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Cambridge, M.L., 1975. Seagrasses of south-western Australia with special reference to the ecology of Posidonia australis Hook f. in a polluted environment. Aquat. Bot., 1:149--161 New ecological data are given for the nine species of seagrasses of south-western Australia, with special reference to Posidonia australis in Cockburn Sound, a polluted marine embayment.

Total annual production of Amphibolis antarctica (Labill.) Sonder ex Aschers. and Posidonia australis Hook.f. were measured for monospecific meadows at adjoining sites in Shark Bay during 1982–1983 and values of 1.6 and 1.0 kg dry weight m−2, respectively, were obtained. Amphibolis antarctica had maximum production in summer, coincident with high light intensities and temperatures, whereas P. australis showed no clear seasonal pattern. Both species had maximum specific growth rates in summer. There were fluctuations in plant tissue concentrations of nitrogen and phosphorus, but standing-stock values for total nitrogen and total phosphorus were relatively constant in the meadows. Amounts of nutrients incorporated into new growth were 49.7 mg total nitrogen and 3.6 mg total phosphorus m−2 day−1 for A. antarctica, and 25.6 mg total nitrogen and 4.3 mg total phosphorus m−2 day−1 for P. australis. The dominance of A. antarctica in Shark Bay, a nutrient-poor environment, is reflected in its higher productivity and this may be related to its more efficient utilization of phosphorus.

The structure of fish communities inhabiting separate meadows of the seagrasses Zostera capricorni Aschers. and Posidonia australis Hook f. in Botany Bay, central New South Wales, was studied over a 2-year period using a number of community parameters. The heterogeneity, species richness and total abundance of fishes were not found to be significantly different between the two seagrass habitats, although more species and more individuals were associated with Zostera. The number of species, abundance of individuals and total biomass were all significantly greater in both habitats during summer, and total biomass was significantly greater in the Posidonia. Ordination of species abundance data separated samples from the two habitats. In general, larger species of the Gobiidae and Syngnathidae were diagnostic of Posidonia and smaller members of these families were unique to, or more abundant in, Zostera. Occurrence, abundance and biomass data were used to identify the 20 dominant members of the fish community in each habitat. About 50% of the dominant fish species associated with each seagrass habitat were of some economic importance. Such species were usually residents or transients and made up most of the biomass in each habitat. Adults of these dominant economically important species were most abundant in both seagrass habitats during summer. Juveniles of several economically important species were present in both Posidonia and Zostera meadows. However, marked differences in time of occurrence, size and feeding behaviour of these juveniles between the two habitats suggest that such species use Zostera and Posidonia meadows sequentially. Differences in the structure of the communities, and utilization of resources by juveniles of several economically important species, appear to be related to variations in the structural complexity of the canopies created by the two seagrasses.

In situ decomposition studies were carried out using litter bags. Losses in dry weight, phosphorus, nitrogen and organic carbon were followed during two experiments, initiated at the times of highest and lowest water temperatures. Initial nutrient concentrations were low for both species, at both times of year, < 1 mg g- 1 total phosphorus, and < 11 mg g- 1 total nitrogen, but were higher in winter than in summer. Rates of loss of all material for both species were similar, and were best described mathematically by a linear equation. The loss rate was 0.5% day- 1, giving a time for complete decomposition of about 200 days. The significance of this fast rate is discussed in relation to the nutrient budget of seagrass meadows, in a nutrient poor environment.

Temporal variation in the concentrations of Cd, Cu, Mn, Ni, Pb and Zn in leaves of the seagrass Posidonia australis was studied at three sites near a lead smelter on the shore of Spencer Gulf, a large hypersaline marine embayment in South Australia, on four occasions from October 1980 to September 1981. Concentrations of Cd, Mn, Pb and Zn of up to 541, 537, 379 and 4241 μg g-1, respectively, were found in leaves collected from the site nearest to the smelter. A substantial temporal variation in the concentrations of these metals in samples from all sites resulted from the combined effect of leaf age and collection strategy. Concentrations of Cd, Cu and Zn in the leaf epibiota were lower than those in the leaves, but the reverse was true for Mn and Ni. The use of seagrass leaves as sentinel accumulators for Cd, Pb and Zn must be based on collections made at the same time of year, or otherwise account for the effect of leaf age on concentrations of the metals in the samples.

