News and Notices‎ > ‎

City determines cause of conditions at Zandvlei Estuary Nature Reserve

posted Apr 20, 2012, 4:27 AM by Damian Gibbs   [ updated Apr 20, 2012, 4:29 AM ]




20 APRIL 2012


City determines cause of conditions at Zandvlei Estuary Nature Reserve


Following extensive tests, the City of Cape Town has determined the reason for the the unusual conditions and fish mortalities currently prevailing in Zandvlei Estuary Nature Reserve.


The City is monitoring the situation at the Zandvlei very carefully and has conducted a number of tests in this regard. The tests and daily observations will continue for the foreseeable future. The City takes this situation very seriously as Zandvlei is an important natural, environmental and recreational asset. It acts as an important estuarine system; a nursery for many marine fish species that are caught by small scale commercial fishermen in the adjacent marine environment; a site for recreational fishing; and a recreational site for water sports. The vlei is also a valuable open space, with many residents living on its banks or enjoying picnics; bird watching; and hiking around its shores. All of these needs are taken into account in the vlei’s management and must be considered whenever issues affecting the vlei arise.   


The latest test results – samples taken on 16 April 2012

Initial test results indicated that the problem was due to a lack of oxygen in the water. It has since been confirmed that this was a symptom of the cause, rather than the cause itself.


The latest test results have confirmed that an extensive algal bloom comprising the speciesPrymnesium parvum, commonly referred to as “Golden algae”, is responsible for the aforementioned conditions.


This species has been identified in samples collected from Zandvlei on 16 April 2012. Of importance is the fact that it was also implicated in a fish kill at Zandvlei in the early 1970s. The Golden algae is classified with the Haptophyceae and does NOT belong to the same family as the so called “Blue-green algae” or Cyanophyceae which are periodically problematic in other water bodies around Cape Town.


When the population of Golden algae increases during a bloom cycle, the water may begin to turn cloudy yellowish, yellow-copper, or a copper-brown tea color. Another sign of P. parvum blooms can be foaming at the water surface if agitated or aerated, such as where there is wave action.


Fish affected by P. parvum toxins behave erratically. They may swim slowly below the surface, accumulate in the shallows, or show no normal avoidance of human presence or other disturbance. They may try to leap from the water to avoid the toxins. If clean water flows into the affected body of water, fish will often accumulate around this freshwater source. Young fish are often more sensitive to the toxins than adults. In the early stages of intoxication, the effects are reversible if the fish can move to uncontaminated water. Ecological impacts will vary depending on the length and severity of the toxic bloom. In larger water bodies with access to fresh water, toxic blooms may not kill all the fish present.


In addition to the toxicity of the algae, the fish at Zandvlei are also affected by diurnal (daytime) depletion of dissolved oxygen levels due to night time respiration by the large algal bloom and all other aquatic organisms within the water body.


Prymnesium parvum blooms are not a public health threat. The toxins that golden algae produce appear to have no negative effect on other wildlife, livestock, or humans. However, as a common sense precaution, dead or dying fish should not be consumed.


Residents are asked to please understand that while it is distressing to see so many fish die, it is a natural event and not due to a toxic spill from a factory or sewage system or pollution. Blooms can occur even in pristine systems that are not subject to urban-driven eutrophication. City staff from all departments are monitoring the area, continuing tests and removing dead fish from the area.


What are the causes of the bloom?

Most algal blooms occur due to favourable environmental conditions such as warm, still weather, lack of natural controls (such as herbivorous fish or zooplankton) and abundant nutrients. The Golden algal bloom probably responded to similar conditions prevailing in Zandvlei as well as the salinity which has been managed at a good level to ensure healthy estuarine functioning through the mouth management protocol.


For a long period Cape Town has experienced no rain and the heavy Easter rainfall would have disrupted the sediment, with additional nutrients washed off the catchment during the Easter rainfall fuelling the bloom.


Zandvlei is located in an urban catchment and receives ongoing high levels of nutrients from its catchment. Zandvlei also does not function as an entirely natural system, due to urban development in the Marina and canalisation of the mouth. This, combined with the increased run-off due to the hardening of the catchment, has resulted in a permanent body of water. This would have contributed to the favourable conditions for the bloom.


