ABOUT SOUTH AFRICAN ESTUARIES

Content prepared by AT Forbes

Definition
Maps and locations of South African Estuaries
Types of Estuaries
Physical characteristics
Ecological characteristics
Management principles
Some key questions
 
 

Definition

In the southern African context the following is a generally accepted definition of an estuary. “It is a partially enclosed, coastal body of water which is either permanently or periodically open to the sea and within which, there is a measurable variation of salinity due to the mixture of sea water with fresh water derived from land drainage” (Day 1980). Such water bodies are therefore linked to a river, stream or other freshwater input at one and to the sea at the other. The absence of a recognizable source of freshwater, such as occurs in Langebaan Lagoon, would exclude any such systems from inclusion in this section although it displays many of the typical estuarine characteristics.
 

Types of Estuaries

According to Reddering and Rust (1990), few southern African systems are true estuaries with only 37 (12.8%) of the 289 river mouths maintaining permanent tidal inlets with the sea. Whitfield (1992) suggested the following classification:


Physical characteristics

The southern African coastline from Maputo Bay in Mozambique in the east to Angola in the west is uniquely characterized amongst southern hemisphere coastlines by the combination of few near shore islands, strong wave action, especially in the southern Cape, and the very small number of sheltered bays. Our estuaries are therefore virtually the only protected coastal habitats which are able to provide, generally in their lower reaches, an environment which combines predominantly marine or near marine salinity’s, shelter from wave action and relatively fine sediments. This combination is preferred by a variety of animal and plant species.

A universal and characteristic feature of estuaries is the variation in physical and chemical conditions, both along the length of the system and over time in any one area.  This time variation often occurs over periods measured only in hours or days for example tidal cycles and differences in daily flow patterns and is therefore much shorter than the annual seasonal cycles of temperature, day length and rainfall.  The magnitude of the daily tidal fluctuations may, however, often be equal to that of annual variations which, means that the organisms living in this highly variable environment, must be able to cope with these changes.  The source of much of this variation, which is a major characteristic of “open mouth” estuarine systems, lies in the daily rise and fall of the tides, currents set up by these tides and fluctuating freshwater inflow.

Estuaries are dynamic systems and virtually any physical or chemical feature associated with them is subject to rapid and sometimes extreme changes. The mouths of South African estuaries unless pinned by some rocky feature tend to meander under the influence of currents, wind and wave action and sediment movement.  Under flood conditions major mouth changes involving thousands if not millions of cubic meters of sand may occur in a few hours. Attempts to secure and stabilize mobile mouth areas, particularly in the case of larger systems are frequently doomed to failure. This was graphically demonstrated at St Lucia during the floods associated with Cyclone Domoina in 1984 when, as shown in the accompanying pictures, a tarred access road to a car park on the northern bank of the estuary mouth which was supported by concrete retaining walls and protective dolosse was completely destroyed.

Estuarine mouths represent dynamic equilibrium. Marine derived sediments are forced into the mouth by long shore drift, wave action and flood tide currents which may ultimately block off the mouth. The scouring effects of floods act as opposing forces and re-set the whole system before the marine driven mouth sedimentation and closure processes start again. Salinity fluctuations are typical and normal and most estuarine species have remarkably wide tolerance ranges, although the effects of low salinities become more severe in the eastern and southern Cape where floods tends to be associated with low temperatures, generally well below 20 oC. Under these conditions the more tropical species such as the mangrove crab may suffer severe mortality as was found by Hill (1975) in the Kleinemonde near Port Alfred. No crab mortalities were noted at St Lucia following Cyclone Domoina when the water temperatures were about 27 oC.

High salinities are less frequent but are nevertheless a feature of west coast estuaries and also St. Lucia during drought years when this very large, shallow system is subject to high evaporation rates. Most estuarine invertebrate animals seem to be able to tolerate salinities up to about 500 ppt (normal seawater is 35‰). Many estuarine fish seem to be able to survive salinities of 60 to 700 ppt; (Whitfield, Blaber & Cyrus 1981). Large scale temperature or salinity induced mortalities of estuarine organisms have therefore been relatively rare and localised to date. However, this can be expected to increase as freshwater abstraction from rivers becomes more intensive and marine exchange or freshwater input does not compensate for evaporation in estuaries with long tidal reaches or extensive shallow areas.
 

