A winter wonderland in the River Ehen

I have just finished another visualisation of the underwater life of the River Ehen, following up my observations in Food for Thought from the River Ehen.   In that post I commented on the prominent brown biofilms that I saw on rocks which, under the microscope, turned out to be mixtures of the green alga Bulbochaete and diatoms.   I have tried to convey this in my illustration, with the long-stalked Gomphonemas risking through the mat of Bulbochaete filaments (many of which end in long hairs) and, on the left hand side, a colony of Fragilaria tenera cells, loosely attached to the green alga.   Our measurements showed a conspicuous increase in the quantity of algae present on the stream bed compared to our previous visit, though we are still puzzling about why this is the case.


The submerged world of the River Ehen, November 2014 showing diatoms growing on and around Bulbochaete: left: Fragilaria tenera, central foreground, Gomphonema truncatum, left foreground: Gomphonema acuminatum.   The Gomphonema truncatum cells are about 40 micrometres (one fortieth of a millimetre) long.

We do know that many of the stream algae found in the UK are able to grow at low temperatures and so thrive throughout the winter.   One reason why we expect the increased quantity at this time is that the grazers which feed on the algae are less active in cold weather so the ratio between algal biomass that is produced and that which is consumed decreases.   Just how effective grazers can be is illustrated by two photographs taken the following month, one of which shows even more algae present at the same site where the samples on which my picture is based were collected. The other was taken about 100 metres upstream on the same day and shows another stone covered with tiny Simuliidae larvae.   These will, in time, develop into the swarms of blackflies that can plague river users, as the adult females of many species feed on human blood.


Left: a stone from the bed of the River Ehen in December 2014 covered with Simuliidae larvae (scale bar: one centimetre) and, right, the head of a Simuliidae larva (scale bar: half a millimetre).

Of course, the abundance of Simuliidae larvae does throw some doubt on the idea that lack of grazing is primary reason for the abundance of algae.   Yet this is the first time we have seen such an abundance of Simuliidae at this particular site (which has sometimes had the greatest quantity of algae of all the sites on the Ehen that we visit) which means that we are puzzling over variability in time as well as in space.   I suspect that the long hairs on the Bulbochaete, as we know that other species with similar structures use these to secrete enzymes into the water that enable them to release phosphorus from organic molecules. My suspicion is that the wetter conditions of autumn and winter flush more of these molecules from the peaty soils of the Ennerdale catchment into the river and these, coupled with the lower grazing intensity, fuel the burst of growth that we see.


A mass of green algae (predominately Bulbochaete sp) smothering stones on the bed of the River Ehen, December 2014.   The effect of the numerous colourless hairs of the algae is to create a translucent “haze” over the top of the mass of green filaments.

Simuliidae larvae are often described as filter-feeders which attach to the substratum and have mouthparts adapted into “fans” that capture particles as they drift downstream. However, I have seen Simuliidae bent double so that they can feed directly on the benthic algae. Look at the difference in the quantity of algae between stones with and without Simuliidae, which were only about 100 metres apart.

We have been visiting the Ehen regularly now for about two years and it continues to surprise us.   Simuliidae were certainly not here in such obvious quantities at this time last year or the year before.   I suspect that we are dealing here with nothing more sinister than nature’s complex dynamics, and it makes the case, again, for a slower and more patient study of the natural world than is often possible with the present grant-awarding system (see “Slow science and streamcraft”).   Two and a half years into my explorations of the River Ehen and I’m still sometimes surprised by what I am seeing.