Zostera Capensis Research Articles

Nine decades of research on Zostera capensis: from foundational science to conservation and resilience

Cape dwarf eelgrass Zostera capensis is the most abundant seagrass in South Africa, with some isolated populations occurring along the East African coastline. Providing numerous ecosystem services that support biodiversity and human wellbeing, it is endangered throughout its distribution. As such, the long-term persistence of Z. capensis is uncertain and novel approaches are required to reverse declines and the loss of associated critical ecosystem functions. Central to this is the need to build on existing scientific knowledge, so that habitat requirements, responses to future global change pressures and the functional significance of Z. capensis from a socio-ecological perspective can be better understood. In this comprehensive review, we summarise 8 key topics from 197 studies published between 1933 and 2023 and present a timely overview of research conducted on Z. capensis. Some of the earliest research that focussed on foundational knowledge included geographical distribution and aspects of seagrass ecology and physiology. In the last 3 decades, the breadth and diversity of research increased to encompass themes particularly relevant to ecological value and conservation, including emerging approaches such as conservation genomics, restoration and assessing the value of seagrass meadows in the context of blue carbon. This review also identifies critical gaps in our understanding of Z. capensis, including the impact of pollutants and sea level rise on seagrass persistence, novel techniques for mapping seagrass meadows, the interaction of microbiomes and seagrass health, alternative sources for upscaling seagrass restoration, increasing management and conservation efforts and engaging with people’s perceptions on the value of seagrass meadows.

Quantifying direct and indirect linkages between seagrasses, environment and associated macrofauna in a temperate lagoon

AbstractSeagrass ecosystems play a vital role in marine environments, providing essential services, including physical habitat that supports a species diversity critical to robust trophic structures. However, seagrass loss due to various influences raises concerns about the persistence of these ecosystems. Seagrass‐associated macrofauna serve as indicators of environmental health, noting change or stability. This study investigated the structural and environmental drivers influencing macrofaunal communities associated with seagrass (Zostera capensis) in a temperate lagoon. Variation in macrofaunal community structure across phenotypically distinct seagrass populations found large‐leaved morphotypes in deeper intertidal stands to support higher species diversity and richness. Conversely, small‐leaved populations in higher shore positions had a greater abundance of desiccation‐resistant gastropods. Seasonal differences were significant, with higher abundances generally observed during spring and summer, while diversity and richness peaked in autumn and winter. Structural equation modelling revealed shoot densities and leaf width to have positive direct effects on macrofaunal abundance, while temperature and tidal exposure produced significant indirect effects. Temperature had a direct effect on seagrass structure and significantly influenced five out of six seagrass metrics measured. Observations of seagrass distribution in the lagoon indicate significant declines in large‐leaved populations with likely adverse consequences for associated macrofauna and related trophic levels.

The viability of seagrass ecosystems for supporting dugong recovery in Kenya

Seagrasses are the primary source of food for dugongs and a good indicator of marine ecosys- tem health. The East African dugong (Dugong dugon) population is listed as critically endan- gered under the IUCN Red List. This study aimed to document the status of seagrass beds and evaluate their potential for supporting dugong recovery in Kenya. A cross-sectional survey was conducted in December 2016 to March 2017, with data gathered through desktop reviews, inter- views, beach surveys and aerial surveys. Seven seagrass species were found at sampled sites, namely Syringodium isoetifolium, Thalassodendron ciliatum, Halophila ovalis, Zostera capensis, Thal- assia hemprichii, Cymodocea serrulata, and Halodule uninervis. Halodule and Halophila seagrass spe- cies are important in the diet of dugongs. Two dugongs were sighted during the aerial survey. The spread of sea urchins, unplanned infrastructure development, Illegal, Unreported and Unregulated fishing, and boat anchors negatively affected seagrass ecosystems and hence dugong distribution in Kenya.

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.

Using transplantation to restore seagrass meadows in a protected South African lagoon.

