Streams and rivers

Most community groups carrying out freshwater biological monitoring focus on streams or rivers. Flowing habitats provide advantages and challenges for freshwater algae, depending on the taxa. A major advantage of flowing water habitats is that the supply of water-borne nutrients (required by algae for growth) is being constantly renewed. However, flowing water presents the risk that algae can be washed off the bed and carried into unsuitable habitats (including out to sea). To reduce this risk, some filamentous algae have holdfast structures that anchor the filament to stable streambed surfaces. The flexible nature of some filamentous algae also allows them to bend with the flow, reducing the drag effect of flowing water. Some filamentous algae form mats that hug the surfaces of stones where frictional forces mean the flow is much slower than a few centimetres above (the boundary layer effect). Many non-filamentous algae are so small that they can establish large populations on submerged surfaces living well within the boundary layer. Some algae can “glue” themselves to stream surfaces using mucilaginous stalks or pads. Other algae form colonies enclosed within a mass of mucilage that may be stuck to the streambed. Not surprisingly many algae thrive better in streams with stony beds than in streams with unstable sandy or muddy beds.

Freshwater algae attached to the streambed are often referred to as “periphyton”. These algae attach to most submerged surfaces, including stones, woody debris and even freshwater invertebrates (examples below).

Of course major flood events in streams and rivers create scouring conditions that will remove a high proportion of the algal community from the bed. This is nature’s way of re-setting the stream/river community, allowing the natural process of community colonisation to start over again. There will always be some algae that survives even the biggest floods (they may have survived on the sheltered side of boulders or they may recolonise from upstream reaches that were not badly affected by the floods) so there is always a source of recolonisation.