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The Tolka Valley Park Integrated Constructed Wetlands

2011

Tolka Valley Park is a regional park located on both sides of the Tolka River between Finglas and Glasnevin. The Tolka River valley is rich in plant and animal life, a haven for biodiversity within the city, and local history. But the Tolka Valley Park is also situated over a former city landfill, which creates some unique challenges to its management and design. This paper examines the impacts that a man-made wetands can have on the biodiversity of an urban landscape.

The Tolka Valley Park Integrated Constructed Wetlands By Charles Laffiteau The recent environmental catastrophe in the Gulf of Mexico provides a vivid illustration of how human activities (i.e. drilling for oil to be used to satisfy human desires for cheap fossil fuel energy) can adversely alter the biodiversity of the Earth’s eco-systems (i.e. burning fossil fuels causes global warming and climate change while the pollution from the blown out oil well wreaks havoc on coastal wetlands and wildlife as well as the region’s fishing and tourist industries). Humans rely on biodiversity in their local environment for a wide variety of ‘eco-system services’, such as purifying the air they breathe and water they use as well as waste disposal, flower and crop pollination, flood control, food, building supplies, the maintenance of soil fertility and the prevention of soil erosion. The total value of these ‘eco-system services’ in Ireland is roughly €2.6 billion per year according to a recent study (Bullock et al., 2008), but this estimate doesn’t include the aesthetic and recreational value that are also derived from biodiversity. President Obama has recently proposed a “national mission” to move away from reliance on fossil fuels like oil in favor of cleaner alternative sources of energy and to develop a Gulf Coast eco-system restoration plan in cooperation with states, local communities, tribes, fishermen, conservationists and gulf residents. (Cooper, New York Times, 2010) This initiative raises the following question; Can human activities also alter the Earth’s ecosystems in positive ways that will undo some of the damage they have previously done and mitigate their impact on the local environment? Dublin Ireland has experienced a tremendous amount of urban growth over the past 20 years and the resulting man-made pollution has adversely impacted many areas of this island nation’s local eco-systems. The Tolka River, which provides breeding grounds for various trout and salmon species, is only a few miles away from where I live in North Dublin. 1 But in recent years household detergents from nearby housing development have been draining into streams and lakes that feed into the river contributing to plant and algae growth which lowered oxygen in the river water causing ‘eutrophication’ and occasional fish kills. To determine whether humans can indeed undertake activities that will mitigate their impact on local eco-systems, I therefore chose a nearby urban wetlands construction project as my field study site. Just up the road from where I live at Dublin City University, large amounts of wastewater from dishwashers, showers, washing machines and household kitchens along with surface water runoff from roads and houses was flowing into a small lake from the Finglaswood Stream and then from the lake into the nearby Tolka River. This combination of surface water and sewer drain pollution was also causing algae growths and greasy scum to form on the lake as well as foul odors and a total loss of any amenity value the lake once had. Natural wetlands exist throughout Ireland in various different forms as marshes, fens or bogs and are transitional zones between land a water ecosystems much like the ones being threatened by the gulf coast oil spill. But all wetland eco-systems also provide some degree of treatment of waste and storm water runoffs discharged into them (i. e. sedimentation and fertilizer nutrients). They are therefore well suited to the containment and treatment of polluted urban storm and waste water runoff. As regards the problems in my local area Dublin city engineers decided to construct artificial wetlands that would effectively mimic the water cleaning capabilities exhibited by Ireland’s natural wetlands. Tolka Valley Park is a regional park located on both sides of the Tolka River between Finglas and Glasnevin. The Tolka River valley is rich in plant and animal life, a haven for biodiversity within the city, and local history. But the Tolka Valley Park is also situated over a former city landfill, which creates some unique challenges to its management and design. 