Ecological characteristics

The fauna and flora of estuaries are typically capable of tolerating the above- mentioned changes. A comparison of an estuarine flora and fauna with that of a neighboring marine reef will show that there are many species, both plant and animal, which are either excluded by these changing conditions or avoid estuaries.  Those that can tolerate the estuarine environment are often very successful and abundant in their chosen environment, e.g. sand prawns Callianassa kraussi and mudprawns Upogebia africana, mullet and fish that feed on the bottom or eat plankton.

A feature of the life styles of a variety of estuarine species is migration. Most of the larger fish species as well as invertebrates such as the estuarine swimming prawns and the mangrove crab Scylla serrata breed at sea where salinity, temperature and oxygen availability are much more constant than in an estuary. This favours the sensitive larval stages which then, at a later stage of development, move to the estuarine nursery grounds for a time to grow and develop into mature animals before migrating back to the sea. In invertebrates such as the swimming prawns or fish like the Cape stumpnose Rhabdosargus holubi there is no return migration. Fish like grunter Pomadasys commersonnii may move repeatedly between marine spawning grounds and estuarine nursery or feeding grounds as at St Lucia. It is incorrect therefore to describe estuaries as breeding grounds, except in the case of resident species and even here it has been shown that apparent residents such as the mud prawn synchronise egg hatching to coincide with ebb tides such that newly hatched larvae are transported out to sea where they develop for a time before returning to the estuary (Wooldridge, 1994). From the above, it is clear that the time of the year that the mouth is closed or open is very important and can have a major effect on the nature of estuarine fauna."

If one considers the major habitats in estuaries it is clear that the different plant types often play a major role. Within the water column the phytoplankton, and microscopic single-celled plants (diatoms), provide a source of food for filter-feeders.  The fixed, much larger plant species are extremely significant, whether in the form of the generally submerged seagrasses, intertidal saltmarshes of temperate regions or the mangroves of more tropical areas.  The development of the seagrasses and to some extent the saltmarshes is greater in areas of clearer rather than turbid water which limits light penetration. Clearly mangroves are not affected by water turbidity except possibly in the very early stages of establishment of propagules (seedlings). Seagrasses can survive in non-tidal conditions where their distribution may be limited by salinity, as in the St Lucia system, but mangroves and salt marshes require a tidal regime for survival. The roots of mangroves cannot tolerate long term immersion in the waterlogged, low sediments where they are rooted.  These macrophytes all contribute to the formation of a three dimensional environment and thereby provide major habitat types for sheltering small fish and invertebrates.

Estuaries depend on a basic input of carbon as a food supply. This may arise from a variety of sources, e.g. local plant growth in the shape of mangroves, reeds, saltmarsh vegetation, phytoplankton or microscopic one celled plants called diatoms which are found on intertidal mudflats. They also depend upon the input of leaves and other plant litter brought in from the catchment or even marine seaweeds washed in by tidal currents. Little of the macrophytic material mentioned above appears to be consumed in the fresh form. Instead, it contributes to the detrital food chain whereby it is colonized by bacteria and fungi which convert the often indigestible carbon material into more easily assimilable carbohydrates and proteins.

Tidal currents play a vital role in dispersing this material throughout the estuary thereby making it available to filter feeders. These organisms are represented by animals such as mudprawns, various mussel species and surface deposit feeders such as crabs which forage during low tide on the material deposited during the preceding high tide, or slack water period.

Fine detrital material in sediments is also used by many of the estuarine mullet species which, are often a very abundant component of estuarine fish communities. These are in turn preyed on by large fish eating birds such as cormorants, herons, fish eagles and crocodiles in systems like St. Lucia.
 

Management principles

Estuaries are well known for their high productivity, high carrying capacity and ability to support, apart from the resident species, a variety of migratory fish, birds and invertebrates. The maximization of this capacity depends on a variety of interacting attributes or features several of which reflect the significance of processes in the catchment and the need for a holistic approach for successful estuarine management.

Biodiversity in estuarine systems is enhanced by a number of factors such as, the size of the system, the habitat diversity, the presence of intertidal areas whether salt marsh, mangrove, sand or mud flats and by the presence of an axial salinity gradient, i.e. a gradient from full seawater at the mouth to freshwater or significantly reduced salinities at the head of the estuary.

It is clear from the preceding sentence that a combination of an open mouth, tidal action and freshwater input are generally positive factors in estuarine functioning.  This should not, be interpreted as implying that the frequent closure of many of our estuarine systems, both large and small, renders them biologically or ecologically insignificant and therefore less deserving of sympathetic development. A variety of fish species as well as invertebrates such as the sand prawn and the mangrove crab all survive quite adequately in temporarily closed systems and may in fact be favoured by the conditions that exist during such periods. The life cycles of many of these species are geared to the natural cycles of opening and closing and arbitrary artificial breaching can adversely influence the successful completion of these cycles.