Seagrass meadows provide valuable ecosystem services but are threatened by global change pressures, and there is growing concern that the functions seagrasses perform within an ecosystem will be reduced or lost without intervention. Restoration has become an integral part of coastal management in response to major seagrass declines, but is often context dependent, requiring an assessment of methods to maximise restoration success. Here we investigate the use of different restoration strategies for the endangered Zostera capensis in South Africa. We assessed restoration feasibility by establishing seagrass transplant plots based on different transplant source materials (diameter (ø) 10 cm cores and anchored individual shoots), planting patterns (line, dense, bullseye) and planting site (upper, upper-mid and mid-intertidal zones). Monitoring of area cover, shoot length, and macrofaunal diversity was conducted over 18 months. Mixed model analysis showed distinct effects of transplant material used, planting pattern and site on transplant survival and area cover. Significant declines in seagrass cover across all treatments was recorded post-transplantation (2 months), followed by a period of recovery. Of the transplants that persisted after 18 months of monitoring (~58% plots survived across all treatments), seagrass area cover increased (~112%) and in some cases expanded by over >400% cover, depending on type of transplant material, planting arrangement and site. Higher bioturbator pressure from sandprawns (Kraussillichirus kraussi) significantly reduced transplant survival and area cover. Transplant plots were colonised by invertebrates, including seagrass specialists, such as South Africa's most endangered marine invertebrate, the false-eelgrass limpet (Siphonaria compressa). For future seagrass restoration projects, transplanting cores was deemed the best method, showing higher long-term persistence and cover, however this approach is also resource intensive with potentially negative impacts on donor meadows at larger scales. There is a clear need for further research to address Z. capensis restoration scalability and improve long-term transplant persistence.

Eelgrass Zostera capensis populations in KwaZulu-Natal, South Africa, harbour distinct genomic signals despite limited geographical distance

Seagrasses are threatened by anthropogenic stressors and climate change, with numerous population declines reported. In South Africa, the eelgrass Zostera capensis is restricted to estuarine environments and has a disjunct distribution and declining status. With the majority of the distribution of Z. capensis on the west and southwest coasts of South Africa, the isolated easternmost populations in KwaZulu-Natal Province (KZN) are of particular interest. Following the extirpation of Z. capensis at Durban and St Lucia, only five populations remain, of which three (in the Amatikulu, Mlalazi and Mhlatuze estuaries) are situated <50 km apart. Previous molecular analyses showed strong population structure between Z. capensis populations, but the geographically close populations on the east coast were not included. In this study, using single nucleotide polymorphisms (SNPs) isolated from 31 individual sampled plants collected from three northern KZN estuaries, we provide evidence for distinct population clusters, with unique evolutionary signals. Zostera capensis in the Mlalazi Estuary has a low level of genomic diversity, likely as a result of a small, dynamic population unable to withstand prolonged freshwater exposure. Our results suggest that conservation efforts need to consider unique population signals even among geographically close populations, in particular within the context of restoration, where genomic compatibility may determine the persistence of restored populations.

A critically endangered estuarine limpet’s only two populations are genomically and morphologically distinct

Abstract Seagrass habitats are declining worldwide, placing several seagrass‐associated animals at risk of extinction. The Critically Endangered limpet Siphonaria compressa is one of the rarest molluscs in Africa, and has been reported from only two disjunctive lagoons in South Africa. Being a highly specialized grazer that lives exclusively on the narrow blades of Cape eelgrass, Zostera capensis, which is itself listed as Endangered in the South African Red List and has decreased in abundance, conservation initiatives are urgently needed to ensure the long‐term survival of S. compressa. Molecular data (sequence data from the mitochondrial genome and single nucleotide polymorphism data of the nuclear genome) and morphological data were analysed to determine whether the two populations are conspecific, with implications for whether translocation between localities could be a viable management strategy to restore either population in the event of a collapse, or to maintain the genetic diversity of each population. Strong evidence emerged for the distinctness of the two populations, including a lack of shared mtDNA haplotypes that indicate an absence of contemporary gene flow, a divergence time that dates to the late Pleistocene, and a number of morphological characters that distinguish their shells. These findings indicate that the two populations are distinct cryptic subspecies. As the two populations occur in different temperature‐defined marine biogeographical regions, they are probably adapted to different thermal environments. Translocations are not recommended, as this management strategy has considerable potential to result in outbreeding depression and exacerbate the extinction risk. Instead, each population should be managed separately, and several alternative conservation measures are discussed, including the protection and restoration of seagrass beds.

