South African Estuaries Research Articles

Assessing the feasibility of mapping changes of ecosystem functional groups in South African estuaries using Landsat and Sentinel images of 1990, 2014, 2018 and 2020

This study evaluates the feasibility of using medium-resolution satellite sensors to monitor changes in the extent of ecosystem functional groups (EFGs) in South African estuaries, for reporting on the 2030 targets of the Global Biodiversity Framework (GBF). Landsat and Sentinel-1 and -2 image collections in Google Earth Engine (GEE) were used to generate output layers for each of the national land cover years—1990, 2014, 2018 and 2020. Image composites of each year’s two growth seasons and one dry season, vegetation indices and topographic data were generated. Changes in the extent and accuracies of three estuarine (mangroves, salt marshes and submerged macrophytes) and three freshwater (forested wetlands, freshwater marshes and large macrophytes) EFGs were calculated and compared to a manually mapped through image interpretation, high-confidence layer. Overall, estuarine EFGs comprised between 10 and 18% of the extent of the EFGs, while freshwater EFGs made up 15% of the extent of estuaries. The overall accuracies of detection of EFGs for 1990 were < 64% compared to the > 71% attained for 2014, 2018 and 2020. In comparison to manual delineations of some of these habitats, the outputs generated from these medium-resolution sensors resulted in overestimation of extent for all EFGs; for mangroves by 115% and for salt marshes and submerged macrophytes by 150–230%. Finer spatial resolution images, and time-series mapping would be critical for improved delineation and monitoring of South Africa’s estuarine habitats.

Ecological networks reveal important taxa for ecosystem function in two subtropical South African estuaries

Biodiversity is essential for estuarine ecosystems' functioning, resilience, and persistence. While biodiversity assessments provide some insight into the ecosystem state, they cannot quantify every aspect of ecosystem-level function. We assessed aspects of estuarine ecosystem function and pinpointed taxa of interest that fulfil specific roles in estuarine ecosystems, using a series of novel food web model ensembles and Ecological Network Analysis. We investigated emergent network properties of two subtropical South African estuaries, the anthropogenically impacted large temporarily closed uMdloti Estuary and the ‘near natural’ predominantly open uMlalazi Estuary, during a drought period induced by the El Niño Southern Oscillation. In addition, important network components were identified to elucidate the importance of estuarine/marine taxa in maintaining ecosystem function. The key findings included (1) estuaries exhibit system-specific functional responses under drought conditions, and (2) estuarine/marine taxa are critical for overall ecosystem function. Both estuaries showed seasonal differences in ecosystem activity and reliance on trophic level I (microalgae and detritus) to fuel the food web, and persistently low cycling (<9.3 %). However, compared to uMdloti Estuary, uMlalazi Estuary showed less variation in mean seasonal activity (1597–2095 mgC m−2 d−1), and higher reliance on detritus to fuel the food web (D:H > 2.02), indicating greater ecosystem functional resilience to external perturbations. uMlalazi Estuary ecosystem function could be attributed to predominantly open mouth conditions maintaining a salinity gradient, and subsequent large contributions to overall ecosystem function from typically estuarine/marine invertebrate taxa. In contrast, the uMdloti Estuary, dominated by freshwater/brackish taxa under prolonged closed mouth conditions, showed greater seasonal variations in mean system activity (1084–2289 mgC m−2 d−1) and generally high reliance on microalgae (D:H < 1.26), indicative of decreased resilience to external perturbations. During this study, a single mouth breach acted as a ’biological reset’ of uMdloti Estuary, enabling recruitment of estuarine/marine taxa, and a temporary recovery in ecosystem function reflected in increased system cycling (FCI >14.05 %) and decreased reliance on microalgae (D:H > 1. 41). These findings suggest that future predicted drought impacts on estuarine ecosystem function may be partially system-specific due to differing estuarine types, anthropogenic impact, and invertebrate community composition. This study provides a scientific link between ecosystem-level indicators and the role of individual taxa within the system, which is a valuable complement to biodiversity assessments and ecosystem state evaluation.