The accumulation is described of N, P, K, S, Ca, Mg, Na, Fe, Zn, Mn and Cu in the developing pericarp and seed of two species of seagrass. Both species showed essentially the same patterns, which resemble those of herbaceous terrestrial plants. There was a close relation between dry matter and nutrient accumulation. N, P, K, Fe, Zn, Mn and Cu accumulated in the fruit against large concentration gradients, with discrimination against Na. Seeds accumulated N, P and trace elements to a greater extent than pericarps and other plant parts; P was apparently retrieved from pericarps to a greater extent than other elements. Calculations were made of the losses of these elements from seagrass meadows in shed fruits.

Reactive phosphate was released into the water over a bed of the seagrass Posidonia australis Hook. f., both day and night. No release occurred over a similar unvegetated area nearby. Estimates of release rate were between 0.055 and 0.17 μg-at. P h−1 g−1 leaf dry wt. It was shown that the plants themselves were not responsible for releasing this reactive phosphate into the water column. It is postulated, instead, that the seagrass bed released reactive phosphate through microbiological remineralization of destritus accumulated from the water column, seagrass leaves and epibiota. The reactive phosphate thus produced entered the water column either by diffusion or by tidal flow mechanisms.

The component hydrocarbons, sterols, alcohols, monocarboxylic, α, ω-dicarboxylic and ω-hydroxy acids of the seagrasses Posidonia australis and Heterozostera tasmanica and a sample of P. australis detritus are reported. The fresh leaves of P. australis and P. australis detritus are characterized by a distinctive distribution of solvent-extractable long-chain monocarboxylic, α, ω-dicarboxylic and ω-hydroxy acids. This distinctive pattern should enable these lipid components along with other distinctive components to be used as chemical markers of the seagrass P. australis . H. tasmanica is characterized by (1) higher relative concentrations of 16:2ω6 and 16:3ω3 than P. australis , (2) the absence of the distinctive distribution pattern of long-chain monocarboxylic and ω-hydroxy acids observed for P. australis , (3) the absence of α, ω-diacids and (4) a lower absolute concentration of ω-hydroxy acids than P. australis .

The hypothesis was examined that increased epiphyte growth was responsible for a reduction in seagrass meadows in Cockburn Sound during the discharge of nutrient-rich effluent. One study site was in a deteriorating meadow near an effluent outfall, the other at similar depth in an unaffected meadow in more oceanic water. Seagrass production at the first site was less than that at the second, with 33% lower growth per shoot and 29% less dense meadow. Water at the former site had higher mean concentrations of chlorophyll and phosphate than the latter, but light reaching the seagrass meadows was not significantly different. Epiphyte loads (as dry weight or chlorophyll per unit leaf area) were 2–8 times higher at the former site. Seasonal changes in epiphyte loads were well correlated with periphyton biomass on glass slides or plastic seagrass. Photosynthesis of leaf segments, with and without epiphytes, was measured using an oxygen meter in the laboratory; epiphyte photosynthetic rates were similar to those of periphyton on plastic, expressed per unit chlorophyll. The percentage reduction in light by known periphyton loads was measured, and used to calculate light reduction by epiphytes in the field, which was estimated to be 63% on average at the first site and 15% at the second. Pooling data for sites and seasons, there was a negative log-linear relationship between leaf production and epiphyte load. The observations provide support for the suggestion that seagrass loss in the Sound may be attributed to enhanced epiphyte loads following nutrient enrichment.

