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Hybridization is common among plants and has been crucial in the evolution of many aquatic plant groups. However, hybrid individuals are often difficult to identify, particularly in the marine environment. We aim to confirm a hybrid origin between two seagrass species for a morphologically-intermediate plant observed in shallow subtidal environments near Perth, Western Australia. Both morphological measures and DNA markers demonstrated a clear case for hybridization among two sympatric Posidonia species from different species complexes: Posidonia australis and P. coriacea. All five samples with the intermediate morphology had the same multilocus genotype (MLG), providing evidence that it resulted from a single hybridization event, followed by local clonal (rhizome) expansion. This result highlights weak reproductive barriers within the genus Posidonia, a trait common among other seagrass genera with sympatric ranges, apparently overlapping morphologies and low phylogenetic divergence.

Desalination has the potential to provide an important source of potable water to growing coastal populations but it also produces highly saline brines with chemical additives, posing a possible threat to benthic marine communities. The effects of brine (0%, 50%, 100%) were compared to seawater treatments with the same salinity (37, 46, 54 psu) for seagrass (Posidonia australis) in mesocosms over 2 weeks. There were significant differences between brine and salinity treatments for photosynthesis, water relations and growth. Germinating seedlings of P. australis were also tested in brine treatments (0%, 25%, 50%, 100%) over 7 weeks followed by 2.5 weeks recovery in seawater. Growth was severely inhibited only in 100% brine. These experiments demonstrated that brine increased the speed and symptoms of stress in adult plants compared to treatments with the same salinity, whereas seedlings tolerated far longer brine exposure, and so could potentially contribute to seagrass recovery through recruitment.

Dredging can have significant impacts on benthic marine organisms through mechanisms such as sedimentation and reduction in light availability as a result of increased suspension of sediments. Phototrophic marine organisms and those with limited mobility are particularly at risk from the effects of dredging. The potential impacts of dredging on benthic species depend on biological processes including feeding mechanism, mobility, life history characteristics (LHCs), stage of development and environmental conditions. Environmental windows (EWs) are a management technique in which dredging activities are permitted during specific periods throughout the year; avoiding periods of increased vulnerability for particular organisms in specific locations. In this review we identify these critical ecological processes for temperate and tropical marine benthic organisms; and examine if EWs could be used to mitigate dredging impacts using Western Australia (WA) as a case study. We examined LHCs for a range of marine taxa and identified, where possible, their vulnerability to dredging. Large gaps in knowledge exist for the timing of LHCs for major species of marine invertebrates, seagrasses and macroalgae, increasing uncertainty around their vulnerability to an increase in suspended sediments or light attenuation. We conclude that there is currently insufficient scientific basis to justify the adoption of generic EWs for dredging operations in WA for any group of organisms other than corals and possibly for temperate seagrasses. This is due to; 1) the temporal and spatial variation in the timing of known critical life history stages of different species; and 2) our current level of knowledge and understanding of the critical life history stages and characteristics for most taxa and for most areas being largely inadequate to justify any meaningful EW selection. As such, we suggest that EWs are only considered on a case-by-case basis to protect ecologically or economically important species for which sufficient location-specific information is available, with consideration of probable exposures associated with a given mode of dredging.

Highly saline brines from desalination plants expose seagrass communities to salt stress. We examined effects of raised salinity (46 and 54psu) compared with seawater controls (37psu) over 6weeks on the seagrass, Posidonia australis, growing in tanks with the aim of separating effects of salinity from other potentially deleterious components of brine and determining appropriate bioindicators. Plants survived exposures of 2-4weeks at 54psu, the maximum salinity of brine released from a nearby desalination plant. Salinity significantly reduced maximum quantum yield of PSII (chlorophyll a fluorescence emissions). Leaf water potential (Ψw) and osmotic potential (Ψπ) were more negative at increased salinity, while turgor pressure (Ψp) was unaffected. Leaf concentrations of K+ and Ca2+ decreased, whereas concentrations of sugars (mainly sucrose) and amino acids increased. We recommend leaf osmolarity, ion, sugar and amino acid concentrations as bioindicators for salinity effects, associated with brine released in desalination plant outfalls.

Climate change induced sea level rise will affect shallow estuarine habitats, which are already under threat from multiple anthropogenic stressors. Here, we present the results of modelling to predict potential impacts of climate change associated processes on seagrass distributions. We use a novel application of relative environmental suitability (RES) modelling to examine relationships between variables of physiological importance to seagrasses (light availability, wave exposure, and current flow) and seagrass distributions within 5 estuarine embayments. Models were constructed separately for Posidonia australis and Zostera muelleri subsp. capricorni using seagrass data from Port Stephens estuary, New South Wales, Australia. Subsequent testing of models used independent datasets from four other estuarine embayments (Wallis Lake, Lake Illawarra, Merimbula Lake, and Pambula Lake) distributed along 570 km of the east Australian coast. Relative environmental suitability models provided adequate predictions for seagrass distributions within Port Stephens and the other estuarine embayments, indicating that they may have broad regional application. Under the predictions of RES models, both sea level rise and increased turbidity are predicted to cause substantial seagrass losses in deeper estuarine areas, resulting in a net shoreward movement of seagrass beds. Seagrass species distribution models developed in this study provide a valuable tool to predict future shifts in estuarine seagrass distributions, allowing identification of areas for protection, monitoring and rehabilitation.

Este artículo contiene 8 páginas, 3 tablas, 3 figuras., Oyster Harbour, on the south coast of Western Australia, supports 3.6e3.9 km2 of seagrass meadows, following the loss of approximately 2.8e3.1 km2 in the 1980s. This small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years. In this study, we sampled three sediment cores from a Posidonia australis meadow and analysed organic matter (OM), organic carbon (OC) and inorganic carbon (IC) contents in bulk sediments, and d13C signatures of OM. The OM and OC contents (mean ± SE) in the cores were 9.07 ± 0.36% and 2.24 ± 0.05%, respectively. The mean IC content was 3.16 ± 0.17%. d13C signatures of the sedimentary OM ranged from 10.01‰ to 13.28‰. Using a Bayesian isotopic mixing model, it is estimated that 57e67% of the OM in the seagrass sediments was derived from P. australis detrital matter. The total carbon (TC) stores in 150 cm-thick seagrass sediments averaged 27.92 kg TC m 2 (10.79 kg OC m 2 and 17.13 kg IC m 2). Based on radiocarbon dating, the mean sediment accumulation rate was 0.0494 cm yr 1, which led to a long-term TC accumulation rate of 8.92 g TC m 2 yr 1 (3.45 g OC m 2 yr 1 and 5.47 g IC m 2 yr 1). Based on historical seagrass cover (3.6 e6.7 km2 during the 1960s to 1980s), the estimated TC stores in 150 cm-thick seagrass sediments at Oyster Harbour would have been 101e187 Gg TC. The eutrophication-driven loss of seagrasses during the 1980s resulted in the absence of OC accumulation capacity amounting to 280e310 Mg OC (over 29 years). The loss of seagrass area could also have resulted in the release of 37e41 Gg CO2, assuming that all of the OC in shallow sediment is remineralised after meadow disturbance. These results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation., This work was supported by an ECU Postgraduate Research Scholarship and a UKM Scholarship for Postgraduate Study awarded to MR. We appreciate the field- and lab-work assistance from colleagues from ECU's Centre for Marine Ecosystems Research and Oceans Institute, UWA.

