One of the sites we visited had a lot of fine, silty material at the margins, washed into the river following floods a few weeks before our visit. There were a few light green patches on this silt which were dotted with oxygen bubbles as the algae made the most of the bright sunlight. Under the microscope the green patches resolved into filaments of the blue-green alga Phormidium (probably P. autumnale or a relative). You can see from the picture that this is a much simpler organism than the Stigonema that we met in the previous post, consisting just of straight, unbranched filaments. However, it is effective at growing around the silt particles, creating a “mat” of algal filaments. The Phormidium filaments are capable of limited gliding motion which means that they can adjust their position to get the maximum benefit from the light.
A patch of Phormidium autumnale (or a close relative) growing on silt at the side of the Atma River, July 2013. The air bubbles are about two millimetres across.
The next time there is a flood in the Atma, these banks of silt will probably be washed away, along with the Phormidium colonies. However, we have seen very similar colonies form more substantial growths in the River Ehen (see post of 24 April 2013), perhaps reflecting a more stable habitat though these, too, could be washed away by the larger floods.
It is often hard to convince people of the importance of algae in lakes and rivers. My work can seem abstract and esoteric but these oxygen bubbles help us put it all into perspective. Put simply, the algae are the engines of rivers, particularly fast-flowing rivers such as the Atma where higher plants cannot get established. They use the sunlight to create simple sugars out of carbon dioxide and water and this, in turn, is the food for the midge larvae and other bugs which are, ultimately, eaten by fish. The oxygen is a by-product of this process but also plays a role in keeping the river healthy. All of the other organisms in the river need oxygen if they are to survive, so there needs to be a source that can constantly replenish the supply. Algae contribute to the yin and the yang of freshwater ecology: capturing the sun’s energy and then balancing this by producing the oxygen that other organisms need to release this energy again for their own needs. Those of us who study algae tend to get bogged down with putting names on all the microscopic shapes we find and too easily forget to explain the role that they play.
A network of filaments of Phormidium autumnale (or a close relative) growing amongst silt particles in the Atma River. The inset shows a single filament (scale bar: 10 micrometres = 1/100th of a millimetre).