Zostera Capensis Research Articles

Causes of rarity and range restriction of an endangered, endemic limpet, Siphonaria compressa

The pulmonate limpet Siphonaria compressa is South Africa's most endangered marine mollusc. It is endemic to just two localities: Langebaan Lagoon on the west coast, and Knysna Estuary on the south coast, and occurs only on the eelgrass Zostera capensis. In Langebaan Lagoon, eelgrass has fluctuated substantially over the last 34 years, and S. compressa has twice approached extinction. S. compressa is largely confined to the lower edge of the eelgrass beds there, being replaced higher up by another small gastropod, Assiminea globulus. We explored the physical and biological factors underlying the limpet's narrow habitat, using field observations, translocations, caging and transplant experiments. Abundance of S. compressa was positively correlated with Z. capensis cover and negatively correlated with shore height. When moved to the upper portions of the eelgrass bed, S. compressa had lower rates of persistence and survival than in the lowest zone. The lower limit of zonation for S. compressa was set indirectly by bioturbation by the sandprawn Callianassa kraussi, which excluded eelgrass from intertidal sandbanks. Transplants of eelgrass into the sandbanks proliferated provided C. kraussi was experimentally eliminated, and supported densities of S. compressa 20-fold greater than in control eelgrass beds, suggesting that high-shore eelgrass beds to which S. compressa is normally confined are suboptimal for the limpet. A. globulus showed patterns opposite to those of S. compressa: its persistence and survival were greatest in the upper zone and it actively avoided the lower sections of these beds and never colonised eelgrass transplanted into sandflats lower on the shore. There was no evidence that competition between S. compressa and A. globulus influenced the zonation or abundance of either species. Rarity of S. compressa and its endangered status seem dictated by its extremely narrow and temporally changeable habitat-range, which is defined by physical stress in the high-shore and bioturbation by C. kraussi in the low-shore. Fluctuations in eelgrass abundance and limitation of S. compressa to just two localities add substantially to the risks of extinction for this embattled stenotypic limpet.

Interactions between Zostera capensis, Callianassa kraussi and Upogebia africana: deductions from field surveys in Langebaan Lagoon, South Africa

Callianassid sandprawns and eelgrasses have been postulated to have mutually negative effects on each other. The relationship between Callianassa kraussi and Zostera capensis was investigated in Langebaan Lagoon on the west coast of South Africa by correlative field surveys. Seven facts point to mutually antagonistic interactions between the species: (1) cover of Z. capensis was negatively correlated with density of C. kraussi; (2) Z. capensis was largely confined to the high shore, whereas C. kraussi occurred in unvegetated sandbanks below the eelgrass beds; (3) at sites where Z. capensis was absent, C. kraussi extended farther up the shore than in its presence; (4) C. kraussi reached a larger maximum size in sandbanks than in beds of Z. capensis; (5) immediately below eelgrass beds there was a 'frontier zone' where above-ground Z. capensis was absent but below-ground root-rhizomes appeared to exclude C. kraussi; (6) sediment penetrability was greater in sandbanks than in eelgrass beds; (7) rate of burrowing by C. kraussi was faster in unvegetated sandbanks than in eelgrass beds. Conversely, the density and sizes of the mudprawn Upogebia africana were greater in beds of Z. capensis than in sandflats dominated by C. kraussi. Although this evidence is correlative, it suggests that Z. capensis excludes C. kraussi because it stabilises the sediment and inhibits burrowing, and that C. kraussi prevents establishment of eelgrass beds by destabilising the sediment and suspending material that smothers the eelgrass. In the process, two distinct habitats are created. U. africana may benefit directly from eelgrass because it requires stable sediment for its semi-permanent U-tubes; but also indirectly because Z. capensis excludes C. kraussi, which may disrupt these feeding burrows. At the head of the lagoon where sediments are very fine and compact, U. africana occurs both in and below the eelgrass bed, and coexists with at least low densities of C. kraussi, although Z. capensis and C. kraussi are still negatively correlated.