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species.

12 páginas, 5 figuras, 4 tablas, 4 apéndices., The study of a Posidonia australis sedimentary archive has provided a record of changes in element concentrations (Al, Fe, Mn, Pb, Zn, Cr, Cd, Co, As, Cu, Ni and S) over the last 3000 years in the Australian marine environment. Human-derived contamination in Oyster Harbor (SWAustralia) started ~100 years ago (AD ~1900) and exponentially increased until present. This appears to be related to European colonization of Australia and the subsequent impact of human activities, namely mining, coal and metal production, and extensive agriculture. Two contamination periods of different magnitude have been identified: Expansion period (EXP, AD ~1900–1970) and Establishment period (EST, AD ~1970 to present). Enrichments of chemical elementswith respect to baseline concentrations (in samples older than ~115 cal years BP) were found for all elements studied in both periods, except for Ni, As and S. The highest enrichment factors were obtained for the EST period (ranging from 1.3-fold increase in Cu to 7.2-fold in Zn concentrations) compared to the EXP period (1.1-fold increase for Cu and Cr to 2.4-fold increase for Pb). Zinc, Pb, Mn and Co concentrations during both periods were 2- to 7-fold higher than baseline levels. This study demonstrates the value of Posidonia mats as long-termarchives of element concentrations and trends in coastal ecosystems. We also provide preliminary evidence on the potential for Posidonia meadows to act as significant long-term biogeochemical sinks of chemical elements., Thisworkwas supported by the ECU Faculty Research Grant Scheme, the ECU Early Career Research Grant Scheme, and the CSIRO Flagship Marine & Coastal Carbon Biogeochemical Cluster with funding from the CSIRO Flagship Collaboration Fund. PMwas supported by a Gledden Visiting Fellowship and AAO by Obra Social “la Caixa”.

The eelgrass, Zostera, is a key habitat for fish productivity in southern Australia, but it is also susceptible to major declines, as in Western Port, Victoria in the 1970s. The fish assemblages in Zostera in Western Port have been relatively well studied, but assemblages in locations with alternative habitats, and their spatial and temporal variability, have been poorly studied. This means that the ability of these locations to serve as refuge areas in periods of Zostera decline is unknown. Two primary sampling methods were used for sampling locations with alternative habitats. Underwater stereo video was used to sample locations with higher water clarity and dominated by Amphibolis, reef-macroalgae and rhodolith bed habitats. A mini otter trawl was used to sample locations with low water clarity and low relief bottom: one location was dominated by Caulerpa habitat and the other was a reference Zostera location. The results showed that many species that have previously been found to be common in Zostera seagrass were also found in Caulerpa habitat, and to a lesser extent with Amphibolis habitat. Most species were associated with multiple plant habitats although some were more specific, such as the Weedy Seadragon, Phyllopteryx taeniolatus, which was only recorded in Amphibolis habitat. Multivariate analysis indicated that fish community structure was very similar between the location with Caulerpa and the reference sub-tidal Zostera location. Smaller fish were more abundant at all locations in autumn reflecting the spring spawning period of most species. Overall, most species were not limited to associating with Zostera habitat and therefore locations with alternative structured habitats may provide a measure of refuge in the face of Zostera decline. This conclusion, however, can only be considered preliminary until there is a clearer understanding of the functional relationship between fish species and these alternative habitats. Any potential refuge value is likely to be limited for species with a specific preference for Zostera such as some pipefish species, or where species have larvae that settle primarily in shallow habitats or in locations dominated by Zostera.

Seagrasses of the genus Posidonia are known to be particularly vulnerable to disturbances because damaged plants are slow to recover and recolonisation by seedlings is rare. Reestablishment of denuded areas by propagules is therefore likely to rely on maximising the survivorship of seedlings or vegetative fragments. This study investigated methods for enhancing growth rates of P. australis seedlings using hormones and nutrients, which may increase establishment success of plants used for rehabilitation. Laboratory experiments compared seedlings treated with different combinations and concentrations of hormones (auxin, cytokinin, gibberellic acid) and nutrients (N, P, K). Growth rates of healthy seedlings were not significantly enhanced by any treatment and negative effects of 30 mg/L auxin and nutrients were evident. Any positive effects of hormones were limited to improving survival of small seedlings with the addition of low concentrations of auxin (5 mg/L) or cytokinin (2.5 mg/L), and 0.1 or 20 mg/L gibberellic acid. A review of hormone effects on seagrasses found few consistent results among studies, in part due to differences in methods, species and hormones used. The few studies that were comparable tended to have conflicting results and we found relatively little evidence for positive effects of hormones on the growth of any seagrass. A final experiment demonstrated that the greatest influence on growth of P. australis seedlings was the initial size of seedlings (just after being released from the fruit). Large seedlings produced significantly larger plants (1.5–2.3 fold for various above and below ground measures) over 232 d than did initially small seedlings. Rehabilitation programmes should therefore use the largest seedlings available to help maximise the chances of success.

Este artículo contiene 11 páginas, 8 figuras, 1 tabla., Seagrass ecosystems are recognised for their role in climate change mitigation, due to their capacity toform organic-rich sediments. The chemical recalcitrance of seagrass organs is one characteristic drivingcarbon storage, but the molecular background of this feature is poorly understood. We assessedmolecular composition changes ofPosidonia australissheaths (SH) and roots plus rhizomes (RR) alonga sediment core, encompassing 3200 cal. yr BP, by means of nuclear magnetic resonance spectroscopy(13C NMR), conventional analytical pyrolysis (Py-GC–MS) and thermally assisted hydrolysis and methy-lation (THM-GC–MS). Significant trends with depth (age) in the composition of both SH and RR remainsofP. australiswere observed from all methods. In general terms, polysaccharides become depleted(degraded) and lignin enriched (selectively preserved) as age increases, and the minor constituents cutin,suberin and condensed tannin are also preferentially depleted during ageing in both fractions. Molecularchanges with ageing were smaller in SH, especially regarding polysaccharides, indicative of a superiorstability compared to RR. The molecular changes observed are most pronounced within the first 75 cmof the record, which reflects the recalcitrance ofP. australisdetritus once it is buried below that depth(corresponding to approximately 700 cal. yr BP). The capacity ofP. australisto act as a long-term carbonsink seems to be mainly related to the resistance of buried lignocellulose materials to decomposition. Theresults on diagenetic effects on the molecular fingerprint of seagrass detritus contribute to our under-standing of carbon sequestration in Blue Carbon ecosystems. Furthermore, data comparison of the meth-ods applied using principal component analysis (PCA) allowed us to identify consistencies, discrepanciesand complementarities, This work was supported by the ECU Silver Jubilee Award. O.S. was supported by the Australian Research Council (DE170101524).