Uncovering sediment pollution sources: A comparative assessment of two South African Estuaries

Richards Bay Harbour is South Africa's largest and busiest deep-water port by tonnage, and this ongoing shipping and port activities will expectedly result in metal contamination issues. The adjacent Mhlathuze Estuary is a nature reserve with a long history with Richards Bay Harbour and may be prone to metal contamination from multiple sources. The study investigates the sources of contaminated sediments in these two South African estuaries arising from their proximity to various anthropogenic activities. Investigating these sources of contamination is crucial for developing effective pollution control strategy since Richards Bay Harbour and Mhlathuze Estuary are regarded as estuaries of national conservation importance in South Africa. To provide further insights, sediment samples were acid digested and quantified for metal concentrations using Inductively coupled plasma-optical emission spectrometry (ICP-OES). Multivariate (SIMPROF and cluster) analysis was used to identify how sampling sites in Richards Bay Harbour and Mhlathuze Estuary were grouped based on sediment metal concentrations. Results of pollution load index for Richards Bay Harbour showed site 4 to be heavily polluted, while site 3 in Mhlathuze Estuary showed moderate pollution. Multivariate analysis (SIMPROF and cluster) further revealed sites related to anthropogenic activities and those that were unpolluted. Engaging this advanced analysis enhanced the ability to delineate potential pollution sources and further helped in the identification of the main contributors to sediment pollution. The findings indicate that port and industrial activities are the major contributors to sediment contamination at the estuaries. The results provide specific insights into the sources of pollution, informing targeted environmental management and policy-making efforts to protect and improve the estuarine environments.

A review of metal pollution in a transformed, urban South African Estuary

Metal pollutants enter coastal water bodies from agricultural, industrial, and domestic wastewater activities amongst other sources. These pollutants are often transported through the food chain to higher trophic levels including organisms such as fish. Ultimately, they may pass to humans that consume contaminated seafoods. As such, it is imperative to understand how metals in estuarine systems are transported across trophic levels. Baseline data on metal concentrations are of crucial importance to evaluate changes over time and amongst areas to inform management and conservation strategies. In this assessment, we compile and discuss a database of metal concentrations recorded in various environmental media in the Swartkops River estuary, as a baseline against which future comparisons can be made. The Swartkops Estuary is subjected to considerable anthropogenic pressure that has led to a deterioration in water quality. We also provide a conceptual model to understand the flow of metals in the estuary. We furthermore identify lines of future research that will address gaps and uncertainties in the existing data and provide recommendations for remediation. Studies on metal concentrations in the water column of the estuary are limited. Studies on metals in sediment suggest that the estuary is not significantly metal contaminated. While studies revealed that plants do accumulate metals and fauna studies show differences in metal concentrations in the tissue of invertebrates and fish. It is difficult to conclude if these fauna are accumulating metals in their tissue compared to historically and if concentrations reflect uptake as a consequence of exposure to metal contaminated water, sediment and food.

Multi-method approach identifies a South African estuary as an important elasmobranch habitat and potential nursery ground

Elasmobranchs play important ecological roles but are highly threatened. Understanding their habitat use and abundance is crucial for the management and conservation of these species and their habitats. While some species are known to use estuaries as vital habitats (e.g. feeding or nursery grounds), this remains an understudied area. A multi-method approach was used to survey the Keurbooms Estuary on the south coast of South Africa, over two years, to quantify the diversity, relative abundance and habitat use of elasmobranchs in this system. The survey methods were baited remote underwater video (BRUV), boat-based visual sightings, and unmanned aerial video (UAV). Opportunistic sightings were also collected. Six elasmobranch species were observed, with significant differences in community composition between the methods. Whereas effort was skewed towards BRUV surveys, the boat and UAV surveys recorded more elasmobranchs more frequently, and opportunistic sightings recorded the highest diversity. The common eagle ray Myliobatis aquila was the most abundant elasmobranch species and the only one recorded year-round; most individuals (93%) were juveniles, suggesting the Keurbooms Estuary may be a nursery area for this Critically Endangered species. The duckbill ray Aetomylaeus bovinus and common smoothhound shark Mustelus mustelus were observed during summer, the latter forming large aggregations. Another three ray species were recorded rarely. This study highlights the benefit of using a multi-method survey approach for shallow ecosystems and reveals the Keurbooms Estuary as an important habitat for elasmobranchs.

