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African Journal of Marine Science 2012, 34(2): 163–180Printed in South Africa — All rights reserved

Copyright © NISC (Pty) LtdAFRICAN JOURNAL OF

MARINE SCIENCEISSN 1814-232X EISSN 1814-2338

http://dx.doi.org/10.2989/1814232X.2012.675041

African Journal of Marine Science is co-published by NISC (Pty) Ltd and Taylor & Francis

A review of the ecology and management of temporarily open/closed estuaries in South Africa, with particular emphasis on river flow and mouth state as primary drivers of these systems

AK Whitfield1*, GC Bate2, JB Adams2, PD Cowley1, PW Froneman3, PT Gama2, NA Strydom4, S Taljaard5, AK Theron5, JK Turpie1, L van Niekerk5 and TH Wooldridge4

1 South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa 2 Department of Botany, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa3 Department of Zoology and Entomology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa4 Department of Zoology, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa5 Council for Scientific and Industrial Research (CSIR), PO Box 320, Stellenbosch 7600, South Africa* Corresponding author, e-mail: a.whitfield@saiab.ac.za

Research in South African temporarily open/closed estuaries that includes studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fishes and birds is used as a basis to review the ecology and management of this estuary type on the subcontinent. Particular attention is given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by riverine and marine events, as well as anthropogenic influences. In addition, the wider implications of these research findings for the management of temporarily open/closed estuaries in terms of freshwater supply are explored, together with the role of government legislation in maintaining the ecological integrity of these important wetland systems.

Keywords: birds, estuarine ecology, fish, hyperbenthos, macrophytes, microalgae, sediments, zoobenthos, zooplankton

Estuarine management involves the management of the activities in and around estuaries that can potentially impact on the health of a system (Heydorn 1979). The primary objectives of estuary management are to maintain a predefined standard of ecosystem health and ecologi cal functioning that will ensure a desired level of estuary services of both economic and social value (Huppert et al. 2003). Although the management of estuaries tends to be geared towards human requirements, most scientists are also very aware of the biological attributes and ecologi cal needs of species occupying these systems (Day and Grindley 1981). Major threats to estuarine health and functioning in South Africa include freshwater flow modifications associated with excessive human abstraction, industrial and agricultural pollution, overexploitation of living natural resources and deleterious activities and development within the estuarine environs (Morant and Quinn 1999).

Individual estuaries display unique physico-chemical charac-teristics, to which biotic components respond in a complex manner (Vorwerk et al. 2008). The high degree of biolog-ical activities characterised by intraspecies and interspe-cies interactions in a number of estuaries that display similar characteristics does not necessarily suggest that the two are the same (Scharler and Baird 2003, Ferreira et al. 2005, Chuwen et al. 2009). Although, there are generic ways that estuaries respond to the environment, each, however, has to

be understood and managed according to the unique physico-chemical, biological, social and economic requirements and values that have been set for it (Dyer and Orth 1994).

Research on South African estuaries in the past has been primarily aimed at permanently open estuaries (Whitfield 2000) as they were under more pressure from water resource development by virtue of their larger river volumes supplying more fresh water for agricultural, industrial and urban development use. Temporary open/closed estuaries (TOCEs) have recently come under similar pressures requiring that more detailed understanding of their functioning is carried out (Perissinotto et al. 2010a). As the dominant estuary type in southern Africa (approximately 70% of functional estuaries on the subcontinent) much needed research on TOCEs has gained momentum over the past decade (e.g. Whitfield et al. 2008).

Separate sections of this review are dedicated to under-standing the management implications of the individual biotic and abiotic components of TOCEs, since each component often entails a unique set of management requirements. However, it is also necessary to take an overall ecosystem perspective and, for this reason, the latter part of this review focuses on providing recommendations for actions that can be taken to mitigate the effects of increased water use from inflowing rivers and the changing mouth behaviour patterns that are increasingly affecting TOCEs.

Introduction

Whitfield, Bate, Adams, Cowley, Froneman, Gama, Strydom, Taljaard, Theron, Turpie, van Niekerk and Wooldridge164

Current understanding of temporarily open/closed estuaries

Although TOCEs have only recently become the focus of research attention, there is already a substantial amount of information available for scientific planning and estuarine management purposes (see Perissinotto et al. 2010b for a comprehensive review). It should also be emphasised that there are distinctive parallels between the structure, functioning and management of TOCEs in South Africa and those same components pertaining to intermittently open estuaries on other continents such as Australia (Roy et al. 2001, Haines et al. 2006).

Hydrology and hydrodynamicsSound catchment management for TOCEs requires that adequate fresh water reaches an estuary, especially to ensure appropriate mouth behaviour for that particular estuary (Hay et al. 2005). River floods, baseflow and tidal water exchange are critical elements in the triggering and

maintaining of open mouth conditions, as well as establishing crucial salinity gradients. In most TOCEs, baseflow during the closed phase is responsible for gradually filling up the basin area behind the sand berm and inundating the supratidal areas around the estuary (Perissinotto et al. 2010b).

In some cases when fresh water is in short supply, management interventions can assist with restoring some estuarine functionality. The mouths of all TOCEs will usually close during the low-flow season for a certain length of time (Figure 1). There is therefore an estuary-specific range of frequency and duration within which mouth closure naturally occurs (van Niekerk 2007). From a management perspec-tive, if closure falls within this range, no mouth manipula-tion is required. However, if closure falls outside the normal range, action of some sort may be required such as artifi-cial breaching or increasing freshwater inflow (van Niekerk et al. 2005). The reverse is also true, in that TOCEs are sensitive to increases in river inflow. An increase in runoff to TOCEs will increase the frequency of mouth opening, which

Figure 1: Schematic diagram illustrating cyclic physical processes in small temporarily open/closed estuaries (TOCEs) on the KwaZulu-Natal coast (modified from Cooper 1991). The average mouth state of most South African TOCEs is predominantly (a) closed, rather than (b) open or (c) closing

African Journal of Marine Science 2012, 34(2): 163–180 165

in turn reduces the ‘stable’ closed phase for the ecology of the estuary (Whitfield 1980a).

In general, the smaller the system, the more sensitive it will be to a modification in river flow (van Niekerk 2007). This implies a knowledge base regarding the runoff regime of the catchment that drives the dynamics of the estuary and an understanding of the potential impacts that changing catchment management practices may have on an estuary. This has been illustrated by a number of previous Department of Water Affairs (DWA) ecological water requirement determinations for estuaries (e.g. van Niekerk et al. 2008). Hence, the hydrological management of a TOCE does not begin or end at the defined boundary of the estuary in question.

Anthropogenic developments have the effect of both reducing natural water flow into estuaries and increasing flood intensity. The former is a result of water abstraction for human, agricultural and industrial use and the latter as a result of sewage effluent discharge or the ‘hardening’ of the immediate catchment through impermeable areas such as roads and concrete aprons. All these structures should be carefully considered in any local integrated development plan (IDP), spatial development plan (SDP) or catchment management strategy (CMS) (van Niekerk and Taljaard 2003).