Plant tissues display high levels of intra-specific variation in composition that strongly affects important ecosystem processes such as rates of herbivory, disease, and decomposition. Near infrared reflectance spectroscopy (NIRS) provides a rapid method to analyse tissue content with the potential for lower costs than conventional chemical analyses, and facilitating the high sample replication required to quantify spatial or temporal patterns in tissue variation. This study aims to determine the effectiveness of NIRS to quantify the ecologically important traits of total phenols, total nitrogen and total carbon in the seagrass Posidonia australis. Robust NIRS models were successfully developed for total phenols and nitrogen content, and used to quantify broad scale geographic patterns of intra-specific variation in these traits across 12 populations of P. australis. Given that only 90 out of 360 samples were required for the chemical analyses used to produce calibrations, NIRS facilitated this research by increasing sample size and reducing time and cost per sample.

Microsporogenesis, megasporogenesis and embryogenesis were studied using Posidonia australis as representative among three sympatric species in the seagrass genus Posidonia ( P. australis , P. coriacea and P. sinuosa ). Each flower spikelet has only one ovary (this is common to the three species). With P. australis the microspore is initially globular then becomes baton-like and finally matures into stick- or thread-like pollen. The mature embryo sac is four-nucleate comprising one polar nucleus, one egg cell and two syngerids, however as the embryo sac becomes mature, only three nucleate cells dominate, comprising one central cell (polar nucleus), one egg cell and one syngerid. After fertilization, the fertilized cell divides into two longitudinally. The basal cell ceases division, vacuolates and may function as a conduit for nutrients to the upper, dividing cell(s). The upper cell firstly divides transversely into two cells and then longitudinally into four cells, that in turn develops into a globular zygotic embryo attached by the basal cell. The globular zygotic embryo morphs into a shuttle-shaped embryo and loses attachment to the basal cell. This developmental sequence also appears to apply to P. coriacea and P. sinuosa .

Addition of nutrients to sediments has been proposed as a means of enhancing transplantation success in seagrasses. The effects of nutrient and iron additions to natural sediments on the growth and morphology of Posidonia australis transplants were evaluated in underwater plots in two contrasting environments: a coastal embayment (Princess Royal Harbour) with sandy sediments and little riverine input, and an estuary (Oyster Harbour) with organic-rich sediments and subject to seasonal river flow from a large rural catchment. Sixty six planting units spaced 1 m apart were transplanted in situ in each location. Nitrogen (N) and phosphorus (P) were added in a randomized factorial design using slow release fertilizer granules at the start of the experiment and repeated every 4–5 months for 2 years. In a concurrent experiment, chelated iron Fe EDTA was added to modify the sediment sulphur cycle . In Oyster Harbour, the addition of N significantly increased leaf N concentrations but reduced total biomass and biomass of leaves. Addition of P significantly increased leaf P concentrations and number of living leaves per transplant, leaf area, leaf length, length of longest rhizome axis and total rhizome length. Combined N + P addition resulted in a significant increase in leaf P concentrations and leaf area per plant only. In Princess Royal Harbour, addition of N produced significant increases in leaf variables (total and leaf biomass, number of shoots and living leaves, leaf area, and leaf length) but there were no significant differences observed in below ground plant parts (rhizomes). Addition of P had no significant effects on any growth measurements. Addition of N + P combined increased number of living leaves and leaf area significantly. δ15N in mature leaf tissue were significantly more negative for N and N + P treatments at both locations. Our results indicated that N limitation was occurring in the coastal embayment, Princess Royal Harbour whereas in the more estuarine Oyster Harbour, P was limiting plant growth. Addition of FeEDTA produced equivocal results at both sites and we suggest these results are confounded by the addition of N and C in the EDTA. We caution the use of nutrient addition to transplants of slow growing seagrasses such as P. australis without a thorough understanding of the nutrient status of the system, estuarine or coastal embayment, in which they are to be transplanted.

Transplant trials of the seagrass Posidonia australis were carried out after loss of seagrasses following eutrophication and increased turbidity in two marine inlets on the south coast of Western Australia. A pilot study in Oyster Harbour measured survival and growth in situ for 4 years. Long-term survival rates were high (96–98%), providing plants were anchored into the sediment. All unanchored plants were lost in the first winter. Following the success of the pilot study, a more comprehensive program began 3 years later with over 500 transplant units collected from either actively growing edges of nearby patches (plagiotropic growth form) or within established meadows (orthotropic growth form). Transplant units from edges expanded at a faster rate compared to units from mid-meadow but increases in shoot numbers were similar. Growth rates in the first 2.5 years averaged 10–20 cm yr−1 horizontal rhizome extension, depending on the source of the transplant units, and 4–12 shoots per initial shoot yr−1, depending on the initial shoot number of the transplant unit. After 5 years, shoot numbers of individual transplants were similar to shoot densities recorded for natural meadows, >500 shoots m−2. Approximately, 10% of transplants from mid-meadow flowered in the first year, whereas transplants from edges flowered only after 5 years. Transplant trials were also established in nearby Princess Royal Harbour at a site selected to test the effect of disturbance by bioturbation from large sand-burrowing worms or by sediment erosion. Survival was lower than in Oyster Harbour, 75–89% in areas with bioturbation but only 14% in areas where sediments were eroded. Growth was poor, This study showed that Posidonia australis could be transplanted with a high degree of success into a protected embayment previously vegetated with seagrass meadows. After 4 years, flower and seed production had occurred, and transplants spaced 1 m apart began merging. By the fifth year, individual transplant units could no longer be distinguished and planted areas resembled a natural meadow. These findings have important implications for restoration of impaired Posidonia habitats in Australia, using conventional low-cost techniques for transplantation once the sources of injury have abated.

Coastal development can impact on ecologically significant seagrasses. We sought to determine whether the construction of a pier and associated boat launching facilities would impact on Posidonia australis within Jervis Bay Marine Park, New South Wales, Australia. Using a modified ‘beyond BACI’ analysis we tested for impacts at three spatial scales; directly beneath the pier deck, within metres of the pier and approximately 50 m from the construction zone. We compared shoot density, leaf length and epiphytic cover prior to the commencement of construction to 7 months and 46 months later. Impacts were highly localised; we detected significant seagrass loss directly below the open-mesh deck of the pier, but not at the scale of metres and tens of metres relative to two non-impacted reference locations. Our data also supported the use of the trigger point approach to initiate management intervention; shoot density beneath of the deck of the pier fell below the 30th percentile, ending 73% lower than the mean after 46 months. If similar developments occur in waterways supporting seagrass species of high conservation significance, then management agencies need to weigh the inevitable loss of a small area of meadow with the advantages the development will provide.

