Imagine, just for a moment, that someone makes a list of the plants growing in a river or any other aquatic habitat and includes a category called “unidentified dicotyledon”. Most botanists would throw up their hands in horror. Yet they probably have a category on their field record sheets for “filamentous green algae” which they use on a regular basis. It takes time, after all, to take a specimen back to the laboratory to check under the microscope and, let’s face it, the identification guides that are available are not very user-friendly and are full of unfamiliar terminology.
A slight variant on this particular sin is to record all the filamentous green algae that you encounter as Cladophora glomerata. You are on a fairly safe bet here because a) it is a very common alga; and, b) no-one is likely to check. However, there are a few algae that can be easily mistaken for Cladophora, especially if you are not paying close attention.
Last week, I did a survey of some streams draining into the River Browney, a tributary of the River Wear in County Durham. Most showed evidence of enrichment which was not surprising as there were small sewage works, arable cultivation and a fish farm within these catchments. And, as a result, it was no surprise to find that Cladophora dominated the stream beds at several sites. One site, however, had thick wefts of filaments which looked and felt like Cladophora but, when viewed under the microscope, were quite different.
Stockerley Burn at Bogle Hole, About half the river bed is covered by thick wefts of filamentous green algae up to about 30 cm in length.
I found three different species of green alga entangled in these wefts. There was some Cladophora glomerata but the most abundant of the three was a species of Oedogonium (see “The River Wear in summer”), characterised by unbranched filaments and cap cells. 66 species of Oedogonium have been recorded from Britain and Ireland but we know little of their ecology. Whilst some forms are common in lowland, nutrient rich rivers and streams such as this one, I have also found Oedogonium in remote, low-nutrient environments such as the River Ehen in Cumbria. It pays to be careful, in other words, and to make sure that your “Cladophora” really is Cladophora. The easiest way to do this is to check for branching using a hand-lens. If you can’t see branching in the field, take a specimen back to the laboratory and check it under a microscope. Some populations of Cladophora are much more sparsely branched than others, so you may simply confirm your original suspicions. Oedogonium is, in fact, a very distant relation of Cladophora, despite their similarity when viewed with the naked eye. Mistaking Oedogonium and Cladophora is equivalent to confusing your best friend with a sea squirt (see “Who do you think you are?”).
More about Oedogonium in the next post.
Filamentous algae in Stockerley Burn: main picture shows the wefts of (mostly) Oedogonium; inset shows cells from a single filament with the cap cells arrowed (scale bar: 10 micrometres, 1/100th of a millimetre).