A COMPARISON BETWEEN FRESH WATER WETLAND AND SALINE WETLAND
The Australian landscape has a natural component which is salt. This salt in the landscape gets into the various water bodies and affects their level of salinity which affects the way of life of the various biota inhabiting these places. Around the world and in Australia it is been identified that the salinization of fresh water is a major environmental issue. This concern of the effect of the gradual rise of salinity in water bodies’ biodiversity has resulted in the investigative studies on the lives of the invertebrate family both in the saline water and in the fresh water. There has been limited information on how the levels of salinity affect the numerous life stages of invertebrates. This paper aims at comparing the lives of invertebrates in saline water and in fresh water. The methodology involves the selection of two sites in the South West of Western Australia for sampling of Vegetation macro-invertebrates and measurement of physio-chemical parameters for water quality. From the results it can be identified that there is a difference in the life of invertebrates in saline areas and in fresh water areas.
Key words: Salinity, invertebrates, salinization, fresh water, concentration of salt
Australia has been facing a major environmental concern in the form of salinity which has specifically affected Western Australia water bodies, biodiversity, infrastructure and agricultural sectors. The rate at which salinity is increasing has been higher as large amount of water contains dissolved salts which comes the expansive landscape. Since the European settlement began most of the vegetation has been cleared up for settlement creating more sediments in the landscape. The millions of hectares in the south-west agricultural region have been harshly affected by salinity as illustrated by Land Monitor (2004). 6.3 millions hectares are projected to develop shallow saline waters over the next 50 years.
Evolution of the inland water in the south of Australia has been majorly influenced by salinity. During the geological time a large portion of the present landscape was covered by seawater as noted by (Drexel & Preiss). Most of the animals and plants in the inland waters show a close relationship with other species in the marine waters such as amphipods, shrimps, and palaemonid. There is a hint of evidence that the natural salinity could have brought about the seen in the salinity tolerance among animal species in the inland waters. (Pyper, 2000) explains how it has been found that the Western part of Australia which had natural salinity has invertebrate which are more tolerant to salinity than those in other places.
Great changes have been noted on the extent of the salinity regime in some of the inland waters. Before the river regulation was in place, the salinity extent in the River Murray located in the South of Australia greatly varied. During drought conditions and periods of low flow, salinities were found to exceed 10,000 mg L-1 as illustrated by (Williams 1999; Anon 1987). Aquatic biota found in South Australia was thus exposed to immensely variable water quality and water levels conditions. This is an indication that current studies on the lives of flora and fauna must take into consideration the past levels of salinity conditions. This exposure to such salinity condition is crucial for studying of the aquatic way of life. However, there has been no evidence that the salinity levels currently exceed what the biota can tolerate which has been associated with the lower River Murray (Williams 1999; Close 1990).
Sediments found in the South Australian surface waters are very high as explained by (Glatz, 1985) with a majority of the saline waters present in the south of Australia. An example is how numerous saline streams are located the catchment areas such as Lake Eyre drainage, Eyre Peninsula, mid north, Flinder Ranges, and around Kangoro island. As most of the water surfaces in south of Australia are temporary, most of the biota get exposed larger amplitude in their chemical and physical parameters than are in the permanent waters as explained by (Boulton & Suter 1986) due to the greater variation of salinity.
Salinization has become a very serious environmental issue in Australia. Even though the dissolved salts are natural components found in the fresh water and other aquatic systems, the high natural salinity levels are having an impact the lives of the biota. Excessive anthropogenic issues increase the concentration of the dissolved salts which have an immense effect on the aquatic ecosystems. By the year 2050 it is estimated that 5.6 million hectares will be at a risk of being affected by dry land salinity. Areas which are already affected salinity have had impacts on the economic, social, and environmental issues. This is a warning of the need to manage the impacts of salinity.
Through the balance of salt loss through drainage and leaching from the catchment and salt input through the rain, the stored becomes stored in the landscape. Regions where rainfall is low and evaporation is high the salts enter into the landscape and are not flashed away. This results in accumulation which is more often than not below the root zone.
In a poorly dissected country, storage of salt takes place aided by the factors such as flat ground thus no clear escape for surface or ground water. Salt also accumulates in area where landscape ‘sumps’ as a major part of the drainage is internal. All this is an indication of low the levels of rainfall and landscape affect the salinity of the water bodies. The saline water bodies then control how most of the invertebrates live within their ecological system. This results in the altering of the ecological structures of the invertebrates which in turn controls their living patterns. Studying on how they live in the fresh and saline water bodies gives an insight on the salinity of the water bodies affect their way of life.