This study investigated within- and among-species variability in the leaf optical properties of eight large-bodied seagrasses, Posidonia australis , Posidonia sinuosa , Posidonia coriacea , Posidonia angustifolia , Amphibolis antarctica , Amphibolis griffithii , Zostera tasmanica , and Zostera capricorni and the small-bodied Halophila ovalis from the east and west coasts of Australia. Leaf spectral transmittance [ T L ( λ )], reflectance [ R L ( λ )], and non-photosynthetic absorptance [ A L (NP)] were measured in order to calculate leaf spectral absorptance [ A L ( λ )] and photosynthetic leaf absorptance [ A L (PAR)]. Leaf spectra for the seagrasses were similar in shape, but variable in magnitude. Greatest among-species differences in leaf optical properties were present across green (500–600 nm) and near infrared (700–750 nm) wavelengths. This suggests differences in seagrass leaf optical properties are mainly due to variations in accessory pigments and leaf functional anatomy (e.g., internal structure, sclerenchyma fibers, etc.). Reflectance of photosynthetically active radiation (PAR) by leaves [% R L (PAR)] ranged from 4.6 to 9.3% with a mean of 6.1 ± 1.2%. A L (NP) was more variable (16 ± 5%) than % R L (PAR) among species and sites. P. coriacea , a seagrass characteristic of high energy environments with the most structurally reinforced leaves, had the highest values of % R L (PAR) (9.3 ± 1.4%) and A L (NP) (29 ± 8%). A L (PAR) values ranged from 45 ± 3% for H. ovalis from a shallow (1 m) open coastal site in Western Australia to 68 ± 3% for P. australis from a deeper (3 m) protected site in New South Wales. The overall mean A L (PAR) of 57 ± 6% for the seagrasses measured here is over 30% lower than the absorption factor of 84%, commonly used in PAM fluorometric studies of seagrasses. Because of the relatively low within-species variability in this parameter, A L (PAR) was significantly higher for the eight large-bodied species (59 ± 6%) compared to the small-bodied H. ovalis (53 ± 5%). This limited survey of seagrass leaf optical characteristics indicates relatively low variability in light absorption across a wide range of leaf morphologies and environments consistent with previous suggestions of constraints on light absorption by aquatic plants.

We used the Imaging-PAM fluorometer to map spatial variability of photosynthesis in three seagrass species, Halophila ovalis, Zostera capricorni and Posidonia australis. Photosynthesis was described by relative photosynthetic rate (PS/50), effective quantum yield (ΦPSII), non-photochemical quenching (NPQ and qN), electron transport rate (ETR) and leaf absorptivity. Photosynthetic patterns were linked to leaf age and light climate but patterns were not consistent across species. Longitudinal heterogeneity in photosynthesis was apparent along the leaves of all three species while lateral spatial heterogeneity was found only across Z. capricorni and H. ovalis leaves. Age of leaf tissue, determined by longitudinal location on the leaf, strongly influenced photosynthetic activity of Z. capricorni and P. australis. A comparison of H. ovalis leaves of differing maturity demonstrated the influence of leaf age on photosynthetic activity, yet a comparison of Z. capricorni leaves of differing maturity showed no leaf-age effects. Variations in stress-induced changes across a seagrass leaf can be used to identify areas or particular regions of the leaf, which are more susceptible to photodamage. Clear evidence of substantial within-leaf heterogeneity in photosynthetic activity (i.e., a two-fold variation in half saturation constant along a leaf of P. australis) has serious implications for use of small sections of leaf for photosynthetic incubations (such as O2 or single-point chlorophyll a fluorescence measurements). © 2004 Elsevier B.V. All rights reserved.

Electron transport rate (ETR) calculations require values of irradiance, effective quantum yield of chlorophyll fluorescence ( Δ F / F ′ m ), the distribution of energy between PSII and PSI and leaf-specific photosynthetic absorptance. We conducted virtually simultaneous replicated measurements of diel changes in Δ F / F ′ m of Posidonia australis in situ, and determined leaf-specific photosynthetic absorptance in vivo using integrating spheres. Leaf-specific non-photosynthetic and photosynthetic absorptance, corrected for spectral reflectance, comprised 16 and 68% of total spectral absorptance (84%); about 7% of photosynthetically active radiation incident on the leaves was reflected. Virtually simultaneous quantum-yield measurements of eight short shoots provided an estimate of among-sample variability in Δ F / F ′ m : generally the standard deviation was ∼10% of the mean except for a few hours around midday when the variability rose to ∼25% of the mean. Our results indicate that leaf-specific non-photosynthetic absorptance, spectral reflectance and shoot-to-shoot variability need to be taken into account when estimating ETR in the field.

Seedlings of the seagrass Posidonia australis are rarely found and little is known about their development. St. Georges Basin, an estuary in southeast Australia, harbours relatively large numbers of P. australis seedlings and patches. Seedlings of varying age were collected from four sites within the estuary. Mature plagiotropic shoots (horizontal rhizomes) on existing beds were tagged to measure vegetative growth and provide a plastochrone interval with which to age the seedlings. The abundance of P. australis seedlings ranged from 0.001 to 0.014 seedlings m −2 . Based on the plastochrone intervals (plagiotropic: 33±0.8 days; orthotropic: 48.0±1.4 days) estimated seedling age ranged from 0.7 to 8.2 years. Both the numbers of shoots and leaf areas were related to seedling age, with increases particularly evident between 2 and 5 years. Of 57 seedlings either collected or observed in situ, only 6 had produced a plagiotropic shoot. Rhizome growth for the seedlings ranged from 5.1±0.5 to 17.1±1.3 cm per year, which was slower than the mean rhizome growth of mature plants (21.4±1.0 cm per year). A development time-line was constructed for P. australis for the first 5 years of seedling growth based on the data collected. On average, production of a second shoot occurred at the age of 2.7±0.3 years, and thereafter approximately one shoot was produced per year. Plagiotropic shoots (horizontal rhizomes) did not begin to develop until 4.3±0.7 years.

Seagrasses in Western Australia grow on open coastlines that are wave and surge exposed, which are known to reduce transplant success. This study investigated the use of habitat enhancement procedures to improve the survival and expansion of Posidonia australis transplants. Limited success transplanting P. australis vegetative propagules (plugs) was achieved after habitat enhancement was implemented, using artificial seagrass (ASG) mats to emulate a "surrogate" seagrass meadow. The ASG mats enhanced one aspect of the habitat (stabilised sediment grain size) but did not suitably stabilise erosion and accretion. Transplant survival was significantly increased (up to 50% survival) in 60% of the sites where ASG mats were present. However, only 8% of the total transplant population exhibited rhizome extension (3.2+/-0.11 mmd(-1)).

