What is a Pond?
The definition of a pond, and in particular how ponds differ from lakes has been the subject of much debate. Here we use the definition of a pond as established by the recently founded Norfolk Ponds Project;
A pond is a small area of shallow (generally <2 m), still, freshwater less than 100 m across. Due to their small size and a lack of wind mixing, ponds are prone to chemical stratification of the water, resulting in high oxygen and pH at the water surface and low oxygen and pH towards the pond bed. In larger shallow water bodies this chemical separation of the water does not usually occur. Ponds can be permanent or temporary, lasting for only a few months in a given year.
Why are ponds important?
- Ponds can provide vital clean freshwater in the countryside and urban areas
- Ponds can be wonderful habitats for aquatic wildlife including plants, invertebrates, amphibians, fishes, birds and mammals
- Ponds provide refuge for over two thirds of Britain’s rarest freshwater wetland invertebrates and are vital reservoirs of aquatic biodiversity (Williams, 2004)
- Good ponds act as stepping stones that allow species to move through the landscape (Joly et al. 2001; Pereira et al. 2011).
- Ponds are of great cultural importance, and can contain important historical, archaeological and palaeoenvironmental information (Prince, 1962; Jeffries, 2011)
Threats to ponds
With changing agricultural practices, especially the enlarging of fields and more intensive land drainage and fertilisation, farmland ponds in particular have been affected by eutrophication (excessive inputs of nitrogen and phosphorus). Typically this effect is most pronounced where ponds have direct connections to agricultural fields via ditches and drainage pipes and where arable land directly abuts the pond edge. Nutrient-enrichment in ponds can result in a loss of aquatic plants and dominance by planktonic algae (making the water green) and filamentous algae -often called “blanket weed” (Declerck et al. 2006; Williams et al. 2010).
Invasive species increasingly threaten pond ecosystems and biodiversity; the number of invasive species present in the UK continues to rise. In ponds one of the most widespread invasive species is New Zealand Pigmyweed (Crassula helmsii), a plant that has spread into the wild from garden centres via garden ponds. C. helmsii often completely swamps ponds and can eliminate native aquatic plants, especially shoreline species. The non-native goldfish (Carassius auratus), is also sometimes found in farmland ponds. Indeed C. helmsii and goldfish are often found together in ponds close to roads and houses suggesting a common garden pond origin. The Goldfish hydridises with the Crucian Carp (Carassius carassius), a species of major conservation concern in Europe. This hybridisation has been shown to widely contaminate Crucian Carp populations (Hänfling et al. 2005).
Since around the 1970s-1980s many farmland pond have become overgrown by trees and bushes. Prior to this period however recent research in East Anglia suggests that farmland ponds were kept open and scrub-free for a range of reasons; (i) cattle and shire-horse watering before grassland was removed from arable crop rotations; (ii) fishing by local people for species like Crucian Carp (Carassius carassius) and Tench (Tinca tinca); (iii) tree removal for firewood; (iv) tree and shrub removal to reduce habitat for Brown Rat (Rattus norvegicus) as a form of pest control; (v) a need to provide work for farm labourers during quieter periods in the agricultural calendar i.e. early winter (Prince, 1962; Rackham, 1986; Sayer et al. 2013). Now many of the reasons for controlling pond succession (and probably many others) no longer apply and the vast majority of farmland ponds are over-crowded by scrub. Such ponds usually contain black, anoxic muds dominated by poorly decomposed leaves and branches and have been shown to be generally poor in species, across multiple biological groups (Sayer et al. 2012).
Pond in-filling due to agricultural land reclamation
Linked to the post-1940s drive for farming efficiency and increased crop production, pond loss has been extensive throughout the UK and much of Europe (Rackham, 1986; Robinson & Sutherland, 2002). Levels of pond loss due to agricultural intensification range from <10% to >90%, with around 50-60% being the average for much of Europe (Wood et al. 2003; Curado et al. 2011). Even in areas where a high density of ponds remain, the ‘pondscape’ has been fragmented and reduced. This has consequences for biodiversity, and for the dispersal of aquatic and terrestrial organisms, many of which rely on ponds providing stepping-stones across the agricultural landscape (Joly et al. 2001, Pereira et al. 2011).
