  Far more than any other aquatic ecosystems, streams and rivers have an intimate and immediate connection with atmospheric processes.  Watching streams and rivers closely makes you aware of how rainfall can be converted into raw physical power.  I can feel it pushing against my waders as I collect samples, even in a small stream such as Croasdale Beck.   So I should not be surprised that it also has the capacity to affect the plants and animals that live in and around the channel.   The problem is that the famous unpredictability of the British weather means that there is also inherent unpredictability in the ecology of our streams.  Just as the British love talking about the weather, so a few of us will always finding something unexpected to talk about when we visit a stream …

Wrote this whilst listening to:    Too Busy Thinking About My Baby and other Motown classics by Marvin Gaye.  And the extremely atmospheric soundtrack to Spencer by Jonny Greenwood.

Cultural highlights:  Spencer, the new film about Princess Diana.   Closer in mood to The Shining than The Crown, which is just fine by me.

Currently reading:  Dancing with Bees by Brigit Strawbridge Howard.

Culinary highlight:  My mainly-vegetarian daughter allows herself one steak per year and I joined her to cook and devour a medium-rare fillet steak from a local organic farm, accompanied by a homemade peppercorn sauce.