TOCEs close naturally when ebb tidal flow decreases below a minimum amount, in conjunction with a reduction in freshwater inflow (Figure 2) and/or an increase in wave action. Any structures (e.g. jetties, bridges and causeways) or activities that reduce natural flow rate (e.g. changes in cross sectional area), or result in excessive vegetation growth, should be carefully examined for their potential to cause an unacceptable alteration in tidal exchange (van Niekerk 2007).

There are a large number of case studies in South Africa where bridges across rivers and estuaries were built with little or no thought to environmental consequences (Burns and Heydorn 1988). The minimum and cheapest structure to convey a road or railway over a river or estuary, while still meeting safety requirements (e.g. withstand a 1:50 or 1:100 year river flood) usually results in a comparatively narrow multispan bridge with long-approach embankments. One of the obvious impacts of bridges on estuaries is that in most cases they stabilise dynamic estuarine channels (Baird et al. 1983). In turn, the combination of stabilised channels and heavy floods being forced through constricted areas leads to the extensive erosion of sediments underneath the bridge span (Figure 3). Erosion beneath a bridge can also occur in conjunction with extensive deposition downstream of the bridge, especially adjacent to the road approaches to the bridge.

Figure 2: Diagrammatic representation of the dominant hydrodynamic process during different TOCE phases: (a) outflow phase, (b) tidal phase, and (c) closed phase (modified from Taljaard et al. 2009)

Whitfield, Bate, Adams, Cowley, Froneman, Gama, Strydom, Taljaard, Theron, Turpie, van Niekerk and Wooldridge166

The changes in flow velocity and related sediment distri-bution can lead to undesirable changes in habitat and biota, especially in the immediate vicinity of poorly designed bridges. For example, Gaigher (1984) attributed the disappearance of the bloodworm Arenicola loveni from the mouth region of the Uilkraal Estuary in the Western Cape province to the building of a road bridge, which not only encroached directly on previous bloodworm habitat but also changed the hydrody-namics of the estuary to the detriment of this species.

The approaches to a multispan bridge are typically built over floodplain vegetation (e.g. salt marshes), which are filled in with rubble and the road constructed on top (Day and Grindley 1981). Solid fill acts as an obstruction to tidal flow and an area of ‘dead’ water can develop on either side of the closed embankments. Such areas act as sediment traps where sandbanks and muddy shoals can develop, often becoming consolidated and vegetated which then reduces the tidal prism of the estuary. The changes in substratum can, in some instances, cause the higher levels of marsh above a bridge to dry out and their valuable production to be lost to the estuary. If tidal flow is restricted as a result of sediment accumulation caused by unsuitable or environmentally insensitive bridge construction, this can lead to premature mouth closure in smaller TOCEs, as for example in the Seekoei Estuary in the Eastern Cape province (Bickerton and Badenhorst 1987).

Sediment dynamics A considerable supply of marine sediment is usually present at estuary mouths for potential transport into the system (Day 1981a). Thus, the amount of marine sediment intrusion into

an estuary is more dependent on the net transport capacity of the ebb and flood tidal flow near the mouth and less on the usually ample amount of sediment available outside the mouth. Generally, there is a relatively small amount of direct marine sediment intrusion due to berm overwash inwards from the sea or aeolian transport from dune sand. In some South African estuaries, however, the amount of marine sediment available near the mouth is very small, which is the case where the shoreline is predominantly rocky. In a few other instances, where the estuary mouth is located in a well-sheltered area, such as in bays and in the lee of points/headlands or reefs, the amount of wave energy available to transport marine sediment towards the mouth might also be limited (Theron and Bornman 2008).

The literature (e.g. Beck et al. 2004) shows that the sediment balance in estuaries is directly affected by river floods and sometimes depends on the changing dominance between flood and ebb tide flows. It is incorrect simply to conclude that the net sedimentation that sometimes occurs in an estuary is necessarily due to a flood tide that has relatively higher flow velocities than during ebb tides; actually the flow path and duration of the ebb flow is different to that of the flood tide (Theron 2007). For most South African estuaries, very little data are available regarding sediment transport or estuarine morphology, and in only a few instances has clear evidence of progressive sedimentation actually been obtained (Cooper 1993).

Estuary mouths always tend to close as a result of sediment accumulation. The reason for this is that turbulent wave action in the surf zone causes sediment to go into

Figure 3: Aerial view of road bridges over the East and West Kleinemonde estuaries in the Eastern Cape province, South Africa. Notice the narrow bridge spans through which river floods are constricted, as well as the extended road embankments across the two estuaries that prevents complete scouring of sediments in the lower reaches during river flooding (photo: AK Whitfield)

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suspension. Tidal inflow carries this sediment into the mouth where the reduced flow velocity and turbulence causes it to settle out, thus blocking the mouth (Theron 2007, Wolanski 2007). Relatively slow-moving water within the estuary due to reduced tidal action is not effective in scouring sediments during the ebb tide and the net result is for estuary mouths to close up due to sediment accumulation. When rivers flood, there is more water leaving the estuary and the outflow velocity through the mouth increases, causing scour and the removal of sediment from the mouth into the sea (Cooper 2001). Thus it follows that the difference between permanently open estuaries (POEs) and TOCEs is driven primarily by differences in the magnitude of the ebb-tidal flows (in combination with the freshwater outflow) versus the wave generated flow. In POEs, the sediment transport due to these three drivers tend to almost be in equilibrium over long time-scales, while in TOCEs the lower river inflow and relatively small tidal prism results in sediment accumulation, thus causing mouth closure.

Haines et al. (2006) examined several basic parameters they considered important to estuarine hydromorphology. These were area, shape, volume, tidal prism ratio, mean annual runoff (MAR), catchment sediment load, and an entrance closure index. In TOCEs that are closed from the ocean for much of the year, the water surface area is dependent on water level, which is strongly influenced by river inflow. In most South African TOCEs, the water level drops considerably when the mouth opens, especially those systems that are ‘perched’ (Cooper 1989). Conversely, those TOCEs with an extensive floodplain can have large water surface areas during the closed phase, provided human structures have not encroached onto the low-lying areas around the estuary.

Water quality The nutrient and water quality characteristics at any point along the length of an estuary are largely dependent on the extent of marine and freshwater influences on the system (Allanson 2001, Figure 4). This, in turn, is determined by the quality and quantity of river water entering the estuary during a given period, the state of the tide and condition of the mouth (Slinger et al. 1994, Figure 4). For this reason, the water quality characteristics of TOCEs are largely determined by the hydrodynamic state existing within an estuary (Snow and Taljaard 2007). For example, if the closed-mouth state coincides with periods of high evapora-tion and zero river inflow, hypersaline conditions can arise within a TOCE (Taljaard et al. 2009). Thus, if the frequency of distribution of hydrodynamic states (Figure 1) is modified through the alteration of freshwater inflow or artificial mouth breaching, the water quality characteristics of TOCEs will also be modified. For example, municipal sewage effluent disposal to estuaries or coastal rivers can increase natural flow, especially where water from outside the catchment is used to augment supply to the urban area supplying the sewage (Perissinotto et al. 2004). This situation has been well documented for the Mhlanga Estuary in KwaZulu-Natal, which now opens much more frequently than in its natural state due to processed sewage effluent from nearby urban areas being directed into the river just above the estuary (Lawrie et al. 2010).