The high level of salinity has been proven to affect aquatic life. Numerous studies have been conducted to support that fact that macro invertebrates fail to survive in saline water environment because of the increase in salt concentration. These organisms are only left to tolerate the conditions, however but for a period before they are affected. The study by Kefford et al (2003) confirms the assertions by showing certain fresh water organisms having lower tolerance levels when exposed to certain levels of salinity. This further indicates that freshwater wetlands have high species richness for macro invertebrates compared to salty water wetlands.
Study Site Description
Two sites in the South West of Western Australia were selected for sampling of Vegetation macro-invertebrates and measurement of physio-chemical parameters for water quality. These two wetlands were Manning Lake and Mount Brown Lake.
Manning Lake is located in the City of Cockburn, in Spearwood, Perth, WA. The climate in Manning Lake area is a warm temperate climate. Manning Lake is considered as a natural and urban wetland that has a freshwater system, a permanent water regime and a basin landform. Manning Lake also has two drains that it receives water from on its northern side. It appears that the soil type in Manning Lake area is Spearwood dune sand, with underlying Tamala Limestone (Conservation Commission of Western Australia 2005). Moreover, Manning Lake is surrounded with a periform Sememniuk Vegetation Classification. Most of the plants in Manning Lake are dependent vegetation like Freshwater Paperbark (Melaleucarhapiophylla and Baumea juncea) and Parrot Bush like Dyandra sessilis (Nature Reserves in Cockburn, 2012). The natures fauna can be found in Manning Lake are a wide range of birds, reptiles and frogs (Nature Reserves in Cockburn, 2012).
On the other hand, Mt Brown Lake is also located in the city of Cockburn, in Henderson, Perth, WA. The usual climate in the area is a moderate warm temperate climate. Mt Brown Lake is considered as a natural lake and an urban wetland that has a saline system, permanent water regime and a basin landform. It appears that the soil type in Mt Brown Lake area is Spearwood dune sand, however it has underlying carbonate and dolomite muds and muddy sands (Conservation Commission of Western Australia 2005). Moreover, Mt Brown Lake is surrounded with a periform Sememniuk Vegetation Classification.
Physio-chemical parameters were measured to assess if any of the two wetlands were significantly different. These included conductivity, pH and temperature which were measured insitu. A water sample was collected and analysed in the laboratory for colour and turbidity.
Macro-invertebrate sampling technique
At various depths the 23 macro invertebrate samples were collected over a 10m transect using a 250µm sweep net as is commonly practised (Davis and Christidis, 1997, 15). The macro-invertebrates were then separated on the basis of size, and treated with 70% ethanol in order to retain all trophic levels. Once they were identified they were classified into Mollusca, Arachnida, Crustacea or Insecta and then further differentiated to more specific levels (Miller, 1983; Davis and Christidis, 1997).
Having collected quantitative numerical data, the analysis therefore had to rely on statistical quantitative methods for data analysis. Calculation of mean and variance was necessary to help generate a figurative overview of the variables under consideration. Further statistical measures were used to develop the significance levels of the data analysed. Correlation was primarily used in determining any association between the variables under consideration. The graphs summarized the comparison of salinity tolerance of various species in the two sample settings.
Figure1: Mean number of species were found in the two lakes.
Figure 1 illustrates the mean number of species found in the two lakes. Cyprididae blue and green pointy showed a significant large number of individuals species in both sample locations. Other macro vertebrate species found in significantly large number included: calanoida and corixidae.
Figure 2: Mean number of total species for Curstacia and Insecta were found in the two lakes.
The graph illustrates the mean number of total species for Curstacia and Insecta found in the two lakes. It can be noted from the graph that the saline environment in Mount brown wetland had high richness for crustacia species compared to insect species. The explanation for the occurrence is given in the discussion section.
Figure 3: Total no.species vs. salinity in the two lakes.
The graph provides a summarized illustration of the total number of species verses the salinity in the two sample locations. Salinity levels in the two sample location were varying therefore it can be seen that most species were generally tolerant to saline environment.
Figure 4: Aquatic plant species were found in the two lakes.
Figure 4 provides a graphical representation of species of aquatic plants found in the two sample locations. It can be noted that most aquatic plant species are not affected by salinity levels. Generally, they have a tolerance for increase in salinity.
Figure 5: Terrestrial plant species were found in the two lakes.
For terrestrial plant species, the tolerance level for salinity varied in the two lakes.
Figure 6: Fringing plant species were found in the two lakes.
Fringing plant species were also found in the two lakes however at varying levels. This implied a difference in tolerance of salinity.
