Temporarily Open/closed Estuaries Research Articles

Methods for predicting berm height at Temporarily Open/Closed Estuaries

This study investigates the berm crest elevation at 20 Temporarily Open/Closed Estuaries (TOCEs) along the wave-dominated, microtidal coastline of South Africa and provides suitable methods of prediction. Several years of berm survey and estuary water level data were analysed for the selected estuaries, resulting in a relatively large record of historical berm crest elevations compared to previous regional datasets. The new dataset was utilised to test and develop multiple methods for the prediction of berm height and consequently flood levels within TOCEs. Long-term wave runup elevations (R2%) were simulated at the estuary berms using existing runup parameterisations to establish the exceedance probability threshold of R2% associated with the maximum berm height (BMAX). It was found that the 5% exceedance probability of runup provided consistent predictions of BMAX at the selected TOCEs with a mean absolute error (MAE) of 0.28 m. A Berm Height Criteria was developed to predict BMAX based on the wave height, beach face slope, sediment grain size and tidal range at the estuary. The criteria-based approach ensures ease of use and is intended as a first estimate of BMAX (with MAE of 0.23 m). Lastly, a stepwise berm growth model was tested according to its potential to track short-term berm growth during inlet closure to breach cycles. The model provides a simplified approach to predict berm accretion based on the overwash potential depth, which is defined as the elevation difference between R2% and the berm height. The use of a site-specific scaling parameter for berm accretion provided predictions of the berm height at breach with MAEs varying between 0.03 m and 0.17 m among estuaries for the short-term scenarios tested. The hydrodynamic and morphological variability displayed by the selected South Africa TOCEs increased the robustness of the results, potentially making the findings relevant to TOCEs in other parts of the world.

Natural nutrient enrichment and algal responses in near pristine micro-estuaries and micro-outlets

Naturally-occurring pristine estuarine ecosystems are rare in modern environments due to anthropogenic encroachment. There are more than 100 outlets around the South African coast arising from streams flowing from small catchments close to the sea. Eight near natural systems were sampled seasonally over the period of a year to acquire baseline information on water quality and chlorophyll a status across a variety of algal guilds (benthic microalgae, phytoplankton and macroalgal cover). Albeit on a much smaller-scale, these systems represent natural surrogates of larger temporarily open/closed estuaries (TOCEs). Inorganic nutrients (ammonium, total oxidized nitrogen and soluble reactive phosphate), phytoplankton and microphytobenthos chlorophyll a, as well as macroagal percentage cover, were measured using standard methods. Algae showed a seasonal trend, with blooms of both micro- and macro-algae occurring during summer, with a dieback recorded in autumn. During summer, only one system had a phytoplankton peak in chlorophyll a above 20μgL−1, while the microphytobenthos concentrations in three of the systems were above 100mgm−2. Summer blooms of green filamentous macroalgae occurred in all four micro-outlets and in one micro-estuary. Using a linear mixed-effects modelling approach, significant drivers for algal growth related to temperature, nutrient conditions, light availability and water residence time, all of which are known to stimulate primary production. The results show that enrichment from natural sources display similar responses from primary producers to mesotrophic and/or eutrophic water bodies, with the exception that they revert to a natural state rather than continue into a degraded state as is the case in artificially enriched systems. This importantly demonstrates how larger temporarily/open closed estuaries, most of which are anthropogenically degraded, might have functioned under a former more balanced state. Some of these larger systems now respond to nutrient enrichment by exhibiting permanent cultural eutrophication.