From the above it becomes apparent that both a reduction and increase or a change in the timing of the natural flow can have serious effects on the hydrodynamic states and, subsequently, on the water quality and biota in TOCEs to the extent that their condition may impact on the manage-ment class allocated to them in terms of the Water Act (RSA 1998a). The implication is that even past activities may have to be reversed in order to make the condition of the resource comply with the Act.

Municipal wastewater discharge and septic tank overflow into estuaries, in addition to modifying the natural flow, can also result in increased mineral nutrient concentrations (Lawrie et al. 2010). This is a concern in TOCEs especially when they are closed and when a long residence time of enriched water can cause eutrophication (Thomas et al. 2005) that can take the form of nuisance or even harmful algal growth under certain circumstances (Wood 2010).

Agricultural activities in catchments can cause high mineral nutrient and pesticide loads to enter estuaries (Heinecken et al. 1983). Together, these can have adverse effects on estuarine biota both in terms of productivity and species diversity. Non-point pollution from agricultural sources (e.g. irrigation schemes), primarily in the form of leached fertiliser residues from irrigation water, enters the river via ground-water (Emmerson 1989). Moreover, the accumulation of these pollutants becomes potentially more threatening to TOCEs than POEs owing to a longer retention period during the closed phase in TOCEs.

Evidence from recent studies has indicated that TOCEs should generally be left to open and close naturally. However, conditions may have become so altered due to a reduction in water supply, or levels of pollution, that it is no longer possible to maintain the natural functioning of the estuary. Under these conditions an artificial breaching of the sand bar at the mouth becomes the best preferred form of manage-ment strategy (Burton 1960, Stretch and Parkinson 2006, Perissinotto et al. 2010a).

MicroalgaeMicroalgae are unicellular algae that are found either on sediments or in the water column (phytoplankton) and form the base of TOCE estuarine foodwebs (Walker et al. 2001, Mundree et al. 2003). These organisms provide nutrition for many of the estuarine invertebrates that are consumed by the larger fauna, including fish and birds. Owing to their size, together with inherent difficulties often related with physiological manipulations and taxonomic identifications, microalgae have not received as much scientific attention as larger estuarine animals and plants (Gama et al. 2005, Perissinotto et al. 2010a). However, it is important for managers to understand that microalgae shape and support the base of the foodweb, i.e. they provide the necessary carbon energy for the primary consumers and therefore are of major importance to the ecological functioning of TOCEs.

Prior to significant human settlement along the South African coast, particularly in the eastern sector of the country, most TOCEs were probably characterised by low macronutrient inputs (Day 1981b). Microalgae receive their nutrition from minerals in the water that arrives from both land and sea (Lucas et al. 1999, Gama et al. 2005, Painting

Whitfield, Bate, Adams, Cowley, Froneman, Gama, Strydom, Taljaard, Theron, Turpie, van Niekerk and Wooldridge168

et al. 2007). Under natural conditions, low levels of nutrients (oligotrophy) were more common in South African TOCEs (Snow and Adams 2007), with very high levels (eutrophy) being a rare event that was probably associated with the periodic congregation of large herbivores in and around estuaries (Faith 2011), or around natural freshettes entering from the catchment. Possibly the major difference between natural eutrophic conditions and those of today is the source and frequency of the supply of nutrients (MacClelland and Valiela 1998). Currently, the major source of additional nutrients entering estuaries is via return flows from agricul-ture, urban runoff and/or treated/untreated municipal effluent (Morant and Quinn 1999, Thomas et al. 2005).

The influence of river inflow on the microalgal biomass and species composition in TOCEs is very important (Froneman

2002, Thomas et al. 2005, Gama 2008). In a number of these estuaries heavy rainfall in the catchment often leads to rapid flushing and scouring that generally resets the estuary through the import of fluvial inorganic and organic matter, and the export of estuarine sediment and organic matter to the marine environment (Vorwerk 2007). Increases in river inflow have been correlated with increases in phytoplankton chlorophyll a biomass, suggesting a close link between terrestrially derived inorganic nutrients and elevated chloro-phyll a (Perissinotto et al. 2010a). However, this is not always the case and factors such as light conditions within the water column, zooplankton grazing rates and settling out of phytoplankton from the water column all influence biomass and primary production by these organisms in TOCEs (Anandraj et al. 2007).

Figure 4: Diagrammatic representation of the dominant nutrient exchange processes during different TOCE phases: (a) outflow phase, (b) tidal phase, and (c) closed phase (modified from Taljaard et al. 2009)

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Isolation of TOCEs from the sea establishes stable water column conditions behind the berm that can lead to eutroph-ication and the proliferation of phytoplankton and epiphytic algae (Perissinotto et al. 2010b). Mouth breaching from increased river inflow ensures the renewal of estuarine water through flushing and maintaining connectivity with the marine environment (Anandraj et al. 2008). Isolation from the sea by the formation of a sand bar at the mouth, together with the reduction in freshwater inflow from the catchment, reduces estuarine phytoplankton diversity by supporting a limited number of flagellate species and non-flagellated forms under low nutrient conditions (Gama 2008).

When an estuary mouth closes after a flooding event, benthic microalgal populations recover and begin to build up over a period of weeks to months (Perissinotto et al. 2002). This benthic material is probably the most important food supply in the estuary because it is the food resource utilised by the primary consumers (Whitfield 1980a). Some TOCEs retain high microphytobenthos populations even when the mouth is open (Skinner et al. 2006). However, the wet-sediment surface area available to these algae during this phase remains limited because of the reduced and fluctuating water level within the estuary. For this reason, if an estuary opens more frequently than natural as a result of manipulations of water inflow or dredging of the mouth, the benthic microalgae may not reach their optimum densities and high levels of diversity (Perissinotto et al. 2004, Anandraj et al. 2008).

Benthic microalgae are most abundant in surface sedi-ments where light is sufficiently high to support photosyn-thesis (Mundree et al. 2003). Because most South African TOCEs are shallow (i.e. average depth ~1 m or less) for much of the water area, light is not often a limitation to productivity. Consequently it is not surprising that benthic microalgae often have a higher biomass in TOCEs than the overlying phytoplankton (Nozias et al. 2001, Perissinotto et al. 2003). In addition, relatively static water conditions during the closed phase result in the settlement of silt and other inorganic loads carried by rivers into TOCEs, such that water clarity generally increases during this phase (Begg 1978), thus favouring high benthic microalgae productivity. Conversely, poor light penetration may occur in those TOCEs that are supplied by humic-stained streams or rivers (Allanson 2001). Under such low light conditions the benthic microalgae are likely to be confined to shallow littoral areas.