The depletion of dry matter, N and P from seeds of Posidonia australis Hook. f. and Posidonia sinuosa Cambridge et Kuo during germination and seedling establishment is described. Seeds of both species showed essentially the same patterns of depletion, which resembled those of terrestrial plants. Seed reserves of N and P were retrieved with an apparent efficiency of ca. 95%, and linear relationships existed between the loss of these nutrients and dry matter from seeds during the first nine months of seedling growth. The distribution of dry matter, N and P amongst plant parts of both species was very similar. Leaves were major sinks for N and P during the first two years of plant development. Leaves contained the highest concentrations of N and rhizomes the highest levels of P. All seedling parts accumulated N and P against large concentration gradients. The environment contributed 100–150 mg N and 17–25 mg P m−2 to seedlings during their two years of growth. In 5-year-old plants, leaf bases contained important reserves of P, and 30–40% of the plant's N and P was associated with dead and morinund tissue. Leaf bases lost 84% of their N and 95% of their P during senescence.

This paper describes quantitative studies on the macro-benthic fauna of seagrass beds dominated by Posidonia australis Hook f. at 9 sites in 5 estuaries and an open ocean site representing the geographical range of this seagrass species distribution in New South Wales. The communities at each site were described using various community indices, numbers of diminant and diagnostic species and feeding categories. Similarities between sites were determined using classification analysis (MULCLAS). A total of 323 species were collected and the fauna was found to be dominated by polychaetes, molluscs and crustaceans, in terms of both species and individuals. Many species (46%) were restricted to single sites and no “typical” Posidonia macro-benthic fauna could be identified. Marine/estuarine dominance appears to be a principal determinant of faunal composition.

Northern Spencer Gulf is the landward extremity of a shallow marine embayment which occupies a structural depression that overlies a major Precambrian lineament. Situated in a warm temperate climate, the low rainfall and high evaporation have created high salinities resulting in an hydrological inverse estuary. Modern sedimentation is dominantly biogenic carbonate to mixed terrigenous-carbonate. It is controlled by a mesotidal regime with occasional storm surges, minor wave activity, and a prolific growth of seagrass in shallow water. The skeletal detritus consists of bivalves, gastropods, forams, echinoids, coralline algae and bryozoa. The subtidal zone between 10–25 m is divided morphologically into two provinces. The wide southern part has a relatively smooth floor, but the northern part is narrower, and the seafloor is either scoured free of loose sediment, or covered with wide belts of megaripples. The subtidal zone between 0–10 m is everywhere dominated by seagrass meadows. The seagrasses are largely Posidonia australis and P. sinuosa and occupy broad depositional platforms, and discrete offshore banks. The seagrass meadows produce and trap mollusc/foram detritus, resulting in the accumulation of very poorly sorted, organically bound structureless carbonate muddy sand. Intertidal and supratidal zone sediments are very extensive. The intertidal zone includes bare sand flats or those covered by the seagrass Zostera . Dense mangroves ( Avicennia marina ) from mean sea level to spring high-tide level are followed progressively by a halophytic (samphire) association and an Atriplex (saltbush) association. Extensive algal mats occur with the halophytes and extend into the mangrove forests. The sediments are muddy and only moderately calcareous. The supratidal zone consists mainly of bare carbonate flats, some stranded beach ridges, and coastal dunes. The carbonate flats contain discoidal gypsum crystals in weakly layered, fenestral, calcitic mud. Coastal changes during historic time are limited, and the dominant sedimentary regime in northern Spencer Gulf is the vertical growth of seagrass areas to form intertidal sand flats.

Zostera?capricorni and Posidonia australis are marine angiosperms which synthesise chlorophyll in the dark (Adamson, Hiller 1981). They are ideally suited to studies of normal chloroplast development because their leaves grow from a basal meristem. This pattern of growth in which cell division and subsequent cell expansion are confined to a discrete, measurable region at the base of the leaf gives rise to a linear array of cells with the oldest at the tip and the youngest at the base. This makes it easy to compare cells of different ages in the same leaf and to monitor changes which are induced by particular treatments.

