We have had over a week of dry weather in the north of England, so the water level in the River Ehen was lower on Friday than I’ve seen it for a long time, although the weather was still cold enough for the occasional flurry of snow. After my last experience plunging my arm into the river and soaking the sleeve of my shirt, I had come prepared today, with a box of vet’s insemination gloves which, though they don’t make the water any less cold, do at least keep my clothes dry.
Even though the trees overhanging the river were bare, the algae on the submerged stones were thriving, and added unexpected dashes of colour to the river bed. Just downstream from the outflow from Ennerdale Water, the top surface of almost all the cobbles had a vivid green colour. Closer examination showed a layer of fine green slimy filaments over a lower, browner and grittier layer. Under the microscope, the green slimy filaments have the spiral chloroplasts characteristic of Spirogyra, albeit a different species to the one I found at Cassop (see post). The lower layer was a mixture of tiny sediment particles and diatoms, mostly Tabellaria flocculosa and species of Gomphonema on long stalks. Other diatoms included Achnanthidium minutissimum growing on short stalks themselves attached to the Gomphonema stalks, some fine needle-like cells of Fragilaria rumpens, also apparently attached to the stalks, and some cells of Brachysira amidst the humic particles trapped within the matrix.
Spirogyra in the River Ehen: the image on the left shows the algae smothering a cobble; the image on the right is a microscopic view. The filaments are 60 micrometres in diameter (= 0.06 mm).
The overall effect is of a microscopic “forest”, in which the Spirogyra forms a “canopy” and the diatoms the “understory” and “herbs”. The mode of attachment of the Spirogyra was not evident, and Floras are vague on the subject, as Spirogyra is more often found as free-floating masses. There is a passing reference to rhizoids in the Freshwater Algal Flora but no illustrations, so my illustration is based instead on a paper by Nagata (Plant Physiology 59: 680-683, 1977) which suggests that terminal cells secrete “sticky” substances which promote adhesion onto surfaces. (Note, too, that the illustration is based on samples collected in October 2012, when the dominant Gomphonema species was G. acuminatum, whereas in February it was a species resembling G. clavatum).
The Spirogyra-dominated community of the River Ehen: a two-dimensional illustration on the left, showing the Spirogyra “canopy” and the diatom-dominated understory (Gomphonema on stalks and zig-zag colonies of Tabellaria), and a three-dimensional diorama on the right.
This community had been present on most of my visits over the previous year. However, in February, there was also a green jelly-like growth on and around the same stones. Under the microscope, this nondescript growth was transformed into beautiful growths of another green alga, Draparnaldia glomerata. As for Spirogyra, this was composed of filaments of cells, though this time the chloroplast is not a spiral and, more significantly, the filaments are branched. There is a main axis of cells, with clusters of side branches occurring at intervals along this. These side branches each branch again, with the final branches terminating in long, colourless “hairs”.
Draparnaldia glomerata from the River Ehen: left hand image shows a macroscopic view; right hand image shows the same organism but viewed at 400x magnification. The main filament is 50 micrometres (one twentieth of a millimetre) in diameter.
Draparnaldia is adapted to living in situations where the nutrient concentrations in the water are generally low. The hairs secrete an enzyme which enables Draparnaldia to obtain phosphorus from organic substances in the water. The peat in the fells surrounding Ennerdale will provide plenty of this, particularly in the winter when rainfall and erosion will be at their peak. Once these natural nutrients are supplemented by dissolved phosphorus in wastewater and from agricultural runoff, Draparnaldia loses its competitive edge and disappears. The presence of so much Draparnaldia in the Ehen is, therefore, a clue to the generally good health of the river.