Distribution of macrophyte species and habitats in South African estuaries

This study investigated the area coverage and species composition of different habitat types in all estuaries and bioclimatic regions along the coast of South Africa. Estuary habitat covers a total area of 95,657ha. Reeds and sedges (14,732.6ha) were the dominant habitat type overall. Supratidal salt marsh was dominant in the cool temperate region (4869.7ha), intertidal salt marsh in the warm temperate region (2509ha) and reeds and sedges in the subtropical region (10,484ha). In the cool temperate, warm temperate and subtropical regions, the estuaries with the largest areas were the Berg (6799ha), Knysna (2038.7ha) and St Lucia estuaries (40,832.8ha), respectively. The number of macrophyte species per estuary varied from one to 53. Macrophyte species richness was higher in estuaries of the cool and warm temperate zone than that in the subtropical zone. Macrophyte species are distributed in 47 families and the dominant families are the Cyperaceae (18), Chenopodiaceae (14), Juncaceae (5), while 15 other families had only one representative species. The number of species in temporarily open/closed estuaries (TOCEs) and permanently open estuaries (POEs) was higher than that found in other estuary types (river mouths, estuarine bays and lakes). Only Juncus kraussii and Phragmites australis occurred in more than half of South African estuaries. The data are collated in an estuary botanical database that can be accessed for monitoring and conservation planning purposes. These findings have already found application in the management of estuaries in South Africa, the National Biodiversity Assessment and the National Estuary Biodiversity Plan.

Subtidal macrozoobenthic community resilience to change following natural mouth breaching of the temporarily open/closed East Kleinemonde Estuary, South Africa

The East Kleinemonde Estuary, on the south-east coast of South Africa, typically opens for only short periods, usually <10 days. During a nine-month study in 2006, marine influence persisted for a near-continuous period of three months. This provided an opportunity to explore potential changes in the structure and pattern of the subtidal estuarine macrozoobenthic community. Salinity changes were minimal after mouth opening, decreasing by c. 4 and 8 in the lower and upper estuary, respectively. Salinity decrease was due to river dominance over tidal influence. Two biotic assemblages, structured by sediment characteristics, typified the community throughout the study: a sand-associated group near the mouth and a mud-associated group upstream. Amphipods were proportionally the most abundant group within each assemblage and on each sampling occasion. At the species level, high variability typified the community, with no trends or patterns observed. However, species richness declined from 27–30 to 23 after mouth opening. The number of species returned to former levels at the time of the final survey and just before final mouth closure. An analysis of the subtidal macrozoobenthos in the East Kleinemonde and other temporarily open/closed estuaries (TOCEs) of similar size in the bioregion indicated that communities were typically composed of relatively few, broadly tolerant euryhaline species with no species unique to TOCEs.

Complex genetic structure of a euryhaline marine fish in temporarily open/closed estuaries from the wider Gulf of Aden

Temporarily open/closed estuaries (TOCEs) are major ecosystems of the Indian Ocean coastal zones. Their functioning is tightly linked to climatic events such as monsoons and storms, and their mouth can close up for prolonged and variable periods of time, thus limiting their connectivity with the marine environment. Two types of genetic markers (i.e., mitochondrial cytochrome c oxidase I (COI) gene and microsatellites) were used to assess the genetic structure of 288 individuals of Terapon jarbua, a widely distributed fish species in the wider Gulf of Aden. Firstly, the hypothesis of panmixia was tested. Then, alternative hypotheses were investigated to explain the population genetic structure of T. jarbua: could it be shaped by (1) regional biogeographic barriers (i.e., Socotra Island vs. mainland Yemen) and/or (2) the particular functioning of TOCEs in relation to the species life cycle and particular physical ocean parameters? On one hand, the polymorphism of the COI inferred (1) a high haplotype diversity and a reduced nucleotide diversity over the whole data set and (2) a “star-like” shape of the haplotype network, thus suggesting a population expansion after local extinctions during the Pleistocene glaciations. On a second hand, the genotyping of eight microsatellites showed a significant genetic differentiation between T. jarbua populations in the wider Gulf of Aden (F ST = 0.035, p < 0.01), and thus, the panmixia hypothesis was rejected. Analyses of molecular variance results did not show any significant structure between Socotra Island and mainland Yemen and thus did not support the role of biogeographic barriers in structuring T. jarbua populations. Significant multi-locus deficits in heterozygotes at particular locations displaying high levels of F IS were recorded. It was suggested that a possible Wahlund effect took place in those TOCEs which could gather several cohorts of larvae stemming from different marine subpopulations over the sampled area. The present study emphasized the uniqueness of each TOCE as a potential reservoir of biodiversity and the urgent need for a better conservation program of those estuaries in the region, in order to avoid habitat fragmentation and permanent closure of those nursery areas by human activities.

