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Diversity and Productivity of Biotic Communities in Relation to Freshwater Inputs in Three Eastern Cape Estuaries

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Diversity and Productivity of Biotic Communities in Relation to Freshwater Inputs in Three Eastern Cape Estuaries FINAL REPORT DIVERSITY AND PRODUCTIVITY OF BIOTIC COMMUNITIES IN RELATION TO FRESHWATER INPUTS IN THREE EASTERN CAPE ESTUARIES bv U.M. SCHARLER, D. BAIRD, P.E.D. WINTER Department of Zoology, University of Port Elizabeth Report to the Water Research Commission 1997 Report No. 463/1/98 ISBN 1 86845 439 8 EXECUTIVE SUMMARY The average rainfall in South Africa (500 mm) is well below the world's average of 860 mm. Of this, only a small part (ca 9 %) is converted into runoff, which is by way the most important water source in the country (DWA. 1986). As a result, water is one of South Africa's scarcest commodities, and therefore receives considerable attention concerning its uses by various interest groups. Over half of the available water is exploited by the agrici 'rural sector, and the remaining part is divided amongst the industry and mining, urban and domestic use, as well as the environment (DWAF, 1997). The environment is estimated to use as big a proportion of water as the sector of industry and mining (between 15 and 20 % of the total), and is stated to include recreational and ecotourism uses, which are acknowledged to be a growing sector with an additional high quality demand (DWAF, 1997). The current political situation, which allows for the upliftment of the big proportion of inherently disadvantaged people, will increase the percentage of water used in the urban and domestic sector, due to increasing numbers of households being given access to quality water. The 'White paper on a national water policy for South Africa' (DWAF, 1997) states an integration of environmental, economic and developmental goals, rather than competition between these interest groups, and generally acknowledges the importance of maintaining the "ecological integrity of South Africa's water resources" and in consequence a healthy environment for South Africa's population. Furthermore, it is stated that "The Bill of Rights also gives all South Africans the right to have the environment protected for the benefit of present and future generations", which amongst other points allows for rivers, dams and wetlands to be included in the management of water in South Africa, which are recognized as 'legitimate' water users. With all these constraints and demands on the country's water resources, 289 estuaries along the South African coast (Reddering and Rust, 1990) are in addition dependent on certain amounts of freshwater inflow via their rivers for their ecological functioning. Estuaries are in need of sufficient water levels in their rivers for a variety of reasons. Firstly, freshwater inflow at the tidal head of an estuary creates a salinity gradient along the longitudinal axes of estuaries. Secondly, if freshwater inflow is not sustained, hypersalinities at the head of estuaries are likely to occur. In general, South African estuaries are flood dominated and mostly the lower reaches are effected by the accumulation of marine sand. If floods fail to occur, no scouring effect will be generated, the estuary silts up, and the estuarine mouth may close eventually. Numerous estuaries have been impacted through increased water abstraction (e.g. Kromme, Kariega, Seekoei, Orange), which has its effect in a decrease of the number and strength of floods, as well as the frequency of occurrence of mouth closures. And lastly, rivers in general carry higher concentrations of nutrients compared to seawater (e.g. Aston, 1980; Funicelli, 1984). Reduced freshwater inflow therefore deprives the estuary of sufficient nutrient input to sustain a certain level of phytoplankton production. Different habitats are created along the length of the estuary due to the presence of a longitudinal salinity gradient, created and maintained by freshwater and seawater mixing, and, a variety of plant and animal species have become adapted to this variable aquatic environment Hypersalinities can be detrimental for the estuarine flora and fauna, mainly through difficultie of coping with osmotic stress as well as unfavourable breeding conditions in these areas. Mouth closure of an estuary inhibits all movement between the sea and the estuary, which is a vital feature for many species spending part of their life cycles in both environments. Nutrient input and phytoplankton production are an indication of the amount of freshwater the individual estuary is receiving. Other biota are equally effected by this feature, and in this study it was hypothesized, that estuaries with a disrupted freshwater inflow sustain a lower biomass, productivity and species diversity of estuarine biota compared to estuaries with adequate freshwater input. Three estuaries were under study during this project, namely the Kromme, Swartkops and Sundays estuaries, which are situated along the Eastern Cape coast. Those three estuaries were chosen for the investigation