Interactions between Zostera capensis and Callianassa kraussi: influences on community composition of eelgrass beds and sandflats

Eelgrass beds have been postulated to support different faunal communities from unvegetated sandflats, in part because of the stabilisation of sediments in eelgrass beds and their destabilisation in sandflats due to bioturbation. More specifically, eelgrass beds have been hypothesised to support disproportionately greater numbers of small, flexible and non-burrowing species, and bioturbated sandflats to support more large, hard-bodied burrowers. These concepts were tested in Langebaan Lagoon on the west coast of South Africa, where beds of the eelgrass Zostera capensis and sandflats dominated by Callianassa kraussi bioturbation are mutually exclusive. In eelgrass beds, sediment penetrability was significantly reduced compared to that in sandflats. Distinctly different faunal communities occurred in the two habitats, and the suite of species characteristic of each was consistent between years and between sites. Overall, density was higher in eelgrass beds than in sandflats. In contrast to previous studies, however, richness and diversity in sandflats were greater than, or equal to, values for eelgrass beds. There is convincing support for the hypothesis that burrowers are more prevalent in sandflats and less prevalent in eelgrass beds. Conversely, body flexibility does not differ between habitats and sizes are greater in the eelgrass beds than in the sandflats.

The distribution and habitat in the Knysna Estuary of the endemic South African mudsnail Hydrobia knysnaensis and the influence of intraspecific competition and ambient salinity on its abundance

The typical habitat of the rare, endemic South African mudsnail Hydrobia knysnaensis is the leaves of upper-shore Zostera capensis within high-salinity salt-marsh pools and channels. In the Knysna estuarine system, it is the numerically dominant member of a guild of six small microphytophagous gastropods; it is absent from lower level and more exposed Zostera meadows, where its place is taken by Rissoa pinna. H. knysnaensis is also here recorded from the nearby Swartvlei Estuary, in the same habitat type. This unusual habitat for a Hydrobia may in part account for the failure of earlier surveys to detect its presence, notwithstanding that it may well locally be the most numerous gastropod in each of these systems. Generally, however, it (and probably other small gastropods) seem to have been confused in estuarine fauna lists with Assiminea. Experiments show that the rate of feeding in H. knysnaensis is curtailed at population densities exceeding 2000–4000m−2 and in salinities below some 10psu. The proportion of non-feeding snails also increases at high population densities and in low salinities. The bearing of these results on whether H. knysnaensis is likely to be the 'Hydrobia sp.' recorded from some other South African localities and on the causes of its rarity are discussed.

First Field Studies of an Endangered South African seahorse, Hippocampus capensis

South Africa's endemic Knysna seahorse, Hippocampus capensis Boulenger 1900, is a rare example of a marine fish listed as Endangered by the IUCN because of its limited range and habitat vulnerability. It is restricted to four estuaries on the southern coast of South Africa. This study reports on its biology in the Knysna and Swartvlei estuaries, both of which are experiencing heavy coastal development. We found that H. capensis was distributed heterogeneously throughout the Knysna Estuary, with a mean density of 0.0089 m−2 and an estimated total population of 89 000 seahorses (95% confidence interval: 30 000–148 000). H. capensis was found most frequently in low density vegetation stands (≤20% cover) and grasping Zostera capensis. Seahorse density was not otherwise correlated with habitat type or depth. The size of the area in which any particular seahorse was resighted did not differ between males and females. Adult sex ratios were skewed in most transects, with more males than females, but were even on a 10 by 10 m focal study grid. Only three juveniles were sighted during the study. Both sexes were reproductively active but no greeting or courtship behaviours were observed. Males on the focal study grid were longer than females, and had shorter heads and longer tails, but were similar in colouration and skin filamentation. The level of threat to H. capensis and our limited knowledge of its biology mean that further scientific study is urgently needed to assist in developing sound management practices.