Inlet mouth phase influences density, variability and standing stocks of plankton assemblages in temporarily open/closed estuaries

Temporarily open/closed estuaries (TOCEs) are systems temporarily closed off from the sea due to the development of a sandbar across the mouth during low rainfall periods. Due to their generally small size and overall higher retention times during the closed phase, these environments are highly susceptible to anthropogenic changes such as catchment degradation, climate change, pollution and changing river flow patterns. Several authors have reported marked fluctuations in biomass and density of biotic communities between the open and closed phases of specific estuaries. This study determines the responses of water column nutrients, plankton density and biomass to inlet phase changes of these systems, for which information was collated on dissolved inorganic nitrogen and phosphorus concentrations, phytoplankton biomass, zooplankton density and biomass from 16 TOCEs in South Africa. Our analyses detected higher phytoplankton chlorophyll-a concentrations, zooplankton density and biomass during the closed phase of these estuaries. The analysis of temporal dynamics of five systems showed that plankton biomass can attain pre-breaching levels 10 days after mouth closure. Estimates showed that plankton standing stocks during the closed phase can be 26 to 10 000 times higher than during the open phase. Our findings have significant implications for the management and conservation of TOCEs, especially the regulation of freshwater inflow and artificial breaching of their mouths, and highlight the vulnerability of these systems to current and expected impacts of anthropogenic and climatic changes on their productivity and functioning.

Effects of Artificial Breaching of a Temporarily Open/Closed Estuary on Benthic Macroinvertebrates (Camacho Lagoon, Southern Brazil)

The mouth dynamics of temporarily open/closed estuaries (TOCEs) play a key role in their overall functioning. In this study, the effect of the inlet state (closed vs. artificially breached) on spatial variability of macrobenthic invertebrates (composition, abundance, and biomass) was assessed in a temporarily open/closed lagoon of South Brazil (28°35′S/48°52′W). Samplings were carried out in two periods during closed (July and November) and open phases (July and November). Additionally, in order to evaluate possible transitory effects of breaching, data obtained during closed and open phases were compared with those samples taken 60 days after the end of mechanical opening of the mouth (January). The artificial breaching markedly changed the dynamic of the benthic environment. After the inlet dredging and bulldozing, total organic content and microphytobenthic biomass were significantly reduced. The disturbance also resulted in a population crash of the macrobenthic invertebrates, with a reduction of 50% in biomass and 90% in density. Following the shock produced by the artificial breaching, most of the macroinvertebrates descriptors recovered, as shown by the univariate and multivariate analysis. However, a benthic community with a significantly different structure emerged. During the study, the macroinvertebrates from inner portions of the lagoon were less variable than those in the middle or near the lagoon inlet. The results of this study showed that the macrobenthic associations of Camacho lagoon were primarily structured by salinity and microphytobenthic biomass, which in turn, were regulated by the state of the inlet.

Predicting macrophyte states in a small temporarily open/closed estuary

Temporarily open/closed estuaries (TOCEs) shift between abiotic states associated with mouth status. The aim of this study was to assess whether macrophyte states could be identified based on the dominant cover abundance of different species representative of specific habitats. A 5-year dataset of monthly macrophyte cover was assessed for the East Kleinemonde Estuary in South Africa. Three macrophyte states were identified: namely open and tidal (predominantly intertidal salt marsh); closed and low water level (predominantly salt marsh); and closed and high water level (with submerged macrophytes). The threshold water level for the change from salt marsh to submerged macrophytes was 1.6 m above mean sea level (amsl). A fourth state where macroalgae were dominant was identified for high salinity conditions (above 30 PSU). It was then possible to examine simulated water level data for different inflow scenarios to determine how often these macrophyte states occurred. Available macrophyte habitat was also calculated for different water levels using a spatial model written in Modelbuilder (ArcGIS 9.3.1). Both methods used to predict available macrophyte habitats are rapid, requiring only information on the elevation range of the main habitats, as well as present distribution and bathymetric maps. These predictive techniques are useful in the determination of the ecological water requirements of small estuaries.