Epiphytes form an important component of the benthic microalgae in TOCEs and they are dependent on the aquatic macrophytes to which they are attached (Howard-Williams and Liptrot 1980). Submerged macrophytes provide optimum areas for attachment, but the stalks of emergent forms (e.g. the common reed Phragmites australis) also provide adequate substrata on which benthic microalgae can attach, thus increasing the total surface area available for the microalgae. Artificial removal of submerged and emergent macrophytes from TOCEs therefore impacts negatively on both macrophyte and epiphyte primary production.

MacrophytesMacrophytes are an important component of the natural environment in and around estuaries, especially TOCEs (Lubke and van Wijk 1988). These visible plants occur as

emergent (e.g. P. australis) and submerged vegetation (e.g. aquatic grass Ruppia cirrhosa) and next to the estuary as fringing plants (e.g. sharp rush Juncus kraussii) (Adams et al. 1992, Steinke 1972). Fringing vegetation provides stormwater control and natural bank stabilisation, in addition to reducing lateral erosion and littoral water velocity during river flooding. Submerged macrophytes in particular provide substrata for epiflora and epifauna (Davies 1982), as well as a foraging and refuge habitat for juvenile fish (Whitfield 1984). Stable conditions during the closed mouth phase will promote the growth of submerged macrophytes (Riddin and Adams 2008). If the mouth is closed for long enough, they can grow and expand to occupy the entire water column where the habitat is suitable, i.e. shallow and calm water with stable salinity.

Extended mouth closure can result in high water levels which causes die-back of emergent plants such as salt marsh and reeds and sedges (Adams and Bate 1996, Adams et al. 1999). An intertidal salt marsh takes approxi-mately four months to become established (Riddin and Adams 2008). Macrophytes in TOCEs are adapted to the fluctuating environmental conditions as they have rapid life cycles. For example, in the East Kleinemonde Estuary in the Eastern Cape, Salicornia meyeriana and Sarcocornia tegetaria were able to produce viable seeds within three and four months of germinating respectively (Vromans 2011). In those TOCEs where freshwater abstraction has caused the mouth to be closed for a prolonged period, it should be allowed to remain open for as long as possible to allow the macrophytes to grow and set seed. In particu lar, care is needed to ensure that the salt marsh is not degraded as this would result in a significant decline in biodiversity (Adams and Bate 1996). In some Eastern Cape estuaries, extended mouth closure due to excessive freshwater abstraction can result in extreme hypersaline conditions (e.g. >70 in the Seekoei Estuary), which also reduces macrophyte diversity (Adams et al. 1992).

The implication for management of macrophytes within TOCEs therefore relates especially to how long these plants can survive prolonged mouth closure with a high water level and high or low salinity (Adams and Bate 1994, 1995). There is concern regarding the length of time that seeds and other propagules can survive abnormal flooding, exposure or inundation. Propagule survival is essential for the resetting of the estuary after the mouth has been breached. Mouth closure for longer than three years would likely result in the decomposition of submerged seeds. Some studies have suggested that storage for longer than a year can decrease seed viability (Riddin and Adams 2009). Loss of propagules could result in a change in the biotic structure of an estuary, which in turn will increase the deviation from the natural state due to the absence of certain species from the system.

Floods that open the mouth are very important because they flush out sediment accreting from the banks, thus preventing sedimentation and reed encroachment into the main channel (van der Elst et al. 1999). Floods also remove accumulated organic matter, thus reducing the possibility of anoxia as a result of decomposition. For example, a flow rate >1 m s–1 is sufficient to remove certain submerged macrophytes and at 0.5 m s–1 growth is significantly reduced owing to mechanical damage (Adams et al. 1999). Floods

Whitfield, Bate, Adams, Cowley, Froneman, Gama, Strydom, Taljaard, Theron, Turpie, van Niekerk and Wooldridge170

and the subsequent reduced salinity profile within a TOCE can also promote the germination of salt marsh plants (Adams and Bate 1994). Perched estuaries can drain almost completely after a flood has breached the mouth, thus exposing previously submerged areas and resulting in significant loss of submerged macrophyte biomass (Riddin and Adams 2008). The natural flooding regime should be maintained as far as possible to ensure persistence of macrophyte habitats.

Overwash (i.e. when seawater flows over the berm from the marine environment into the estuary) may result in the introduction of propagules and vegetative plant fragments from adjacent estuaries. Overwash increases the salinity of the estuary, which is important in maintaining salt-tolerant communities such as salt marsh (Riddin and Adams 2010). Overtopping (i.e. when estuarine water flows over the berm into the sea) of the sand bar following freshwater flooding and the subsequent opening of the mouth causes the removal of plant material and nutrients from the estuary to the sea. During the open phase, tidal action is respon-sible for the redistribution of plant wrack within an estuary (Whitfield 1988) and may even move seeds and propagules to new areas where they can germinate.

Knowledge of what macrophytes occur in a TOCE under natural conditions, as well as the optimum growth require-ments and tolerance ranges for these species, is very important from a management perspective. This will permit predictions of change under varying hydrological conditions, e.g. changing mouth status. The life cycle and seasonality of the plants are also important so that timing and frequency of any artificial mouth breaching will be appropriate. Very high water levels just prior to breaching and the rate of change during opening events are important in terms of the subsequent habitat availability to faunal assemblages. Hence, managers need to know if the changes taking place are natural or in response to some anthropogenic disturbance. For example, an increase in some plant components (e.g. encroachment by the common reed P. australis in response to either increased sedimentation or localised nutrient inputs), or a decrease in other components through catchment mismanagement or excessive freshwater abstraction (e.g. the disappearance of the aquatic grass Potomogeton pectinatus under conditions of elevated salinity), can indicate that a particular TOCE is deviating from the ‘natural’ plant succes-sion patterns for that estuary.

Houses built too near an estuary can alter the distribution and density of macrophytes, especially if there is nutrient input into the system from septic tanks and fertiliser wash. This causes an increase in littoral reed growth (Human and Adams 2011). Residents often remove vegetation, especially reeds and submerged macrophytes in order to improve their view and boating access respectively. These actions do not consider the ecological role of such habitats and should therefore be prevented (Wood 2010). Boats and jetties influence macrophytes directly through anchor deployment, mooring and propeller damage, or indirectly through increasing turbidity, fuel or oil spillage, shoreline erosion and the introduction of alien species. Where uprooting of macrophytes occurs, the rate of organic production can be decreased and bank erosion increased, especially in those TOCEs where power boats are active.

However, the negative effects of boating can be minimised by zoning the estuary for different activities and regulating speed limits.

