Biology Fieldwork

A Level

Method

Sweep netting in a stream

2. Method

The flow of energy through a freshwater ecosystem can be studied by sampling freshwater invertebrates. Use a random sampling technique within the water body, stream or pond.

Equipment

  • A sampling tray
  • 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. Water-borne diseases such as Weil’s disease need to be considered and care taken to ensure hands are sanitised after working in freshwater. Wear waterproof trousers and wellington boots. The site should be visited by a member of staff to check it is safe to use.

Summary of procedure

  • Carry out sampling of freshwater invertebrates within a represenative part of the stream or pond. Use kick sampling or stone washing methods for streams, or sweep netting for ponds. 
  • Identify invertebrates to the lowest taxonomic level as possible. This will help determine their trophic level.
  • The number of individuals of each species needs to be recorded.
  • Information about the feeding type and biomass of each species can be obtained using secondary sources such as the internet, scientific journals or field guides

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. 

Measuring productivity

Productivity is the rate at which energy passes through each trophic level in a food chain. It is measured in units of energy per area per period of time, typically kJ per m2 per year.

  • Gross Primary Productivity (GPP) is the rate at which plants convert light energy into chemical energy
  • Net Primary Productivity (NPP) is GPP minus respiration loss. NPP represents the energy available to the primary consumer

`"Net Primary Production (NPP)" = "Gross Primary Production (GPP)"-"Respiration (R)"`

This can be rearranged as

`"GPP" = "NPP"+"R"`

Measuring productivity in freshwater

Productivity can be measured in a freshwater pond or lake over a period of time using two glass bottles filled with pond water.

`"GPP" = "NPP"+"R"`

The first glass bottle is used to calculate NPP. Take a clear glass bottle filled with pond water that contains phytoplankton (algae). Measure the amount of dissolved oxygen present. Then leave it for 24 hours. Measure the amount of dissolved oxygen present again.

The amount of oxygen in the water should increase as a result of photosynthesis being carried out by the algae. However, since those algae are also respiring and thus using up some of the oxygen in the water, you are actually measuring is the net photosynthesis or net primary production.

`"NPP" = "Final concentration of oxygen in light bottle"-"Initial concentration of oxygen in light bottle"`

The second glass bottle is used to calculate R. Take an identical sample of water and put it into a darkened bottle, which excludes all light and therefore prevents photosynthesis taking place. Measure the amount of dissolved oxygen present. Then leave it for 24 hours. Measure the amount of dissolved oxygen present again.

The amount of oxygen in the water should decrease as a result of respiration being carried out by the algae. No photosynthesis will take place as there is no light.

`"R" = "Initial concentration of oxygen in dark bottle"-"Final concentration of oxygen in dark bottle"`

Gross Primary Productivity can be calculated using these two values:

`"GPP" = "NPP"+"R"`