Vegetative fragment recruitment of both Posidonia australis and Posidonia coriacea was observed on Success Bank, Western Australia, beginning in November 1993 ( n =106). Recruitment of vegetative fragments was defined as attachment to the substrate and subsequent rhizome extension. Rhizome extension occurred in 31% of all recruited P. australis vegetative fragments, yet no extension was detected in recruited P. coriacea vegetative fragments. In shallow water ( P. australis vegetative fragment recruits extended at an average rate of 0.78±0.02 mm per day, but did not survive for more than 10 months. This extension rate was slightly less than in situ meadow rhizomes of established P. australis plants (1.01±0.19 mm per day) from the same depth. The majority (78%) of deep water (10–12 m) P. australis vegetative fragment recruits survived >10 months, with rhizomes extending at slower rates (0.41±0.02 mm per day) than shallow water recruits. No in situ seagrasses grew at the deep site for direct comparison.

The canopy structure and reproductive phenology of the seagrasses Posidonia australis Hook. f. and Posidonia sinuosa Cambridge and Kuo were studied at Warnbro Sound, near Perth, Western Australia. Canopy and inflorescence height, shoot density, leaf area, leaf biomass, inflorescence density and density of other reproductive units were obtained from six (0.04 m 2 ) cores of each of the species. Leaf area and biomass were divided into 5 cm height classes and constructed into an in situ canopy profile. Vegetative shoot density was 913±45 shoots m −2 in the P. australis canopy and 1.475±140 shoots m −2 in the P. sinuosa canopy. Total leaf area index of the P. australis canopy (4.1±0.1 m 2 m −2 ) was less than that of the P. sinuosa canopy (7.0±0.2 m 2 m −2 ). There were fewer flowers in the P. sinuosa canopy (10±2 flowers m −2 ) than in the P. australis canopy (16±2 flowers m −2 ). Mean inflorescence length was 36.5±1.2 cm in the P. australis canopy and 13.4±4.7 cm in the P. sinuosa canopy. Leaf area index at flower height was significantly higher in the P. sinuosa canopy (0.70±0.07 m 2 m −2 ) than the P. australis canopy (0.25±0.02 m 2 m −2 ). Reproductive phenology was studied from surveys of tagged inflorescence in situ. Anther dehiscence in P. australis canopy occurred from mid August to mid September and in the P. sinuosa canopy from early to late September. Pollen/ovule ratios of P. australis were similar to that of P. sinuosa . Pollen viability was longer in P. australis than in P. sinuosa . The similar pollen production but low leaf area around flowers, longer pollination period of P. australis flowers result in greater pollination success than for P. sinuosa flowers.

Changes in seagrass coverage in Cockburn Sound from 1967 to 1999 were assessed from aerial photographs using modern mapping methods with the aim of accurately determining the magnitude of change in hectares of seagrasses between 1967 and 1999 and to set up a baseline for future monitoring of seagrass loss in Cockburn Sound. Firstly, coverage and assemblages of seagrasses in Cockburn Sound were mapped using the best available aerial photographs from 1999, rectified to a common geodesic base with comprehensive groundtruth information, and with a semi-automated mapping algorithm. Then the same technique was used to map historical seagrass coverage in Cockburn Sound from aerial photographs taken in 1967, 1972, 1981 and 1994. The seagrass coverage in Cockburn Sound has declined by 77% since 1967. Between 1967 and 1972, 1587 ha of seagrass, were lost from Cockburn Sound, mostly from shallow subtidal banks on the eastern and southern shores. By 1981, a further 602 ha had been lost. Since 1981, further seagrass losses (79 ha) have been restricted to a shallowing of the depth limit of seagrasses, localised losses associated with port maintenance and a sea urchin outbreak on inshore northern Garden Island. There has been no recovery of seagrasses on the eastern shelf of Cockburn Sound after nutrient loads were reduced in the 1980s, suggesting that this shallow shelf environment has been altered to an environment not suited for large-scale recolonisation by Posidonia species. © 2002 Elsevier Science B.V. All rights reserved.

A seismic survey of southern Jervis Bay (NSW, Australia) carried out in the late 1960s resulted in 11 bare circular areas, or ‘holes’, within an otherwise continuous Posidonia australis seagrass meadow. The area of these holes were mapped using a series of historical aerial photographs, to determine horizontal spreading rates and to estimate recovery times for P. australis. Between the years 1972 and 1997 the total area occupied by the 11 holes decreased by 56%, from 7606 to 3307 m2, due to the spread of P. australis rhizomes from the surrounding bed. Recovery was slow but consistent, although further damage to some bare areas was evident between 1981 and 1989. Spreading rates varied between time periods and between holes, ranging from negative estimates during the period of damage, to a maximum rate of 58 cm per year. Mean spreading rate (±SE) estimated for the 11 holes over the 25-year period was 21(±2) cm per year. Assuming that this mean rate of spreading continues and that there is no further damage to the holes, we estimate that the holes should be completely revegetated at various times over the next century, the earliest being 2034 and the latest 2071.

Changes in seagrass cover on Success and Parmelia Banks, Western Australia between 1965 and 1995 were mapped from aerial photography using changes in the distribution and size of phototonal categories. Aerial photography from 1965, 1972, 1982 and 1995 was used to determine the temporal and spatial changes in seagrass distribution. Aerial photography was rectified using photogrammetric techniques, and manually interpreted at a scale of 1:10 000 to determine the extent of seagrasses and unvegetated sands. The 1995 data were also ground-truthed to determine the seagrass species assemblages. The mapping of seagrass cover using phototones is only feasible for species of seagrasses with a dense leaf canopy. The species that fit this criteria, and that have high cover on Success and Parmelia Banks, are Amphibolis antarctica, Amphibolis griffithii, Posidonia australis, Posidonia sinuosa and Posidonia coriacea. The results of this analysis indicates that the percentage of seagrass cover on Success Bank has increased from 21% (507 ha) in 1965 to 43% (1036 ha) in 1995, whereas on Parmelia Bank the percentage of seagrass cover has remained relatively constant with 46% cover in 1965 (735 ha) and 44% in 1995 (699 ha). The east, central and western regions of Success Bank have all shown an increase in seagrass cover from 1965 to 1995. On Parmelia Bank the seagrass cover on the western region has increased. Whereas, the seagrass cover on the eastern region has decreased. On both Success and Parmelia Banks it appears that the majority of seagrass growth between 1975 and 1995 has been in assemblages that are predominantly single species or mixed species meadows of A. griffithii and P. coriacea. These taxa have previously been considered to be relatively static, not known to colonize over large areas, but this study shows that on Success and Parmelia Banks they are actively colonizing at rates that can be mapped at a scale of 1:10 000.