Intertidal Gleaning Exclusion as a Trigger for Seagrass Species and Fauna Recovery and Passive Seagrass Rehabilitation

This study evaluates gleaning exclusion as an approach for the rehabilitation of seagrass ecosystems and as an option for important intertidal resource management that contributes to the social well-being of communities. The monitoring of seagrass plant and invertebrate recovery after the implementation of gleaning exclusion was conducted over 50 plots of 5 m × 5 m each, which were settled in the seagrass meadow of NW Maputo Bay, Mozambique. The exclusion experiment was designed to compensate for the important loss of seagrass in the area due to gleaning activity characterized mainly by digging and revolving sediments to collect mostly clams. Results showed that, in general, seagrass plant shoot density started having significant positive recovery after five months: three months for Halophila ovalis, five months for Halodule uninvervis, and much more time (&gt;six months) for the IUCN Red List endangered Zostera capensis. For invertebrates, 194 individual invertebrates were collected belonging to 13 species. Solen cylindraceus was the most dominant edible invertebrate species in the local community, and Dosinia hepatica for non-edible species. The result of the experiment showed a positive recovery in the abundance and diversity of invertebrates. The results support previous findings, suggesting that the installation of a no-take zone can enhance the health of an ecosystem. Therefore, to limit the violation and conflicts of the no-take zones, the creation of alternative activities for harvesters and the flexibility of restrictions are vital. Further investigation should be considered to obtain an effective management of the zones, including documentation of species, gleaning practices, and an effective restoration of seagrass meadows.

Investigating transplantation as a mechanism for seagrass restoration in South Africa

The extent of seagrasses has declined globally, with restoration through transplantation seen as an important tool for reversing population loss, yet restoration studies for African seagrass species are scarce. This study investigated the use of different planting patterns (straight‐line, compact, and star) and core sizes (11, 18, and 25 cm Ø) in transplanting ecotypes (intertidal and subtidal) in the predominantly open Knysna and temporarily closed Klein Brak estuaries in South Africa. Cores of the endangered seagrass, Zostera capensis, were transplanted in two experimental repeats per ecotype and core size along transects to investigate survival of plants post‐transplant. No significant differences were observed among core sizes, patterns, or ecotypes in the Knysna Estuary, but cores with the compact pattern had better survival rates in the Klein Brak Estuary. Holes left by the smallest cores in donor sites recovered faster through sediment deposition compared to larger holes. Our study demonstrated that seagrass restoration in South Africa is challenging due to limited suitable habitats and strong environmental variability in estuarine ecosystems. It is important that careful consideration, including of genomic diversity and population structure, as well as ecological similarity between donor and recipient populations must be made for each restoration site when assessing its restoration potential. Effective “no‐take bait zones” and upstream catchment management will be important for protecting potential donor meadows.

Heavy metal compartmentalisation in salt marsh and seagrass of the urbanised Swartkops estuary, South Africa

Wetland plants are naturally exposed to high metal concentrations, and often have mechanisms to prevent metal toxicity. This study compared metal concentrations in seagrass (Zostera capensis) and salt marsh (Spartina maritima, Salicornia tegetaria) — to determine their niche as metal sinks. Samples were collected in each season over a year at five sites in the estuary and analysed using a Total X-Ray Fluorescence (TXRF) spectrometer. Spartina maritima and S. tegetaria accumulated in their roots, and displayed little translocation to leaves (BCF = 1–14; TF[leaf/root] < 1). Contrastingly, Z. capensis showed high uptake to its leaves (TF[leaf/root] = 1–8; Mn, Zn, Cr, Pb, Ni, Cu). Spartina maritima and S. tegetaria were identified as good phytostabilisers (BCF > 1, TF[leaf/root] < 1). Compartmentalization was unique to each species, and considering their ecosystem service importance, more plant species should be analysed to estimate their ecological value for management purposes.