Ostracoda and Foraminifera as bioindicators of (aquatic) pollution in the protected area of uMlalazi estuary, South Africa

To mitigate ecological and health risks, implementing a comprehensive multidisciplinary monitoring strategy is imperative. This approach aims to effectively identify and record potential declines in water quality and ecological conditions. Utilizing cost-effective and efficient monitoring tools is crucial, especially for developing nations. Despite the previously reported uMlalazi River's pristine status within a protected natural reserve at South Africa's eastern coast, our findings challenge the assumption of its cleanliness, emphasizing the need for ongoing proactive monitoring. Here we reassess the pollution levels and ecological status of aquatic life of the river, and use this to enhance the indicator value of microfauna in South Africa. We analysed 25 surface sediment samples from the uMlalazi estuary, covering a salinity range from oligohaline to euhaline, with a focus on marginal marine Ostracoda and Foraminifera as potential indicators. All samples contained Ostracoda and Foraminifera, with the exception of two. Among the identified ostracod species, there were 17 species belonging to 14 genera. Typical taxa are the brackish water species Perissocytheridea estuaria, Sulcostocythere knysnaenis, and Australoloxoconcha favornamentata. We identified 19 Foraminifera species from 16 genera, with dominant taxa such as Ammonia sp., Quinqueloculina sp., and Miliolinella sp. Three distinct assemblages were observed: A) Ammonia sp. and Quinqueloculina sp., with very low diversity and abundances in general, located along the river course at stations exceeding Pollution Load Index (PLI), which indicates deterioration of sites quality; B) Ammonia sp., Quinqueloculina sp., and Sulcostocythere knysnaenis associated with higher salinity and lower PLI; C) Ammonia sp., Quinqueloculina agglutinans, and Cribroelphidium articulatum located in mudflats with minimal PLI. Our findings align with the commonly observed diversity trend, which indicates reduced species diversity corresponding to elevated pollution levels. Notably, the examined samples revealed a range of Foraminiferal Abnormality Index (FAI) up to 23%, exhibiting anomalies such as multiple tests, changes in coiling, and abnormal chamber shapes. Geochemical analysis indicates that the catchment is subjected to substantial anthropogenic pressure, as evidenced by elevated concentrations of heavy metals, sulphur, and microplastic. Sugarcane farming, urban sewage, titanium mining, and fish farming are the primary sources of pollution in the catchment area. Ongoing investigations in South African estuaries are expanding our dataset and will contribute to a better understanding of the species-specific responses of Ostracoda and Foraminifera to anthropogenic pressure.

Investigation of alkyl, aryl, and chlorinated OPFRs in sediments from estuarine systems: Seasonal variation, spatial distribution and ecological risks assessment

Estuaries in South Africa are very important for biodiversity conservation and serve as focal points for leisure and tourism activities. The organophosphate flame retardants (OPFRs) levels in these aquatic systems haven't been documented in any studies as of yet. Due to the negative effects of persistent organic pollutants in South African estuaries, we examined the occurrence of eight OPFRs in sediments of two estuaries by studying their spatiotemporal distribution, season variation, and ecological risks. The Sundays Estuary (SDE), a semi-urbanized agricultural surrounding system, recorded an ∑8OPFR concentration in sediments that ranged from 0.71 to 22.5 ng/g dw, whereas Swartkops Estuary, a largely urbanized system, recorded a concentration that ranged from 0.61 to 119 ng/g dw. Alkyl-OPFRs were the prevalent homologue in both estuaries compared to the chlorinated and aryl groups. While TBP, TCPP, and TCrP were the most abundant compounds among the homologue groups. There was no distinct seasonal trend of ∑8OPFR concentration in either estuary, with summer and autumn seasons recording the highest concentrations in SDE and SWE, respectively. Ecological risks in the majority of the study sites for the detected compounds were at low (RQ < 0.1) and medium levels (0.1 ≤ RQ < 1) for certain species of fish, Daphnia magna and algae. However, the cumulative RQs for all the compounds had ∑RQs ≥1 for most sites in both estuaries, indicating that these organisms, if present in both estuaries, may be exposed to potential ecological concerns due to accumulated OPFR chemicals. The scope of future studies should be broadened to include research areas that are not only focus on the bioaccumulation patterns of these compounds but also find sustainable ways to reduce them from these estuarine environments.

