Environmental changes at verloren vlei

Verlorenvlei is a coastal lake and swamp system connected to the sea by a small estuary at Elands Bay, 180 km north of Cape Town. Environmental investigations at Verlorenvlei have provided evidence of environmental changes since the last interglacial. Subfossil shell deposits at 3.8-6 m above vlei-level on the southern side of Verlorenvlei are considered to date to the last interglacial. When the sea-level was higher, the vlei mouth was open to the sea and wider than the existing mouth. The vlei would then have been a marine lagoon like Langebaan, open to the sea. The mollusc assemblage suggests a sandy substrate as it consists of a dominant infaunal bivalve element with Dosinia lupinus being the most abundant. Six of the twenty-five species of mollusc are indicative of warm waters with minimum temperatures about 17-18°C. During the cold late Pleistocene there is evidence that the original mouth of Verlorenvlei lay slightly further to the north than the existing mouth. Bathmetric studies indicate that the mouth must have been in a different location at some time in the past, for the 5 m depth of the vlei indicates that there must have been a deep channel to the mouth for the basin of the vlei to have eroded to its present form. The present channel has a sill of rock 300 m from the mouth which would have prevented erosion to a lower level. The borehole logs of the Railway Bridge crossing the present channel indicate deposits of grey sand and seashells down to a depth of —; 30 m below which clayey sand extends down to a depth of — 34 m, a few metres north of the present channel. Archaeological investigations by Professor Partington in the Elands Bay cave revealed a changing sequence of faunal remains reflecting corresponding palaeoenvironmental changes during the Eustatic rise of sea level in the late Pleistocene. The order in which estuarine and marine elements appeared in the cave sequence appeared to document the Terminal Pleistocene rise in sea-level. The period from 11 000 to 9000 BP provided a dramatic increase in marine elements. An apparently wave cut shelf at about 1 m to 1.5 m above sea-level exists around the shores of Verlorenvlei probably representing an early Holocene higher water-level. Subfossil molluscs found below the bed of the vlei on the south side may be related. The writings of many early travellers contain references to conditions at Verlorenvlei during the 17th and 18th centuries. During the present century the vlei has been changed significantly by the development of a series of causeways and bridges across the channel to the sea limiting the migration of estuarine and marine fish. On a short term or seasonal basis striking changes in salinity take place as a result of variation in river inflow (brackish), rainfall, sea-water inflow from the mouth and evaporation. A range from 1.0 to 74.0% has been recorded with consequent biotic fluctuations.