AGULHAS NATIONAL PARK – A VERY SPECIAL PLACE
– Marine and Coastal Ecologist
Some observations prepared for the Kogelberg Branch of the BotSoc, following a field trip in August 2016
It is hard to think of a more spectacular location than that of the Agulhas National Park (ANP) situated right at the southern tip of the enormous African Continent. It is furthermore of interest that the ANP represents a transitional area between the sub-tropical East Coast and the West Coast with its more temperate, mediterranean-type climate. Plant and animal life – on land and in the sea – have adapted to these transitional conditions.
The Marine Component of the ANP
The climate of the southern tip of Africa is determined by the mixing of two prominent oceanic current systems – the Agulhas Current (which has its origin in the South Equatorial and Mozambique currents) flowing southwards from the equator, and the upwelling of cold water of Antarctic origin as part of the West Coast Benguela Current System. The Antarctic water is forced to the surface when it encounters the continental shelf of the southern African coastline, especially when surface water is moved offshore by south-easterly winds which are prevalent during summer.
Mixing of these two massive oceanic systems is therefore largely determined by the underwater topography of the southern portion of African Continent. Of particular significance in this regard is the Agulhas Bank – the exceptionally wide continental shelf area off the southern tip of Africa. This deflects the Agulhas Current from the coast into deeper water in the form of huge gyres and this promotes mixing with cooler waters of Atlantic Ocean origin. Because of these ongoing mixing processes, a sharp physical demarcation between the Indian and Atlantic oceans is not possible. This is illustrated by the figures below. The first is a schematic indication of the main current systems of southern Africa – note also the width of the continental shelf at the southern tip of Africa. The second figure shows a satellite photo utilising infra-red imaging. Warm sea temperatures are indicated in orange, mixing of warm and cold water by yellow and green, and cold water by blue.
An indication of changes in environmental conditions in this transitional area, is the eastern border of distribution of the West Coast kelp forests near Cape Agulhas. The main kelp species concerned are Ecklonia maxima and Laminaria pallida. The same applies to organisms associated with this Kelp Ecosystem such as the West Coast rock lobster, Jasus lalandii, and several fish species.
The Terrestrial Component of the ANP
These oceanographic conditions determine the climate at the southern tip of Africa. This has a profound influence on the type of vegetation found here. In this regard it is interesting to note that the dominant sub-tropical East Coast fore-dune binding species, Scaevola thunbergii and Ipomoea brasiliensis, are absent in the ANP, but occur slightly further eastwards at Stilbaai. Similarly deep-rooted species binding fore-dunes on the West Coast, e.g. Blombos (Metalasia muricata) and Dune Gonna (Passerina rigida), were not seen on the fore-dunes in the ANP although they occur further inland in this area. More ubiquitous species such as the endemic sea-wheat (Agropyron dystichum) and Marram grass (Ammophila arenaria), as well as the sea-pumpkin (Arctotheca populifolia) and Tetragonia decumbens, are common on fore-dunes in the ANP and give stability to them. However, these initial observations are in need of more detailed follow-up investigation. Fairly broad-brush mention is made of the dune asteraceous fynbos in South African Flower Guide No 8, Southern Overberg (Mustard, Cowling and Albertyn, 1997), but these authors don’t distinguish between fore-dune and hind-dune vegetation.
An interesting geomorphological feature of the coast of the ANP are the pebble beaches. These appear to occur in association with inshore marine lagoon areas protected by reefs further offshore. The profusion of beautifully shaped pebbles of various sizes, rounded by surf abrasion, are typical of these steeply-sloped beaches. Under stormy conditions one can actually hear the pebbles being ground by wave action. This environment is too rough for the growth of rooted beach vegetation but a profusion of small, colourful algae can be seen washed up amongst the pebbles. The variety of algae is also of interest in this transitional area and is worthy of further study.
The coastal plain inland of the shoreline provides further examples of diversity in the ANP. It is characterised by Bredasdorp limestone formations deposited between 25 and 10 million years ago when sea levels were much higher than at present. Huge chemical solution hollows (underground caves) have developed within these limestone formations and hold much of the underground water of this region. In places the limestone deposits are covered by younger alkaline wind-blown sand, and finer muds and sands – especially in the Zoetendals Vallei area (SA Flower Guide No 8). This range of substrates contributes enormously to the biodiversity of the ANP. The authors of SA Flower Guide No 8 describe these various habitat types under the headings: acid sand proteoid fynbos; limestone proteoid fynbos; neutral sand proteoid fynbos; ericaceous fynbos; wet and dry restioid fynbos; renosterveld; coastal strand and rocky shelf vegetation; and wetlands. This is an excellent and authoritative record and there is little point in trying to summarise the vast amount of information in this publication within the scope of the current brief overview. However, the following needs to be noted:
• The Kogelberg Branch was privileged to have Emmarentia De Kock of the ANP lead an outing to various components of the coastal plain.
• The wide range of habitats and vegetation types is reflected by a similar variety of avifauna, insects and other organisms occurring in the ANP.
• The wetlands and vleis, with Zoetendalsvlei the most prominent, contribute to the occurrence of prolific freshwater aquatic fauna in this region.
This brief overview is concluded by quoting from the website of the ANP:
“The area around the southern-most tip of Africa, often referred to as the Agulhas Plain, has rich natural history and cultural features, which make it worthy of national park status. The Agulhas Plain is of international significance due to its rich plant biodiversity, with species richness values equalling those of tropical rain forests. It has approximately 2000 species of indigenous plants including 100 which are endemic to the area and over 110 Red Data Book species. Consequently, the Agulhas Plain is a very important component of the Cape Floral Kingdom, the smallest and richest of the world’s six plant kingdoms.
The Agulhas Plain is unique in that a wide variety of wetlands occur in the area, contributing to a high diversity of wetland plants and aquatic invertebrates. This is also home to the endangered Cape Platanna and the micro frog. In addition these wetlands attract a host of water birds, with over 21 000 migrant and resident wetland birds estimated to occur in the area annually. The coastline supports a rich marine and intertidal life, with breeding sites of rare coastal birds such as the African Black Oystercatcher and Damara Terns. The nearby islands are home to a variety of seabirds and seals. In spring and early summer southern right whales frequent the waters of the Agulhas coast.
Besides its ecological importance, the Agulhas area has a rich cultural heritage. A reconnaissance of the area has established the presence of significant archaeological sites along the coast. The discovery of stone hearths and pottery, together with shell middens, link the archaeological deposits with the era of Khoisan migration and settlements. The Agulhas area also provides history of a different kind – numerous shipwrecks of the early explorers attempting to conquer the wild seas off the southern tip of Africa, dot the coastline. Many national monuments are found in the area, such as the historical Cape Agulhas Lighthouse, which has been in operation since 1849. In addition, historical buildings such as the water mill at Elim and certain homesteads reflect the European influence in the history of the region”.
A visit to the ANP should therefore be high on the bucket list of all who are interested in the natural history and ecology of southern Africa. It is an area worthy of more detailed research, especially in terms of adaptations of animal and plant life in this highly interesting transitional region.
A more fully illustrated PowerPoint presentation on the Agulhas National Park is available for viewing if required.