Eutrophication often occurs if an estuary receives urban or agricultural runoff. This can lead to increased epiphyte, phytoplankton and macroalgal growth that reduces light availability to submerged macrophytes. Mass accumula-tions of filamentous green algae can reduce the water quality of estuaries, not only by depleting the oxygen in the water column upon decomposition but also causing anoxic sediment conditions when large mats rest on the sediment under low flow conditions (Howard-Williams and Liptrot 1980). Increased growth of macroalgae and microalgae, especially epiphytic forms, may cause shading out of submerged macrophytes. In addition, increases in nutrients also indirectly influence shading by increasing water column plant growth that will have a detrimental effect on micro -phytobenthos production, an important source of food for higher trophic levels.

From the foregoing, it is apparent that managers should prevent excessive nutrient input, clearing of riparian zones, and infrequent or too frequent mouth breaching (Wood 2010). Monitoring the estuary by means of aerial photographs allows an assessment of changes over time (Colloty et al. 2000, Russell 2003) and is essential for good management.

Zooplankton and hyperbenthosZooplankton and the hyperbenthos are conspicuous components of the invertebrate fauna in TOCEs and form an important link between primary producers (microalgae) and the larger predators, mainly fish and birds (Wooldridge 1999, Mbande et al. 2004, Allan and Froneman 2008). Zooplankton comprises small invertebrate species such as copepods (e.g. Pseudodiaptomus hessei) and the larval forms of benthic invertebrates (e.g. veligers of the bivalve Musculus virgiliae) that occur throughout the water column. The hyperbenthos consists mainly of larger invertebrates such as the shrimp Palaemon peringueyi and swimming prawns (e.g. Penaeus indicus) that live just above the substratum.

The hyperbenthic and zooplankton community structure, abundance and biomass within temporarily open/closed estuaries reflect the inflow of fresh water and mouth phase (Schlacher and Wooldridge 1995, Froneman 2002, 2006, Kibirige and Perissinotto 2003). Under conditions whereby an elevated macronutrient supply increases phytoplankton, total abundance and biomass of the zooplankton and hyperbenthos increases, reflecting greater food avail ability (Froneman 2004a). The inflow of fresh water into the estuary, particularly during the closed phase, is thus critical in maintaining the increased abundances and biomass of the zooplankton and hyperbenthos within the system (Bernard and Froneman 2005, Kibirige et al. 2006). During the closed phase of the estuary, the zooplankton community is dominated numerically and by biomass of true estuarine species such as the calanoid copepods P. hessei and Acartia natalensis (Perissinotto et al. 2000, Froneman 2004b).

Breaching events are typically associated with a decline in the abundance and biomass of zooplankton and the hyper-benthos (Froneman 2004b, 2008). The observed pattern can be related to the outflow of biomass-rich estuarine waters

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into the marine environment. Additionally, the temporary loss of submerged macrophytes as a habitat for selected hyperbenthic organisms during the open phase probably also contributes to the decline in abundance and biomass within in the estuary (Henninger et al. 2009). Importantly, breaching events do provide an opportunity for marine-breeding species in the estuary to return to the marine environment and is the primary mechanism whereby the larvae and post larvae of marine-breeding species can recruit into the system (Wooldridge 1991, Perissinotto et al. 2010b). As a consequence, breaching events are typically associated with an increase in the zooplankton and hyperbenthic diversity due to the immigration of the larvae of marine-breeding species into the estuary during the tidal phase (Kibirige and Perissinotto 2003), with biomass peaking during the closed phase (Whitfield 1980a).

From the above it becomes apparent that river flow and estuary mouth state in TOCEs are primary drivers for both the zooplankton and hyperbenthos within these systems. Consequently, the wise management of these abiotic drivers is of crucial importance to the health of the zooplankton and hyperbenthos.

ZoobenthosZoobenthos comprises those invertebrates living on and in the sediments of an estuary, with some burrowing species occurring more than 50 cm below the surface, e.g. the sand prawn Callianassa kraussi and bivalve Solen capensis. The zoobenthic community structure of TOCEs is strongly driven by the prevailing sediment type, especially in Eastern Cape systems (Teske and Wooldridge 2003). Thus estuarine sand and mud fauna have distinct compositions, even when the overlying physico-chemical conditions are similar (Teske and Wooldridge 2004). Obviously, the muddy sediments carried by rivers into TOCEs are vital to perpetuating this zonation, as is wave action and tidal sorting of coarser sediments in the lower reaches of these systems during the open phase.

It would appear that the zoobenthos of TOCEs is well adapted to freshwater flushing events when salinity during the outflow phase is the equivalent of those in the outflowing river. However, once the floodwaters have dissipated (usually in a matter of hours to days), a tidal regime commences which brings saline water back into the estuary (Whitfield et al. 2008). The zoobenthos, which comprises mainly euryha-line estuarine species, is well adapted to the oligohaline to polyhaline salinity ranges usually experienced in South African TOCEs, although mortality of selected species (e.g. the bivalve Solen cylindraceus) has been recorded in certain systems experiencing hyperhaline conditions (Perissinotto et al. 2010b).

Breaching of TOCEs due to river flooding often has a short-term negative effect on the zoobenthos, particularly surface-dwelling invertebrates, due to sediment scour that is followed by prolonged aerial exposure of the estuarine sediments (Whitfield 1980a). Research has shown that mouth breaching often results in a population crash of the invertebrates and that populations only recover once the mouth closes and all the sediments of the TOCE are inundated (De Decker and Bally 1985). Hence, frequent artificial breaching is likely to have negative consequences, particularly if the breaching events occur at consecutive

short intervals, thus allowing the zoobenthos insufficient time to recover from the extreme physico-chemical conditions during the mouth outflow and tidal phases.

IchthyoplanktonEstuarine populations of many marine-spawning fish species (e.g. Cape stumpnose Rhabdosargus holubi and longarm mullet Valamugil cunnesius), like certain invertebrate species in estuaries, also require the mouth of TOCEs to open at least once per year for larval exchange. Recruitment is maximised when opening events coincide with adult spawning periods, predominantly over the spring and early summer months (Whitfield 1998). Research has shown that moderate amounts of water are required to flow into the surf zone to get quite large migrations of larval fish into a TOCE (Strydom 2003a). Fresh water flowing from the estuary into the sea results in an increase in surf-zone productivity (van Ballegooyen et al. 2007, Vorwerk 2007), which in turn has a beneficial effect on larval fish survival and recruitment into estuaries (Strydom 2003a). Up to 80% of all fish recruitment into Eastern Cape TOCEs by estuary-associated fish takes place predominantly during the late larval stage (Strydom et al. 2003).

Larval recruitment into estuaries does not require a major open phase to be successful (James et al. 2007). Marine overwash events associated with rough seas have been shown to facilitate recruitment of marine-spawned larval and early juvenile fish (Cowley et al. 2001). Similarly, a mere trickle of water out of an estuary into the sea can be sufficient to allow successful recruitment into an estuary (Harrison and Cooper 1991). Such a semi-closed state can be achieved in some TOCEs by a continual base flow. The actual flow rate required to achieve such a semi-closed state, however, varies from system to system. In many estuaries, this semi-closed state may cease as a result of freshwater abstraction from the river feeding the estuary, thus denying larval fish oppor tunity for extended recruitment.

