Introduction
The Integrated Constructed Wetland (ICW) at Tolka Valley Park in Finglas, Dublin, was created in 1999 as a novel way of treating the polluted Finglaswood Stream that was polluted by misconnected domestic drains. This project was jointly initiated by Dublin City Council Parks and Landscape Services Division, Drainage
division and National Parks and Wildlife Service. Wastewater from dis
hwashers, showers, washing machines, kitchen waste,
oils, car washings together with surface water runoff from roads and houses
was flowing to an artificial pond in the park, via the now culverted Finglaswood Stream, before ultimately discharging into the nearby Tolka river. Pollutants from the surface water sewers were resulting in algal growths,
floating greases and milky scum, foul odours, and a total loss of any amenity
value that the pond may have had (Collins, 2007). Phosphates and Ammonia
were also contributing to eutrophication, a term used to describe the excess
application of nutrients, in the Tolka river. This phenomenon results in reduced oxygen concentrations in the river and accelerates ‘weedy’ plant and algal growth that blocks light and chokes streams – thereby creating a hostile environment for salmonid fish, including Salmon (Salmo salar) and Trout (Salmo trutta) (Giller, 1998). The subsequent construction of the ICW has been a success on many fronts:
attenuating pollutants, reducing malodours, enhancement of the pond’s amenity value, and creation of new wetland habitat for local biodiversity. This study focuses on the biodiversity of the ICW and aims to quantify how
successful the project has been in creating new habitats. The term ‘biodiversity’ is a contraction of the words ‘biological diversity’ and is
used to describe the enormous variety of life on Earth, including variations
between species, habitats and genes. All life depends on biodiversity for its
support systems and humans particularly rely on it for ‘eco-system services’,
such as purifying air and water, waste disposal, pollination, flood alleviation,
food, building and clothing materials, as well as maintaining soil fertility and
preventing erosion. In fact the total value of these services was placed at €2.6
billion per annum in Ireland by a recent study (Bullock et al., 2008). Not
included in this cost is the aesthetic and spiritualvalue that we derive from
biodiversity. The ICW provides an excellent example of how biodiversity provides these
ecosystem services, in this case attenuating pollutants and purifying water. The reeds and sedges that were planted in the wetland remove particulate
matter by physically filtering the wastewater, pathogens quickly die in its
cooler waters, while chemical pollutants such as phosphates, nitrates and
ammonia are absorbed by micro-organisms that adhere to the plants’
surfaces (USEPA, 2000), creating nutrients that are in turn taken up by the
growing mass of vegetation. The ecology of a constructed wetland can
be expected to be similar to that of a natural wetland. Upon construction, a
small number of selected species are chosen as the pioneer species. These include Common reed (Phragmites australis), Bulrushes (Typha sp)., Yellow iris (Iris pseudacorus), and Sedges (Carex sp). However, from these small beginnings, the wetland gradually becomes colonised with numerous other species. Some of these will be
wetland specialists and not only plants, but animals too, including invertebrates, mammals, birds, fish and amphibians. This invasion is to be welcomed as increased biodiversity has been proven to result in greater ecosystem functioning and consequently higher performance in terms of its capacity to remove pollutants (Otte, 2003). The Wetland Habitat
The wetland presents a unique habitat to which an array of specialist species
are adapted. The constant presence of water is the greatest contributing
factor to the ability of certain species to colonise the ICW. However other factors are also important, particularly the flow regime, the wetland shape and volume, and the type and nature of the vegetation (van der Valk, 2006). The flow of water is slow to stagnant in many places and this reduces physical
friction so that animals and plants do not need to ‘hold on’ in the way that they
do in a river system. Slow flow also results in a low level of dissolved oxygen
in the water as there is little interaction between the water body and the
atmosphere above. Dissolved oxygen principally comes from passive diffusion
and this occurs slowly and will influence water near the surface much more
than at deeper levels since there is very little mixing. This is a major limiting
factor for many aquatic organisms so that only those that are specially
adapted can survive. Unlike in a lake, where the deep, still water results in a
temperature profile from warm to cold, the shallow water of the ICW is more
uniform. The shape of the wetland is designed to be broad and shallow. The relatively
small volume to surface area ratio means that water can be close to ambient
temperature with little buffering capacity. Increases in temperature reduce
water’s capacity to dissolve oxygen and so places extra stress on aquatic
organisms. Another important factor for biodiversity in the ICW is the nature of the
vegetation. Different species of tall reeds and sedges influence light levels,
water temperatures, oxygen concentrations and water chemistry in their own
way. This results in a complex mosaic of conditions across the wetland that
increases the diversity of microhabitats to the benefit of biodiversity. The ICW
in Tolka Valley Park, which has distinct patches of different species of vegetation, will therefore result in greater biodiversity than one which relies on only one species. This is good not only for nature, but also its functioning capacity. Methodology and Constraints