The aim of this study was to investigate characteristics of some south-western Australian seagrasses that might be useful for further investigation with respect to restoration. Although seeds and fruits of seagrass species may be useful for producing propagules for planting, their collection is based on knowing when they become available. Posidonia australis, P. angustifolia and P. coriacea flower profusely annually, but P. sinuosa does not. Once collected, the fruits dehisce in a few days and are ready for planting. It was found that the best way to plant Posidonia seedlings is in clusters in Growool® blocks but the rhizomes do not spread quickly. Single Posidonia seedlings, when planted out, were not successful and neither were naturally colonizing seedlings of Posidonia sinuosa in unexposed unvegetated areas. Successful planting of seedlings and sprigs of Amphibolis in matting needs months of calm weather for them to spread out from their attaching matting which must be very securely attached to the substratum. Little success was achieved in growing Amphibolis. In a search for Halophila ovalis seeds, few were found because of the patchy nature of their fruiting and small size. Germination was slow but 63% germinated within 6 months.

Posidonia australis seed predation experiments conducted in three seagrass habitats ( P. australis , Posidonia sinuosa , Halophila ovalis ) and bare sand in Two Peoples Bay, Western Australia, showed higher predation rates in seagrass than bare sand, supporting general conclusions from two previous predation studies in Western Australia. However, much higher rates were noted in H. ovalis , compared to previous observations of very low rates in H. ovalis on Rottnest Island, Western Australia. We attribute these differences to gammaridean amphipods (family Lysianassidae) that were present in a detrital layer within the H. ovalis in Two Peoples Bay. Our data from Two Peoples Bay continues to add to the growing body of information showing high seed predation rates in most seagrass habitats by a diverse group of crustacean species.

A hierarchical sampling scheme was used to describe variation in the timing and intensity of flowering of multiple meadows of two species of seagrasses: Posidonia australis and Zostera capricorni. Sampling was stratified across a range of spatial scales: among sites and depths (P. australis only) within individual meadows, among meadows within the same estuary and among estuaries separated by tens of kilometres on the coastline of Central New South Wales, Australia. Four surveys of P. australis were undertaken between 1992 and 1994. Surveys of Z. capricorni were done over a single reproductive season; between September 1992 and March 1993. Flowers of P. australis were found only in August 1993, when they were present in all nine of the meadows examined. Densities of flowering shoots varied significantly among estuaries and at smaller spatial scales within individual meadows. The timing of this unusual flowering event coincided with unseasonably cool autumnal water temperatures. Reproductive shoots of Z. capricorni were also recorded in each of the nine meadows surveyed, but the timing and magnitude of peak abundance varied widely among estuaries and exhibited considerable patchiness within individual meadows. The results suggest that the timing and intensity of flowering of Z. capricorni and P. australis are affected by processes that operate over at least three spatial scales. Initiation of flower production appears to be triggered by regional (tens to hundreds of kilometres) changes in environmental conditions, such as water temperature or photoperiod, whereas the abundance of flowers varied significantly among estuaries and may be influenced more by both meso-scale (kilometres) processes within estuaries and by local conditions within each meadow (metres to tens of metres).

Above-ground primary production and nutrient fluxes (N and P) were investigated for two species of seagrass, Posidonia sinuosa Cambridge et Kuo and P. australis Hook. f. from Warnbro and Cockburn Sounds over an annual cycle, at sites ranging in depth from 0.5–10 m where P. sinuosa formed either single-species stands or co-occurred with P. australis . Annual leaf primary production ranged from 600 to 900 g m −2 yr −1 in P. sinuosa and 900–1100 g m −2 yr −1 in P. australis , and epiphytes on the leaves produced 130–160 g m −2 yr −1 . In some patches, flowering shoots and fruits also made a substantial contribution, up to 160 g m −2 yr −1 . Annual above-ground productivity (dry weight production per unit ground area) of Posidonia spp. (600–1300 g m −2 yr −1 ) is similar to that of Amphibolis antarctica (Labill.) Sonder et Aschers. ex Aschers. and A. griffithii (Black) den Hartog, two species from the other genus of large seagrasses in south-western Australia, but only 30 to 50% of that of the kelp Ecklonia radiata (C. Ag.) J. Agardh. (3500 g m −2 yr −1 ). Nitrogen and phosphorus incorporated annually into new leaf tissue ranged from 9–17 g N and 1.1–1.7 g P m −2 yr −1 , respectively, depending on species and site. Estimates of annual nutrient losses via leaf detritus ranged from 5–9 g N and 0.4–0.7 g P m −2 yr −1 , compared to maximum losses of 1.2 g N and 0.4 g P m −2 yr −1 via the fruits at the highest density of flowering shoots (223 m −2 ). Thus, annual nutrient losses via leaf detritus represent a considerable proportion of the nutrients incorporated annually into new growth, indicating a lower degree of nutrient conservation than might be expected in a low nutrient environment.

To assist in formulating a model of seagrass growth, the photosynthesis-irradiance (P-I) relationships of Posidonia sinuosa, Posidonia australis, Amphibolis griffithii and Amphibolis antarctica were determined by oxygen flux measurements at a range of light intensities and water temperatures. Net and gross photosynthesis and respiration rates generally increased in response to increasing temperature within the range 13–23°C and the observed trends suggest that the optimum temperature range for photosynthesis is between 18°C and 23°C for P. sinuosa and 23°C or greater for the other species. The geographic distributions of the seagrass species studied here are generally consistent with their respective metabolic responses to water temperature. In general, the genus Amphibolis has higher maximum photosynthetic rates, and these are reached at lower light intensities, than Posidonia. Comparisons of the metabolic rates and critical light requirements of P. sinuosa between summer and winter suggest that ambient environmental conditions rather than seasonal changes in physiology are primarily responsible for the seasonal differences in growth rates that occur in the field. P. sinuosa plants growing near their natural lower depth limit are morphologically different and their maximum photosynthetic rates are reached at lower light intensities than plants growing in shallower water, but genotypic rather than phenotypic adaptation is suggested as the cause.

Oxygen exchange of leaf segments of the seagrass Posidonia australis Hook, F. was used to assess the sources of inorganic carbon (Ci) available to photosynthesis. Under light saturation (500 μmol m−2 s−1) net photosynthesis varied from 953 nmol m−2 s−1 at pH 7.5 to 82 nmol m−2 s−1 at pH 9.0. There was no affect of pH on respiration. Only at pH 9.0 was it necessary to invoke an uptake of HCO3− based on an unstirred layer of 50 μmol. A model is discussed in which the state of equilibrium between the four forms of Ci in and around the cell wall is determined by the rate of OH− production from the dehydration of HCO3− and the rate of OH− efflux away from the leaf surface.