Large-scale environmental signals in seagrass blue carbon stocks are hidden by high variability at local scales

Increasing focus on nature-based climate change mitigation and adaptation strategies has led to the recognition of seagrasses as globally significant organic carbon (Corg) stocks. However, estimates of carbon stocks have been generally confined to a few regions, with few African studies represented in global datasets. In addition, the extent to which biogeographical and environmental variation shape carbon stocks in marine vegetated environments remains uncertain. For South Africa, Zostera capensis is the dominant seagrass species with limited mapping and quantification of its Corg stocks. Here, we measured Z. capensis Corg stocks at six South African estuaries spanning ∼1800 km of the cool-temperate to subtropical marine environmental gradient. Targeting the intertidal zone of the upper and lower estuary reaches, we collected Z. capensis sediments to a depth of 50 cm and measured the Corg, with the median Corg stock estimated at 24.11 Mg C ha−1 (40.4 ± 53.02; mean ± SD). While this is lower than the global average, these data demonstrate that Z. capensis ecosystems are important contributors to blue carbon stocks in the region. Measured Corg stocks showed significant differences between sampling sites for estuaries; however, we did not detect significant differences between estuaries due to high intra-estuarine Corg variability. Examination of biogeographical regions, terrestrial and marine environmental variables as drivers of Corg variability revealed that annual mean sea surface temperature may explain variation in Corg stocks. Furthermore, we found evidence of signals of biogeographical regions and precipitation driving some of the variability in Corg stocks; however, this requires further investigation. Overall, our estimates for Z. capensis add to ongoing national and global efforts to quantify seagrass Corg stocks across environmental and biogeographic gradients to better determine their contributions as nature-based solutions to climate change.

Occurrence of larval fishes sampled by drifting light traps in the lower reaches of a South African estuary.

The tidal occurrence of larval fishes was investigated in the permanently open Kowie Estuary on the warm-temperate coast of South Africa. Larval fishes were sampled in the mouth region using two drifting light traps deployed on the ebb and flood tides every second night for two consecutive 14-day periods, coinciding with the dark moon phase. A total of 553 larval fishes were caught, representing nine families and 26 species, of which Blenniidae and Clupeidae dominated. The prevalence of different estuarine association fish guilds was also tide-specific. Marine and estuarine species, such as Omobranchus woodi, were more dominant during flood tides, while marine straggler species, such as Sardinops sagax, which are not dependent on estuaries, were dominant on the ebb tide. Marine estuarine-dependents were only present during flood tides, potentially indicating ingress and entrainment within the estuary. The results confirm that light trap catches yield a different composition of species compared to towed ichthyoplankton net studies. Additionally, drifting light traps allow for better targeting of species with a phototactic response and reduction of incidental catch. Consequently, a mixture of gear is encouraged for more comprehensive surveys of larval fish occurrence.

Differential sampling in the assessment of conservation and biodiversity merit: a comparison of the seagrass macrofauna in three nearby South African estuaries

To what extent is the relative biodiversity of some flagship conservation sites a result of differential attention? Knysna estuarine bay is the topmost ranked South African estuary for conservation importance and biodiversity. It is also one of the most intensively studied, and hence differential sampling effort could partly be responsible for its apparent relative richness. To assess the extent to which this might be true, identical sampling area, effort and methodology were employed to compare the benthic macrofauna of one specific major Knysna habitat (Zostera capensis seagrass beds) with equivalent ones in two nearby lesser-studied estuaries, the Keurbooms/Bitou and Swartvlei. Investigation showed all three localities to share a common species pool, but different elements of it dominated the shared habitat type in each. The seagrass and adjacent sandflat macrobenthos proved just as biodiverse in unprotected Keurbooms/Bitou as in the Protected Area of Knysna, but that in Swartvlei (also a Protected Area) was impoverished in comparison, presumably consequent on mouth closure and the prevailing lower salinity. Despite marked geomorphological and hydrological differences, all three estuaries share a suite of unusual faunal elements and such particularly close faunal similarity suggests the importance of historical biogeographic processes. The analysis emphasises the need for caution when assessing the relative conservation importance or other merits of different individual systems in a data-limited environment.