In many cases when agricultural ponds are filled-in, a damp depression or crop mark is left behind (above), marking the location of these lost habitats for many decades. These are ‘Ghost Ponds’, and rather than being extinct habitats they have great ecological potential, as recent research shows that re-excavation brings to the surface seeds and eggs from the past pond, which are often viable (see later section on “Restoring lost Ghost Ponds”).
A summary of all relevant Statutory Regulations, UK legislation and Planning Policy Guidance relevant to ponds and the species which inhabit them is provided by Defra here.
Principles of pond restoration
- Clean and clear is good. Water free from effluents, fertilisers and farm chemicals is vital to a healthy clear water pond. Ponds need to be buffered from run-off and fertiliser inputs from surrounding agricultural land.
- Mosaic of sunny and shady ponds. A healthy pond landscape will contain a mosaic of ponds with different shade levels including many open sunny ponds which contain abundant water plants. Sunlight is essential for aquatic plants to grow. Currently most ponds in the UK are heavily overgrown by trees and scrub, so more open-canopy ponds are urgently needed.
- Not all ponds need restoration. Ponds in woodland, ponds surrounded by old and valuable trees and ponds with important associated bog and fen habitats are worthy of conservation in their own right and should not be disturbed.
- All shapes and sizes are great. There is no perfect pond shape or size so when undertaking restoration the contours of the original pond should always be preserved and celebrated. By not working to a pre-described “ideal pond” formula, differences between ponds are maintained which should lead to increased biodiversity in the pond landscape.
- Leave ponds to natural colonisation. Ponds should be left to natural colonisation by plants and animals. Research shows water plants to return quickly following restoration from dormant seed banks. It is important to avoid the stocking of plants from garden centres and nurseries which can lead to major problems with invasive species.
- Conserve the historic nature of ponds. Some ponds began their life as medieval moats and fishponds, ornamental features in post medieval parkland or duck decoys and, as a result, are important historic features worthy of conservation in their own right. Restoration of historic ponds should be carefully considered and specialist advice sought. It may be necessary to leave ponds, their banks and important sediments within them undisturbed.
Restoration in practice
Restoring overgrown farmland ponds
There is general agreement amongst the scientific and nature conservation communities that biodiversity will be optimised where pond landscapes contain ponds at different stages of succession with varying levels of shade/tree cover. To achieve this goal, as most UK agricultural ponds are currently in a highly overgrown, state there is a need for many more ‘open’ plant-dominated ponds with low shading. Pond restoration by scrub and mud removal affords an excellent practical means of achieving this goal.
It is important to recognise that not all overgrown ponds are good candidates for restoration. In fact there are many instances where pond restoration should not be undertaken. These include the following situations:
- The pond is situated in woodland – many UK ponds are found in woodland patches and this pond type has its own characteristic flora and fauna that is highly worthy of conservation in its own right.
- Where a pond is shaded by several old and important trees, especially oak, we would not propose that management should take place – these kinds of tree should never be removed from ponds and where they dominate the pond surrounds a pond is best left alone.
- Where a pond has high quality bog or fen habitats present – these areas will likely harbour rare and important plant and invertebrate species and in this situation management is inappropriate
Before an overgrown pond is tackled it is vital to make a plan of action. [Link to section on pond restoration approaches]
Choosing ponds for restoration
Before restoring a pond it is important to consider the following:
- Are protected species present? If the pond that you are planning to restore supports a protected species (e.g. Great Crested Newt), the manner and timing of the work will need to be planned carefully to reduce disturbance.
- Does the pond have any archaeological interest? The shapes, slopes and bases of ponds are important and very old ponds may contain sediments of archaeological and environmental interest. Where ponds are old and have associated archaeological and historical interest, e.g. moats, medieval fishponds, specialist expertise and advice should be sought before undertaking pond restoration. Local councils and associated historical, monument and archaeological services (e.g. Historic England) are a good starting point.
- Is the pond connected to other waterways? Inflow pipes and ditches that connect ponds to agricultural fields and other waterways typically result in polluted ponds, whilst increasing the potential for colonisation by invasive species (e.g. American Signal Crayfish). The best ponds for restoration are isolated ponds with no inflows. Restoration should aim to buffer ponds from the farmland by use of grass headlands.