Isopods of the genus Lynseia have been found burrowing (leaf mining) in meadow forming seagrasses (Posidonia) from Western Australia. These small (1.5–1.8 mm) isopods burrow beneath the epidermis consuming the mesophyll, forming a linear mine of similar width to the isopod along the leaf lamina. At Rottnest Island in January 1991, 75% of Posidonia australis Hook. f. and 66% of Posidonia sinuosa Cambridge and Kuo leaves were burrowed by isopods. Isopods were found in leaves of all sizes, but number of burrows, number of isopods and the length of burrows increased in older leaves. In P. australis, there were 3.15 ± 0.1 burrows and 2.6 ± 0.1 isopods per burrowed leaf, in P. sinuosa there were 2.9 ± 0.1 burrows and 2.48 ± 0.2 isopods per burrowed leaf. Isopods were mobile, vacating older burrows and forming new ones. Male and female isopods were found in pairs within burrows and young isopods were then brooded within the burrows, forming new branches from the parent burrow. Within monospecific stands of P. australis and P. sinuosa there were 2950 and 2280 isopods m−2 respectively. Mean length of individual burrows was 22 mm in P. australis and 28 mm in P. sinuosa. Total burrow length per leaf (mean ± SE) was 69.6 ± 3.6 mm in P. australis and 81.7 ± 4.7 mm in P. sinuosa. The consumption of leaf tissues and the resulting lines of weakness along burrow lines may have negative effects on the seagrass. Burrows facilitate entry of water, bacteria and fungi to the blade, leading to localised tissue death. Epiphytes may proliferate along burrow lines, owing to surface roughness or solute loss associated with the burrow. Burrows also split and increase fragmentation of leaves in storm events. Loss of photosynthetic area due to epiphytic growth and fragmentation of the leaves may reduce the growth of Posidonia. These potential effects of lynseid seagrass leaf miners may be important if the seagrass meadow was already under stress.

A technique is described for measuring the metabolic response of seagrasses to different light intensities and water temperatures in the laboratory. The observed depth distribution of the seagrass species can be related to their light requirements and metabolic rates. The photosynthetic efficiency and maximum photosynthetic rates of Amphibolis griffithii (Black) den Hartog were significantly higher than those of Posidonia sinuosa Cambridge & Kuo and Posidonia australis Hook.f. The optimum temperature range for net photosynthesis of Posidonia sinuosa was 18–23°C and photoinhibition occurred at relatively low temperature and high light intensity. Maximum gross photosynthetic rates of P. sinuosa in an attenuated light regime simulating that experienced by plants in the field were higher than in a uniform light field. The respiration rates of the leaves were four to seven times higher than those of the root/rhizome component in the Posidonia species. There was no evidence for physiological adaptation to low light environments, using P. sinuosa from different depths and with different epiphyte loadings.

Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae (Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm−2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm−2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O2 cm−2 h−1 and 2.0 μg O2 cm−2 h−1, respectively. Daily net productivity was 14 μg C cm−2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m−2 day−1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Rhizomes of Posidonia australis Hook. f., collected from Botany Bay on the eastern coast of Australia, were analyzed for extractable carbohydrates. The concentration of extractable carbohydrate in the stelar tissue of the rhizome was significantly higher than that in the surrounding cortex. Reduced concentrations of extractable carbohydrate reserves were found in juvenile tissue and tissue adjacent to the apical meristem. Unexpanded internodes contained higher concentrations of extractable carbohydrate than expanded internodes. Tissue from equivalent positions along both the lateral and parental axis of a rhizome branch showed similar extractable carbohydrate levels and had similar patterns of internodal length. The pattern of carbohydrate storage within the rhizomatous tissue of P. australis is closely related to the age and health of the plant.

Long-term changes of seagrass meadows ( Posidonia australis Hook. f. and Zostera capricorni Aschers.) in Botany Bay, Australia, were investigated using historical aerial photography (1930–1985), field observations (1970–1987) and in situ sediment coring for fibrous remains of Posidonia . Aerial photography and field observations indicated that P. australis had undergone a steady decline in distribution in Botany Bay over the past 40 years. Between 1942 and 1984, 58% (257 ha) of Posidonia was lost from the bay's southern foreshores. By 1987, the once continuous meadow of Posidonia consisted of a number of fragmented beds. The decline occurred during a period of industrial and residential development in the catchment, which included the dredging of the bay's entrance to allow passage of large container ships. The dredging caused an increased wave height in many parts of the bay, and resulted in deterioration and erosion of seagrass beds, particularly during storm events. As an example of this process, the effect of minor storms in reducing leaf biomass of Posidonia by 70% is reported. Grazing by the sea urchin, Heliocideras erythrogramma (Valenc.), between 1980 and 1986 was responsible for smaller losses of Posidonia . The existence of fibrous remains in sediment cores offered circumstantial evidence that Posidonia had a much wider distribution in Botany Bay prior to the earliest aerial photography. A history of poor catchment management, uncontrolled effluent disposal and widespread dredging is suggested as the cause for these earlier, unquantified losses. Over the same period (1930–1987), Z. capricorni was found to have undergone cyclical fluctuations in area throughout the bay, and had colonised many sites that were previously vegetated with Posidonia .

Experimental seagrass transplant trials were carried out to assess the feasibility of a proposed large scale transplant programme in Botany Bay, Australia. A variety of methods of transplanting Zostera capricorni and Posidonia australis at three depths were evaluated. In addition, small scale historical seagrass mapping was carried out to determine site suitability. 1800 seagrass ‘units’ were transplanted. For both species and at all depths, survival of transplants was highest when simple wire anchors were used. Although the transplanting seemed successful and plants showed some initial growth, they were destroyed by storms after only 4 months. Consequently, restoration of the seagrass meadows in Botany Bay is considered impractical without accompanying major engineering works to reduce the effect of storm erosion, which has been exacerbated by previous large scale dredging and port construction. The applicability of seagrass transplanting as a means of compensating for habitat loss is also discussed.

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.

Stratigraphic and chronologic studies of the coastal sediments of northern Spencer Gulf in the vicinity of Redcliff are described. Two Pleistocene marine units are recognised beneath an extensive Holocene peritidal sequence. The Holocene sequence is dominated by sediments of the Posidonia australis seagrass facies which form a distinctive carbonate bank fringing the coastline. Peritidal sedimentation commenced prior to 6600 radiocarbon yrs B.P. and a + 2.5 m higher relative sea level existed from about 6000 until around 1700 yrs B.P. The subsequent relative fall to present level probably resulted from local tectonic uplift. After the regression, mangrove and samphire colonization, and beach ridge progradation, occurred over the former seagrass bank.

Two new species of seagrass, Posidonia sinuosa and Posidonia angustifolia, from southern Australia, are described. These species are sympatric with the broad leaved Posidonia australis Hook. f., under which name they were previously known as “narrow leaved” forms. Morphological, anatomical and ecological data are presented as evidence for the separation of the species. The principal differentiating characters are the shape of the epidermal cells, the position of fibre cells in the leaf and leaf sheath, and the presence around the rhizome of straw-like fibres, derived from the disintegrating leaf sheaths. The species also differ in their habitat requirements, growth patterns and depth distribution.