Diversity and distribution of seagrasses around Inhaca Island, southern Mozambique

Nine seagrass species were identified around Inhaca as well as a more narrow form of Thalassodendron ciliatum (Forsk.) den Hartog occurring in rocky pools at the north east coast. Seagrasses were mapped and grouped in seven distinct community types (in order of areas covered): Thalassia hemprichii/Halodule wrightii, Zostera capensis, Thalassodendron ciliatum/Cymodocea serrulata, Thalassodendron ciliatum/seaweeds, Cymodocea rotundata/Halodule wrightii, Cymodocea serrulata and Halophila ovalis /Halodule wrightii; with the ninth species Syringodium isoetifolium occurring in three of these communities. A dichotomous identification key is presented. Seagrasses covered half of the whole intertidal area around Inhaca Island and diversity was also high at community level. Transects at macro and micro scales showed zonation of species which may be due to tidal gradients or topographic variation. Cluster analysis indicated ecological dissimilarities between co-dominant species. A decline of Zostera capensis has recently been seen outside the local village. Baseline studies like this are important for coastal zone management in developing countries, where large future changes in seagrass cover can be expected.

Food web structure in three contrasting estuaries determined using stable isotope (∂13C) analysis

Food web structure in three contrasting estuaries, the freshwater-deprived Kariega, the freshwater-dominated Great Fish River and the temporarily open/closed Kasouga estuary, along the south-east coast of southern Africa was investigated in summer (1–4 November) 2000 by employing stable carbon (∂13C) isotope analysis. Results of the analysis indicated that there were distinct differences in the primary sources of carbon utilised by zooplankton in the three estuaries. In the Kariega estuary two distinct zooplankton feeding guilds were identified, the first having moderate ∂13C values ranging between −16‰ and −18‰, the second having more enriched ∂13C values varying between −10‰ and −12‰. Zooplankters of the first guild utilised carbon derived mainly from the microbenthic algae, whereas those of the second guild appeared to obtain theirs from eelgrass, Zostera capensis, and its associated epiphytes. In the temporarily closed/open Kasouga estuary, zooplankton had isotopic values ranging between −17.8‰ and −18.1‰. The most likely source of carbon for the zooplankton in this estuary is the extensive microphytobenthic alga stocks usually recorded in the system. In contrast to the freshwater-deprived estuaries, the depleted isotopic values of the zooplankton samples (−28‰ to −26‰) from the Great Fish River estuary suggest that the primary source of carbon consumed there was derived from phytoplankton. Data obtained in this study are in agreement with those of previous studies conducted in the region and highlight the importance of freshwater input in determining the trophodynamics in estuarine systems in southern Africa.

Patterns of Abundance of Seagrasses and Associated Infaunal Communities at Inhaca Island, Mozambique

This study was conducted at Inhaca Island, south Mozambique, and focused on three different seagrass associations, dominated respectively by Thalassodendron ciliatum, Thalassia hemprichii and Zostera capensis. The main objective of this study was to compare the three seagrass associations, in what concerns vegetal and associated infaunal communities biomass. The approach was a nested design, which intended to provide information on seasonal effects and variability at decreasing spatial scales. Nested ANOVA was performed for all biological and porewater parameters and a cluster analysis of different biotic and abiotic parameters was performed. Nutrient concentration was consistently higher during the dry period as well as mean values of organic matter. Consistently lower values of chlorophyll a (chl a) were obtained at T. hemprichii meadow sites and the highest values were found at Z. capensis meadow. Phaeopigments have shown an opposite pattern when compared to chla, and average values were consistently higher during the dry season. There were no net differences of seagrass standing crop during both sampled seasons, however at meadow level major differences were found, and higher biomass was obtained at T. ciliatum meadow, followed by T. hemprichii. Mean values of epibiota have shown that higher biomass was obtained at T. ciliatum meadow, the remaining meadows presented very low values. Meiofauna biomass showed fluctuations between seagrass meadows, stations and also between seasons. Macrofauna biomass showed clear fluctuations between seasons, being strongly more homogeneous during the dry season and highly variable in the wet season. ANOVA results have shown different significance at the different levels of the analysis, suggesting different spatial and temporal patterns and interactions between parameters. The dendrogram produced two main groups of variables, the first one comprised of OM, phaeopigments, nitrites, nitrates, meiofauna, grain size and chl a. The second group comprises pH, macrofauna, ammonia, both seagrass components and epibiota on above-ground seagrass fraction. These two groups of parameters may correspond to different functional components of the seagrass ecosystem. The sediment biotic structure seems largely dependent on detritus-based chains, and has a high degree of spatial variability. On the other hand, macrofauna seems to be more related to the direct influence of seagrass standing crop, probably caused by the increased habitat complexity provided by higher seagrass biomass.

