The Estuary Essay Sample
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When fresh river water mixes with sea water in a semi-enclosed basin, an estuary is formed. The formation of estuaries can also occur in drowned river valleys (Owen, 1992, p.3). A variety of plant and animal species live in the estuary as they are provided with a wealth of living space, food and shelter. Many native flowering plants and seaweeds are included in the flora of the estuaries, where the
where the plant species are distributed based on their salinity tolerances. Salt marshes form towards the high-tide level where the vegetation of the salt marsh consists of three zones. The lower marsh consist of “tall sea rush or mixtures of herbaceous plants” whereas herbaceous species such as the buck’s horn plantain and Selliera live in the mid-marsh zone.
The upper salt marsh is formed by taller species such as jointed rush, tall fescue and coastal ribbonwood (Jones & Marsden, 2005, p.11). The animal species that are found in estuaries can either be shore-dwelling, benthic or pelagic, where the benthic invertebrates that reside on shore permanently, dominates the shore animals.
The estuary is also used as a feeding and stopping-off point for birds during their migration. An example is the Manawatu Estuary, where native birds such as the Spotting Spoonbills and the endangered Wrybills, and migratory birds i.e Godwits, use the estuary as a place to rest and a place to hunt for food (Leckie, 2006, 2005). Besides that, the Manawatu Estuary is also used by the flounder and white bait as their breeding grounds (Electra, n.d.). The estuary food webs are quite easy to understand as they are mainly based on detritus, which is the matter produced by the decay or disintegration of an organic substance, and the micro-organisms associated with it (Jones & Marsden, 2005, p.13). The food web below shows an example of a food web in an estuarine ecosystem.
Estuary Food Web.
Fig 1. (Estuaries – Estuary Food Web, n.d.)
The food web begins with energy entering the ecosystem through the conversion of the sun’s energy by the phytoplankton. It can be seen in the food web that when the plants and animals die, they become detritus, which are eaten by the zooplankton, fish, invertebrates and birds. The humans capture the fishes living in the estuarine ecosystem for food.
Estuaries are also useful to humans as they are used as harbours and places to get food (Jones & Marsden, 2005, p.13). Most of New Zealand’s town and cities have developed around estuaries, such as Auckland, Wellington and Christchurch. This shows the importance of estuaries as they are used as a place to obtain fish for food, but also acts as a food supply to humans and also as harbours.
Introduction of Spartina
Three different species of the Genus Spartina can be found in the estuaries around New Zealand which are Spartina anglica, S. alterniflora and S. x townsendii. Spartina was first introduced into New Zealand in 1913 where S. x townsendii was the first species of Spartina to be introduced into New Zealand, which was taken from England and planted in the Manawatu River. S. x townsendii is a sterile hybrid that is not able to produce seeds. S. anglica was then introduced in 1928.The establishment of S. x townsendii in Invercargill had been very successful. However, as S. x townsendii was sterile, S. anglica was further introduced by Thames in 1950 as it was fertile. Both S. anglica and S. x townsendii failed to be established in the northern part of North Island, which was why S. alterniflora was introduced in 1955, which proved to be successful in Northland, more than S. anglica (Partridge, 1987, p.573).
Spartina has a massive root system which includes about one metre deep anchor roots and feeding roots that forms “a dense mat near the surface of the substrate” (Environment Waikato Regional Council, n.d.). It has the ability to trap sediments as it forms intertidal meadows that builds up the estuary bed, which leads to the change from bare tidal flat to pasture in the estuarine environment (Te Ara Encyclopedia of New Zealand, n.d.). This was the reason why Spartina was introduced into New Zealand, for the reclamation of the estuary land. This benefits the people as they are able to use the extra land for farming to raise livestock as a source of food and able to establish a place to live in (Department of Conservation, 2008). Eventual negative impacts of Spartina
Although Spartina was introduced into New Zealand to reclaim estuary land, it also had negative impacts on the environment. The large amount of sediment trapped by Spartina raises the level of the ground above the high tide mark. This in turn destroys the intertidal habitat, reducing estuaries and shallow harbours into thin drains surrounded by rank grasslands, therefore leading to a loss of biodiversity (Marlborough District Council, n.d.). Dense stands are also formed by Spartina in intertidal zones, which results in the wading birds and spawning fish losing their habitats, which then leads to negative impacts on recreational fisheries and seafood resources (Environment Waikato Regional Council, n.d.). As stated above, migratory birds use the estuary as a feeding and stopping-off point. The birds that feed on open mud are affected the most by the planting of S.anglica, as the feeding areas are removed and the feeding time is reduced (Department of Conservation, 2008). The birds also need a clear and wide space to be able to detect predators, and S. anglica deprives them off this.