The budget and fate of organic carbon from the leaves of the seagrass Posidonia australis Hooker f. were studied in a small sunken river valley in Port Hacking, New South Wales. Standing stock and leaf growth were measured over 12-month periods. Estimated average relative leaf growth was 2.3 mgC g dry wt. −1 day −1 . Estimated losses totalled 2.6 mgC g dry wt. −1 day −1 of which 48% was in the form of dissolved organic carbon, while grazing by herbivores (3%), leaves floating off (12%) and sinking leaves (37%) accounted for the remainder of the carbonaceous material lost from the seagrass leaves.

Twelve species of seagrass were found in Shark Bay (26°S, 114°E), forming some of the largest seagrass meadows reported. The distribution of the species within the bay and descriptions of typical habitat types are given. The area is dominated by Amphibolis antarctica (Labill.) Sonder ex Aschers. which covers 3700 km2, approximately 85% of the area covered by seagrasses, with smaller areas of Posidonia australis Hook. f. (200 km2). Smaller seagrasses occupy an additional 500 km2, which includes patches of high species richness, with up to 9 species within a few m2. Factors influencing the distribution of seagrasses within Shark Bay are discussed.

The vegetative structures of Posidonia sinuosa Cambridge & Kuo have been studied and are compared to those of Posidonia australis Hook. f. and, where possible, the ecological significance of structures is considered. Each shoot of P. sinuosa bears 1–2 leaves which grow more slowly than those of P. australis. The leaf cuticle has a uniformly low electron density and is not porous in appearance. The elongated leaf blade epidermal cells have a rugulose surface and sinuose outline, but have no cell wall ingrowths. Microfibrillar arrangement, in the inner portion of the distal wall and the distal portion of the radial wall in leaf epidermal cells, appears to be a source of mechanical weakness resulting in longitudinal splitting of the leaf blades when they are subjected to the stress of strong wave action, so that the species is usually restricted to calmer habitats. Only a few fibre bundles are present in the parenchyma tissues of leaf sheaths, so that P. sinuosa does not form marine fibre deposits. Anatomy and ultrastructure of rhizome and root in both P. sinuosa and P. australis are similar. Posidonia sinuosa dominates habitats with limited sand influx which appears to be related to its capacity for limited rhizome growth.

1. 1. The lipid components of three animals, the rock crab Nectocarcinus integrifons , the rock flathead Platycephalus laevigatus and the southern garfish Hyporhamphus melanochir , feeding in the seagrass beds at Corner Inlet, Victoria, Australia have been examined in detail in order to provide further information on seagrass community structure. 2. 2. Biological marker compounds detected within animal gut content material were used to recognize dietary sources and then utilized by community members. 3. 3. Both H. melanochir and N. integrifons have been shown to ingest and to varying degrees incorporate seagrass lipid material, thus further confirming the importance of seagrass carbon in the Corner Inlet environment. 4. 4. The southern sea garfish H. melanochir is observed to remove C 18 PUFAs (polyunsaturated fatty acids) from ingested seagrass material. 5. 5. Seagrass sterols are altered during incorporation into the lipids of this fish. 6. 6. Lipid-rich digestive juices play a role in the digestive processes of all three animals. 7. 7. Components tentatively identified as (NMI) (non-methylene interrupted) fatty acids have been detected in the lipids of the garfish H. melanochir and the crab N. integrifons . 8. 8. The fecal material of all three animals represent possible sources of these lipids (NMI acids) in Corner Inlet sediments. 9. 9. Based on lipid compositional data, N. integrifons feeds on Posidonia australis detritus and associated epiphyte material. 10. 10. The removal of both plant and epibiota cellular lipids along the digestive tract of the crab was observed, although structural components such as long chain mono- and α,ω-dicarboxylic acids, which have been previously recognized as seagrass marker lipids are not directly absorbed.

This study examines whether relationships between abundances of fishes and decapods and the leaf height and density of seagrass determined from small scale field experiments within a seagrass bed were also evident at a wider spatial scale, i.e. among beds within a bay. Separate comparisons were made for common species of fishes and decapods associated with the seagrasses Zostera capricorni Aschers, and Posidonia australis Hook f. Of the 23 significant responses by species to leaf height or density in the small scale experimental study, 15 were not repeated at the wider scale. Abundances of species with inconsistent responses at the two scales were not correlated meaningfully with other major environmental variables such as sediment size, area of seagrass bed or distance from the ocean. We reconcile the high incidence of effects of leaf height and density within beds, and their scarcity among beds, with a model of how postlarvae settle into, and move within, seagrass beds. The model holds that (1) larvae of fishes and decapods are distributed patchily when ready to settle, (2) larvae do not discriminate among beds when they settle, and (3) individuals do not leave a bed soon after settling but do redistribute within the bed to select microsites favouring survival.

Epiphyte biomass on Posidonia australis Hook f. and artificial seagrass leaves was analysed in relation to distance from leaf tip at different time periods following the autumnal shedding. The distribution pattern indicated no significant difference in the exponential nature of biomass increase. Similarities between epiphyte biomass distribution on real and artificial leaves is consistent with the hypothesis that biomass distribution on real leaves is not influenced significantly by metabolic interactions between host plant and epiphytes. Although a time component is involved in epiphyte biomass accumulation, the major factor in determining epiphyte biomass distribution is the relative position along the leaf.

The fish community present in a 400-m 2 area of seagrass meadow dominated by Posidonia australis Hook f. was quantitatively sampled during two springs and two autumns by enclosure and poisoning with rotenone. Thirty-nine species were collected and these were classified into permanent residents (∼ 50% of total numbers), temporary residents (∼ 30%) and transients (∼20%). Approximately 30% of the total numbers of fish collected belonged to species of some economic importance. Species diversity indices were calculated and compared between seasons, but no obvious differences were found. Shannon's H ′ (heterogeneity index) ranged from 2.11 to 2.56, Gleason's d (richness index) ranged from 3.08 to 5.05, and scaled H (eveness index) ranged from 0.59 to 0.74. Stomach contents of all fish were analysed by an estimated volumetric method. Agglomerative polythetic classification to the average diets of all species collected indicated 7 basic feeding groups within the community. Many species preferred crustaceans as food (this latter group comprising approximately 50% of the overall community diet), and several species consumed seagrass (approximately 10% of the overall community diet). Upper water column dwellers were mainly microcrustacean feeders; canopy dwellers fed primarily on crustaceans, seagrass and algae; benthic dwellers consumed mainly crustaceans and polychaetes.

The ash, carbon, hydrogen, nitrogen, protein and proteinic and free amino acids content of Posidonia australis Hook.f. has been assessed and compared with data obtained from similar analyses on Posidonia oceanica (L.) Delile. Selected comparisons have also been made with Cymodocea nodosa (Ucria) Aschers., Thalassia testudinum Banks ex Konig, Halophila stipulacea (Forsk.) Aschers., Zostera marina L. and Zostera noltii Hornemann. Amino acid content has also been compared with that recorded forlucerne, and hay composed of several species of Gramineae.