How healthy is your pond?
1. Introduction and Overview
PSYM, the Predictive SYstem for Multimetrics, (pronounced sim) has been developed to provide a method for assessing the biological quality of still waters in England and Wales. The method uses a number of aquatic plant and invertebrate measures (known as metrics), which are combined together to give a single value which represents the waterbody's overall quality status. The method uses the following steps:
- Simple environmental data are gathered for each waterbody from map or field evidence (area, grid reference, geology etc.).
- Biological surveys of the plant and animal communities are undertaken and net samples are processed.
The biological and environmental data are entered into the PSYM computer
- uses the environmental data to predict which plants and animals should be present in the waterbody if it is undegraded,
- takes the real plant and animal lists and calculates a number of metrics.
Finally the program compares the predicted plant and animal metrics with the real survey metrics to see how similar they are (i.e. how near the waterbody currently is to its ideal/undegraded state). The metric scores are then combined to provide a single value, which summarises the overall ecological quality of the waterbody. Where appropriate, individual metric scores can also be examined to help diagnose the causes of any observed degradation.
2. Survey Methodology
Pond PSYM has currently been developed for use in the Summer season (June, July, August), and is based on assessments of both macroinvertebrate and macrophyte assemblages. Pond PSYM can be used on ponds and small lakes up to about 5 ha in area in England and Wales. The method can, in theory, be used to assess the quality of seasonal ponds, but in practice it "over-predicts" for ponds which are highly seasonal (i.e. which dry hard every year), and is best restricted to ponds which are either permanent, or semi-seasonal (i.e. which dry occasionally in very hot years).
2.1 Environmental Data
The methods used to collect the main predictive variable data are briefly outlined below.
reference and altitude
- Grid reference: six figure reference, taken from 1:50,000 or 1:25,000 OS maps, input into the model as Easting and Northing (100 km cell reference followed by 3 figures). Alternatively, pinpoint the grid values using the utility on the right of this page.
- Altitude: in metres above sea level, taken from 1:50,000 or 1:25,000 OS maps.
- pH: measured either (i) in the field in a bucket of water taken from a representative area of the pond, or (ii) using a water sample collected in the field and analysed later in the laboratory. For laboratory analysed samples, use acid washed bottles stored in a cool place after collection (e.g. cold box) and analyse within one day of collection.
- Pond area: this is the area lying within the outer edge of the pond. The pond dimensions can be measured using a tape, or by careful pacing. A small sketch can help to make this estimate. For large ponds it can be easier to use an OS map outline, with the dimensions checked in the field.
- Pond overhung: the percentage of the pond area which is directly overhung (e.g. by trees, scrub etc.).
- % of pond edge grazed by livestock: the percentage of the perimeter of the pond to which livestock have active access. Note that if cattle, sheep, horses etc. are not grazing at the time of the survey, their presence can be detected by other features such as poaching of the ground.
- Pond base: the rock type underlying the pond (beneath the sediment). This can often be assessed directly in the field, or be determined using a geology map. In the field, push the handle of the pond net through the sediment into the base. Exact measurement is not necessary, only broad categorization into one of three percentage categories: 1= 0%-32%, 2=33%-66%, 3=67%-100%. Lined ponds are categorised as 3.
- Inflow: whether or not the pond has a surface inflow. This can be a direct or indirect inflow from a river, stream, ditch, spring or seepage. The inflow can be dry at the time of the survey.
- Emergent plant cover: the percentage of the pond covered by emergent plant species. The term "emergent plant species" includes all species listed as emergents on the wetland plant recording sheet. It includes these species regardless of their habit at the time of the survey (e.g. some emergent species may be growing predominantly under water at the time of the survey). It does not include any other species e.g. terrestrial species or plants specifically defined as "submerged" or "floating-leaved" plant species on the wetland plant recording sheet. Estimates of the percentage cover of emergent plants should be made for the whole area withi the outer edge of the pond, not the current water area. The cover of sparsely growing stands of plants (e.g. occasional bulrush plants with much open water between), should be estimated as if they were growing closely together. The easiest way of doing this is to imagine all emergent plants pushed together on one side of the pond, with an estimate then made of what proportion of the pond this covers.
A conductivity test can also be undertaken to measure the total quantity of chemicals dissolved in the water. If the conductivity is 500-1000 + µS/cm, it is usually a sign of some kind of pollution. Perfectly clean water pond would have a figure of 100µS/cm or less. This metric is not used in the PSYM calculations.