The effects of wastewater discharges on the functioning of a small temporarily open/closed estuary

Wastewater discharges affect the functioning of small temporarily open/closed estuaries (TOCEs) through two main mechanisms: (1) they can significantly change the water balance by altering the quantity of water inflows, and (2) they can significantly change the nutrient balance and hence the water quality. This study investigated the bio-physical responses of a typical, small TOCE on the east coast of South Africa, the Mhlanga Estuary. This estuary receives significant inflows of treated effluent from upstream wastewater treatment works. Water and nutrient budgets were used together with biological sampling to investigate changes in the functioning of the system. The increase in inflows due to the effluent discharges has significantly increased the mouth breaching frequency. Furthermore, when the mouth closes, the accumulation of nutrients leads to eutrophication and algal blooms. A grey water index, namely the proportion of effluent in the estuary and an indicator of the additional nutrient inputs into the estuary, reached high values (≳50%) during low flow regimes and when the mouth was closed. In these hyper-eutrophic conditions (DIN and DIP concentrations up to 457 μM and 100 μM respectively), field measurements showed that algal blooms occurred within about 14 days following closure of the mouth (chlorophyll-a concentrations up to 375 mg chl-a m −3). Water and nutrient balance simulations for alternative scenarios suggest that further increases in wastewater discharges would result in more frequent breaching events and longer open mouth conditions, but the occurrence of hyper-eutrophic conditions would initially intensify despite more frequent openings. The study indicates how water and nutrient balance simulations can be used in the planning and impact assessment of wastewater treatment facilities.

Verification of a conceptual model of water quality for small temporarily open/closed estuaries: East Kleinemonde Estuary, South Africa

The dynamics of the world’s larger permanently open estuaries have been well documented, but much less is known about the complex dynamics of smaller temporarily open/closed estuaries (TOCEs). Seventy per cent of South Africa’s 250 estuaries are classified as TOCEs, and many are less than 50 ha. A conceptual model of the water-quality dynamics of TOCEs has been proposed, primarily considering three hydrodynamic states (closed, semi-closed and open). This paper uses data from the East Kleinemonde Estuary, a small TOCE in the warm-temperate region of South Africa, to verify the model, focusing on physico-chemical parameters (salinity, temperature, pH, dissolved oxygen and turbidity) and inorganic macronutrients (dissolved inorganic nitrogen and dissolved inorganic phosphate) for the open and closed states. The results generally fitted the model in terms of the behaviour of the physico-chemical parameters and macronutrients in the water column. This knowledge provides a first approach to predict the water-quality dynamics in small TOCEs, thereby ensuring that the often limited resources are targeted at resolving uncertainties at appropriate temporal and spatial scales. TOCEs are not unique to South Africa and also occur along other coasts (Mediterranean and south-western Australia) where this type of model could also be applied.