Biomass distribution and leaf nutrient concentrations and resorption of Thalassia hemprichii at Inhaca Island, Mozambique

Biomass and nutrients (C, N and P) of the seagrass Thalassia hemprichii were compared for the Northern Bay (NB) and Southern Bay (SB) of Inhaca Island, southern Mozambique. T. hemprichii is one of the most dominant seagrass species in the Indian Ocean, although still poorly documented in Africa. Its mean biomass varied between 787.4 ± 233.8gDWm−2 and 1876.8 ± 389.4gDWm−2 for NB and SB, respectively. For the associated seagrass species, Cymodocea rotundata, Halodule wrightii, Halophila ovalis and Zostera capensis, they corresponded up to 8.8% of the T. hemprichii biomass. Leaf nutrient concentrations showed a clear gradual decrease with age. This varied from 3.03% to 1.7% for N, 0.25% to 0.07% for P in NB and from 3.3% to 1.7% for N, 0.27% to 0.1% for P in SB. Mean C:N:P ratios, normalised for biomass, were 545:35:1 and 509:35:1 for NB and SB, respectively. The leaf resorption efficiencies, were 9.5% and 63.9% for nitrogen and 44.4% and 63.2% for phosphorus in NB and SB, respectively.

Stable isotope (∂13C) composition of the food web of the temperate Kariega estuary (Eastern Cape)

Stable isotope (∂13C) composition of the food web in the temperate Kariega estuary in summer and winter was analysed to determine the trophic relationships between 19 taxa. There was no seasonality in the stable isotope ratios of the dominant producers and consumers in the estuary. Two distinct zooplankton assemblages were identified. The first assemblage, comprising the different developmental stages of the copepods Acartia longipatella, A. natalensis and Pseudodiaptomus hessei and the mysid Mesopodopsis wooldridgei, had intermediate ∂13C values ranging between −16 and −18‰. These values indicate a carbon source relatively depleted in carbon, most likely particulate organic matter derived mainly from the salt marsh and riparian vegetation. The second zooplankton assemblage, consisting of the caridean shrimp Palaemon peringueyi, the cumacean Iphinoe truncata and the amphipod Grandidierella lignorum, had ∂13C values (range −9 to −12‰) enriched in carbon. The most likely sources of carbon for these zooplankters were the eelgrass Zostera capensis, its associated epiphytes and/or Spartina maritima. The isotope compositions of the seven most abundant fish species were also divided into two groups. The first group, comprising Atherina breviceps and Gilchristella aestuaria, had isotope values ranging between −14 and −15‰, suggesting that they consume mainly zooplankton. The ∂13C values of the second group, comprising Rhabdosargus holubi, Liza richardsonii, L. dumerilii, Caffrogobius natalensis and C. gilchristi ranged between −11 and −12‰. These values suggest that these species assimilate a carbon source enriched in carbon derived either from eelgrass, Zostera capensis, its associated epiphytes or S. maritima. Results of the study suggest that there are two main carbon pathways for plankton and nekton in the Kariega estuary, carbon derived from the eelgrass and its associated epiphytes and carbon which has its origins in the salt marsh riparian vegetation and zooplankton.

Biomass dynamics of seagrasses and the role of mangrove and seagrass vegetation as different nutrient sources for an intertidal ecosystem

