A survey of the quality of water flowing from micro-system types to the ocean, along the subtropical east coast of South Africa, showed a wide variation in the concentrations of total nitrogen, ...phosphorus and phytoplankton biomass in the different systems located, in many cases, only a short distance from each other along the coastline. The origins of the high phytoplankton growth indicate pollutants caused by the land-use in this highly populated coastal region. The main agricultural activities in the area are sugarcane, permanent orchards, and forestry. The levels of N and P in the water varied from 'good' to 'poor', i.e., TN 0.15-3.99 mg·L-1, TP 0.02-0.33 mg·L-1 and chlorophyll-a from 0 to almost 45 mug·L-1. Rapid coastal population densification appears to have been the cause of the pollution levels measured for total nitrogen, phosphorus, and phytoplankton biomass. Most of the micro-systems with a total modified peri-catchment above 80% were enriched by both TN and TP. While the hypothesis tested was that the main cause was residential development (e.g., septic tank effluent), it was not possible to show any statistical significance to support such a specific conclusion. Although these systems are small individually, the great number along the coastline warrants recognition as important sources of freshwater inflow and nutrients to the marine environment.
In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ...ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary
was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year
period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and
invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit
into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river
flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15-25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure
(5-15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water
through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order
to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.
Epipelic diatom flora was sampled around the South African coast between the Olifants Estuary, on the cool Atlantic Ocean northwest coast, and the St. Lucia Estuary, on the Indian Ocean northeast ...coast. Altogether, 333 taxa were identified with 14 being ubiquitous, as they were found in the cool temperate, warm temperate, and subtropical areas, as well as in St. Lucia Estuary situated close to Moçambique. There was little difference between the epipelic diatom species present in intertidal and subtidal areas and, because many of the species have a high tolerance to salinity, with some being found in conditions ranging from freshwater to a salinity of more than 150 psu, it was concluded that many of the species sampled do not appear to be reliable indicators for assessing salinity in South African estuaries. Although there was a wide spread of diatoms across all of the estuaries around the coast, the greatest species similarity occurred between the Olifants, Great Berg and Breede estuaries, suggesting that the Breede Estuary, normally considered to fall within the warm temperate region, may be more similar to the cool temperate type estuaries. Data also showed that there was very little similarity between the diatom flora in the rivers flowing into estuaries and the diatom flora in the estuaries.
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.
Impoundments and abstraction from rivers have reduced the amount of riverwater flowing into estuaries. This has necessitated research on the riverwater requirements of these important ecosystems. A ...study was initiated on the structure and biomass of microalgae in the Gamtoos estuary (Eastern Cape Province, South Africa) in order to improve our capability to predict changes and determine the acceptability of reduced riverwater input. The mean annual runoff of the Gamtoos River has been halved by major impoundments. Agriculture in the Gamtoos estuary valley contributes high concentrations of nutrients through groundwater seepage and an extensive sub-surface drainage system. This study determined the responses of microalgae to the volume of river discharge. A strong longitudinal salinity gradient was present at water flow rates ranging from 0·3 to 30·5m3s−1. There was strong vertical stratification at flows above 1m3s−1. Nitrate concentration increased from 1·9±0·2μM (at 0·3m3s−1) to 83·6±13·8μM (at 30·5m3s−1). Phytoplankton biomass was highest in the oligo- and mesohaline reaches of the estuary and was strongly correlated to turbidity. Phytoplankton chlorophylla was highest at flow rates of 0·8 (47·5±4·5μgl−1) and 1·16m3s−1(49·9±7·3μgl−1). Subtidal and intertidal benthic chlorophylla were highest at flow rates of 1·0 (57·7±0·4kg) and 1·16m3s−1(8·7±0·1kg) respectively. A riverwater input rate of ∼1·0m3s−1appears optimal for phytoplankton and benthic microalgal growth. At this flow rate, the estuary acts effectively at removing excess nutrients from the water before it discharges to the sea.
