Ponds with fluctuating water levels

Smooth newt by Philip Precey

Some Suffolk ponds appear to dry up whenever there is a hot summer while others have water levels that drop steeply downwards every year. Although it is natural for there to be some fluctuation in water levels, there are times when action or a deeper understanding of your pond's history is useful.

The following scenarios and answers are designed to help those worried about water levels.

Stickleback by Bill StevensonSCENARIO: My pond is stocked with expensive fish

Consider an emergency top up with well water only if there are no restrictions on water use to keep your fish alive. But unless you really want the fish, consider how much better the pond might be without these predators.

If your pond dries out annually a more permanent solution is probably required. It may be that the clay pond lining has a crack in it, or an artificial liner has a leak, or that the pond is simply too shallow and needs deepening.

SCENARIO: My pond dries up every year

Specialist species will make the most of any water. For instance, although animal hoof prints are small and the length of time water may actually stand in them can be extremely temporary, plants such as the toad rush will colonise them, and some invertebrate species, such as the caddisfly, will breed in them. The invertebrates confined to temporary pools or ponds are usually unable to cope with competition or predation from other animals in permanent water.

Your pond is considerably bigger than a hoof print, so it is worth how thinking how many species it may be harbouring before you take action to top up or deepen your pond.

SCENARIO: My pond dries up to five years in 10

First, it is worth considering how old these so-called ‘ponds’ are? And were they ever intended as ponds for storing water? Quite possibly not. Many waterbodies we call ponds today originated as a by-product from excavating some material (such as marle to improve the local soils, or clay to build with) and the hole left fills up seasonally, depending on surrounding water levels.

Historically, people may have used the resulting ‘pond’ to pot the occasional winter duck, but little more. However, the older these seasonal ponds, the more valuable for wildlife they often are - gradually more species (plants, invertebrates, mosses etc) have evolved to these conditions and will have colonised the site, and simply could not cope with competition or predation from other animals such as fish if these ponds were to become ‘permanent’.

These temporary ponds are now considered some of the most important with the greatest overall value for rare species such as the fairy shrimp which, like other crustaceans, survive the dry period as eggs buried in the mud, but are wiped out in permanent ponds with fish and insect predators.

Similarly, great crested newt and frogs like these conditions as there are fewer potential predators such as fish at critical times of year. Many newts and frogs will have left the pond by the time it dries up in July or August, to start moving along rough cover before hibernating away from the pond.

If the pond is shaded, the trees will be drawing considerable water, shading the pond and casting leaf litter. Coppicing and de-silting may be a solution.

SCENARIO: My pond only dries out occasionally

Like ponds which dry out regularly, the occasional drought and drying out of the pond need not be a disaster for wildlife. Indeed, for frogs, toads and newts, a drought year may help reduce their predators and whilst the drought year may not be particularly productive for them, the following year may be a comparatively safe one for their vulnerable tadpoles.

The problem, however, for these ponds is that vegetation growth is likely to be considerably more luxuriant than for those that dry out completely. Shallow waters with depths of less than 1m are susceptible to colonisation by emergent plants and need far more management as stands of reedmace or branched bur-reed can quickly colonise a pond, and open water is quickly covered over. Stock access may help by their grazing of young palatable growth, but more vigorous management

Great crested newt by Bill StevensonSCENARIO: My pond doesn’t dry out, but it fluctuates a lot

The possibly unsightly beach of exposed pond margin, created by fluctuating water levels, natural erosion or even livestock trampling, is an asset to a water body. And ponds which have a constant level all year are usually of least value for wildlife. This naked pond edge - sometimes encrusted with algae, mud or just exposed, unvegetated pond substrate - often appears of little obvious importance but its value for wildlife has probably been much neglected and for many invertebrates this area of bare ground is essential for basking, hunting, burrowing or nesting.

Different substrates will support very different species

• Soft organic mud on a very gently sloped pond edge in summer hosts a variety of wildlife. Swallows and house martins can be seen collecting mud for nesting, butterflies can be seen resting and sipping at the water’s edge and the flies of several families are found on the mud surface; with the larvae of many species living in the surface layers below.

• Firmer and less organic muds host a different community of invertebrates with those living on the surface and others in burrows where the mud can support them. Sandy margins host an entirely different collection sometimes supporting characteristic seashore species far inland.

• Vertical and near-vertical pond banks can have their own specialised fauna. Cracks and crevices provide shelter and nesting sites - and, if south-facing and dry they can provide nesting sites for solitary bees and wasps.

• Structure of the littoral zone is important too - variety will attract more species. Mud is valuable, but mud, stones and bits of wood or a few tussocks of sedge will be even more valuable.

Mallard by Bill StevensonMud, sand and pebble beaches will disappear

Unless wave erosion (on large ponds) or ducks prevents plants from becoming established, the bare muddy, sandy or pebbly beach will disappear. A good way to ensure that a large number of plants and invertebrates thrive along your pond margins, is to manage sections of the pond margins differently to provide a mosaic of bare mud and a range of different marginal vegetation.

How you manage these sections will depend on pond size and available “tools”:

• Cutting (and pulling of rhizomatous species) of the margins will ensure that margins are not taken over entirely by tall, rank, competitive species and that smaller grasses, sedges and marginal plants can have a chance.

• Scraping with machine may create some firm, vertical vegetation-free banks quickly on large ponds where hand-labour is not an option.

• Grazing with access from special bays can be very valuable as poaching and limited dunging creates excellent conditions for different plant and invertebrate species.

A shallow pond with an average depth of 1m may lose 0.5m depth due to a dry summer - it may be good for the “beach margin species”, but in nutrient-rich waters this does effectively double the concentration of the remaining nutrients in the water and may lead to other problems such as excessive algal growth and unsuitable conditions for many desirable aquatic species. For more information on algae, see our pond problem section.