Any biodiversity survey is limited by a number of factors. A wetland, like all
natural habitats, is a dynamic system. Species come and go with the passing of the seasons. Certain organisms are nocturnal or prefer particular times of
the day (birds for instance). Some are only obvious when the sun is shining,
such as butterflies, dragonflies and damselflies, while over the years different
species will colonise new ground and other will become extinct. Consequently, there is no one perfect time for which to do a survey. At a global level biodiversity is now here near being fully catalogued, and new
species are being discovered all the time. However, even at a local level it is practically impossible to fully inventory the biodiversity of a site, no matter how small. One experiment in Norway found that the number of different types of bacteria in a single gram of soil from a Beech forest could be between 4,000 and 5,000 species! Even if each species was laid out before the observer it would take a great deal of taxonomic expertise to identify the many types of ant, spider, hoverfly, or moth that may be present. For instance, there are 929 known species of aquatic flies in Ireland and the group is so vast that specialists in the subject tend to confine themselves to smaller families within the group (Ashe et al., 1998). Despite these constraints it is possible to assess the level of biodiversity on a site by using indicator species. These are groups of species that are obvious, relatively easy to identify and about which a lot is known. In Ireland, as elsewhere, the principle group of indicators for
assessing habitat type and function is the vascular (or higher) plants (as opposed to the bryophytes: mosses and liverworts). They are easily recognisable, stationary, and a lot of data has been gathered about them over the decades. Other groups can, and are, frequently used. These include the birds, excellent environmental indicators as not only do they fulfil the criteria above but their populations are extremely dynamic and sensitive to change. Butterflies and Odonates (the family of Dragonflies and Damselflies) are also useful as they are colourful, popular and their diversity is not so bewildering that anyone could learn to identify the commoner species without any specialist equipment. This survey therefore concentrated on the vascular plants and notes were made of other distinctive species as and when they were encountered. Two site visits were made on May 15th and June 11th 2008. These dates are well within the optimal time periods for conducting vegetation surveys (NRA, 2006). All species of vascular plant were identified. Other terrestrial species were not specifically surveyed for but were noted when encountered including: birds; butterflies; dragonflies and damselflies. Sampling for aquatic invertebrates was carried out using a standard net and were studied on the bank side before being returned to the water. Vascular plants are those with dedicated organs for the transport of water and food and include trees, flowers, grasses & rushes, ferns and horsetails. "Because the surveys took place during the day, the presence of bats was not
recorded. While the site does not contain any suitable roosting sites for bats, it
could well provide a foraging ground where these mammals could hunt for
moths and other night-flying insects. Indeed the wetland could provide a diversity of habitats for moths but again, being nocturnal, these were not surveyed for this report." Results
Flora
The presence of different species of flora is particularly dependant on the depth of water at different areas of the site. The cells of the ICW are designed to be shallow so that a continuous stand of tall reeds and sedges will
predominate. In contrast, the pond is too deep for most of its area to support
these plants. These ‘reedy’ areas are species-poor, being dominated by
homogenous patches of single species. Around the edges the water levels are less consistent, being drier in many places, and here a much wider diversity of species can be found. There are also some small patches of disturbed ground and this has prompted its own community of plants. For the purposes of this survey, the plant communities can be divided into two groups: those in the centre of the cells; and those around the edge. Because the pond is artificial it has quite steeply sloping edges. This feature means that there is a thin band of wetland vegetation which comprises elements of both the centre and the edge. The Centre
It is this part of the ICW that does the ‘heavy lifting’ in terms of pollutant attenuation in the wetland. Here the reeds and sedges that were originally
planted have established themselves and are comprised of the following
species:
Yellow iris (Iris pseudacorus) is a very distinctive wetland plant with its broad green leaves and brilliant yellow flowers. Sedges (Carex sp). A variety of sedges were planted including Greater pond
sedge (Carex riparia), Bottle sedge (Carex rostrata) and Greater tussock sedge (Carex paniculata). These are among the most abundant plants to be found in the wetland and are
dominant at early in the year when the taller reeds are still emergent. Bulrush (Typha sp.) can be distinguished by its distinctive brown, velvety seed-
heads. Common reed (Phragmites australis) is one of the more common species in
the wetland and is the one most commonly used for the construction of Reed
Bed Wetlands in Europe (they are avoided in the US where they are considered to be an invasive species). Their stems can grow to enormous heights, up to 3.5 meters, which grow anew every year. In winter, the seemingly dead stalks, are home to a multitude of grubs and insects that hibernate in their hollow interiors. Marsh-marigold (Caltha palustris) is a relation of the buttercup and is conspicuous with its bright yellow flowers and shiny green leaves.
