Biology Fieldwork

A Level

Method

Swan

2. Method

Before you start

The distribution and abundance of freshwater invertebrates can be measured by randomly sampling in two different micro-habitats within the water body, or systematically along the length or width of a stream or pond. 

Equipment

  • A sampling tray - a pale coloured tray is best, as it contrasts with the brown/green invertebrates in the sample.
  • D frame net
  • Identification guides, such as the FSC Freshwater Name Trail fold-out chart
  • Hand lens, plastic spoon and/or pipette
  • Equipment for measuring abiotic factors (e.g. velocity, depth, water temperature, water pH, oxygen, nitrate content, light)

Safety

  • Care should be taken when working in freshwater environments.
  • Waterborne diseases, such as Weil’s disease, need to be considered. Care must be taken to ensure hands are sanitised after working in freshwater.
  • Wear waterproof trousers and wellington boots.
  • Do not work by yourself.

Summary of procedure

  • Carry out a pilot survey to identify contrasting habitats, such as two sections of a stream which differ in some way, such as pools compared with riffles. Control as many other variables as you can, such as altitude, rock type, human disturbance.
  • Carry out sampling of freshwater invertebrates in each area, using kick sampling or stone washing methods for streams or sweep netting for ponds. 
  • Identify invertebrates to the lowest taxonomic level as possible.
  • Record the number of individuals of each species or estimate abundance if they are in large numbers, such as water fleas (Daphnia sp).
  • Take sufficient samples for your data to be statistically representative. For many statistical tests, 10-30 samples are needed in each area.
  • Collect abiotic data at the same time as your biotic data, ensuring sufficient repeats for the statistical test you wish to carry out.
  • Return all invertebrates to the habitat and make sure all the equipment is rinsed clean and sanitised

Why sample?

It is not practical to record every single invertebrate. Instead you can take a representative sample.

If you are comparing two or more areas, take several random samples at each area. If you are investigating the effect of an environmental gradient, take a stratified sample. Here are some examples of sensible sampling strategies:

  • Investigation question: is there a difference in freshwater invertebrate species between the open water and the vegetated parts of the pond.
    Sampling strategy: 5 sweep net samples are taken in open water, and 5 sweep net samples in vegetated parts of the pond.
  • Investigation question: what is the impact on freshwater invertebrates of the sewage plant outfall on the river?
    Sampling strategy: 10 locations are chosen at 1km intervals along the river, including upstream of the sewage outfall. At each location, 2 sweep net samples are taken.

Disturbance sampling

During the day many freshwater invertebrates are generally found in the substrate (stones and mud) at the base of the stream, river or pond.

The idea of disturbance sampling is to dislodge the invertebrates from the substrate and trap them in a net. You can then take them out of the net for counting and identification. There are several techniques for disturbance sampling:

Stone washing

In shallow still or slow-moving water with a stony bottom, lift a sample of stones out of the water and quickly put each one into a net. Wipe or scrub the stones to disturb any invertebrates, which will be trapped in the net.

Kick sampling

Suitable for shallow running water with a gravel or muddy bottom. Hold a fine-mesh net in the direction that you are facing. This should be downstream of where you are standing. Use one foot to kick the bottom of the stream, dislodging the substrate in the direction of the net. Animals dislodged from the substrate will be washed into the net.

As sampling disturbs the substrate, always take the first sample at the lowest point upstream, then work back upstream.

Make sure you standardise:

  • The time you spend kicking at each site (e.g. 30 seconds)
  • The area of stream bed that you sample at each site

You could place a 50cm x 50cm quadrat on the stream bed, and only kick within the area of the quadrat. If the mouth of the net is smaller than the quadrat, you may need to kick more than once. By standardising the time spent kicking and the area disturbed, you can make a direct count of the density of invertebrates. If you standardise just the time spent kicking, you make a relative estimate of abundance only.

Sweep netting

In still water such as a pond, or if the water is too deep to enter safely, stand on the bank and vigorously sweep a net in the water, ideally tracing out a figure-of-eight shape.

Make sure you standardise:

  • The time you spend sweeping at each site (e.g. 30 seconds)
  • The volume of water that you sample at each site

You could place a bottomless plastic dustbin in the water, to limit a known volume of water, and only sweep the net within the area of the dustbin. By standardising the time spent sweeping and the volume of water swept, you can make a direct count of the density of invertebrates. If you standardise just the time spent sweeping, you make a relative estimate of abundance only.

Surber sampler

The horizontal frame is placed on the bed with the mouth of the net facing upstream. Use a hand trowel to disturb the substrate within the frame. The net bag can then be emptied into a white tray for identification and counting.

Identifying invertebrates

Add some water from the stream or pond to a white plastic tray. Invert the net and empty the contents fully into the tray. After you have emptied the contents, wash the net through with pond or stream water before using it again.

Leave the tray to settle for about a minute. it will be much easier to identify the animals once you see them moving. Use a spoon or pipette to move different groups of animals into holding trays. Try to identify and count as many of the animals as you can. Use FSC freshwater invertebrates guide to aid identification.