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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.

Lakes, ponds and puddles

Many algae are planktonic, i.e. they float or actively swim in the open water of lakes, ponds or puddles, and some have the ability to accumulate at the surface of still or slow-flowing waters. Planktonic algae are often referred to as “phytoplankton”. These groups have various mechanisms to prevent them sinking too deep (where there may be insufficient light for photosynthesis) such as gas vacuoles within the cells, propulsion mechanisms like flagellae (whip-like tails in various groups) or raphes (in diatoms) allowing these groups to swim/glide through the water, or simply the combination of small size and neutral buoyancy.

Planktonic algae that discolour the water are often seen as undesirable, but they are important components of aquatic food chains, providing food for zooplankton and small invertebrates, that in turn provide food for important fish stocks. Planktonic algae are also used extensively in wastewater treatment ponds because they can absorb nutrients and undesirable contaminants, binding them into less harmful organic matter.

Many algal groups live around the shallow margins of lakes, wetlands and ponds, including groups that are not well suited to open-water habitats. Many types of filamentous algae can attach to (or become entangled amongst) the bed or to aquatic plants in these shallow zones where there is always sufficient light for photosynthesis. These algae create the green or brown “fuzz” often seen on submerged surfaces in marginal habitats.

The effect of light on algae

Algae, like higher plants generally need light for photosynthesis (there are only a few exceptions where some protozoan algae can live without photosynthesis).

Algal blooms

Too much light can contribute to too much algae (undesirable blooms) if other conditions such as nutrient levels, streambed type and flow regime allow. The removal of shade-producing riparian vegetation is therefore one factor that contributes to the likelihood of undesirable blooms.

Algae in the food chain

Natural bush-covered streams have quite low light levels and they tend to support low algal biomass, but these habitats can still support a range of beneficial algae (often thin diatom films) that provide food for freshwater invertebrates (often grazing mayfly nymphs), which in turn provide food for fish.

The artificial shading of streams through piping eliminates almost all algae from that section of stream. This prevents algae from providing food for grazing invertebrates, and it prevents algae from performing their natural “decontamination service” (the uptake of nutrients and other contaminants from the water column).