This article presents a historical overview of estuarine microphytobenthos (MPB) research in South Africa published over the period 1950 to the present, highlighting major milestones, challenges and ...estuarine management problems, as well as future research needs within the South African context. The studies that were covered comprise peer-reviewed books and journal articles on relevant research conducted in any estuarine environment during the period reviewed. There was a general increase in MPB research outputs over the decades, from only two publications in the 1950s, to over 20 outputs between 2010 and the present. Whereas the MPB studies in South Africa cover a broad spectrum of themes, the research priorities in these works have changed from taxonomically biased studies to those of ecologically based research. Research in the 1950s and 1960s was exclusively taxonomic in nature, with the first ecological investigation incorporating MPB being produced in the 1970s. By the 1980s, ecological studies dominated the research outputs and this trend has persisted to the present. The 1990s and 2000s saw the rise of research into the role of fresh water as a driver of MPB dynamics, whereas the period 2010 to the present saw an increase in more diverse ecological themes, ranging from an autecological investigation to food-web studies and the assessment of multiple drivers of MPB dynamics. However, the majority of studies have focused on either diatoms or estimates of overall MPB biomass. Moreover, there is a regional underrepresentation that runs broadly along biogeographic lines, with the bulk of the work having been conducted in the warm-temperate and subtropical zones of South Africa. Challenges and future research needs for the region are outlined, as is the need to expand MPB research to include other aspects of the biology and ecology of this flora.
The purpose of this study was to determine whether there was a sufficiently high residual salt load in the dry sediments of the St Lucia Estuary to cause salinity problems should it later fill up ...with either freshwater or seawater. The estuary lakes have suffered the effects of a severe drought since 2002 with the result that many areas were dry, and the salinity of the residual water varied between 4 psu and up to five times that of seawater. Measurements of the salts content in the sediments to a depth of 20 cm showed that more than 2 million tonnes of salt was held in this layer of the sediment in 2006. Recent management of the estuary (since 1970) has ensured that the mouth was not artificially opened. This was to prevent the inflow of seawater, with its salts, that would otherwise enter while the drought was in place. The results of the sediment salinity data showed that if the drought had been broken and the lake area filled with rain and river water, the resulting salinity would be about 6 psu. In March 2007, Cyclone Gamede in the Indian Ocean off the east coast of South Africa produced a wave climate at sea that resulted in the mouth breaching; thus introducing an estimated 12 million tonnes of salts. The high salinity in the system resulting from this breach is expected to have an adverse effect on the ecology of the system, whereas the residual salinity in the sediments would not have caused an environmental problem. If the estuary and lake system were to fill completely with seawater, the residual salts together with seawater will raise the salinity to an initial value higher than 40 psu, which will have the effect of suppressing much of the important submerged vegetation that is vital for sustaining juvenile fish in the system. Many of the large fauna will also suffer from a shortage of freshwater.
The Olifants Estuary is one of only three permanently open estuaries on the West Coast and is ranked third in terms of conservation importance of all estuaries in South Africa. It has the largest ...supratidal (143 ha) and floodplain (797.1 ha) salt marshes in the country. Intertidal salt marsh covers 91.94 ha, reeds and sedges 60.05 ha and the submerged macrophyte
Zostera capensis 47.72 ha. Correspondence analysis revealed intertidal, supratidal, floodplain and terrestrial plant community types within the studied estuary. The species composition of these communities was the same for the lower, middle and upper reaches of the Olifants Estuary, with the exception of the supratidal community type in the upper reaches that consisted of halophytes with a lower salinity tolerance range rather than the typical dense monospecific stands of
Sarcocornia pillansii. The environmental variables that had the greatest influence on the distribution of the dominant salt marsh species,
S. pillansii, included soil moisture, distance from the estuary, elevation above mean sea level, and depth to the water table. The most important ecological driver for salt marsh vegetation, especially along the arid West Coast of southern Africa, is moisture. The low rainfall and irregular occurrence of advection sea fog increases the importance of a shallow (<
1.5 m) saline (<
35 psu) water table in the floodplain as a source of moisture during dry periods.
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