This study aimed at proving the hypothesis “Freshwater urban wetland, Manning Lake, will have higher invertebrate family richness than a saline urban wetland, Mount Brown Lake due to lower concentration of salt in the water”. Based on the data analysis above, it can be argued that there is a higher species richness of invertebrates in freshwater lake than in salt water lake. Other similar studies have also supported this assertion and the information is primarily used to help protect aquatic ecosystem from the effects of salinity. Salinity has been identified to have pervasive influence on every major aspect of the lives of invertebrates. Salinity has been identified as major environmental concern in Australia. The values that trigger salinity in freshwater bodies can be identified and used to manage its impact. However, salinity tolerance information is first required in order to manage the impact of salinity on invertebrates.
Based on the data analysis, the SSD values for invertebrates from the manning lake and mount brown lake were broadly similar. Considering the salinity values less than 9.9 mS, it can be noted that there was no major differences among the categories of EC (This is demonstrated in figure 6 in the result section).
These differences were in relation to the variations in the categories. However, it is also apparent, according to figure 5, that if sample numbers increased, the differences in salinity values will start will start emerging in the different categories of EC. Therefore, this has implication of species richness per sample. As salinity increases, there is a growing tendency of a type of species to be present in the sample. Similarly, an increase in EC tends to reduce occurrence of a pool of species. 1
Using mathematical techniques of quadratic and exponential functions, it was noted that species reduction that was projected by SSD was similar to the reduction of salinity levels in the EC categories. In most cases, the primary focus of the managers of these wetlands is to protect species by at least 90% and to maintain the levels of salinity to be more than 15 mS/cm. However, these goals are currently not prioritized in the two sample locations. The usefulness of SSD and its factors for safety are therefore not limited by using SSD to trace the richness in species up to the required level in salinity2.
Both the quadratic and exponential functions produced the same safety factor and this can be attributed by the fact that it is possible to use a quadratic function to approximate an exponential function3. The preference of exponential functions in biological theory is based on the fact that they have an extra degree of freedom compared to quadratic functions. In relation to this research study, the use of exponential functions produced a better match for the percentage of salinity levels than quadratic functions. This confirms that in biological theories, the use of exponential functions yield better and accurate results than when quadratic functions are used.
Many similar studies conducted, which focus on over large ranges of salinity, have reported negative correlation between richness of species in every unit sample and the levels of salinity. However, the studies that focus on slight ranges of salinity have found that the richness of species is poorly related to salinity and in some certain cases the two factors are completely not correlated. Merchant (1997) was able to establish a lack of correlations between species richness for every sample and salinity. This is despite the fact that invertebrate community is largely associated with salinity4. Other studies that used different sets of data found that there is a relationship between the community structure of invertebrates and the salinity. But, these studies still tend to confirm a lack of relationship between the two factors, salinity and species richness.
The zero correlations between the salinity and species richness in every sample can be explained using the insufficient changes in salinity in relation to variation in biotic and abiotic factors. This variation is said to affect the richness of species per sample. Table 1-4 shows are used to indicate the lack of correlation between salinity and species richness. It is however, problematic to make comparison of the per sample species richness without referring to the sample curve of species. The measure of species richness can differ because of differences in shape of abundance distribution and differences in the examined individuals. The richness of species in every sample is a quantity relating to density and Gatelli and Cowel (2009) notes that this quantity should not misunderstood or mistaken for richness of species5.
Communities of macro-invertebrate tolerate salinity levels differently (see figure 3). However, as earlier mentioned, there is a negative relationship between increasing levels of salinity and species richness. This is because different organisms are meant to adapt to saline aquatic environment while others are meant to adapt to fresh water environment6. As a result, as the level of salinity rises, growth of fresh water biota and species richness tends to be on a decline. This is demonstrated in the graph on figure 5. These assertions can be attributes to the results found in table 27. According to studies conducted in WA, crustaceans were reported to have a general tolerance of increases in salinity levels compared to insects8.
Conclusion of discussion
Fresh water invertebrates were found to have different levels of tolerance to salinity. The findings are consistent with earlier studies on the topic but different set of data. The large variation in salinity tolerance for species such as crustaceans and mollusks can be linked to the fact that these organisms can either be classified as marine or fresh water9.There is a blurred distinction between the classification of fresh water invertebrates and salt water invertebrates and there is need for more research studies to be conducted in order to establish a clear distinction of the two10. The broad range of levels of salinity determined in this study shows that some fresh water invertebrates can survive to short term exposure of high levels of salinity. The salinity concentration near that of sea water is tolerable by most fresh water organism. However, toxic levels of salinity cannot be tolerated by fresh water macro invertebrates11. This can be noted in the figure 3 in the result section. This study finally provides advancement in availability of significant information that would help protect ecosystems from the adverse effects of water level salinity.
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