The long period of warm, dry weather has had a pronounced effect on our rivers. Whilst out rowing on the River Wear last week, I noticed bright green patches at several points along the edges and many small brown leaf-sized mats floating down the channel. The Wear at Durham is held back by a series of weirs, built originally to serve mills, so the current here is slower than in most parts of the river, which may have helped these to proliferate here.
The River Wear at Durham, showing the weir impounding the river beside the Old Fulling Mill. July 2013.
The green patches were either free-floating or entangled amongst other vegetation at the side of the river and had a rough feel when I rubbed a portion between my fingers. I could just make out, with my naked eye, individual unbranched filaments. Under the microscope, these were revealed to belong to the genus Oedogonium. As is the case for several other green algae that we have met, identification of species within this genus requires reproductive organs, yet these are rarely seen in natural populations. The filaments of this particular population were about 45 micrometres wide, with individual cells between one or two times as long as broad. There appear to be many small chloroplasts though, in fact, these are all part of a single “reticulate” (net-like) chloroplast which is spread around the cell’s perimeter. One other diagnostic property of Oedogonium is the presence of “cap cells” (arrowed in the diagram below). About one in ten cells have a series of rings at one end. These are “scar tissue” arising from when the cell has divided.
a. Floating mats of Oedogonium in the River Wear, July 2013 (collected close to the left foreground in the picture above); b. low magnification view of the tangle of filaments; c. single filament of Oedogonium showing the “caps” (arrowed). Scale bar: 50 micrometres (1/20th of a millimetre).
Oedogonium was not the only green alga floating in the Wear at this time. I also found some growths of an alga whose tubular growth form bore a superficial resemblance to a sausage skin. This alga used to be called Enteromorpha – a comment on the superficial resemblance to an intestine. However, it has recently been reclassified into a genus called Ulva. Ulva is very common in estuaries and in the marine intertidal zone but a couple of forms are also found in freshwaters too They can be found attached to surfaces, via tiny rhizoids, but, in rivers at least, they are more commonly found free-floating or trapped amongst other vegetation. The hollow tubes act as a buoyancy aid by trapping the oxygen produced from photosynthesis.
Neither Oedogonium nor Ulva flexuosa are new records for the River Wear but it is unusual to find quite so much here in Durham. The weather is the most likely explanation, but they serve as a useful reminder of how rivers subtly change their character as the year progresses.
Ulva flexuosa growing in the River Wear at Durham, July 2013.