The input of organic matter and nutrients produced by mangrove and seagrass vegetation in the intertidal bay on Inhaca island, Mozambique, was estimated. Mean mangrove tree height was 2.20 m, diameter at breast height was 6.4 cm and density was 6047 trees per hectare. Above-ground biomass of mangrove was calculated at 107 t ha −1 and annual litterfall at 6 t ha −1. Above- and below-ground biomass, shoot density, leaf size, number of leaves per shoot, production and production/biomass (P/B) ratio of three different seagrass species, Cymodocea serrulata, Halodule wrightii, and Zostera capensis, was measured. Shoot densities were low and plants small. Total dry weight (DW) biomass was 18–198 g m −2, with higher values in summer, except for Z. capensis. Production was 0.08–0.62 g DW per day, 2.2 times higher in summer, compared with winter, with highest values recorded for C. serrulata. The P/B ratio varied from 3.2 to 5.8. The decomposition decay constant K of mangrove and seagrass leaves was 0.010–0.017; t 50 values were 69 and 41 days, respectively. Decomposition was faster at Saco, compared with the Banco area. Mean litter biomass was higher at Saco, than at Banco (0.24 and 0.05 g ash-free DW per square metre, respectively), with significant differences for litter type, season and substrate. The ratio of mangrove to seagrass litter stock was equal to expected values, based on production and decomposition. Local differences in litter stock could be explained by tidal current patterns. Litter production was estimated at 0.08 and 0.10 g ash-free DW per square metre, respectively, for the mangroves and seagrasses. The annual input of mangroves into the system was estimated at 330 t C, 6 t N and 0.6 t P, whilst seagrass produced more nutrients, 490 t C, 30 t N and 2 t P. This emphasised the importance of seagrass vegetation as the dominant nutrient source of the system.

Micro-PIXE mapping of sodium and chlorine in selected seagrasses

The distributions of sodium and chlorine in leaf cross sections of the seagrasses Zostera capensis and Thalassodendron ciliatum were studied using the true elemental imaging system (Dynamic Analysis) of the NAC nuclear microprobe. Analyses were performed using protons of 0.6 and 1.5 MeV and experimental conditions were compared in detail. Using protons of 0.6 MeV is recommended for mapping of Na, Mg and Al.

Casparian band-like structures in the root hypodermis of some aquatic angiosperms

Casparian band-like structures were detected in the root hypodermis of Potamogeton pectinatus L., Ruppia maritima L., Zostera capensis Setchell, Z. marina L., Z. japonica Aschers and Graebn, Halophila ovalis (R.Br.) Hook f. and Nymphaea capensis Thumb, but not in Egeria densa Planch, Eichhornia crassipes (Mart.) Solms and Lema minor L. The bands stained with the fluorochrome berberine hemi-sulphate which is specific for Casparian bands. They also contained suberin, resisted acid digestion and had an ultrastructure resembling Casparian bands of the endodermis. They were always associated with suberin lamellae, and blocked apoplastic transport into the interior tissues of the roots. Symplastic transport through the root tissues, however, was not affected by the bands' presence.

The tolerance to desiccation of the submerged macrophytes Ruppia cirrhosa (Petagna) grande and Zostera capensis setchell

The effect of short-term desiccation (0–120 min) on Zostera capensis and Ruppia cirrhosa, two dominant submerged macrophytes in South African estuaries, was measured using chlorophyll fluorescence ( Fv Fm ratio). The Fv Fm ratio for Zostera leaf tissue decreased to a constant minimum level after 30-min exposure. The Fv Fm ratio for Ruppia leaf tissue decreased as the degree of desiccation (exposure time) increased. Zostera was 75% desiccated after 30 min, whereas Ruppia was 63% desiccated. Zostera recovered better than did Ruppia after 24 h of resubmergence. After 4 days of resubmergence both plants in all exposure treatments showed complete recovery, i.e. the Fv Fm ratios had increased to the initial values. This recovery was related to the rapid growth of new leaves and not the recuperation of desiccated leaves. Neither species recovered from a desiccation treatment lasting 1 wk. A daily exposure of 5 h did not effect Zostera leaf production but was lethal for Ruppia. Ruppia is more sensitive to desiccation and Zostera possibly outcompetes Ruppia in tidal marine habitats.