Biomass and productivity of the seagrass Posidonia australis have been investigated (1977-78), at sites in Botany Bay (N.S.W.), Jervis Bay (N.S.W.), Port Hacking (N.S.W.) and Spencers Gulf (S.A.). Leaf biomass, which represented 20—25% of total biomass, ranged from 100 to 600 g dry weight per m2, and varied seasonally depending on water turbulence and growth rate. Leaf productivity was found to range from 0.6 to 5.2 g dry weight per m2 per day, depending on site and season. Rhizome and shoot production in Botany Bay was estimated to be 0.12 to 0.20 g dry weight per m2 per day. The turnover time (time to replace mean biomass) was found to be 93—117 days for leaves, and 11—19 years for the rhizome and shoot portions.

The stationary volumetric elastic modulus (epsilon(s)) of the leaf cells of three seagrasses (Halophila ovalis (R.Br.) Hook, Zostera capricorni Aschers, and Posidonia australis Hook f.) was evaluated from estimates of epsilon(s) plus intracellular osmotic pressure (epsilon(s) + II(i)) and II(i). The estimates of (epsilon(s) + II(i)) were made using a linear displacement transducer to measure very small changes in thickness of leaf tissue produced by changes in external osmotic pressure (II(o)). epsilon(s) increases with increasing turgor pressure in each of the species and the maximum values of epsilon(s) are: 22 megapascals for H. ovalis, 17 megapascals for Z. capricorni, and 51 megapascals for P. australis.There is a hysteresis in thickness changes versus changes in II(o) which indicates a hysteresis in the relationship between volume and turgor pressure. The hysteresis results in epsilon(s) being different for swelling and for shrinking cells over the same range of II(o) and this may be important in other aspects of plant-water relations.A new design of an apparatus employing a linear displacement transducer for measuring very small changes in tissue thickness is described. The new design has the advantages of virtually frictionless movement and a precision of 0.05 micrometer.

The leaves of four angiosperm species, the mangrove Avicennia marina, the samphire Sarcocornia quinqueflora, the seablight Suaeda australis and the seagrass Posidonia australis, were sampled from three locations from the south-east coast of NSW. Mean total arsenic concentrations (mean ± SD) in dry mass for all locations were A. marina (0.38 ± 0.18 to 1.2 ± 0.7 µg g−1), S. quinqueflora (0.13 ± 0.06 to 0.46 ± 0.22 µg g−1), S. australis (0.03 ± 0.06 to 0.05 ± 0.03 µg g−1) and P. australis (0.34 ± 0.10 to 0.65 ± 0.26 µg g−1). Arsenic concentrations were significantly different between species and locations but were consistently low compared with marine macroalgae species. Significant relationships were found between arsenic and iron concentrations for A. marina, S. quinqueflora and P. australis and a negative relationship between arsenic and zinc concentrations for S. quinqueflora. No relationship between arsenic and phosphorus concentrations was found in this study. Angiosperms contained predominantly inorganic arsenic in the water-extractable and residue fractions with minor concentrations of DMA in the water-soluble fraction. P. australis also contained dimethylated glycerol and phosphate arsenoriboses. The presence of arsenobetaine, arsenocholine, trimethylated glycerol arsonioribose and an unknown cation in P. australis is most likely due to the presence of epiphytes on fronds. There is no evidence to suggest that angiosperms produce arsenobetaine as arsenic is mostly present as inorganic arsenic. In conclusion, marine angiosperms only accumulate low arsenic concentrations and uptake appears to be dependent on iron uptake but not phosphorus uptake. Marine angiosperms mainly cycle inorganic arsenic with little biomethylation of arsenic occurring. Copyright © 2007 John Wiley & Sons, Ltd.