TOCEs, such as the Mhlanga in KwaZulu-Natal and the East Kleinemonde in the Eastern Cape, provide a nursery to new stocks of larvae and juveniles after each opening event (Whitfield 1980b, Strydom et al. 2003). Every year that goes by without opening is a loss of this potential nursery function. Artificial opening events should only be consid-ered when anthropogenic change has drastically altered flood and natural opening events and then they should be based on historical information of river floods, baseflow, etc. A long period of closure (years) and loss of productivity and water quality are indications that artificial breaching may be necessary. However, artificial openings are not going to benefit the productivity of larval fish as much as natural openings because freshwater input plays a major role in driving productivity (Perissinotto et al. 2010a). Artificial openings only partially deal with the problem and if an artifi-cial opening is deemed necessary in times other than during a prolonged natural drought, it is a clear indication that the natural freshwater supply needs to be addressed.

River baseflow is essential, even during times when the mouth is closed, because it brings in nutrients that fuel primary productivity which in turn serves as the food resource for young fish that are using the estuary as a nursery (Whitfield 1985). Large floods also serve to supply

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nutrients to the estuary and nearshore marine environment and permit fish recruitment while the mouth is open (Vorwerk 2007). Natural opening events drive the nursery function of the estuary, which is essential to the survival of many estuary-dependent fish such as spotted grunter Pomadasys commersonnii and white steenbras Lithognathus lithog-nathus, which are important fishery species (Pradavand and Baird 2002).

Seepage of estuarine water through the sandbar into the marine environment during the closed phase, or via overflow when the system is very full, plays a potentially important ecological role. Cues from the estuary enter the marine environment and possibly aid in the accumulation of larvae off the closed mouth prior to a potential opening event (Strydom 2003a, James et al. 2008a). This in turn supple-ments recruitment of larval fish into the estuary, especially during the spawning season when larval numbers are high in the surf zone (Whitfield 1989, Strydom and d’Hotman 2005). Overwashing of seawater into a TOCE is also an opportunity for the ingress of fish larvae and other planktonic organisms and nutrients from the surf zone (Cowley et al. 2001, Froneman 2004b).

Management of TOCEs requires an understanding of life cycles of fish, particularly the economically important species, and understanding the role that fresh water and estuaries play in these life cycles. Managers should familiarise themselves with basic estuary function and the use of scientific informa-tion on which to base management decisions (Wood 2010). Mismanagement of catchment land around estuaries, the estuary itself and the freshwater supply has major repercus-sions for the success of coastal fish populations dependent on estuaries.

If marginal vegetation is removed and artificial channelling is introduced, problems often follow. Steep-sided channels increase current velocity along the margins and exclude larval fish from their preferred habitat, i.e. shallow littoral areas. The natural shallow area of estuary margins is the ideal shelter habitat for larvae and juvenile fish of marine, estuarine and freshwater origin (Strydom 2003b, Ellender et al. 2008). Power boating disturbs sheltering young fish and certain structures (e.g. jetties and retaining walls) alter the natural state of marginal habitats, which affects the nursery function of the estuary. Similarly, bridges and jetties adversely affect potential nursery habitats for fish by changing currents and causing unnatural sand scour and deposition (Wood 2010). Although jetties can provide larval and juvenile shelter for some species of fish (e.g. oval moony Monodactylus falciformis), this advantage is outweighed by the resultant reduced littoral habitat that is important to other species (e.g. flathead mullet Mugil cephalus).

FishesDiverse and abundant populations of marine-spawning juvenile fish in TOCEs are maintained by periodic connec-tions with the sea that usually follow river flooding and breaching of the berm barrier (Harrison and Whitfield 2006). These connections allow for the emigration of adults to the sea and the recruitment of larvae and juveniles into the estuary (Whitfield 1998). However, the abundance of certain taxa (e.g. R. holubi, M. falciformis and various mugilid species) in TOCEs can also be ascribed to their

ability to recruit during marine overwash events (Cowley et al. 2001). Prolonged closed phases with limited overwash events are ideal for estuarine-spawning species due to their ability to breed within TOCEs that have stable physical (e.g. water level) and physico-chemical (e.g. salinity) conditions (Bennett et al. 1985, Lukey et al. 2006). Prolonged closed phases can result in marine species becoming isolated and unable to reach the sea to spawn (Reddy et al. 2011), but this is currently a rare occurrence in most South African TOCEs.

Anoxic conditions created by excessive inputs of organic matter, or as a result of excessive algal growth following hypertrophic conditions, have been reported to result in major fish kills in certain KwaZulu-Natal TOCEs (Begg 1978, Figure 5). Under such conditions artificial breaching of a TOCE is justified and has been undertaken in the past (Wood 2010, Figure 6). However, artificial breaching should only be consid-ered if the undesirable conditions are a result of anthropo-genic influences and not natural circumstances, e.g. a fish kill affecting certain marine species has been recorded in the closed East Kleinemonde Estuary in the Eastern Cape, but this was caused by a combination of low salinity and low water temperature and was a natural event (PDC unpublished data) and artificial breaching under these conditions would not normally be recommended.

The benefits to fish of river flooding in a TOCE are that it permits the recruitment of estuary-associated marine larvae and 0+ juveniles, as well as facilitating the emigration of subadult and adult marine fish that can only spawn at sea (Harrison and Whitfield 1995). However, a deep, open mouth is not the only way that an estuary can benefit through a connection to the sea. Prolonged overtopping, although a rare occurrence in most non-perched TOCEs, does provide opportunities for marine-spawning species to recruit into these perched TOCEs (e.g. along the Tsitsikamma coast) and, depending on the depth of the outflowing water, larger individuals of certain species may also emigrate from the estuary during such a phase (AKW unpublished data).

The overwash of marine water into closed estuaries during very high tides, and often associated with sea storm conditions, provides another mechanism for the movement of fish between a TOCE and the sea. There is conclusive evidence that a number of species make use of overwash conditions to recruit into South African TOCEs (Cowley et al. 2001, Vivier and Cyrus 2001). As a consequence, those species that make use of overwash events as a recruitment strategy tend to dominate the species composition in this type of estuary (James et al. 2007).

The infilling of estuaries with excessive sediment due to poor catchment management practices is a problem with regard to the maintenance of habitat diversity (van der Elst et al. 1999), especially for large fish within TOCEs. Although shallow littoral areas are important nursery areas for small fish (Becker et al. 2011a), so too are the deeper areas as a habitat for larger fish (Becker et al. 2011b). Loss or reduction in either habitat due to catchment or estuarine misman-agement would be deleterious for the overall ichthyo-faunal balance within a particular system.