Blue carbon stocks in southern Africa's Endangered seagrass Zostera capensis

Blue carbon research, although rapidly expanding, still requires site- and species-specific assessments to inform decision-making and policy formation. In this study, C stocks were assessed in the Endangered seagrass Zostera capensis. in the Knysna, Swartkops and Nxaxo estuaries, South Africa. Mean biomass C storage was 2.39 ± 1.42 Mg C ha−1 (similar to the global seagrass average), while 177.65 ± 122.29 Mg C ha−1 was estimated in the top meter of sediment. Sediment organic C content was 0.99 ± 0.45% — markedly lower than the seagrass global average but within the range reported for other Zostera species — and displayed significant spatial variability within and between estuaries. Sediment C was particularly high in the eutrophic Swartkops, likely due to excessive wastewater discharge introducing organic matter to the estuary's seagrass habitats. Based on the C stocks determine in this study, it is estimated that approximately 315 970 ± 217 111 Mg C could be stored in South Africa's 1755 ha of Z. capensis habitats. However, these important blue carbon habitats face multiple threats, particularly climatic events like droughts and floods, and anthropogenic eutrophication. This study has provided the first comprehensive C stock assessment for Z. capensis and contributes to global blue carbon datasets. Furthermore, this work has reinforced the importance of protecting the Endangered seagrass by highlighting the important ecosystem service it provides through climate change mitigation and adaptation.

De novo assembly and annotation of the transcriptome of the endangered seagrass Zostera capensis: Insights from differential gene expression under thermal stress.

Seagrasses are important marine ecosystem engineers but anthropogenic impacts and climate change have led to numerous population declines globally. In South Africa, Zostera capensis is endangered due to fragmented populations and heavy anthropogenic pressures on estuarine ecosystems that house the core of the populations. Addressing questions of how pressures such as climate change affect foundational species, including Z. capensis are crucial to supporting their conservation and underpin restoration efforts. Here we use ecological transcriptomics to study key functional responses of Z. capensis through quantification of gene expression after thermal stress and present the first reference transcriptome of Z. capensis. Four de novo reference assemblies (Trinity, IDBA-tran, RNAspades, SOAPdenovo) filtered through the EvidentialGene pipeline resulted in 153,755 transcripts with a BUSCO score of 66.1% for completeness. Differential expression analysis between heat stressed (32°C for three days) and pre-warming plants identified genes involved in photosynthesis, oxidative stress, translation, metabolic and biosynthetic processes in the Z. capensis thermal stress response. This reference transcriptome is a significant contribution to the limited available genomic resources for Z. capensis and represents a vital tool for addressing questions around the species restoration and potential functional responses to warming marine environments.

The role of seagrass meadows in the accumulation of microplastics: Insights from a South African estuary

Microplastics are widespread in marine ecosystems, where they threaten biota and human wellbeing. Seagrasses may act as natural filters of microplastics due to their particle trapping abilities, yet little is known about the extent of microplastics in the sediment of seagrass beds. The aim of this study was to compare microplastic accumulation in the sediments of Zostera capensis meadows with adjacent bare sediments at a small spatial scale (~5 km) in the Knysna estuary in South Africa. No significant difference in total microplastic counts were found between seagrass and unvegetated sediments. However, fibre microplastic counts differed significantly between the two locations at which samples were collected. This may be due to pollution levels and proximity to larger human population densities. Importantly, our study reveals variability in microplastic abundance and type even at small spatial scales, an important consideration for the design of future studies aiming to monitor for microplastics.

Conservation implications of herbicides on seagrasses: sublethal glyphosate exposure decreases fitness in the endangered Zostera capensis.

Worldwide seagrass populations are in decline, calling for urgent measures in their conservation. Glyphosate is the most widely used herbicide globally, leading to increasing concern about its ecological impact, yet little is known about the prevalence or impact of glyphosate on seagrasses. In this study, we investigated the effect of sublethal glyphosate exposure on the endangered seagrass, Zostera capensis, to identify effects on growth, photosynthetic pigments and leaf morphology as measures of seagrass fitness. Seagrasses were exposed to a single dose of a commercial glyphosate formulation-ranging between 250 to 2,200 µg/L. After three weeks, the median leaf area decreased by up to 27%, with reductions of up to 31% in above ground biomass (p < 0.05). Photosynthetic pigment concentration showed no significant difference between groups. The observed effects on biomass and leaf area were seen at glyphosate levels below the regulatory limits set for surface water by several countries and may negatively affect the long-term resilience of this ecosystem engineer to additional stressors, such as those associated with climate change and anthropogenic pollution. As such, glyphosates and other herbicides that are washed into estuarine and marine ecosystems, pose a significant threat to the persistence of seagrasses and are important factors to consider in seagrass conservation, management and restoration efforts.