Background Seagrasses are clonal marine plants that form important biotic habitats in many tropical and temperate coastal ecosystems. While there is a reasonable understanding of the dynamics of asexual (vegetative) growth in seagrasses, sexual reproduction and the dispersal pathways of the seeds remain poorly studied. Here we address the potential for a predominantly clonal seagrass, P. australis, to disperse over long distances by movement of floating fruit via wind and surface currents within the coastal waters of Perth, Western Australia. We first simulated the dominant atmospheric and ocean forcing conditions that are known to disperse these seagrass seeds using a three-dimensional numerical ocean circulation model. Field observations obtained at 8 sites across the study area were used to validate the model performance over ~2 months in summer when buoyant P. australis fruit are released into the water column. P. australis fruit dispersal trajectories were then quantified throughout the region by incorporating key physical properties of the fruit within the transport model. The time taken for the floating fruit to release their seed (dehiscence) was incorporated into the model based on laboratory measurements, and was used to predict the settlement probability distributions across the model domain. Results The results revealed that high rates of local and regional demographic connectivity among P. australis meadows are achieved via contemporary seed dispersal. Dispersal of seeds via floating fruit has the potential to regularly connect meadows at distances of 10s of kilometres (50% of seeds produced) and infrequently for meadows at distances 100 s km (3% of seeds produced). Conclusions The spatial patterns of seed dispersal were heavily influenced by atmospheric and oceanographic conditions, which generally drove a northward pattern of connectivity on a regional scale, but with geographical barriers influencing finer-scale connectivity pathways at some locations. Such levels of seed dispersal infer greater levels of ecological and genetic connectivity and suggest that seagrasses are not just strongly clonal. Electronic supplementary material The online version of this article (doi:10.1186/s40462-015-0034-9) contains supplementary material, which is available to authorized users.

Seagrasses of the genus Posidonia can form an irregular seascape due to erosional processes exposing thick walls of organic matter-rich soils. However, little is known about the location and characteristics of these particular formations. Here we provide comprehensive estimates of organic carbon (Corg) storage in P. oceanica and P. australis meadows, while providing insight into their location and mechanisms of formation, and highlighting future research directions. Erosional reef escarpments are restricted to shallow highly productive P. oceanica meadows from the Mediterranean Sea and P. australis meadows from the Indian Ocean, and sustain the existence of Corg-rich deposits in surrounding meadows. The thickness of the mat escarpments can reach up to 3 m and their length can vary from few to hundreds meters. Mechanisms of formation appear to differ among sites, from naturally-induced escarpments by wave action and/or tidal flow to human-induced escarpments by dredging activities. The inter-twined remains of seagrass shoots within the sediment matrix consolidate the sandy substrate and hold the exposed Posidonia mat escarpments together, maintaining a semi-rigid structure. This phenomenon is unusual but of exceptional importance in marine biogeochemical cycles, revealing the largest Corg sinks among seagrasses worldwide (ranging from 15-176 kg Corg m-2 in 2 m-thick mats accumulated at 2-249 g Corg m-2 yr-1 over 300 to 3000 yr).

The marine angiosperm Posidonia australis Hook f. is always submerged and the leaf cells are able to exchange ions, gases and water with the surrounding seawater. The base of the youngest lamina is surrounded by sheaths from older leaves and a gradient in cell osmotic pressure existed from the base of the lamina enclosed within the sheath to the emerged zone at the top of the sheath. For plants grown in seawater, the cells at the base of the lamina had an osmotic pressure of 1.34 MPa (seawater = 2.54 MPa); the osmotic pressure increased with distance along the lamina to the emerged lamina value of 3.09 MPa. The osmotic gradient was accounted for by cell concentration gradients of Na+ (73 mol m-3 increasing to 412 mol m-3), K+ (91 mol m-3 increasing to 279 mol m-3) and Cl- (62 mol m-3 increasing to 578 mol m-3). Gradients also existed in the cell concentrations of sucrose and amino acids. It is proposed that, within the solution enclosed by the sheath, a standing osmotic gradient is created by ion uptake from the sheath solution. Leaf growth was unaffected by salinities from 13‰ to 57‰ and net photosynthesis was unaffected by reduction in salinity from 34‰ to 19‰. The cells of the leaves and rhizome adjusted their osmotic pressure by changes in Na+, K+ and Cl- concentrations such that turgor varied only between 0.67 and 1.52 MPa over a range in external osmotic pressures from 0.83 to 3.89 MPa. The tolerance of P. australis to changes in salinity in the absence of severe physical disturbance is due, largely, to the sheath and to the osmotic pressure gradient.