Managers need to realise that estuaries are an economic asset that provide a range of goods and services (Lamberth and Turpie 2003). They should also know that estuaries,

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Figure 5: Fish kill in the Mdloti TOCE in KwaZulu-Natal in February 2004, prior to the controlled artificial breaching of the mouth directed at flushing the anoxic waters out of the system (photo: NT Demetriades)

Figure 6: Controlled artificial breaching of the Mdloti Estuary mouth in KwaZulu-Natal on 11 February 2004 in response to the development of anoxic conditions and a major fish kill within the system (photo: NT Demetriades)

including small TOCEs, are important nursery habitats for many fish, some of which are targeted fishery species (James et al. 2008b). Fish returning to sea during mouth-opening events are extremely vulnerable to illicit harvesting. It is therefore vital that estuary managers understand the laws that govern fishing activities in and around estuaries and, most importantly, that they ensure that these laws are enforced (Hay and McKenzie 2005).

BirdsSome 345 000 non-passerine waterbirds occupy South Africa’s estuaries during summer, these numbers being dominated by migrant waders and terns from the Palaearctic region of the Northern Hemisphere (Hockey and Turpie 1999, Martin et al. 2000). In winter, the numbers of South African resident species such as cormorants, gulls, egrets, spoonbills and resident waders often increase when they return from their breeding areas (Heÿl and Currie 1985). Several species also breed in estuaries, mainly in spring and summer in the Eastern and Western Cape and in winter and spring in KwaZulu-Natal (Berruti 1983), their popula-tions remaining relatively stable year round.

Much of our current understanding of estuarine birds comes from the work that has been done on larger open systems. This is not surprising since these systems also support the highest numbers and diversity of waterbirds (Hockey and Turpie 1999). This is due to the presence of large intertidal areas that attract a high diversity and number of waders (Martin and Baird 1987). In comparison to open estuaries, TOCEs tend to be small, lack intertidal feeding areas and often lack salinity gradients. As a consequence, these estuaries tend to support a lower diversity and fewer numbers of birds (Scott 1954).

Of the 160 or so species of birds that use estuaries, about half feed on invertebrates and a quarter feed on fish, and most of the remainder are herbivorous (Hockey and Turpie 1999). In contrast to tidal estuaries, wader numbers and diversity in TOCEs tend to be low and the avifauna is

usually dominated by a small number of resident piscivorous species (Terörde 2008). These mainly include wading pisciv-ores such as herons and egrets, swimming piscivores such as darters, cormorants and grebes, and aerial predators such as kingfishers, fish eagles and terns. Waterfowl (ducks, coots, etc.) are not particularly common in TOCEs, except those with low salinity and aquatic macrophyte beds or in those that feature adjacent floodplain pans, e.g. West Kleinemonde Estuary in the Eastern Cape.

The avifauna of TOCEs can be significantly affected by the changes that result from periodic estuary breaching (Terörde 2008, Froneman et al. 2011). When the TOCE is full, it tends to be dominated by species such as herons, egrets and kingfishers. After mouth breaching, fish are sometimes stranded at high densities in the shallow water remaining and there is often a temporary influx of piscivorous birds, e.g. terns. Waders may visit the area for short periods during the open phase, but the exposed sand and mud flats are not particularly rich feeding grounds for these birds as they are in permanently open estuaries (Martin and Baird 1987).

If TOCEs are to be managed to the benefit of avifauna, their management is probably best directed towards the maintenance of healthy fish populations, since these systems tend to be most important for piscivores. Thus, annual breaching is desirable in order to replenish juvenile marine fish species stocks within TOCEs and this would necessitate adequate riverine inflow to these systems. Regular inundation of the TOCE floodplain will also be favourable for birds, and the shallow productive waters will provide ideal foraging areas for the associated bird fauna.

Apart from the effects of mouth state, the main factors affecting birds within these systems are habitat altera-tion and human disturbance. Estuary banks that are important to kingfishers can be eroded by boat wakes. Vegetation alongside the estuary, which provides roosting and perching sites for piscivorous birds, is sometimes removed for residential development. Jetties replace this function to some extent, but only for the less shy species.

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Human disturbance on foot and in boats can have a signifi-cant impact on bird numbers, although some species develop a greater tolerance to these activities than others. Management for the benefit of birds would therefore also ensure the retention of good stretches of natural fringing habitat, wake-free zones and a restriction of boating activi-ties to non-petrol motors and paddles.

Estuary–marine interactions

When fresh water and seawater mix in estuaries there is an enrichment of the system and primary productivity has been shown to increase (Scharler et al. 1997, Froneman 2002). In permanently open estuaries, the richest area is at the mixing interface, known as the river estuary interface (REI) zone (Bate et al. 2002, Snow et al. 2000). In most small TOCEs there is little or no REI because, during the closed phase, there is a minimal longitudinal salinity gradient (Whitfield et al. 2008) and when the mouth opens the water inside the estuary flows directly into the surf zone (Figure 1) with little or no mixing occurring within the estuary during the outflow phase (Perissinotto et al. 2004).

In recent years it has become increasingly apparent that the outflow phase of TOCEs is of considerable value to the functioning of the nearshore region where increased primary productivity results from the mixing of river water, estuary water and seawater (Vorwerk 2007). The increase in surf-zone productivity is important because it adds to the total capacity of the nearshore zone to support valuable coastal fish stocks (Lamberth et al. 2009). Consequently, the accumulated benefits of numerous TOCEs flowing into the coastal zone (especially along the eastern seaboard of South Africa) are likely to be substantial in terms of the productivity and fisheries in these coastal waters (van Ballegoyen et al. 2007). Management of TOCEs should therefore be directed towards ensuring that breaching events occur naturally at appropriate times of the year, thus facilitating maximum benefits to both the estuaries and the coastal zone.

Managing the freshwater supply and mouth state of TOCEs

Setting the ecological water requirements for estuariesThere are six ecological categories for indicating the present ecological status of an estuary. The Water Act (RSA 1998a) requires that sufficient water of suitable quality be delivered to estuaries to ensure that they retain a predefined level of condition or health. This minimum ecological status or category is decided on the basis of an estuary’s importance and present condition as well as other demands on the influent water. The ecological categories into which estuaries may fall range from A to F, with Category A being pristine or near pristine and F being severely degraded (DWAF 1999). Under the Water Act, the management category, which describes the state of health in which the estuary will be managed in future, cannot be lower than a D category. Once the management category and corresponding ecologi-cal reserve have been set, then estuary managers have to work within the limitations of this allocation (Taljaard et al. 2003). Since TOCEs are particularly sensitive to changes in freshwater inputs, this may have an important bearing

on the use of complementary management strategies such as artificial breaching. Examples of TOCEs that have already had their ecological reserves determined include the Mhlanga in KwaZulu-Natal (Perissinotto et al. 2004) and East Kleinemonde in the Eastern Cape (van Niekerk et al. 2008).