Experimental effects of warming and epiphyte grazing on the ecophysiology of two seagrass morphotypes

Seagrasses form important ecosystems providing essential provisioning and regulating services. They exhibit acclimative properties to contend with environmental change while intrinsic responses allow seagrasses to persist in sub- and supra-optimal temperatures, and are critical to understanding their ability to cope with a changing climate. An indoor mesocosm approach was used to investigate the effects of warming, i.e. mean summer (24 °C) and extreme (30 °C) temperatures, and epiphyte grazing on large-leaved and small-leaved morphotypes of a dwarf eelgrass (Zostera capensis) from a temperate lagoon habitat. Both morphotypes displayed similar responses under extreme temperatures with greater losses observed in shoot densities and total biomass compared to summer mean temperatures. However, higher photosynthetic rates and accumulations of carbon, nitrogen and phenolics were observed in small- compared to large-leaved plants. This suggests that small-leaved populations possess a larger propensity to store resources - a potential for resilience. Large-leaved populations appeared to have channelled resources towards growth suggesting a greater susceptibility to stress from e.g. fouling. Epiphyte biomass increased under extreme temperatures and, even though grazing had a positive effect on seagrass biomass under summer mean conditions, grazers did not regulate algal growth under extreme temperatures which may have been too rapid for grazer control. These findings indicate that seagrasses will be negatively affected by extreme warming with ensuing reductions in biomass, increasing their susceptibility to stressors. However, acclimation strategies including carbon and nitrogen stores provide high resilience potential and attest to a system's ability to cope with stress.

A field experiment exploring disturbance‐and‐recovery, and restoration methodology of Zostera capensis to support its role as a coastal protector

Seagrass degradation in the western Indian Ocean is of serious concerns with numerous severe climate events that has impacted Mozambique, as well as South Africa in recent time, highlighting the need for re‐establishing these critical habitats. The following paper present a field experiment from Mozambique concerning disturbance‐and‐recovery effects of Zostera capensis, a highly impacted seagrass species in the region. We undertook trials mimicking disturbance of meadows in terms of digging to collect clams, a common practice in Maputo Bay. Our results indicated that once such anthropogenic impacts are halted, Z. capensis meadows restore itself after 12 months of impact exclusion. In addition, we also conducted seagrass restoration experiment using a manual method, the so‐called plug method which, demonstrated to improve the survival rate with 75%, which has large implications for seagrass transplantation projects that are both time consuming and financially costly. The results from this study will further guide the appropriate management to build the best practice in seagrasses meadows restoration and awareness. It is clear from this study that a combination of both impact exclusions and active restoration is bound for best results of seagrass management actions.

Characterizing the distribution of the critically endangered estuarine pipefish (Syngnathus watermeyeri) across its range using environmental DNA

AbstractThe effective management of rare and threatened species, especially in areas where population sizes have diminished, relies on knowledge of their population size, threats, and distribution. Robust mapping of distribution presents a particular challenge in aquatic environments for cryptic species, especially those with low abundance. Environmental DNA (eDNA) approaches can offer improved detection rates of many rare and threatened species when compared with traditional sampling approaches. In this study, we developed and optimized a targeted eDNA assay for the critically endangered estuarine pipefish (Syngnathus watermeyeri). eDNA sampling and seine netting were undertaken at 39 sites across the historical range of S. watermeyeri in the Eastern Cape of South Africa in 2019. At each site, five water samples were collected for eDNA analysis (n = 195) along with three seine netting hauls (n = 117). Habitat and environmental data were collected at each location to explore what physical and biotic parameters might correlate with pipefish presence/absence. We successfully detected S. watermeyeri in two estuaries (Kariega and Bushmans) using both survey methods. Importantly, the positive detection rate of eDNA (66.7%) was four times that of seine netting (16.7%), highlighting the value of eDNA as a monitoring tool for rare and cryptic species. Null detections in the Kasouga, East Kleinemonde, and West Kleinemonde estuaries add to the growing body of evidence that the estuarine pipefish has been extirpated from these locations and is now only found in two estuarine systems. The occurrence of S. watermeyeri was found to be highly dependent on the cover of submerged macrophytes such as Zostera capensis (eelgrass). By providing a more complete picture of the conservation status of this critically endangered species, this work facilitates the development of a long‐term monitoring program and the identification of priority conservation areas.

