Having written about a population of Oedogonium that I found in an enriched lowland stream a couple of weeks ago (see “The perplexing case of the celibate alga”), I arrived at the River Ehen last week to find much of the stream bed covered with a dense growth of filamentous green algae that, once again, turned out to be composed largely of Oedogonium. This soft water site differs in many ways from Stockerley Burn, but a species of Oedogonium seems to be thriving here too. Looking back at my notes, I see that I also found it at the same location this time last year. The quantities are such that colleagues are concerned for the health of the pearl mussels which also grow here (see “Pearl mussels in the River Ehen”).
Wefts of filamentous algae (mostly Oedogonium) on the bed of the River Ehen, July 2014. Note, too, the flocculant material on the surfaces of the boulders.
The photomicrographs of Oedogonium also show very nicely the reticulate (net-like) nature of the chloroplast. You can also see two cells of the diatom Tabellaria growing epiphytically on the lower filament.
Photomicrographs of Oedogonium from the River Ehen, July 2014. Note the cap cells (arrowed) and the reticulate (net-like) chloroplasts. Scale bar: 25 micrometres (1/40th of a millimetre).
We do not yet have a definitive explanation for the proliferation of Oedogonium in the River Ehen. The most likely explanation is that the river levels have been low for some time and this means that the gradual accumulation of algae proceeds without the sloughing that occurs when the river is in spate. The longer the algae can grow without disturbance, the higher the biomass. When there is a spate and stones are rolled over, the algae that were on top of the stone are deprived of light and gradually die and decompose. This means that the space between the stones of the substrate, through which oxygen-rich water flows to sustain the young pearl mussels becomes a fetid aquatic “compost heap” which will, in turn, extract this oxygen and make life for the young pearl mussels more difficult. As this is the last healthy pearl mussel population in England, my colleagues in conservation bodies are probably the only people in a region largely dependent on tourists to sustain its economy who are hoping for rain.