Reproductive biologies of Psammogobius knysnaensis and Caffrogobius gilchristi in a warm-temperate South African estuary

Reproductive biologies (spawning period, length at maturity and fecundity) of the Knysna sand goby Psammogobius knysnaensis and the Prison goby Caffrogobius gilchristi were investigated between September 2018 and August 2019 in the warm-temperate Sundays Estuary, South Africa. The two species reproduced during the same period (spring and summer) with peak reproduction occurring in January. P. knysnaensis egg production was higher than C. gilchristi and was not affected by fish size. Although C. gilchristi egg production increased with fish size, this was not significant. P. knysnaensis reached LM50 at smaller sizes than C. gilchristi. Reproductive periods were likely coupled with prey availability, as phytoplankton and zooplankton abundances are highest in warm-temperate estuaries during this period. The ability of these gobies to reproduce at small sizes (< 40 mm TL), which is an adaptive trait of many r-selected species, underpins their high abundance in South African estuaries as they are well adapted to stochastic environments.

Benthic macrofauna communities reflect eutrophic condition in a low-inflow estuary

Nutrient enrichment drives potentially severe changes in low-inflow and temporarily closed estuaries where tidal mixing and freshwater flushing is limited, and bottom-water anoxia occurs frequently. Our research assesses the influence of nutrient enrichment by comparing macrobenthos community metrics in a persistently hypereutrophic South African estuary. This is then compared to published literature for an estuary with a similar catchment size and mean annual runoff, but which experiences limited anthropogenic nutrient input. Macroinvertebrate abundance and richness were higher in an estuary with limited anthropogenic nutrient loading, and this varied seasonally for both estuaries. Oxygen saturation had a significant influence on taxonomic level abundances, with lower oxygen levels in the polluted estuary reducing macroinvertebrate abundances. Macroinvertebrates tolerant of organic pollution (e.g., Oligochaeta) were also more abundant in the polluted estuary. This study highlights the effects of increased nutrient loading in low-inflow estuaries and confirms the supposition from several previous studies that macrozoobenthos could be used as a responsive tool from a bioindicator perspective to monitor eutrophication.

Life cycles and challenges for fish species that breed in South African estuaries.

The literature currently recognizes four guilds of estuarine resident fish species, namely solely estuarine, estuarine and marine, estuarine and freshwater, and estuarine migrant. In this review the life cycles of actual representatives from these four guilds are assessed to determine whether the current definitions, which have never been formally tested, are appropriate to fish species resident in South African estuaries. Detailed information and diagrammatic life cycles are provided for the selected species covered by this review. A potential new estuarine resident guild category is also identified, namely, those taxa that are primarily estuarine but also have subpopulations recorded in both adjacent marine and freshwater habitats. The full range of reproductive characteristics employed by estuary resident species is examined, ranging from live bearers, pouch and nest brooders, to a suite of oviparous taxa that attach their ova to estuarine rocks, shells, and submerged vegetation, all of which assists with larval retention within the estuarine environment. The small size and early reproductive maturity of most estuarine resident species is highlighted, with reduced vulnerability to predation in shallow, sheltered, often turbid estuary waters offering considerable protection during spawning events when compared to the open ocean. In addition, these small fish would not have to move considerable distances at any stage of their life cycle, since egg, larval, juvenile, and adult stages all occur in the same place. The existence of contingent subpopulations within many estuarine resident species is noted, physico-chemical stresses on these species are highlighted, and the eurytopic nature of these small fish taxa emphasized.

A preliminary study into the detection of fish environmental DNA in selected South African estuaries

Environmental DNA (eDNA) is a powerful technique for the non-invasive and comprehensive monitoring of entire ecosystems. Despite its many advantages, the use of eDNA in South Africa as an assessment tool for fish biodiversity is largely underutilised. This study was performed as an initial screening to assess the efficiency of the eDNA method for characterising fish communities in South African waters using the metabarcoding approach. Water samples were collected from eight estuaries and vacuum filtered using 0.45 μm nylon membrane filters. DNA was extracted from the filters, and sample extracts underwent PCR assaying using MiFish primers for the 12S rRNA gene, prior to being sequenced on the Illumina MiSeq platform. The resulting sequence data produced 1 574 092 raw reads, which resulted in 119 operational taxonomic units (OTUs) after filtering and clustering. Taxonomic assignment of the OTUs produced four different taxonomic groups and one group representing unidentified sequences. The group with the highest number of OTUs (54%) was assigned to fish taxa consisting of 12 families and 22 genera identified across all estuaries, with only 16 taxa identified to species level. As this was a preliminary study, few samples were collected, and therefore several recommendations for a more comprehensive study design are provided. Overall, these results support the hypothesis that the eDNA metabarcoding approach can efficiently detect fish species in South African estuarine environments.

