A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system

  • AK Whitfield South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
  • JB Adams Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • GC Bate Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • K Bezuidenhout Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • TG Bornman Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • PD Cowley South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
  • PW Froneman Department of Zoology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
  • PT Gama Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • NC James South African Environmental Observation Network, Private Bag 1015, Grahamstown 6140, South Africa
  • B Mackenzie South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
  • T Riddin Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • GC Snow Department of Botany, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
  • NA Strydom South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
  • S Taljaard Council for Scientific and Industrial Research, PO Box 320, Stellenbosch 7600, South Africa
  • AI Terörde Percy Fitzpatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa
  • AK Theron Council for Scientific and Industrial Research, PO Box 320, Stellenbosch 7600, South Africa
  • JK Turpie Percy Fitzpatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa
  • L van Niekerk Council for Scientific and Industrial Research, PO Box 320, Stellenbosch 7600, South Africa
  • PD Vorwerk Department of Zoology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
  • TH Wooldridge Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa

Abstract

In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15–25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure (5–15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.

Keywords: East Kleinemonde Estuary; estuarine biota; mouth state; physical conditions; river flow; salinity regime

African Journal of Marine Science 2008, 30(3): 453–473
Published
2008-12-17
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X