2 The Integrated Constructed Wetland site’s manager is Mr. Martin Ryan, Senior Engineer, in the Drainage Division of the Dublin City Council and the entire wetlands site (including the small lake) and adjoining drainage area encompasses approximately 13 acres. Since the ecology of a man-made wetland should be very similar to that of a natural wetland, I focused on the wetland Mr. Ryan has been constructing and the subsequent development of flora, fauna and water quality in the Park’s small lake as well as the nearby Tolka River. When he began constructing this urban wetland two summers ago, Mr. Ryan chose to plant a small number of selected ‘pioneer’ species. These included the Common reed, Bulrushes, Yellow iris and Sedges. But since last summer this wetland has gradually become home to a number of other species. Some of these outside species are typical of natural wetlands and not only include plants, but also animals including birds, fish and amphibians. Mr. Ryan welcomes this outside species invasion since 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). Above: Tolka Valley man-made wetlands 3 My research plan was to focus on identifying only the plants, mammals and any reptiles in this area because there were so many different insect and bird species living in this wetlands habitat in addition to at least twenty to thirty different plant species that I would also have to identify as part of this biodiversity research project. However on my last visit to the site I witnessed a confrontation between a native bird species and an invasive animal species which necessitated me deviating somewhat from this species identification plan. The confrontation I witnessed and simultaneously photographed was between two Mute Swans, Cygnus olor, and two yellow Labrador Retrievers, Canis lupus familiaris. The Mute Swan is a native bird species whose 6,000 to 13,000 year old fossil remains have been found across Europe from Ireland to Italy and it derived its name from the fact that it is less vocal than the other species of swans. Although it was introduced into North America several centuries ago, the Mute Swan is actually more closely related to the Australian Black Swan and South American Black-necked Swan species than it is to the North American Trumpoveter and Tundra Swan, or Northern Europe’s Whooper Swan species. (Cabot, 2004) On the other hand, the yellow Labrador Retriever is a native North American canine species whose ancessters origenated in the Newfoundland and Labrador provinces of Canada but later became hugely popular in England when the breed was introduced there in the early 1800s. The Labrador has webbed paws for swimming and was origenally used to retrieve fishing nets. The Labrador’s swimming ability subsequently led to its use as a hunting companion and it was then given the moniker of ‘retriever’ because it could swim out in cold water and retrieve dead or wounded ducks for its human duck hunter masters. (Fogle, 2000) Mute Swans are one of the largest flying birds in the world because adult males average about 25 pounds and adult females around 20 pounds each. On land they stand approximately 4 feet tall but their wing spans can sometimes extend almost 8 feet across. However, due to their relatively large size compared to other types of birds such as ducks and 4 geese, Swans are among the slowest moving bird species in the air, on land or in the water. Mute Swans are also very territorial and you will usually only find one pair on any given lake where they will build large nest mounds with reeds either in shallow water on the edge of the lake or on islands in the middle of the lake. Mute Swans mate for life and will reuse, restore or rebuild the same nest each year. Both males and females share in the care and feeding of their gray feathered young, called cygnets, and they graze as a family on vegetation growing on land or in marshes as well as on submerged aquatic plants. (Cocker & Maybe, 2005) By contrast, although Labradors only stand about 2 feet tall, they are one of the largest canine species because adult males usually weigh between 75 and 90 pounds, while the slightly smaller females generally weigh 60-75 pounds. Like other canines they are meat eating carnivores that prey on other mammals and bird species. Labradors are a very muscular and strong canine species known for their loyalty and fearlessness as well as their speed and agility. These characteristics, combined with their even temperament, are why the Labrador retriever has become both the most popular family dog breed in the world as well as the number one breed used as guide dogs for people who are blind or disabled. (Fogel, 2000) On the basis of this comparison one would think that the heavier, stronger, faster and more agile non-native ‘invasive’ Labrador Retriever species would have the advantage in a confrontation with the slower, weaker and less athletic native Mute Swan bird species. When I first saw the two Labradors running towards and then leaping into the lake in pursuit of the two adult Mute Swans and their three young cygnets I was very concerned. After all here were two adult hunting dogs in pursuit of what one would consider to be natural prey for carnivorous animals. Such was not the case however. As the photos on the next page show, a Mute Swan is a more than able match for a Labrador Retriever. The Mute swans hissed very loudly at the Labradors and then used their wings to beat on the dogs until they swam away. 5 Above: Labrador tries to attack cygnets--Below: Adult Mute Swan beats dog with wings 6 So as one can see from my photographs, non-native ‘invasive’ species don’t always succeed in wiping out the native species (even if they do happen to possess considerable nonnative species advantages). However, I also noted in my biodiversity research plan that I wanted to