- Is it safe to carry out a pond restoration? The activities associated with pond restoration carry many potential risks. Personal health and safety should never be compromised when carrying out a pond restoration.
- How do I prevent the transfer of invasive species? Be wise to the threat of invasive species. Tiny plant fragments from non-native plants that are transferred to ponds can lead to big problems. Be careful when restoring your pond and make sure that all contractors and volunteers are aware of invasive species issues and Check, Clean, Dry procedures. – Key links: http://www.nonnativespecies.org/checkcleandry/ & http://www.nonnativespecies.org/beplantwise/
- Is there somewhere to put the sediment? Sediment removed from ponds on farmland can be spread on agricultural land to replace manufactured fertilisers to improve or maintain soil. The soil should be spread out thinly next to the pond from which it has been removed and away from the buffered margin. This U10 waste exemption is applied for from the Environment Agency.
It is recommended that this kind of pond restoration be undertaken in two stages:
Trees and scrub are typically cleared from the south and west sides of a pond with some tree cover left to the north and east. This allows a newly restored pond to receive the morning sun thus stimulating aquatic plant development. Restoration is best undertaken over September-January, especially late summer when the ground is dry and pond levels are low. This period also avoids breeding seasons for amphibians and birds.
Before any use of a digger it is a good idea to start the process of restoration by removing trees/bushes and, depending on the size of the pond, this may take from ½ to 1 day. Wood can be removed by a small team of volunteers working closely with one or two chainsaw operators. This team of volunteers can be variously employed in dragging out branches cut down by chainsaw and removing brambles and small bushes to assist the chainsaw operator’s movement around the pond.
Trees should be cut down low or removed altogether (by subsequently removing root balls using the digger) with the latter reducing the need for more frequent subsequent management. Valuable countryside trees such as oak and elm should not be removed, although in some cases coppicing and removal of overhanging branches is appropriate. Large pieces of wood, which are too much for a human to lift, can be taken out of the pond using a digger at a later stage. Very large dead trees that have fallen into the water should also be removed by digger. In some cases, for use as a platform to assist with sediment removal in the pond centre, the digger operator may decide to leave some pieces of large cut wood in the bowl of the pond. As a general rule, to achieve the maximum ecological benefit, large-scale tree removal is necessary.
With regard to the removed wood, create separate piles of potential firewood and separate piles of brash. At a later stage all the brash can usually be heaped into one large pile using the digger, or if available a teleporter (cherry picker). Trees that are left in piles will generally rot away and the wood pile will shrink down to nothing in a few years. Such wood piles likely afford good habitat for amphibians. It is generally a good idea to get most of the tree removal and wood piling done before any digging begins.
Once the pond is sufficiently cleared of trees, and if deemed necessary, it is possible to start the mud removal. Mud removal from small to medium-sized ponds can be confidently undertaken with good end results using a 360˚digger. As a general rule the following best practice principles apply:
- It is not necessary to remove all the mud from a pond, but to have a significant beneficial effect, at least one third or a half of the ponds area should be de-silted.
- In general, as soon as underlying clay or marl is seen on the bucket, it is a good idea to stop digging and move to another patch. The key aim should be to remove the highly organic, poorly decomposed leaf/twig dominated sediments that build up in overgrown ponds; these muds are poor in seeds and generally less hospitable to aquatic plant colonisation and growth. Underlying clay should be left intact.
- The original shape and dimensions of the pond should not be changed, thus preserving important natural/cultural and in some cases archaeological heritage. By sticking to the original contours of the pond, and not working to a pre-described “ideal pond” formula, differences between ponds are maintained which should in turn enhance biodiversity in the pond landscape. It may be necessary to use bog mats beneath digger wheels/tracks to ensure the original contours are not damaged.
It is important to make a plan for sediment removal before the digging starts. Use the shallow access route of the original excavators as the main way in thus allowing for easier movement of the digger around the pond and the machine does not get bogged down.