Harvesting of pond weed, particularly in the Marina canals may also have been a contributing factor as this plant is very abundant and shades the water column – which would normally exclude microscopic algae. Decomposition of pieces of cut pond weed in the water would have contributed nutrients to the system.


This species of algae could be part of the natural species assemblage of Zandvlei and it is possible it has always been present in low numbers. It is also possible that dormant spores in the sediments became active. Cells of any algal species can be transported between water bodies on the feathers of water birds or even on boats.


How long will it take for the Zandvlei to recover?

The bloom will probably last several weeks and it is not known whether Zandvlei will “reset” itself (in terms of the composition of the algal community) immediately or over several weeks or even months. It is likely that declining temperatures and winter rains will play a role. Eventual die-off of the algal bloom will probably cause continued oxygen depletion. It is also possible that a different algal species or species assemblage will replace the Golden algae.


It is not possible to determine how long it will take for the fish community to recover. Recovery could take place due to in-migration of freshwater fish from upstream areas. When the mouth is open, fish can also move into the estuary from the sea. Any fish that survive the current conditions will also contribute to recovery.


The way forward

The bloom cannot be stopped and should be left to run its course. Winter rain may assist with reducing salinity which could retard algal growth. Opening the mouth in the immediate future could make the situation worse since there will not be good sea water inflow or flushing and dropping water levels will cause the algae and any toxins to be concentrated within the smaller volume of water. Inflows of sea water would probably help to sustain the bloom since this species thrives in saline conditions. The next scheduled opening of the mouth in early May will be reconsidered depending on prevailing weather conditions and the state of the present algal bloom.


City staff as well as those from partner agencies will continue to monitor the situation. The City’s staff have been on site over the last two weeks on an almost 24-hour basis. This has now been reduced to daily during light hours, with monitoring taking place on a daily basis. Today, 19 April 2012, numerous pelicans and seagulls were feeding in the vlei; the sand prawns were active and there were no reports of fish die-offs (several mullet were seen alive). Staff from the Environmental Resource Management Department are on site assisting with relocation of live fish and removal of dead fish.


Enforcement at the Zandvlei

The cards of four Peace Officers were taken away as part of an investigation which was completed late last year. This investigation was initiated following a media and public alert in August 2011. The re-issuing of the law enforcement cards is in process. Alternative measures were put in place to ensure that law enforcement continued at Zandvlei.


Under the City’s Vlei By-law a permit is required to fish at Zandvlei. No fishing may be done off the bridges or at night. The bag limit for marine species is covered by the Marine Living Resources Act, which is enforced by the Department of Agriculture, Forestry and Fisheries.


Over the last couple of weeks, the Environmental Resource Management Department’s main objective at the Zandvlei has been to monitor and prevent illegal fishing. Metro Police and the South African Police Services cleared the area on various occasions. It must be noted that the Zandvlei area is very large, so it is not possible to prohibit illegal fishing completely but it has been reduced to manageable levels. During this time, there was also legal fishing taking place.


The City is doing all that it can to monitor the Zandvlei, preserve its biodiversity and the integrity of the natural system. Residents are asked to please assist the City by reporting any suspicious behaviour immediately to the Call Centre on 0860 103 089 or to the Biodiversity Management South Region office number on 083 499 1717. Please include the exact time of the incident, the registration of the vehicles involved, number of people involved, and any other details that may prove useful.


There are also discussions with the City’s Safety and Security Directorate on training law enforcement officials as Marine Inspectors, along with the adoption of a new Vlei By-law to regulate the use of the City’s inland waters. This process has been initiated and a draft by-law was advertised. However, the draft requires extensive revision and re-advertising.


Challenges in the management of the Zandvlei

It is important to note that the Zandvlei is an estuary which requires three elements to function optimally: a healthy population of pondweed; tube worm (which grows on hard structures); as well as salinity.


The City’s teams of scientific experts are focusing on the main challenges contributing to the situation currently seen in Zandvlei: the quality of water entering Zandvlei from the catchment; the pond weed harvesting; assisting with the optimal functioning of the estuary (which could include dredging and moving of the rubble weir); and addressing the lack of flushing in the Marina da Gama canals.