The distribution of littoral fish associated with eelgrassZostera capensisbeds in the Kariega Estuary, a southern African system with a reversed salinity gradient

Eelgrass Zostera capensis has expanded its distribution in the Kariega Estuary mainly in response to the reduced salinity gradient in that system. Seine-net sampling showed that Atherina breviceps, Rhabdosargus holubi, Glossogobius callidus and Mugilidae were the most common fish taxa in the Zostera beds. Fish species richness was maximum in the lower reaches, but highest densities (17,3 individuals·m−2) occurred in the upper reaches. There was increased domination by marine opportunist species in the lower reaches, but estuarine taxa predominated the middle and upper reaches. There was a significant similarity between the species in the middle and upper reaches of the estuary, but not between the lower and middle, or lower and upper reaches. No significant correlation was evident between fish density and salinity, temperature or turbidity. The distributional patterns of most fish associated with Zostera beds in the Kariega Estuary were similar to other estuaries in the Eastern Cape with normal axial salinity gradients, indicating that factors other than salinity influence fish distribution in estuaries.

The Ecological Implications of Tolerance to Salinity by Ruppia cirrhosa (Petagna) Grande and Zostera capensis Setchell

The Ecological Implications of Tolerance to Salinity by Ruppia cirrhosa (Petagna) Grande and Zostera capensis Setchell

Effects of varying salinity on leaves of Zostera capensis Setchell. I. Ultrastructural changes

The ultrastructural morphology of leaf blade epidermal cells of the seagrass Zostera capensis Setchell was found to change with salinity. Epidermal cells of plants exposed to high salinity (40%0) compared with those exposed to low salinity (2%0) were characterized by the presence of complex wall ingrowths, a highly invaginated plasmalemma, mitochondria closely associated with the plasma membrane, many chloroplasts, an increase in the width of an electron-dense layer in the outer tangential wall, and smaller vacuoles. In addition, the relative area of mitochondria and chloroplasts increased with salinity. Leaf blade epidermal cells of plants exposed to very high salinity (60%0) showed signs of damage.

The Influence of River Impoundment on the Estuarine Seagrass Zostera capensis Setchell

Photogrammatic methods spanning a 47-year period were used in conjunction with field studies to document the influence of altered catchment hydrology on the dynamics of Zostera capensis Setchell in the Kromme estuary, South Africa. Reduced freshwater inflow and associated lack of sedimentary disturbances, stable salinities and reducted turbidities are considered to have contributed to the increased Z. capensis population. Over the last nine years there has been a fourfold increase in the total standing biomass of Z. capensis (...)

The Distribution of Estuarine Macrophytes in Relation to Freshwater

A continual flow of freshwater into estuaries was shown to create a salinity gradient along which different macrophyte communities were distributed. In systems with no freshwater inflow there was no spatial separation of macrophytes and salt marshes and the submerged macrophyte Zostera capensis, normally associated with the marine end of estuaries extended into the upper reaches. In small estuaries, salinities fluctuated because of the major influence of periodic small floods. During the study period the small estuaries studied were in a stable brackish condition (13-28 ppt.) and dominated by dense Ruppia cirrhosa communities which are normally associated with lower salinity regions (...)

Distribution, abundance and productivity of benthic invertebrates at the Berg River estuary, South Africa

Twenty-five benthic invertebrate species were identified from samples taken monthly over 17 months at four sites on the Berg River estuary, South Africa. Gastropods and polychaetes dominated the macrofauna in terms of both numbers and biomass. Abundance of the dominant species fluctuated in response to seasonal growth of eelgrass Zostera capensis and filamentous alga Cladophora sp. Differences in distributions of invertebrates on the estuary were attributed to differences in physical properties of the substratum and in vegetation cover. Hydrobia sp., Ceratonereis erythraeensis and C. keiskama were the most important species in terms of biomass and accounted for an average of 75% of total biomass at all study sites. Biomass peaked during the austral winter, early spring and again in autumn. An increase in biomass in winter was due to somatic production, whereas spring and autumn increases were attributed to recruitment of juveniles following reproduction. Mean annual biomass for the whole estuary was 19·36 g m −2, and mean annual production 87·58 g m −2 year −1, yielding a net P B ratio of 4·52. Production and P B ratios of invertebrates in estuaries and coastal lagoons at temperate and subtropical latitudes were positively correlated with mean annual ambient temperature and negatively with distance from the equator. Production data are lacking from tropical estuaries.