The ideal place to put the sediment is on a nearby arable field where it can be heaped up and subsequently spread out and ploughed in. The highly organic pond mud makes for a good fertiliser. It is important not to place any mud on agri-environment margins and in order to avoid this, a tractor and trailer is often useful so that mud can be taken to the heap with minimal spillage. In cases where there is no grassland margin around the pond, if appropriate, mud can be spread around the pond. As a general rule it is a good idea to keep the mud and wood piles separated. Once the bulk of the mud has been dealt with, to help achieve future plant colonisation in the shallow margins of the pond, it is a good idea to scrape the pond edges down to remove any loose and fluid mud.
What to expect post-restoration?
While pond restoration by sediment removal has been widely practiced, it has only rarely been studied. After several farmland pond restorations in Norfolk, it is possible to make a number of generalisations with regards to the response of ponds in terms of physical, chemical and biological changes. A typical sequence of events runs as follows:
- Restoration will reduce water levels due to the removal of much water-laden mud, but just a few days afterwards water-levels will rise as ground water pushes up into the new, deeper pond.
- After a few weeks it is highly likely that the pond will undergo an algal bloom, although some ponds do not do this. Algal blooms can occur at any time until perhaps April-May. Frequently a month or two after restoration the pond water will clear and if the weather is warm, even in winter, aquatic plants will start to emerge at the pond edges.
- By late spring-early summer it is typically for several aquatic plants will be growing in the pond especially Stoneworts (Chara app), Pondweeds (e.g. broad-leaved pondweed (Potamogetan natans) and Water-crowfoots (Ranunculus aquatilis). In addition the pond edges should start to colonise with plants such as Branched Bur-reed (Sparganium erectum) and Celery-leaved Buttercup (Ranunculus sceleratus). Amphibians will be evident during this period. The Great Crested Newt (Triturus cristatus) often turns up in restored ponds after an absence of many years.
- By mid summer (July-August) restored ponds frequently support a dense aquatic vegetation with near complete coverage of the dredged area highly likely. Indeed plants will often exactly follow where the digger bucket has been. At this stage more species are likely to have colonised the open waters of the pond, especially duckweeds. The pond will be a magnet for dragonflies by mid-summer and several species can often be seen at any one time.
Unless a pond is severely affected by nutrient-enrichment it should have clear water throughout year two and in the summer, water plants should extend into the non-dredged area such that they cover most of the pond. It is possible that some plant species present in year 1 (e.g. Stoneworts) may not occur in year 2 and it is equally likely that new, previously unrecorded species, will turn up. By this stage marginal vegetation is likely to be much more fully developed. In addition willow and/or alder re-growth will be very noticeable and some localised shading of the pond margins is possible.
After a few years (perhaps 5-10 depending on pond size) trees and scrub will quickly invade the edges of the pond and water plants will eventually decline. At this stage decisions need to be made with regards to pond management i.e. do trees need to be coppiced back? In this respect decisions should be undertaken with consideration of other ponds in the landscape. Ultimately pond management should seek to create a mosaic of ponds differing in terms of shade levels.
Pond Surveying & Monitoring Methods
- The National Pond Survey (NPS) Method
- The PSYM method
- The Rapid Survey Method
- The National Pond Monitoring Network has been established by the Environment Agency and Freshwater Habitats Trust, with the support of UK government agencies and NGOs.
Restoring lost “Ghost Ponds”
While re-excavating ghost ponds is not always desirable (if the original in-filling was successful, and the ‘ghost pond’ is now under productive agriculture), in many cases it can be a way of creating new good quality ponds. Where ghost ponds are located along field margins or in set aside land, their re-excavation can be conducted with very little impact on farm practice. Ghost ponds in the middle of fields can become equally good ponds for wildlife, and in cases where an attempt at filling-in a pond has left a boggy, unproductive patch of field, re-excavating the pond can be a good option; re-digging the pond can help with field drainage, and as long as a 6m buffer is left around the pond, the site is likely to become a high quality habitat, suitable for inclusion in agri-environment schemes.