These and other challenges will be investigated under the Estuary Management Forum, due to be set up next week in partnership with CapeNature.


Opening of the vlei mouth

The mouth was opened earlier than scheduled, i.e. on 29 March instead of 4 April, as there was an accumulation of large fish in the outlet channel attempting to leave the estuary. The fish were responding to natural cues governing movement in and out the estuary, but this could have been exacerbated by the pending algal bloom.


The mouth then remained open for slightly longer than usual until the Spring tide. The mouth was closed on Easter Monday (9 April). The next scheduled opening of the mouth in early May will be reconsidered depending on prevailing weather conditions and the state of the present algal bloom.


While the mouth is open fish move freely in and out to sea to feed and spawn, but are sometimes “trapped” in the vlei when the mouth is closed again. Prior to 29 March the mouth of the estuary had been closed since 9 March 2012. At this time of the year, the fish respond to strong natural cues to move out to sea and are known to congregate near the mouth in anticipation of Spring tides, “waiting” for the mouth to open. There were numerous reports that many large fish had been trying to exit the vlei recently and were being poached illegally from the outlet channel by opportunistic fisherman. The opening of the mouth on 29 March thus enabled the fish to naturally move through the mouth out to sea safely. At the time there were also reports of increased fish activity in False Bay with accompanying cautions regarding possible increased White Shark activity.


The mouth of Zandvlei is managed on an ongoing basis according to a formal mouth management protocol which was developed using extensive information and recommendations from studies that have been undertaken in the estuary over the years. As per the protocol, formal approval from the Provincial Department of Environmental Affairs and Development Planning in terms of NEMA regulations was obtained; along with the support and approval of the CAPE Estuaries Programme Manager. Opening and closing the mouth is done in order to balance water depth and salinity in the estuary. Water depth management is important so as to prevent flooding of residential properties and also to ensure that conditions are not too shallow as this would hamper recreational activities. The mouth management protocol also very importantly allows intrusion of saline water from the sea which is critical for ecological requirements of the estuary and its biota. When the mouth is open fish are able to move into / out of the estuary according to the natural cues governing their feeding and breeding life cycles.


Mouth management is undertaken according the a schedule determined by the annual tide tables with the mouth being opened before and then closed during the peak of the Spring high tide in order to “capture” a good inflow of sea water.


Ongoing monitoring has shown that the current mouth management procedure has resulted in a significant improvement in water quality and biotic assemblage of the vlei – the recent fish kill, low oxygen levels, and algal bloom thus being viewed as localised seasonal occurrence and not necessarily pointing to a permanent “collapse” of the vlei.



While the mouth management process effectively controls water levels which are evident throughout the system including the Marina canals, there is fairly limited saline movement into the canals. This is due to their artificial design and construction. Many of the canals are blind ending and it is a fact that the water in these areas becomes very stagnant at times (particularly in the later summer/early autumn). The City is exploring ways to improve circulation.



Zandvlei has several rivers which enter its northern area via urbanised catchments. These include the Westlake Stream, Keysers River and Sand River. Pollutants such as sewage and other forms of contamination typical of urbanised areas and the agricultural lands located high in the catchment entering the storm water canal systems are also present. The vlei is also compromised by waste such as litter entering the storm water canal systems. 


There have in the past been sewage spills into the vlei, which increased the background nutrient levels particularly in the Marina da Gama canals. The City is investigating additional, and more effective, litter catch nets to capture litter before entry into the vlei as well as a catchment approach to the management of solid waste and other pollution.



Pond weed is an aquatic plant that grows in shallow wetlands and is tolerant of moderately low salinity levels. It is capable of rapid growth and can cover water bodies in short time spans. Its growth is encouraged by nutrient enrichment of water. It is important to note that the resilience that Zandvlei has shown to date is largely due to the presence of the pond weed (which improves water quality through filtering and uptake of nutrients), however the excessive growth of this plant is impacting on recreational uses of the vlei and there is a programme in place to harvest and remove it. This programme also sets down parameters for the amount of weed that can be cut and also allows for areas where there is no cutting.  This is to ensure that a healthy population remains.