Ecological risk assessment of metal pollutants in two agriculturally impacted estuaries

A focused diagnosis of ecosystem health in two South African estuaries (Kromme and Gamtoos) was conducted. Four pollution indices were used, i.e., geoaccumulation (Igeo), ecological risk (RI), contamination factor (CF) and pollution load index (PLI), to assess toxicity levels of metal contaminants in relation to background values. The Igeo results (11.1 %) can be classified as contaminated, with Cd, the only element with high values in both estuaries. Likely sources (herbicides, pesticides) of Cd are used in the agricultural dominated catchments. There was a high concentration of Mn (13.4 ± 2.51 and 12.3 ± 1.13 μg·g−1) and Fe (1289 ± 243 and 1291 ± 130 μg·g−1) at site 4 for Gamtoos and Kromme estuary respectively compared to the other metal elements. Although results indicate low metal contamination, with increasing global anthropogenic pressure, continuous monitoring should be prioritised to assist in managing estuarine systems that support a wide range of socio-economic and ecosystem services.

How drought affects range and variability of environmental conditions in two subtropical South African estuaries

Globally estuaries are under increasing pressure from human development impacts including the growing effects of climate change that already cause major changes in freshwater inflow. One of these are drought events. Although they are regular occurrences and are projected to become progressively more significant in many parts of the world, their realised impact on the estuarine biome requires better understanding and quantification. In this study, we analysed the environmental niche variability along horizontal and vertical estuarine axes during the dry and wet season of a drought and non-drought period, in two contrasting subtropical estuary types in South Africa: the temporarily closed uMdloti Estuary and the predominantly open uMlalazi Estuary. We found quantitative support for the hypothesis that extended periods of drought lead to a decreased niche availability for estuarine organisms over both space and time, expressed as narrower salinity ranges, less habitat with amicable dissolved oxygen (DO) concentrations, and more frequent higher temperature. Drought caused a more homogenous salinity environment in both estuaries, more specifically extremely low salinity throughout the temporarily closed estuary, but increased salinity in the predominantly open estuary. DO levels were more variable during drought conditions and included periods of hypoxia in both estuaries. They were more stable at elevated levels (>5 mg/L) during non-drought conditions. The opposite was true for pH, for which we measured lower values and larger vertical ranges during non-drought conditions when rainfall and tidal exchange were more pronounced. Higher temperature value density peaks were apparent from the drought period, especially pronounced in the temporarily closed uMdloti Estuary. The physico-chemical environment was typically more estuarine during the non-drought period, and drought caused both similar (temperature, DO, pH) and different (salinity) responses in the two estuarine types. Drought-induced conditions can span several generations for estuarine and estuarine-associated species. It is important to quantify niche variability for a range of estuary types to subsequently understand drought effects on estuarine biodiversity.

Juvenile sparids (Rhabdosargus holubi) consistently select structurally dense vegetated habitat in nursery seascapes.

Seagrass habitats provide structural complexity in coastal estuarine and marine environments, which offer fish optimal foraging grounds and refuge from predation. However, seagrasses are some of the most threatened ecosystems globally, with anthropogenic activities such as population growth and environmental degradation leading to the fragmentation, thinning, and loss of these habitats. Rhabdosargus holubi is one of only a few vegetation-associated marine fish species in South African estuaries. Although field studies have shown a strong association with seagrass over other aquatic vegetation for the juveniles of this species, habitat choice has never been empirically tested. Here, we used artificial vegetation units to test habitat choice (different structural complexities) for this species. We also tested whether habitat choice is influenced by a predatory threat, with fish preferentially selecting dense habitat in the presence of a predator and whether this effect may be more apparent in smaller individuals. We found that R. holubi significantly prefer greater structural complexity over less complex habitats, in both the absence and presence of a predator and for both small and large juveniles, showing that R. holubi actively choose more complex structures and are attracted to the structure per se irrespective of the threat of predation. This study highlights the importance of dense seagrass as nursery areas for this species and demonstrates how the loss of these habitats could impact the nursery function of estuaries.