The presence of suspected ghost ponds can usually be verified by looking at historic OS or tithe maps of the area (available on-line for many areas, or from the County Council e.g. in Norfolk
The process of re-excavating a ghost pond is similar to that for mud removal from an overgrown pond. The key difference however is that you must ensure you leave the historic pond sediment, and the seeds which this contains, within the newly excavated pond. The historic pond sediment is easy to identify – it is fine and silty in texture, and almost black in colour, compared to the much coarser, lighter coloured top soil.
As with an overgrown pond, where digging should stop once the marl layer is reached, with a ghost pond digging should stop once you hit the black pond sediment layer. As long as a reasonable amount of this black sediment is left in the pond, it is fine to dig down to the marl layer in some areas, creating a mixture of deep and shallow water zones, but always following the contours of the old buried pond.
Because ghost ponds and restored ponds rely on similar methods for their re-colonisation – both contain historic seeds which are re-activated by removing either the mud or top soil above them – both pond types follow similar patterns as they recover.
An important complimentary strategy to pond restoration that has the same aim of increasing the amount of clean water ponds is the creation of new waterbodies. Large or small, ponds are easy to make and wildlife will turn up within days or even hours of a pond being completed. The Freshwater Habitats Trust (FHT) has begun to make networks of new ponds as part of the Million Ponds Project – with the ultimate aim of getting back to the million ponds that once enriched the British landscape 100 years ago.
Pond creation in the the countryside, is now supported by detailed contained in the Million Ponds Project toolkit, which includes detailed information and advice on pond design and planning. FHT also provides a step by step guide as to how to make a garden pond.
Key Pond Restoration Messages
Curado, N., Hartel, T. & Arntzen, J. W. Amphibian pond loss as a function of landscape change – A case study over three decades in an agricultural area of northern France. Biol. Conserv. 144, 1610–1618 (2011).
Declerck, S. et al. Ecological characteristics of small farmland ponds: Associations with land use practices at multiple spatial scales. Biol. Conserv. 131, 523–532 (2006).
Hänfling, B., Bolton, P., Harley, M. and Carvalho, G.R., 2005. A molecular approach to detect hybridisation between crucian carp (Carassius carassius) and non‐indigenous carp species (Carassius spp. and Cyprinus carpio). Freshwater Biology, 50(3), pp.403-417.
Jeffries, M. J. Ponds and the importance of their history: an audit of pond numbers, turnover and the relationship between the origins of ponds and their contemporary plant communities in south-east Northumberland, UK. Hydrobiologia 689, 11–21 (2011).
Joly, P., Miaud, C., Lehmann, A. & Grolet, O. Habitat Matrix Effects on Pond Occupancy in Newts. Conserv. Biol. 15, 239–248 (2001).
Pereira, M., Segurado, P. & Neves, N. Using spatial network structure in landscape management and planning : A case study with pond turtles. Landsc. Urban Plan. 100, 67–76 (2011).
Prince, H. C. Pits and Ponds in Norfolk. Erdkunde 16, 10–31 (1962).
Robinson, R. a. & Sutherland, W. J. Post-war changes in arable farming and biodiversity in Great Britain. J. Appl. Ecol. 39, 157–176 (2002).
Rackham, O. The History of the Countryside. (Phoenix Press, 1986).
Sayer, C. D. et al. The role of pond management for biodiversity conservation in an agricultural landscape. Aquat. Conserv. Mar. Freshw. Ecosyst. 22, 626–638 (2012).
Sayer, C., Shilland, E., Greaves, H., Dawson, B., Patmore, I., Emson, D., Alderton, E., Robinson, P., Andrews, K., Axmacher, J. and Wiik, E. Managing Britain’s ponds–conservation lessons from a Norfolk farm. British Wildlife, 25, pp.21-28 (2013)
Williams, P. Comparative biodiversity of rivers, streams, ditches and ponds in an agricultural landscape in Southern England. Biol. Conserv. 115, 329–341 (2004)
Williams, P., Biggs, J., Crowe, A., Murphy, J., Nicolet, P., Weatherby, A., Dunbar, M. Countryside Survey: Ponds Report from 2007. Countrys. Surv. Tech. Rep. (2010).
Wood, P. J., Greenwood, M. T. & Agnew, M. D. Pond biodiversity and habitat loss in the UK. Area 35, 206–216 (2003).