2.2 Invertebrate Surveying
The pond invertebrate survey methods used for PSYM are based on standard three minute hand-net sampling methods developed for the National Pond Survey (Pond Action, 1998).
All the main mesohabitats in the pond are sampled so that as many invertebrate species are collected from the site as possible. Examples of typical mesohabitats are: stands of Carex (sedge); gravel- or muddy-bottomed shallows; areas overhung by willows, including water-bound tree-roots; stands of Elodea, or other submerged aquatics; flooded marginal grasses; and inflow areas. As a rough guide, the average pond might contain 3-8 mesohabitats, depending on its size and complexity.
The three-minute sampling time is divided equally between the number of mesohabitats recorded: e.g. for six mesohabitats, each will be sampled for 30 seconds. Where a mesohabitat is extensive or covers several widely separated areas of the pond, the sampling time allotted to that mesohabitat is further divided in order to represent it adequately (e.g. into 6 x 5 second sub-samples).
Each mesohabitat is netted vigorously to collect macroinvertebrates. Stony or sandy substrates are lightly "kicksampled" to disturb and capture macroinvertebrate inhabitants. N.B. deep accumulations of soft sediment are avoided, since these areas typically support few species and collecting large amounts of mud makes later sorting extremely difficult. Similarly, large accumulations of plant material, root masses, and the like should not be taken away in the sample: the idea is to dislodge and capture the animals without collecting an unmanageable sample. Note: the three-minute sampling time refers solely to "net-in-the-water" time, and does not include time moving between adjacent netting areas around the pond.
Amphibians or fish caught whilst sampling are noted on the recording sheet and returned to the pond. However, neither contributes towards the PSYM score.
A further 1 minute (total time, not net-in-the-water time) is spent searching for animals which may otherwise be missed in the 3-minute sample. Areas which might be searched include the water surface (for whirligig beetles, pond skaters etc.) and under stones and logs (for limpets, snails, leeches, flatworms etc.). Additional species found are added to the main 3-minute sample.
Record findings in the columns on the field sheet as follows. If present and so included in ASPT calculation, record in the "ASPT" column, if a dragonfly or alderfly family also record in the "OM" column, or if a Coleoptera family or beetle in the "Cole." column.
2.3 Plant Surveying
The aim of plant recording is to make a complete list of wetland plants present within the outer edge of the pond. The field recording sheet gives a definitive list of the plant species regarded as "wetland". Terrestrial plants and wetland plants growing outside the outer edge of the pond are not recorded. The wetland plant recording sheet includes submerged, floating-leaved and emergent macrophytes, and these groups are used separately in analysis.
Pond macrophytes are surveyed by walking or wading the entire perimeter of the dry and shallow water areas of the waterbody. Deeper water areas are sampled either using a pond net or by grapnel thrown from shallow water or from a boat. Most wetland plants are readily identifiable using a hand lens. However, with a few species (especially fine-leaved Potamogeton and Callitriche spp.) it may be necessary to remove a small amount of plant material for later microscopic examination and confirmation.
Record macrophyte species found on the wetland plant recording sheet.
3. Data Processing
The recording sheets for the environmental data, plants and invertebrates surveys exist in electronic format in Microsoft Excel, as well as the web version linked here. The benefits of the electronic format are that they automatically calculate three plant metrics and three invertebrate metrics:
- Average score per taxon (ASPT)
- Number of dragonfly (Odonata) and alderfly (Megaloptera) families (OM)
- Number of beetle (Coleoptera) families (COLEOPT).
- Number of submerged and emergent plant species only (excludes floating)
- Number of uncommon plant species (rarity score of 2 or more)
- Trophic ranking score for aquatic and emergent plants (TRS). The TRS is the sum of the trophic scores for each plant present, divided by the total number of plants which have a trophic score. Note that some plants do not have a trophic score.
The derived plant and invertebrate metrics defined above, along with the environmental data, are then fed into the PSYM Macro to derive a final PSYM score, expressed as a percentage. The higher the score, the better the waterbody's quality status. The PSYM score is placed into four categories, which reflect the ecological quality of the pond. The categories are:
- 0 - 25% is very poor
- 26 - 50% is poor
- 51 - 75% is fair
- 76 - 100% is good
The aim should be to increase the PSYM score year on year. This can be achieved by a number of factors, not least of which is the annual pond maintenance undertaken during the winter months. A Pond Management Plan formalises the maintenance activities.