On the basis of the above arguments it is possible to draw up a list of actions or activities which should be avoided or prevented because of their potential for immediate or long-term damage to estuarine systems.

A major adverse influence in South African estuaries is accelerated sedimentation because of human activities in the catchment. This results in a progressive and generally irreversible loss of aquatic habitat. The only way of counteracting this, is to encourage agricultural practices which reduce erosion. Any activity which reduces or restricts tidal action in a normally tidal system should be discouraged as this interferes with a great variety of conditions or functions ranging from oxygen availability to larval dispersal.Modification of freshwater inputs through abstraction affects scouring, mouth closure patterns, salinity levels and gradients and nutrient inputs. Wherever possible, this should be prevented or at least catered for through an integrated water release management plan for major dams. Artificial breaching of sandbars at arbitrary times can be disruptive to normal patterns of migration associated with seasonal rainfall. Any form of artificial mouth management should form part of a comprehensive, holistic management plan for the estuary and catchment

Dredging can have potentially disastrous effects through erosion, re- mobilisation of pollutants from sediments with accompanying increases in turbidity, burial of organisms and general habitat destruction. Any dredging project should be subjected to a proper environment impact assessment and it would require a permit from the Department of Mineral and Energy Affairs

Uncontrolled exploitation of the living resources of estuaries through fishing or bait collection with its associated habitat disturbance in the context of the relatively small total area of South Africa’s estuaries can have serious effects. These activities are subject to regulations set by the various Provincial conservation authorities and the enforcement of these controls is very important.

Wetlands in the context of estuaries include salt marshes, mangrove swamps, intertidal sand and mud flats as well as reed beds are all integral components of estuarine environments.  Excavation, reclaiming or draining these areas contribute to a loss of estuarine function which is often irreversible except at great expense. All these activities must be subjected to a proper environmental impact assessment before being permitted or undertaken.

All forms of pollution are a major potential problem in estuaries and should be avoided and reduced at all costs. The reason for this is that because of the estuary’s position as the final recipient of catchment input as well as tidal inflows from the sea. Depending on the nature of the pollutant, its concentration and degradibility, effects may vary from negligible to disastrous. Organic wastes from sewage or from sugar or paper mills lead to oxygen depletion which is reversible, although obviously lethal to many species in the short term, and sustained input of excessive amounts will result in a permanently lifeless system.  Oil is also organic and hence will ultimately break down although this is very slow in anaerobic estuarine muds. Its sticky nature and blanketing effect adversely affects and may kill birds and any other animals or plants with which it comes in contact. Inorganic wastes from industrial activities include all manner of toxic materials including the heavy metals.  Some of these toxins may become concentrated in higher predators including man.  Outflows of coolant water from power stations may have lethal effects in warm estuaries where animals or plants are already living at temperatures close to their upper lethal limits.

Unlike the above which can all be considered as “point sources” of pollution which can be identified and possibly dealt with, agricultural fertilizers and pesticides, which are also potential pollutants, enter rivers in a diffuse fashion with run-off water. The fertilizers can induce blooms of aquatic vegetation, which subsequently die and rot causing oxygen depletion. Some pesticides such as DDT, Dieldrin and other chlorinated hydrocarbons may persist for years and be passed up the food chain due to their tendency to accumulate in fat deposits.
 

Some key questions (issues to consider)

The following points or questions are provided as aids to the assessment of a planned structure or activity in terms of its potential impact on the estuarine environment or on normal estuarine functioning.The severity or significance of the impact would have to be assessed in relation to the nature of the particular estuarine system in question taking into consideration aspects such as size, rarity, conservation status or presence or absence of similar areas in neighbouring estuaries or in the same geographical area. Some questions that should be asked are as follows:
 


It should be noted that an estuarine manager or intending user should be aware of events beyond the actual bounds of the estuary and particularly in the catchment, which could ultimately impinge on the estuary. The sort of questions that then become particularly relevant include the following:
 

Estuarine habitats around the world are under threat because of human pressures, usually in the immediate area but at times because of abusive land use patterns in catchment areas hundreds of kilometers away. The inherent attractiveness and productivity of many estuarine areas sows the seeds of their own ultimate destruction through port and urban development, pollution or over-exploitation of their living resources. This has already occurred on a frightening scale in regions of the world richly endowed with estuarine environments.  In the southern African situation where the estuarine habitat is such a small proportion of the total coastal environment it is even more incumbent on us to be aware of the nature and value of this very special environment.

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