Three species of leatherjackets, Monacanthus chinensis, Meuschenia freycineti and Meuschenia trachylepis, averaged 34% of the total biomass and 27% of the total numbers of fishes in a Posidonia australis seagrass habitat near Sydney. Monacanthus chinensis was dominant, comprising 22% of the total biomass and 18% of the total numbers in this fish community. All three species were omnivorous, consuming considerable amounts of seagrass and algae as well as animal material. However, only the encrusting fauna and epiphytic algae of the seagrass appeared to be actually digested. Other foods of all three species included hydroids, molluscs, crustaceans and polychaetes. Meuschenia freycineti consumed the largest quantities (65%) of seagrass, Monacanthus chinensis consumed the largest amount (40%) and greatest variety of animal foods, and Meuschenia trachylepis took the smallest quantity (12%) and number of animal foods and the greatest amount (55 %) of algae. All three species were found to be highly dependent on the encrusting fauna, epi- phytic algae and other epifauna and infauna of this seagrass habitat, and the importance of preserving Posidonia beds is therefore stressed.

Feeding habits of P. laevigatus were investigated between February 1980 and November 1981 as part of a larger study on the structure of food chains supporting commercial fishes in the seagrass beds of Corner Inlet, south-eastern Australia. The main food components were crabs, predominantly Nectocarcinus integrifons, and juvenile fishes. Most dietary items were inhabitants of the seagrass zone. Small (25-33 cm) rock flathead showed a preference for fish, squid and shrimp whereas larger (> 33 cm) individuals consumed mainly N. integrifons together with some fish. Seasonal variations in diet and feeding intensity occurred. A decrease in feeding intensity in winter was accompanied by a shift in diet from N. integrifons to fish and squid. A second decline in feeding intensity corresponded to the apparent spawning period. The rock flathead and its main prey, N. integrifons, both showed a rhythm in locomotory activity correlated with nocturnal or crepuscular high tides whereas small fish prey were mostly active during daylight.

The distribution, biomass and density of various beds of P. australis have been determined for Botany Bay (N.S.W.). Comparisons have been made with sites at Broken Bay, Port Hacking and Jervis Bay and these include observations of other seagrass beds (Zostera capricorni, Heterozostera tasmanica, Halophila ovalis and Halophila decipiens). The P. australis plants observed were all of the broad-leaved (1.0-1.2 cm) variety. The length of leaves appears to be correlated with depth and with degree of exposure to wave action. The distribution of P. australis in Botany Bay is almost entirely restricted to the southern side and is limited to a maximum depth of - 3 m compared with -7 to -9 m at other sites. The growth of plants in well-established beds seems to be as vigorous in Botany Bay as elsewhere but flowering is poor and infrequent and the overgrowth of epiphytes is very great at certain seasons. Clearing experiments in Botany Bay indicate that recolonization by surrounding plants or seedlings is extremely slow. Transplant experiments indicate that mature plants will survive transplanting to cleared sites on both the northern and southern sides of the Bay. Evidence for a much more extensive distribution of P. australis in Botany Bay in times past and the possible effects of pollution are discussed.

Stable carbon isotope determinations have been used to obtain a general background to food chains being studied by a range of chemical and biological techniques. δ13C values indicate that animals (δ13CC - 11.4‰ to - 14.9‰), including two locally important commercial fish, Platycephalus laevigatus (rock flathead) and Hyporhamphus melanochir (southern sea garfish), from the Corner Inlet seagrass and non-seagrass communities are dependent to varying degrees upon seagrass (δ13C - 7.0‰ to - 9.3‰) and benthic algae for their carbon source. The largest changes in δ13C values in the food chains is at the point involving seagrasses and their direct herbivores. The latter have more negative δ13C values (H. melanochir - 12.1%o, isopods - 11.4%o). Little or no change in δ13C values is apparent at the higher trophic levels (carnivores - 13 .0‰ to - 14.9‰). Epiphytic material on the fresh leaves of the two seagrass species in the Inlet is depleted in 13C when compared with the seagrass leaves. An opposite effect is observed for epiphytic material on Posidonia australis detritus.