There is no accepted definition of a lake but these are generally recognised as bodies of standing water greater than 0.25 hectares. The important characteristics in determining their dragonfly fauna are surface area, water chemistry, altitude, exposure and fish populations. The presence of fish can have a profound influence on the dragonfly fauna. Fish are voracious predators of dragonfly larvae, also high densities of coarse fish can often lead to eutrophication of the system due to the increased nitrogen input from fish faeces. Once the system becomes eutrophic this condition is often maintained as the fish continually stir up the mud.
Lakes can be classified by their trophic status.
Oligotrophic lakes are base-poor, usually upland sites that support few plants. The only dragonfly species that regularly occurs in this habitat is the Common Blue Damselfly Enallagma cyathigerum.
Eutrophic lakes tend to occur in lowland areas. They have high levels of nutrients that often results in algal blooms, which cloud the water depriving aquatic plants of light and under extreme circumstances lead to the deoxygenation of the system. The only species that regularly exploit this habitat are the Common Blue-tailed Damselfly Ischnura elegans and the Azure Damselfly Coenagrion puella, large dragonflies are generally absent.
Mesotrophic lakes are intermediate between the two classes and tend to have a higher diversity of Dragonfly species.
Brackish lakes have low levels of saltwater input. A few species of dragonfly can tolerate this habitat, the Emperor Anax imperator and the Lesser Emperor Anax parthenope were recorded for the first time in Ireland in August 2000 (see News) at Lady's Island Lake, Co Wexford which is a brackish lagoon.
In addition to these general categories, there are also two rare types of lake, marl lakes and Turloughs.
Marl lakes are base rich (calcareous) and nutrient poor and are characterised by clear water, deposits of calcium phosphate (marl) and charophytes (calcareous algae). They are characteristic of the limestone areas in central and western Ireland. They are the main habitat for the Black-tailed Skimmer Orthetrum cancellatum.
Turloughs are seasonal lakes that occur in areas where there is low-lying limestone that is flooded in winter due to the increased level of the water table. This then dries out in summer when the water table is lower. They are characteristic of western Ireland occurring in counties Mayo, Galway and Clare. The Scarce Emerald Damselfly Lestes dryas and the Ruddy Darter Sympetrum sanguineum are characteristic of this habitat. Both species can complete their lifecycle within a year and lay their eggs above water into vegetation, hatching when the water levels rise.
|Nelson, B., Thompson, R. & Morrow, C., 2000 (October 5). [In] DragonflyIreland http://www.ulstermuseum.org.uk/dragonflyireland/|