Blue carbon and nutrient stocks in salt marsh and seagrass from an urban African estuary

Halophytes in estuaries are effective sinks of carbon, nitrogen, and phosphorus. Blue carbon (BC) is carbon stored in coastal habitats such as mangroves, salt marshes and seagrass. The objectives of this study were to firstly assess the biomass and sediment C stocks in salt marsh (Spartina maritima and Salicornia tegetaria) and seagrass (Zostera capensis) habitats of the Swartkops Estuary, South Africa. Secondly, we applied the nutrient pollutant indicator (NPI) to assess the nutrient status of the estuary. Six replicate sediment cores of 1 m (summer) and 0.5 m (winter) per plant species were collected at six sites. Six replicates for biomass of each species were harvested at each site during summer and winter. Biomass and nutrient dynamics showed that there were distinct seasonal differences in the magnitude of C, N, and P stored in the plants. The sediment was the dominant C pool and differed spatially with creek sites storing more C. Out of the three species, S. maritima stored the most C (224 ± 19.1 Mg C ha−1 in sediment, 16.7 ± 1.2 Mg C ha−1 in biomass), followed by S. tegetaria (207 ± 3.5 Mg C ha−1 in sediment, 4.3 ± 0.4 Mg C ha−1 in biomass) and Z. capensis (180 ± 18.4 Mg C ha−1 in sediment, 2.1 ± 0.7 Mg C ha−1 in biomass). The N:biomass ratios were low in both seasons for Z. capensis, S. maritima and S. tegetaria (0.08, 0.02 and 0.04, respectively) indicating consistent eutrophic conditions in the estuary. Resultantly, the plants displayed a significantly lower below-ground standing biomass highlighting the potential variations of BC storage in eutrophic estuaries.

Contributions of Wetland Plants on Metal Accumulation in Sediment

Wetlands, and especially salt marshes, are well-known sinks of metals, which limit toxic amounts of metals from entering the food chain. This study investigated metal concentrations (Cr, Cu, Fe, Mn, Ni, Pb, Zn) in a highly urbanised estuary, and compared vegetated rhizosediment (Salicornia tegetaria, Spartina maritima, and Zostera capensis) with bare sediment, in a depositional and non-depositional site, in the intertidal zone of the Swartkops Estuary. The samples were collected at two sites along the middle and lower reaches of the estuary and analysed using a Total X-ray Fluorescence (TXRF) spectrometer. It was found that the rhizosediment contained more metals and that metal concentrations in the sediment decreased as follows: S. tegetaria &gt; S. maritima &gt; Z. capensis &gt; bare sediment. Although metal accumulation was similar in bare sediment for the depositional (Site B) and the non-depositional site (Site A), the rhizosediment displayed higher metal accumulation in the depositional site (Site B). However, regardless of site-specific depositional tendencies, rhizosediment displayed higher metal accumulation than bare sediment. These results indicate that vegetated sites and vegetated depositional sites should be the focus of monitoring metals in estuaries around the world.

The impact of a Reno mattress installation on an adjacent seagrass meadow and its macrofauna in a South African estuary

Armouring of coastlines and the banks of estuaries to protect property against erosion can have a negative impact on natural habitats and their communities. Within estuaries, one of the most threatened habitats is seagrass meadows. The impact of newly installed Reno mattresses on an adjacent Zostera capensis meadow and associated fauna was investigated in the Keurbooms Estuary, South Africa. The impact on Z. capensis was studied by determining percentage cover and the length of blades before and after placement of Reno mattresses. At the impact site, percentage cover and blade length decreased during the installation phase, however, recovered shortly after installation was completed. At the control site no changes in eelgrass percentage cover were noted. Invertebrate and fish species richness, diversity and abundance were compared in the eelgrass meadows before and after the installation. Whilst invertebrate species richness increased significantly after the installation at the Impacted site, there was no significant difference at the Control site. There was also no significant difference in invertebrate diversity and fish species richness and diversity at the Control and Impacted sites before and after installation. This suggests that the installation had no net negative impact at the Impacted site. Within a month of installation, the Reno mattresses were colonised by two invertebrate species (Amphibalanus amphitrite and Littoraria sp.). Although the introduction of Reno mattresses at the study site created a hybrid habitat that continued to provide a habitat for fish and invertebrates, we suggest that the use of these artificial structures should be in combination with natural vegetation (e.g. as a hybrid habitat) and not one that replaces natural habitat. There is limited information regarding the ecological impacts of using Reno mattresses in estuaries and this study provides new information on their ecological efficacy thus providing valuable insights in future coastal erosion control practices.