Choosing a fieldwork location

A suitable site must be found that is easy to access and safe, with no issues regarding access rights. Ideally displaying all the seral stages of freshwater succession within a relatively small area. Many freshwater habitats can be fragile, please be mindful of this when visiting the site(s).

The raised peat bog at Malham Tarn field centre in North Yorkshire provide a perfect opportunity to experience the entire serial stages of freshwater succession. The Yorkshire dales national trust carry out regular conservation work on the raised peat bog to prevent reed growth and reduce the drying effect.

Investigation aims

There are two broad types of hypotheses that you can investigate freshwater plants

a comparison in the ground vegetation in two areas which differ in some way e.g. close to open water vs some distance away from the open water.
an investigation into ground vegetation along an environmental gradient
this can be spatial (e.g. the effect of a slope at the edge of a pond) or temporal (e.g. across an ox-bow lake inland)

Equipment

Students can carry out the fieldwork in groups of 2-4. Each group will need

A quadrat – either frame or point.
Two ranging poles and clinometer.
Equipment for measuring abiotic factors
Trowel and labelled plastic bags for taking soil samples.
Identification guides, such as the FSC fold-out chart.

Safety

Care should be taken when working near water.

Beware of water borne diseases such as Weil’s disease.
Wear sturdy footwear and beware of slippery and other uneven ground.

Procedure

Carry out a pilot survey to identify a section of the area where there is a 30m stretch along an environmental gradient or tow areas that differ.
Carry out an interrupted belt transect or random sampling. You can use a frame quadrat or a point quadrat. This should be done at regular intervals, perhaps every 3 metres giving an stratified sample. Or at 10 randomly generated coordinates within a sample grid. At each site vegetation data should be collected to allow for the calculation of percentage cover.
Use ranging poles and a clinometer to collect slope profile data. Take readings for every significant break of slope.
Collect abiotic measurements such as soil and micro climate data at regular intervals. This could be the same time and locations that you collect vegetation data.

Fieldwork

There are two broad types of hypotheses that you can investigate freshwater palnts

a comparison in the ground vegetation in two areas which differ in some way e.g. close to open water vs some distance away from the open water
an investigation into ground vegetation along an environmental gradient
this can be spatial (e.g. the effect of a slope at the edge of a pond) or temporal (e.g. across an ox-bow lake inland)

For a comparison of ground vegetation in two areas, construct a random sampling strategy. You need to justify your choice of sampling locations. Use quadrats (either frame or point quadrats) in each sampling location. There is always a minimum number of samples that must be taken if you are performing statistical tests on your results.

Measuring abiotic factors when investigating freshwater plants

Generally the most useful abiotic factors to record are soil characteristics.

Remove vegetation with a trowel. Take a small sample of soil (a quarter of a trowel full is plenty) from 15cm below the surface of the vegetation and seal it in a polythene bag straight away to prevent it from drying out. Put a label marked in pencil inside the bag to show the location and time. Plastic 35mm film pots (becoming much rarer now that digital cameras are widely used) are a useful alternative to polythene bags.

Collecting soil samples by digging holes is destructive and you must have permission from the landowner before you start. Only remove a small amount from each place – not a trowel-load. Replace any vegetation you have removed. Wash your handsthoroughly after carrying out soils work.

Here are some of the most useful soil factors that you can measure:

1. Soil pH

A chemical pH test can be carried out in the field, or on the soil samples in the lab.

Put 1 cm of soil into a test tube
Add 1 cm of barium sulphate to the test tube (this attaches to fine particles and makes them sink leaving a clear layer above it)
Add enough distilled water to bring the level half way to the top of the test tube
Add two pipettes full of indicator solution
Seal the test tube with a rubber bung and shake well (make sure that the contents are mixed thoroughly)
Allow the test tube to stand until the soil has settled out leaving a coloured solution
Hold the tube next to the colour chart and decide which pH colour it most closely matches.
Dispose of the soil in the bin.

2. Soil texture

The soil texture is determined by the proportion of the soil made up of sand, silt and clay. A quick way of judging soil texture in the field is outlined in the OPAL Soil Survey (pdf).

3. Soil moisture content

Soil water content is measured by weighing the fresh soil sample, drying the sample, then weighing the dry soil. The difference between the two figures is the soil moisture content. It can be expressed as a percentage of the mass of the fresh soil sample.

The most effective way to dry the soil sample is to use an oven (a microwave oven will do), but if this is not available then you could leave the soil sample to dry overnight.

Find the mass of a heat resistant crucible.
Half fill the crucible with soil from the soil sample and then weigh it again.
Place the crucible in an oven at a temperature just above the boiling point of water (ideally around 105°C) until it is dry.

You can tell when it is completely dry by weighing the crucible and soil, returning it to the oven for 10 minutes and weighing again and so on, until there is no change in weight after 10 minutes. Don’t be tempted to turn the oven up to make the drying take place more quickly. You will only begin to burn off the humus in the soil and won’t then be able to find out either the weight or humus content accurately. Now record the weight of the crucible and dry soil. Find the diference between the dried soil and the fresh soil, and use this to calculate the % moisture content of the fresh soil.

4. Humus content

Soil humus content is measured by weighing a dry soil sample, burning the humus in the soil, then weighing the soil left. The difference between the two figures is the humus content, which can be expressed as a percentage of the mass of fresh soil sample.

It is usual to measure for soil humus content after you have measured for moisture content, so that you have figures for the mass of fresh soil and also samples of dry soil.

The most effective way to burn off the humus is to use a furnace that is capable of reaching a temperature of 550°C. If this is not available, a Bunsen burner can be used instead.

If using a furnace, place the crucible and dry soil into a furnace at 550°C for 15-30 minutes, until all the humus has been burnt off.

Or if using a Bunsen burner Using a tripod and gauze sheet, heat the soil sample in a crucible above a full-flame Bunsen burner for 30 minutes to 2 hours, until all the humus has been burnt off.

Record the weight of the crucible and soil and calculate the percentage of humus in the soil. The mass of dry soil, rather than the mass of wet soil, is used to calculate the percentage humus content. Using dry soil mass allows you to compare soil samples taken on different days.

Purchase ‘freshwater’ identification guides