This leads to the increase in emigration and mortality of the birds living in the estuary as they are more prone to being eaten by the predators. The dense mats of Spartina also cause silt to accumulate that will affect the marine life in mud flats severely as it raises the level above the high tide mark. The intertidal zone is destroyed because of this, and the marine life is affected as they have lost their habitat. Spartina and the accumulated silt can also smother the small plants and animals. This goes on to affect the population of shellfish, fish and birds further up the food web (Fig.1) and puts the whole food web out of balance (Northland Regional Council, n.d.). Because of this, Spartina has since been classified as a total control pest plant throughout New Zealand by the Department of Conservation, where councils such as Environment Waikato, Horizons and Northland have been trying to eradicate Spartina Control/Management methods and their impacts
As Spartina has been classified as a total control pest plant in New Zealand, plans have been introduced to eradicate the species. Spartina is now “banned from sale, propagation, distribution or commercial display” (Marlborough District Council, n.d.). This is to prevent the numbers of the Spartina population from increasing and also to stop the spreading of the plant. Manual methods are also introduced. The physical removal of Spartina has been attempted on many scales, which includes “hand pulling, digging out individual plants using hand implements and the use of mechanical excavators” (Department of Conservation, n.d.). Hand removal with tools can be successful but it takes a long time, is very labour intensive and would cost a lot if it was attempted on a large scale (Department of Conservation, n.d.). It is also not suitable in some situations as Spartina can have very dense root mat.
However, it is important to make sure that no rhizomes fragments are left behind to be carried away by the tide and current while removing Spartina as it grows back easily from root fragments (West Coast Regional Council, n.d., Convention on Biological Diversity, n.d.). Mechanical excavation has also been attempted in a large scale in the Avon-Heathcote Estuary, but it was unsuccessful. The reason may be because mechanical excavation is not efficient as it leaves residual root material and broken root fragments, which can be spread further by water movement (Department of Conservation, n.d.). Besides that, Spartina has been used as cattle crop in Northland previously but it is not recommended by the Department of Conservation. Although through grazing the Spartina biomass is reduced, it does not kill the plants (Department of Conservation, n.d.).
Negative impacts are also caused by grazing in estuaries, as the trampling produces rhizome fragments, which can be transported downstream where they may grow and spread (Convention on Biological Diversity, n.d.). The most common method used to eradicate Spartina is by using herbicides because it is easy to use and cost effective (West Coast Regional Council, n.d.) . Gallant (active agent Haloxyfop), is the most common herbicide used in New Zealand as it safe to use in the presence of native species, such as mangroves and Juncus sp. It also does not leach, or filter through the soil and breaks down quite fast, killing Spartina easily(Convention on Biological Diversity, n.d.). However, Gallant NF Herbicide also has a downside. It has a “high toxicity to fish and moderate toxicity to aquatic organisms”, where if spilled into waterways, fish and aquatic organisms living in the waterways risk being poisoned and may lead to dieing (Fluoride Action Network, n.d.).
It is important to control and eradicate Spartina as in the future, it will cause a lot of problems to New Zealand estuaries. In Waikouaiti River in Otago, the salt marsh behind a band of S. anglica on mudflats below salt marsh is in a slight depression as too much sediment has been trapped (Partridge, 1987). Spartina thwarts seaward drainage through this process, which will eventually lead to flooding by trapped backed-up water (Partridge, 1987, p.574). If Spartina is not eradicated, it will lead to flooding in the future. Navigational channels may also be blocked by Spartina infestations. It will also cause a further loss of wildlife in the estuary, especially in fish, birds and invertebrates which will break the balance of the food chain in estuaries. Spartina will also continue to displace native plants in estuaries, such as Zostera capricorni which is seagrass in New Zealand estuaries. Seagrass provide important shelter and food sources for fish, crabs, waterfowl and other marine life which leads to a loss in habitat and food source for these marine organisms.
If Spartina is not taken care of, it will also impact on recreation. As stated before, Spartina raises the levels above the high tide mark, where the intertidal zone will be ruined. This will lead to negatively impacting on activities such as fishing, hunting, boating, bird watching, botanizing and shellfish harvesting which are all dependant on the extant intertidal ecosystem, which is happening in Washington but could potentially proceed to occur in New Zealand (Washington State Department of Agriculture, n.d.). No biological control has been found to be effective in controlling Spartina, but Washington State is currently researching the possibility of using an insect, Prokelisia marginata, to control Spartina alterniflora (Department of Conservation, n.d.). Although this research will take a long time, there is a possibility that a biological control will be found to help control Spartina in the future.
In the estuarine areas in the Nelson-Tasman area and Manawatu Estuary, attempts of controlling Spartina have been made. In the Manawatu Estuary aeriel spraying of Spartina has been carried out just recently in February, where in the Nelson-Tasman area, attempts to control Spartina continuously have been going on for more than 30 years. The numbers of the Spartina population has decreased to a low level, but small bands of Spartina are still found where they are sprayed with the Gallant herbicide. This has lead to the Department of Conservation and the Tasman District Council to believing that eradication of Spartina will be made possible and can become an option (Department of Conservation, 2008, 2010).
Spartina must be controlled before it further disrupts the balance of the estuarine ecosystem. Eradication of Spartina has not been achieved in New Zealand, but can become a possible option. The estuary plays a very important and complex role in the life of the coast, as it acts as a breeding and feeding ground for fish and birds and a site for marine farms (Te Ara Encyclopedia of New Zealand, n.d.). New Zealanders have failed to appreciate the value of estuaries in the past, but now that the importance of estuaries have been recognised, it is important to make sure that Spartina is controlled to protect the estuarine ecosystem.
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