The recovery of microalgal production and biomass in a South African temporarily open/closed estuary, following mouth breaching

Mouth breaching is a recurrent event in temporarily open/closed estuaries (TOCEs). Such disturbances result in flushing and sediment scouring, reducing the microalgal biomass stock. The depletion of these microalgae may have negative repercussions in the form of depleted stocks of commercial fish, game fish, crustaceans and mollusks. The aim of this investigation was therefore: (1) to monitor the recovery of microalgal biomass and production following a breaching event; and (2) to determine the key environmental parameters influencing primary production during the open and recovery phases. Phytoplankton and benthic microalgal production was measured ( 14C-uptake method) successively during the closed, open and recovery phases of the Mdloti TOCE (South Africa). Upon breaching, 94–99% of microalgal biomass was washed out to sea through flushing and sediment scouring. A temporary recovery of phytoplankton and benthic microalgal biomass was observed during the open phase, but this was not sustained because of continual flushing and scouring of the sediment. During the re-closure (recovery phase), microalgal biomass immediately increased, reaching pre-breaching levels 35–40 days following the breaching event. In contrast to biomass, autochthonous pelagic primary production reached a maximum level (341 mg C m −2 h −1) during the open phase. Pelagic primary production normalized to biomass (P B) significantly increased during the open phase. This is attributed to a favorable combination of optimum light conditions, high influx of macronutrients and high water temperatures (33 °C). Similarly, benthic primary production normalized to biomass (P B) peaked during the open phase (35 mg C mg chl- a −1 h −1). Multivariate analysis showed that major variations in primary production were mainly controlled by temperature, dissolved inorganic nitrogen (DIN) to phosphorus (DIP) molar ratios (water-column and pore-water) and light extinction ( K d), all of which were regulated by the state of the mouth.

A comparative study of microalgal production in a marine versus a river-dominated temporarily open/closed estuary, South Africa

Temporarily open/closed estuaries (TOCEs) account for almost 71% of the total number of South African estuarine systems. To date the dynamics of microalgal production and biomass in TOCEs is poorly understood. A survey was therefore conducted in two TOCEs along the KwaZulu-Natal coast: the river-dominated Mdloti and the marine-dominated Mpenjati. The aims of this investigation were: (1) to compare the variability of benthic and pelagic microalgal production and biomass in these two contrasting systems; (2) to determine the key environmental parameters influencing primary production in these TOCEs. Results show a similar pattern of primary production and biomass in both estuaries. High benthic microalgal biomass with low primary production and low phytoplankton biomass with high rates of pelagic primary production were observed in both estuaries. Possible explanations for this include: (a) optimum light conditions in the water column, compared to the sediment surface; (b) high impact of grazing by zooplankton in the water column; and (c) settling of phytoplankton. Significant correlations were observed between primary production and environmental parameters ( K d, PAR, temperature, DIN and DIP), during the open and closed phases of both estuaries. A BIOENV (PRIMER) analysis showed that patterns of primary production in both estuaries were influenced primarily by interactions of multiple independent parameters. Comparisons between measured hourly pelagic primary production rates at the Mpenjati and Mdloti and the composite parameter ( P c) of Cole and Cloern (1987) revealed highly significant relationships ( r 2 = 0.8; p < 0.001) between these differently derived variables.

Response of microphytobenthos to flow and trophic variation in two South African temporarily open/closed estuaries

Variations in microphytobenthic biomass were investigated in relation to major environmental factors in the Mdloti and Mhlanga temporarily open/closed estuaries (TOCEs), South Africa, from March 2002 to March 2003. Both estuaries receive different volumes of treated sewage waters. This has caused eutrophication and an increase in the frequency of mouth opening, particularly at the Mhlanga. No significant differences were found in microphytobenthic chl a concentrations between the two estuaries. At the Mdloti, microphytobenthic biomass varied considerably, with values ranging from 1.33 to 131 mg chl a m -2 during the open phase, and from 18 to 391 mg chl a m-2 during the closed phase. At the Mhlanga, microphytobenthic biomass ranged from 7.0 to 313 mg chl a m-2 during the open phase, and from 1.7 to 267 mg chl a m -2 during the closed phase. Unlike in the Mdloti, the higher microphytobenthic biomass values at the Mhlanga were not always associated with the closed mouth state. For the duration of the study, the Mdloti functioned as a typical temporarily open/closed system, with prolonged open and closed phases, while the Mhlanga behaved more like a permanently open estuary than a TOCE (it lacked a period of prolonged mouth closure).