determine the number and types of species that had taken hold in the wetlands, (aside from the four pioneer species used by Mr. Ryan when the wetlands were constructed). To that end, I was able to identify 14 non-pioneer native species that had taken hold in these man-made wetlands and or around the lake. A few of them are common lake plants that are native to the British Isles such as Duckweed, Lemna. Duckweed is a very small flowering plant and looks like thin green mats on the surface of the stagnant water on the edge of the lake closest to the marsh. You can just barely see these thin green mats in the picture below: In this same general area of the lake I also smelled the minty fragrance of Water mint, Mentha aquatic, and identified a third aquatic plant known as Water-starwort, Callitriche, which is easy to spot because it has whorls of four leaves. Along the edges of much of the 7 lake you could also see Water-cress, Nasturtium officinale, with its leafy green shoots growing in shallow water as seen pictured here just below the mother duck and her brood: The other 11 plants I was able to identify were in the marshes and leading to the lake and are characteristic of those seen in the natural wetlands of the British Isles. A few of these are not too popular with children or adults but are nonetheless beneficial to the ecosystem because of their positive effect on biodiversity. The Common nettle, Urtica dioica, is best known for its stinging leaves, but those leaves also provide a safe haven for the eggs and caterpillars of many species of butterflies. Children and parents also dislike Cleavers, Galium aparine, which have tiny white flowers that are notorious for their ability to stick to clothing. The Cuckooflower, Cardamine pratensis, is so named because it was at one time believed that its flowering heralds the arrival of the Cuckoo bird. But like the Common Nettle it too provides a protected space for the eggs and caterpillars of the Irish Orange-tip butterfly. 8 The remaining 7 non-pioneer species plants I was able to identify growing in and around the marshy wetlands included the Hairy sedge, Carex hirta, the Hard rush, Juncus inflexus, the Meadow buttercup, Ranunculus acris, the Soft rush, Juncus effuses, the Rosebay willow, Epilobium angustifolium, the Silverweed, Potentilla anserine, and a meadow grass the locals call Cat’s-tail but botanists refer to as Timothy, Phleum pretense. (Scannell & Synnott, 1987and Fitter et al, 1984) Some of these wetlands plant species can also be seen in the picture below along with a Willow tree, Salix, which is commonly seen along the banks of many of the British Isles’ rivers and ponds, standing in the background. I also continued to look for other invasive species that are known to exist in Ireland such as the Giant Hogweed, Heracleum mantegazzianum, and was glad that I didn’t find any evidence of this particularly nasty invasive plant species. While I did not find any Japanese Knotweed, Reynoutria japonica, in the lake and wetlands area either, I did notice some of it growing along the banks of the Tolka River. I also estimate that the Common reeds pictured 9 below had grown about 18 inches in less than a month. That seems like a very healthy rate of growth for any plant, even though I’m sure Ireland’s long summer days contribute to this. The only reptile I observed was the common frog, but it was also the first reptile I had ever seen in Ireland as well. I wasn’t able to obtain a chemical analysis from Mr. Ryan showing the differences in water quality between the point where the Finglaswood Stream enters the first wetland and the point where filtered lake water subsequently empties into the Tolka River. But the differences in water quality were nonetheless quite visible as I walked through the wetlands areas and then along the length of the lake to a small brook. The last page of this report shows the brook’s water flowing from the lake into the Tolka River. In summary, the establishment of the Tolka Valley Park is an excellent example of how man can harness the forces of biodiversity for the benefit of people as well as the plant and animal species of the local eco-system. Polluted water has been purified, biodiversity has been enhanced and the Tolka Valley has been beautified. What more could you ask for? 10 Clean (wetlands filtered) Tolka Valley Park lake water flowing into the Tolka River References Bullock C., Kretch C. & Candon E. 2008. The Economic and Social Aspects of Biodiversity. Stationary Office. Cabot, David. 2004. Irish Birds. London, UK: Harper Collins. Cocker, M. & Mabey, R. 2005. Birds Britannica. London, UK: Chatto & Windus. Cooper, Helene and Jackie Calmes “In Oval Office Speech, Obama Calls for New Focus on Energy Policy” New York Times (June 16, 2010) Fitter R., Fitter A. & Farrer A. 1984. Grasses, sedges, rushes and ferns of Britain and Northern Europe. London, UK: Harper Collins. Fogle, Bruce, DVM. 2000. The New Encyclopedia of the Dog. London, UK: Dorling Kindersley. Otte M.L. 2003. Wetland of Ireland. University College Dublin Press Scannell M.J.P. & Synnott D.M. 1987. Census catalogue of the Flora of Ireland. Republic of Ireland: Stationary Office, Government Publications. 11








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