of freshwater inflow on various abiotic and biotic parameters, since they differ in the amount of freshwater they receive from their rivers. Mouth closures do not occur in any of the three estuaries, neither on a temporary nor on a permanent basis. The Sundays estuary has a continuous freshwater inflow, despite two major impoundments, the Van Ryneveld Pass dam and the Darlington dam, in its catchment area. Both dams are situated in the upper half of the river, and water is supplemented via the Orange/Fish/Sundays transfer scheme. The Kromme estuary, on the other hand, is deprived of freshwater. The Kromrivier and Impofu dam in its catchment area have a combined storage capacity of over 130 % of the mean annu 1 runoff. The Impofu is situated only 4 km from the tidal head of the estuary. As a result, the Kromme estuary only receives freshwater if both are full and in addition heavy rains lead to an overflow at both dams. During drier-periods therefore, the estuary is freshwater starved. The Swartkops estuary claims an intermediate position in terms of freshwater inflow between the Sundays and Kromme estuaries. The only major impoundment on the Swartkops river, the Groendal dam, is not believed to significantly reduce freshwater flow to the estuarv. Ill These three estuaries with different river flow regimes were studied on a comparative basis. Set objectives were to: 1. assess the freshwater inflow volumes and the resulting salinity regime (Chapter 3) 2. quantify the volume of freshwater necessary to create and maintain a salinity gradient along the longitudinal axis (Chapter 3) 3. investigate the nutrient status and the phytoplankton biomass in relation to freshwater inflow (Chapter 4) 4. establish links between freshwater inflow into the individual estuaries and the biomass, productivity and species diversity of their estuarine biota (Chapter 5) During the study period, the rate of freshwater inflow was measured to be lowest in the Kromme estuary. The general freshwater inflow pattern ranged from very low to completely absent, and only once was a high flow of over 8 m3<sec"' measured at the Impofu dam 4 km from the tidal head of the estuary. Freshwater inflow into the Sundays estuary was continuos and the least variable of the three estuaries, whereas the Swartkops estuary takes an intermediate position between the two former systems. The longitudinal salinity gradient in the three estuaries was a direct reflection of the freshwater inflow pattern. During the study period, the gradient from mouth to the head in the Kromme estuary was only about 5 ppt (from 35 to 30 ppt), in the Swartkops estuary salinities dropped in the upper reaches to a mean value of 15 ppt, whereas in the Sundays estuary salinities were at a mean of ca 10 ppt in the upper reaches. The Mike 11 hydrodynamic model was applied to define the freshwater volumes needed to create and maintain a longitudinal salinity gradient in the Kromme and Swartkops estuary. In the Kromme estuary. 2 x 106 m3 per annum are allocated to the estuary, which is considered to compensate for the evaporative losses of the estuary. Various freshwater inflow scenarios with said amount were conducted by EMATEK (CSIR), Stellenbosch, to create a longitudinal salinity gradient in the Kromme estuary. Results from this study showed, that a freshwater inflow of 0.5 nr'-sec"1 for a period of 1 month would be necessary to create a gradient, which would amount to a total of 2.6 x 10° m3. One major release from the Impofu dam of 2 x 106 m3 would serve the same purpose. To maintain the longitudinal gradient, a frequent release of smaller amounts would be necessary. Judging from the existing freshwater inflow pattern in the Swartkops estuary, comparatively smaller amounts of additional freshwater would be necessary to create and maintain a longitudinal salinity Gradient throughout. IV All three estuaries were investigated for their concentrations of various nutrients, which included phosphate, nitrate, nitrite, ammonia and total particulate nitrogen. Nutrient concentrations in all three estuaries were a direct reflection of the freshwater input into the systems. The overall lowest annual nutrient input was measured for the Kromme estuary (0.22 tons P-PO4 p.a., 6.6 tons of dissolved inorganic nitrogen p.a.). A higher input of all dissolved inorganic nitrogen compounds (22.9 tons p.a.) as well as very high phosphate input (6.5 tons P-PO4 p.a.) was measured in the Swartkops system. Pollution originating mainly in the Uitenhage area is responsible for these high phosphate loads. The Sundays estuary receives higher amounts of phosphates (1.8 tons P-PO4 p.a.) than the Kromme estuary, but concentrations are not nearly as high as in the Swartkops estuary. Nitrogen was abundantly supplied to the Sundays estuary (92.9 tons p.a.) and mainly in the form of nitrate. The Kromme. Swartkops and Sundays rivers were in most cases the major nutrient suppliers to their estuaries. Once the Kromme river reaches its estuary, it contributes to an elevation of all measured nutrients not only in the upper reaches, but throughout the whole estuary.
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