The Influence of Macroclimatic Drivers on the Macrophyte Phylogenetic Diversity in South African Estuaries

The geographical distribution of plants is influenced by macroclimate and dispersal limitations, which have led to lineage isolation and subsequent diversification within and across various environmental gradients. Macroclimatic variables in coastal wetlands influence plant species and lineages across biogeographical boundaries. This study aimed to determine the influence of macroclimatic variables on species and phylogenetic richness in South African estuaries. Open-source chloroplast DNA barcoding sequences, species distribution and climatic data layers were used to determine the relationship between species richness, MPD, MNTD and each bioclimatic variable individually. Temperate species richness and phylogenetic diversity were positively correlated with temperature bioclimatic variables whereas subtropical and tropical species were associated with increases in precipitation. Phylogenetic niche conservatism is evident in malvids and rosids which are restricted to tropical and subtropical regions due to their physiological adaptations to tropical climates. Caryophylales was mostly associated with temperate regions. Poales and Alismatales showed wide distributions that is likely attributed to traits related to wind pollination and hydrochory, rapid, clonal, and high reproductive output, tolerance to stressful conditions, and intraspecific genetic diversity. The findings highlight the importance of considering macroclimate and phylogenetic factors in understanding the distribution and diversity of coastal wetland plants.

Assessing the application of the revised Remane Model to fish species in a fluvially dominated cool-temperate southern African coastal system

Estuaries are ecologically important areas which support a variety of aquatic species, particularly marine and estuarine fish species. This study represents a case study of the Orange River and Estuary (ORE) to understand patterns in fish assemblages and diversity trends that were compared to the revised Remane Model predictions in an estuary with poor marine fish species richness. A total of 30 species were recorded in the River continuum that comprised 14 freshwater, 10 marine and six estuarine species. Species diversity revealed seasonal variation in fish assemblages during the high-flow and low-flow seasons but not inter-annually. The results revealed that species diversity is lower in higher salinity areas when compared to low salinity areas. These patterns are consistent with the biogeographical trend of decreasing species richness along the South African coast from east to west, however, inconsistent with Remane predictions. The extremely low marine fish richness at its lower end and the extremely large freshwater influx at its upper end are the primary causes of the inconsistency. This may reflects the unsuitability of the Remane model for the Orange Estuary. In comparison to similar river-dominated South African estuaries, the ORE has a low marine species richness. When compared to more conventional South African estuaries, the ORE has a unique biotic environment with low fish species richness of estuarine-associated marine species adjacent to the Benguela upwelling zone and therefore the region does not offer suitable habitat for such species. As a result, the ORE is not a good candidate to test the Remane Model. The data does, however, confirm the left-hand part of the Remane model, which shows a decline in the fish species richness of freshwater fish species as salinity rises towards mesohaline and polyhaline levels.

Spatio-temporal relationships between larval fishes and zooplankton in cool-temperate estuaries of South Africa emphasizing the importance of mesohaline zone interactions

A conceptual model for cool-temperate South African estuaries has been proposed to explain the interactions between zooplankton and larval fishes based on small-scale biological processes. Larval fishes and zooplankton data from nine south and west coast South African estuaries were analyzed in terms of the cumulative density of all species and the density dynamics of the dominant taxa in each group. Larval fish and zooplankton density were positively correlated and varied across seasons, peaking simultaneously in summer, although zooplankton also peaked during the stable conditions of the closed phase of some estuaries. Plankton density was positively correlated with salinity and water temperature and negatively correlated with turbidity. The mesohaline zones of estuaries are highlighted as supporting plankton productivity maxima. These results were attributed to the summer peak in plankton productivity coinciding with the lower rainfall, higher temperatures, and clearer waters that are often typical of the dry summers on the south and west coasts of South Africa. Correlation between selected predator, competitor, and prey species supported the hypothesis that estuaries exhibit extensive reciprocal trophic relationships permitting an ideal feeding environment, which maintains a positive relationship between overall larval fish and zooplankton abundance.