There is no accepted definition of a pond but these are generally recognised as small water bodies, less than 0.25ha. They are usually shallow and can often be temporary. The temporary nature of ponds is often critical as they are not large enough to support fish and consequently invertebrates dominate the fauna. They can also be important habitats for amphibians. In Ireland ponds occur naturally in fens, bogs, heaths and in sand dune systems. Most ponds are artificial such as flooded peat cuttings on cutover bogs.
On intact and active bogs the ponds are generally acidic and nutrient poor (oligotrophic or dystrophic) as they receive all their water from rainfall. They are characterised by Sphagnum mosses and a limited range of higher plants such as bog bean Menyanthes trifoliata and sedges Carex spp.
In Ireland species diversity of these ponds is low and the fauna is dominated by large dragonflies; Common Hawker Aeshna juncea, Black Darter Sympetrum danae and Four-spotted Chaser Libellula quadrimaculata. In the SW acid pools support the few populations of both the Downy Emerald Cordulia aenea and the Northern Emerald Somatochlora arctica. The only damselfly commonly found in these acid pools is the Large Red Damselfly Pyrrhosoma nymphula.
Ponds in fens, sand dunes and heaths have water which is less acidic and more nutrient rich as it is partly derived from ground water . Plant diversity is greater in these ponds including pondweeds (Potamogeton) and water lilies and marginal vegetation of sedges, especially the bottle sedge (Carex rostrata) and related species. Stoneworts (Chara spp.) can be common in very base-rich pools such as in sand dunes. Ponds in these situations are classified as either mesotrophic or eutrophic according to the amount of nutrients in the water. Visually mesotrophic ponds have open, botanically diverse swamps and fens, clear water and are often surrounded by low intensity farmland or semi-natural biotopes. Eutrophic ponds generally have very lush often mono-dominant swamps. The dominant species are those indicative of eutrophic conditions such as Phragmites, Typha and Lemna spp. The water often appears murky due to suspended sediments and algal growth. In extreme cases anoxic mud is present.
The fauna of these ponds differ. mesotrophic ponds support most Irish species of dragonfly. The species of acid ponds coexist with additional species. Typical species include the Variable Damselfly Coenagrion pulchellum, Brown Hawker Aeshna grandis, Hairy Dragonfly Brachytron pratense and Ruddy Darter Sympetrum sanguineum.
As ponds become more eutrophic Odonata species diversity decreases, and large dragonflies in particular become less common. The Azure Damselfly Coenagrion puella becomes more common at the expense of the Variable Damselfly Coenagrion pulchellum with increasing eutrophication. In very eutrophic ponds only very tolerant species such as the Blue-tailed Damselfly Ischnura elegans may be present.
|Nelson, B., Thompson, R. & Morrow, C., 2000 (May 2). [In] DragonflyIreland http://www.ulstermuseum.org.uk/dragonflyireland/|