Open Access Reports

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Browsing Open Access Reports by Author "Allanson, BR"

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    Limnological criteria for management of water quality in the Southern hemisphere
    (National Scientific Programmes Unit: CSIR, 1984-12) Hart, RC; Allanson, BR
    The report presents the proceedings of an international workshop on the management of Southern Hemisphere inland waters held in Wilderness, South Africa, in July 1984. It includes an introduction and seven chapters dealing sequentially with catchment management, stream regulation, pollution, salinization, turbidity and suspensoids, eutrophication, and fisheries. Given that limnology is concerned with understanding that diverse array of intrinsic and extrinsic factors which together govern the physical, chemical and biological characteristics of lakes, reservoirs and rivers, each chapter attempts to assess the specific or general role it makes to the holistic management of water quality in the Southern Hemisphere in general (excluding Antarctica and the sub-Antarctic islands) and the more arid latitudes in particular. In so doing, attention is drawn to the relevant use of terminology and definitions which, when used precisely, will materially decrease the semantic confusion which exists between the science of limnology and its application in the maintenance of surface water quality. The chapters also represent a concensus of viewpoints between limnologists and managers in which a considerable amount of tedious descriptive detail has been very largely replaced by highlighting those processes considered essential not only in the implementation of modern limnological principles to water management practise, but also in drawing up research priorities.
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    Limnology and fisheries potential of Lake le Roux
    (National Scientific Programmes Unit: CSIR, 1983-12) Allanson, BR; Jackson, PB
    The fisheries potential of the reservoir in the Orange River has been determined by the combined studies of ichthyologists and limnologists. The combination has been synergistic. It has been shown that the physical structure of the reservoir, coupled with its short retention time of less than one year and the rapid attenuation of light by the high suspensoids, are together responsible for the low summer temperature of the lake. Because of this the overall metabolism of the lake is low. It is argued that this is the primary factor in control of the reservoir's production of fish. Included in this is the effect of rapid light attenuation upon the photosynthesis of phytoplankton and therefore the production of immediately assimilable plant cells such as diatoms which are grazed by filter feeding zooplankton. Dense blooms of Microcystis aeruginosa do occur during the second half of summer but they do not seem to increase the standing stock of zooplankton upon which the principal fish with a pelagic life history stage, the smallmouth yellowfish, feed in the subadult stage. This important food fish selects the large zooplankters individually, depending on vision to catch its prey. The maintenance of high suspensoid concentrations therefore reduces the efficiency of foraging. At age 2+ years a return to a more inshore feeding mode is made when considerable natural mortality, up to 80% by the end of the 3rd year, occurs. Annual breeding takes place regularly, confined almost exclusively to the flowing Orange River at the lake's upper end. In contrast the other important food fish, the Orange River labeo, spawns irregularly depending on more local rainfall; therefore strong year classes are only produced in some years. Total annual yield is provisionally assessed at 150 - 250 tons, giving, at R 1/kg, a minimum value of R 150 000 per annum. It is therefore recommended that a commercial gill-net fishery be established, primarily for yellowfish, with a mesh size chosen to optimize the capture of subadults up to 30 cm long to harvest these before they die of starvation. Larger fish would remain and form the basis of a recreational angling fishery. This management proposal is unorthodox but essential if the productivity of the lake is to be used economically. Recommendations include the careful monitoring of the commercial catch in conjunction with continued assessment of the response of the stocks to this policy by fisheries biologists of the Cape Department of Nature and Environmental Conservation.
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    Limnology of the Touw River floodplain
    (Cooperative Scientific Programmes, CSIR, 1983-12) Allanson, BR; Whitfield, AK
    This report is the summary of a larger two-volumed work originally presented to the Coastal Lakes Working Group in 1981. The floodplain is in essence a series of small, shallow lakes, running in an east-west direction between the village of Sedgefield in the east and Wilderness in the west. They have been formed by segmentation of an earlier barrier lagoon associated with the Touw River. They have kept their connection with the Touw River estuary and so form part of its floodplain. The physical and chemical structure of the lakes is dominated by tidal influence when the estuary mouth is open and by erratic flooding of the Touw River and the Duiwe River, the latter flowing into Eilandvlei and which during floods is responsible for marked but short-lived pulses of phosphate and nitrate derived from agricultural activity on the upland catchment. Normally the lakes are pristine and constitute, together with their associated reed covered margins, an important wetland for a variety of animal forms, particularly birds. The aesthetic quality of this coastal region and the floodplain is under threat by the demands of human activity associated with urban development.
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    Swartvlei (CMS11)
    (CSIR, National Research Institute for Oceanology, 1983-12) Whitfield, AK; Allanson, BR; Heinecken, TJE
    Swartvlei ranks among the most intensively studied of all Cape estuarine systems. The hydrology of the Swartvlei catchment has been investigated by then the Department of Water Affairs, hydrographic survey have been conducted by the Coastal Engineering and Hydraulics Division of the National Research Institute for Oceanology, geological and geophysical investigations have been made by the University of Cape Town Marine Geosciences unit and trace metal studies have been carried out by the National Physical Research laboratory of the CSIR. The Swartvlei system comprises two main sections, the estuary and an upper lake. In this report the term Swartvlei refers to the lake-like portion and the estuary to the area down-stream of the railway bridge.