Managing mouth stateOne of the most significant hydrodynamic management interventions for TOCEs is the artificial breaching of the mouth (van Niekerk et al. 2011a). The artificial breaching of estuary mouths can be managed under the National Water Act (RSA 1998a), Environmental Impact Assessment regulations under the National Environmental Management Act (NEMA; RSA 1998b) or the National Environmental Management: Integrated Coastal Management Act (RSA 2009). Morant and Quinn (1999) considered that in South Africa, the need to artificially breach the mouths of estuaries is the unique byproduct of poor urban planning.

Artificial breaching is commonly undertaken because of low-lying developments within the estuarine floodplain (Heydorn and Tinley 1980). When these become flooded there is a necessity to prematurely breach in order to prevent flooding of properties, public amenities, access roads or farmland (Huizinga and van Niekerk 2001). In addition, a high water level when the mouth is closed can result in potential pollution of estuarine waters from septic tanks and sewage systems (Huizinga et al. 1997). Breaching is sometimes also desirable for maintenance of the fish nursery function and other processes, particularly with unusually long periods of closed mouth state (Perissinotto et al. 2010b). Human pressures to undertake artificial breaching of a TOCE arise mainly when significant infrastructure becomes impacted by prolonged inundation (Huizinga et al. 1997). However, the decision to breach should only be taken when more than just a few properties are affected and there is adequate river inflow to maintain tidal exchange once the outflow phase has been completed.

The need for breaching may also arise if freshwater input is reduced to the extent that the frequency of natural breaching is unacceptably reduced or that extreme hyper haline conditions are beginning to develop within a closed system. Unfortunately, such artificial breaching of a TOCE is often associated with limited ‘head’ of water behind the berm and hence poor scouring of sediment from the estuary during a breaching event (Beck et al. 2004). Repeated low-level artificial breaching will almost certainly lead to a gradual shallowing of the TOCE due to sediment accumulation, particularly in the lower reaches.

In general, management actions should prevent, as far as possible, anthropogenic interference in natural estuary processes (Morant and Quinn 1999). Under most circum-stances, TOCEs should be left closed, for years if necessary. Artificial mouth breaching without simultaneous river inflow may result in the introduction of large volumes of saline water that will alter estuarine community composition by favouring species tolerant of high salinity, which can modify foodweb dynamics. If a manager believes that a mouth should be breached artificially, then consideration should be given to the season, roughness of the sea, the state of the tide (spring or neap), and whether some additional water

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from an upstream dam can be provided during and after breaching to increase the duration of opening and facilitate the scouring of sediment by the outflowing water (Hay et al. 2005, van Niekerk 2007).

Artificial breaching is generally not advised as a solution to persistent pollution-related problems (e.g. eutrophica-tion) as these should be fixed at source. However, there are instances where artificial breaching should be conducted (e.g. where accidental spills or leakages might threaten human and/or ecosystem health). Conversely, there are instances where oil and diesel spills from grounded ships might enter nearby estuaries that are open and, under these circumstances, artificial closure of the mouths of such systems would be justified (Moldan et al. 1979).

Minimising the impacts of surrounding developmentsDevelopment in and around South African estuaries is governed by various acts, such as the Local Government: Municipal Systems Act (RSA 2000), Sea-shore Act (RSA 1935), Environmental Impact Assessment regulations under the National Environmental Management Act (NEMA; RSA 1998b), and the National Environmental Management: Integrated Coastal Management Act (RSA 2009). The geographical boundary of estuaries in South Africa has recently been upgraded to the 5 m above mean sea level (MSL) contour and no development or structures should be permitted below this level. Disturbance of river/estuary banks should also be prevented and destabilisation of vegetation or dunes near estuaries should be disallowed, because this can lead to long-term changes in sediment dynamics and the hydromorphology of estuaries (van Niekerk et al. 2011b).

The 5 m contour is an easily available reference line as it is indicated clearly on the 1:10 000 orthographic maps series (or GIS layer) available from Chief Directorate: Surveys and Mapping (Mowbray, Cape Town). In most cases, the 5 m MSL contour also allows for the inclusion of a buffer zone of terrestrial vegetation that represents the transition between terrestrial and coastal ecosystems (Figure 7). Water levels within TOCEs can often reach levels of between 2.5 and 3.5 m MSL during the closed phase or during major river flood events. In addition, the 5 m MSL contour provides a buffer zone that can allow for an estuary boundary to be elevated in the future if sea level rises due to climate change (van Niekerk et al. 2011b).

There is a need to protect the environment against unnecessary damage as well as developers and investors against loss. Planning of developments should not be based on average conditions but on extreme events that may occur only once in 50 or even 100 years. The massive damage to private property and infrastructure in KwaZulu-Natal during the floods of 1987 (Perry 1989) provides an excellent example of the consequences of siting developments and infrastructure on estuarine floodplains (Fuggle and Rabie 1992). Therefore, no development should be located below the 1-in-100 year river flood level, or at least not below the geographical boundary of an estuary (5 m MSL) if flood lines are not known (Perissinotto et al. 2010b).

To facilitate and improve the management of estuaries, the Integrated Coastal Management Act (RSA 2009) calls for the development of a national estuarine management protocol and individual estuarine management plans. The

Department of Environmental Affairs (DEA), in consultation with the Department of Water Affairs (DWA), is in the process of developing a national estuarine management protocol aimed at providing a framework for aspects that have previously been seen to be beyond the scope of the various sectors or departments (van Niekerk and Taljaard 2003). A generic framework for the preparation of individual estuarine management plans has been developed with the following key components: the development of a situation assess-ment; the setting of a vision and strategic objectives; the evaluation of management strategies to achieve the vision and objectives; the preparation of an estuarine zone plan and the establishment of operational objectives; the identifi-cation of management action plans for different management sectors; the implementation of management action plans; and monitoring (van Niekerk and Taljaard 2003, Taljaard and van Niekerk 2009).

Conclusion

Research in South African TOCEs has reached the stage where the successful input of valuable information for estuary management plans is a viable option. Although much of the literature is published in scientific journals and therefore not always readily accessible or easily interpreted by environmental managers, there is an increasing body of knowledge that has interpreted the science for direct use by management agencies (e.g. Whitfield and Bate 2007, Perissinotto et al. 2010b, Wood 2010). We hope that this review has built upon the above foundations, thus making a contribution to the future management of South African TOCEs.

Acknowledgements — This paper is part of the output from work undertaken in WRC Project K5/1581, and reported in part in Whitfield and Bate (2007), with funds provided by the Water Research Commission (WRC). The authors thank the various

Figure 7: An example of the delineation of the 5 m contour around a typical KwaZulu-Natal TOCE and the location of existing developments (e.g. roads, houses and bridges) that may be affected by that delineation (after Forbes and Demetriades 2010)

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organisations that have supported the respective researchers over the study period, as well as the National Research Foundation (NRF) for providing postgraduate student bursaries. The use of photographs taken by Nicky Demetriades, and helpful comments by two anonymous reviewers on an earlier draft of this paper, are also gratefully acknowledged.

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Manuscript received June 2011; accepted September 2011