Biology Fieldwork

A Level

Discussion

Swan

3. Discussion

Effects of environmental factors on freshwater invertebrates

Velocity

Velocity (the speed of the water) has a major impact on freshwater invertebrates. Adaptations of species to survive in high velocity channels include:

  • a streamlined body shape (low and flat)
  • being a strong swimmer
  • being anchored to the river bed
  • the ability to burrow into sediment

Substrate

The substrate is the material on the river bed and in the river banks. It is made up of inorganic matter (silt, sand, pebbles and rock) and organic matter (coarse or fine particulate matter, such as dead leaves and animals).

For many freshwater animals, the most important feature of the substrate is the size of the spaces between different particles, such as pebbles or sand.

These gaps provide:

  • cover from predators,
  • areas of reduced velocity and
  • a place where organic material becomes trapped.

Animals that live in spaces less than 0.5 mm in size, between sand grains, are called meiofauna and can be abundant.

Alkaline substrate, such as chalk or limestone, are important for molluscs. Dissolved rock releases a large amount of calcium necessary for shell growth.

Plants are unlikely to take root unless the substrate is soft sediment.

Temperature

The temperature in a freshwater habitat can vary according to the following:

  • altitude,
  • the time of day
  • the time of year

Local climate, microhabitats and vegetation on the river banks can often influence the temperature of water.

Oxygen

Freshwater animals use oxygen that is dissolved in water. Freshwater can be depleted of oxygen, or it can be saturated and even super-saturated.

Oxygen content at saturation depends on temperature, and the amount of dissolved oxygen decreases as water temperature rises.

Super-saturation of water can occur in turbulent streams, often where plants are actively photosynthesising.

Oxygen depletion is usually the result of the respiratory activity of organisms following organic pollution.

Oxygen is essential for most life in freshwater.

Some organisms, including invertebrates, can live anaerobically (with little or no oxygen) in the mud at the bottom of a pond.

These may be permanent anaerobes, e.g. some types of bacteria, or organisms able to survive short periods without oxygen, such as chironomid midge larvae.

The majority of organisms in freshwater habitats need a steady supply of oxygen. If the temperature of the water rises, a lack of dissolved oxygen will result in an eventual lowering of biodiversity.

pH

pH varies enormously between ponds and lakes. This can be due to the bedrock, topography and even species of plant present. 

Carbon dioxide disolves in water to produce carbonic acid. A pH test kit, measuring the levels of carbonic acid can be an indirect measurement of the level of CO2 available for photosynthesis.

The ions in carbonic acid dissociate over short periods of time to yield hydrogen and hydrogen carbonate. The tracking of pH levels in a pond over a 24 hour period will demonstrate this fluctuation well. At dawn, the water will be acidic due to the high level of carbon dioxide released through respiration by all the living organisms present. However, as the sunlight becomes available, photosynthesis occurs and the level of carbonic acid decreases.

By mid-day the water may become increasingly alkaline, until the sun sets. The absence of sunlight temporarily halts photosynthesis, so only respiration occurs in organisms overnight. The pH level decreases accordingly.

Some animals are specific to calcium-rich, alkaline water, like the White-Clawed Crayfish (Austropotamobius pallipes). Water snails, such as Lymnaea stagnalis also need calcium for their shells, so will be often found in mineral-rich water.

Mineral ion availability

There are several major sources of nutrients in ponds and lakes.

Sources include:

  • water flowing into the pond or lake, carrying dissolved minerals from rocks that the water flows over or percolates through,
  • soil washed from riverbanks and riverbeds,
  • dissolved minerals in rainwater,
  • nutrients derived from organic matter

The dissolved minerals present will depend very much on the geology of the landscape the water is moving through. It's worth noting that in uplands of granite and other igneous rock, mineral content may be quite low.

Calcium is used by both plants and animals in freshwater habitats. It is needed for bone formation in fish, cell walls in plants and shells in molluscs.

Nitrogen enters in the form of ammonia or nitrates. The latter may be due to run-off from agricultural land and can lead to the habitat suffering from an overload of nutrients (eutrophication).

Phosphate occurs naturally in small amounts and combines with iron to form ferric phosphate, precipitating to the benthic region. However, phosphate is now a significant pollutant of water entering from farmland and will also result in eutrophication.

Organic matter is an important nutrient source for life in freshwater. Decomposing plants and animals in the pond release valuable nutrients. This process is vital for the recycling of materials within freshwater ecosystems. This is called autochthonous material - materials obtained through recycling within the waterbody.

Leaves falling from trees in autumn may be blown significant distances, but the moment they touch the surface tension of freshwater habitats, the leaves stop, sink to the bottom of the pond and gradually decompose. Nutrients derived from matter originating outside freshwater are called allochthonous.

Light intensity

The primary source of energy in freshwater habitats is sunlight. However, the amount of sunlight which penetrates the water can vary due to:

  • time of day
  • season
  • depth
  • turbidity of the water (how clear the water is)
  • the amount of cloud cover
  • the altitude of the habitat

On the equator in the middle of the day, light rays from the sun hit the surface of water at a right angle. Of the total available light, 99% is absorbed by the water. Very little light is reflected back from the surface.

However, as latitude increases, and as the angle becomes more acute, the amount of reflected light increases, while the amount absorbed decreases. At 45 degrees it is about 90% absorption. At 10 degrees about 25% is absorbed.

If water is crystal clear, light may penetrate over 40 metres down into a lake. However, the intensity of light at this depth will be extremely low, just a few percent of what it was at the surface. By 10 metres depth, two thirds of the available light has disappeared.

Light Wavelength

The wavelength of light is also important. On land, yellow-green light (the middle bands of light) are of least importance to plants, whilst blue and red light (the two extremes of the visible spectrum) are the most important. Blue and red light can penetrate water effectively, but will not be able to travel far down into the water column.

Green light can pass down into deeper water. This means that green plants will appear in the upper regions of the photic (light) zone, as this is where light levels are most suitable for photosynthesis.

Some plants, especially algae, will be able to survive lower down because they produce specialised 'accessory pigments' to pick up other wavelengths of llight at greater depths.

Turbidity may be generated by organic matter in the form of detritus from decay, or it could be in a peaty area where humic acid flows into the lake. This can create very dark water. Alternatively, high nutrient levels will encourage high densities of plankton to grow.

Pools and riffles 

Many upland streams feature a sequence of pools and riffles. These are often no more than a few metres in length, so they are practical to investigate for fieldwork.

Some simple differences that you might explore are shown in the table. Note that these are generalisations only. If you do not find these results in your fieldwork, it does not mean that you river is 'wrong'.

Factor

Pool

Riffle

Water velocity

faster

slower

Substrate

smaller particles: pebbles and sand

larger particles: stones

Temperature

marginally higher (but not always)

marginally lower (but not always)

Oxygen

lower: water is not turbulent

higher: water is turbulent