The management of this weed in Zandvlei has been undertaken by the City since the construction of the Marina da Gama in the 1970s. Initially this was done by two custom built “weed harvester” machines.


These were built more than 30 years ago and are now unreliable with a large amount of downtime due to malfunctions, breakages and scarcity of spare parts. One of these machines has subsequently been moved to Little Princess Vlei where it is operated by City Parks, and thus Zandvlei is currently served by a single machine.  As a result, consistent removal of pond weed is not being effectively conducted and the pond weed has grown exceptionally rapidly, particularly in this year’s very warm summer conditions.


This is compounded by the fact that the constant nutrient enrichment of the vlei is encouraging the prolific growth of the pond weed. To address the pond weed issue, the City is looking at the following options:


·       obtaining a new up-to-date aquatic harvesting machine;

·       obtaining a crane truck to reduce the transport costs and improve efficiency of the process;

·       reducing the nutrients entering the vlei system;

·       investigating current weed harvesting plans and improving them where possible.



A rubble weir at a present level of 0,6m above mean-sea-level (average height of the ocean's surface, halfway point between the mean high tide and the mean low tide) across the mouth of Zandvlei controls the water levels to provide reasonable recreational opportunities and allow large scale rain events to pass through the system without causing flooding of residences. It has succeeded in providing optimal salinity levels to ensure healthy estuarine functioning.


In terms of detailed studies undertaken in 2000, it was determined that 0,5 to 0,6m would be the optimal operating level. In the 1990s the rubble weir was operated at higher elevations reducing the inflow of sea water and preventing adequate fish migration which proved to be detrimental to the health of the system.


The rubble weir also serves a secondary function of providing protection to a sewer line just north of the road bridge as well as ensuring that severe local scouring does not further destabilise the walls of the outlet channel. Removal of the sewer line and replacement with a rising main and pump station is technically feasible but would require a capital input of about R10 million which excludes a continuing operating budget.  A feasibility study needs to be done to determine if removal of the rubble weir would assist with flushing of the system. It could have other impacts on water depth; the ecological functioning and on the recreational use which is an important part of the vlei.


Mechanical dredging to remove accumulated river and sea derived sediments as well as organic sediments accumulated in the Marina canals is being investigated as a short-term solution, but will first require an Environmental Impact Assessment (EIA) approval from the Department of Environmental Affairs and Planning to ensure that all ecological processes are adequately addressed. This will increase the saline water entering the vlei and the flushing.  


More information about Golden algae

Prymnesium parvum is a microscopic (about 10 µm), flagellated alga that is capable of producing toxins that can cause fish kills. This algal species is found worldwide and is most often associated with estuarine or marine waters, but it can also occur in inland waters that have a relatively high mineral content. This organism is known for seasonal harmful algal blooms wherein cell densities increase rapidly with the accompanying presence of potent toxins. The algal blooms do not always form toxins however, and the onset of toxic conditions can be difficult to predict.


Golden algae produce a number of these toxins, collectively known as “prymnesins”. The toxin adversely affects gill-breathing organisms such as fish, bivalves, crayfish, gilled amphibians, and also some species of plankton. The toxin damages the permeability of gill cells, which lose their ability to exchange water and absorb oxygen and bleed internally, resulting in death of the organism by asphyxiation.


Although the algae can exist in waters without causing harm, under certain environmental conditions, P. parvum can gain a competitive edge over other algal species, and algal blooms can develop. The presence of blooms of P. parvum does not necessarily mean the algae will produce and secrete prymnesins into the water. The alga can remain dormant as cysts in the benthic sediments of water bodies for many years until favourable conditions for their proliferation exist.





Issued by: Communication Department, City of Cape Town


Media enquiries:


For operational enquiries: Julia Wood, Manager: Biodiversity Management, Environmental Resource Management Department, City of Cape Town, Tel: 021 514 4155 or Cell: 084 464 9153


Dalton Gibbs, Manager: Area South, Environmental Resource Management Department, City of Cape Town, Tel: 021 713 0510


For any other enquiries: Alderman Belinda Walker, Mayoral Committee Member for Economic, Environmental and Spatial Planning, City of Cape Town, Tel: 021 400 1299 or Cell: 083 629 8031, E-mail: