The cycle of seagrass life: From flowers to new meadows.

Understanding sexual reproduction and recruitment in seagrasses is crucial to their conservation and restoration. Flowering, seed production, seed recruitment, and seedling establishment data for the seagrass Posidonia australis was collected annually between 2013 and 2018 in meadows at six locations around Rottnest Island, Western Australia. Variable annual rates of flowering and seed production were observed among meadows between northern and southern sides of the island and among years. Meadows on the northern shore consistently flowered more intensely and produced more seeds across the years of the survey. Inter-site variation in clonal diversity and size of clones, seed production, wind and surface currents during pollen and seed release, and the large, but variable, impact of seed predation are likely the principal drivers of successful recruitment into established meadows and in colonizing unvegetated sands. The prolific but variable annual reproductive investment increases the probability of low levels of continuous recruitment from seed in this seagrass, despite high rates of abiotic and biotic disturbance at seedling, shoot, and patch scales. This strategy also imparts a level of ecological resilience to this long-lived and persistent species.

Modelling seagrass growth and development to evaluate transplanting strategies for restoration.

BACKGROUND AND AIMS: Seagrasses are important marine plants that are under threat globally. Restoration by transplanting vegetative fragments or seedlings into areas where seagrasses have been lost is possible, but long-term trial data are limited. The goal of this study is to use available short-term data to predict long-term outcomes of transplanting seagrass. METHODS: A functional-structural plant model of seagrass growth that integrates data collected from short-term trials and experiments is presented. The model was parameterized for the species Posidonia australis, a limited validation of the model against independent data and a sensitivity analysis were conducted and the model was used to conduct a preliminary evaluation of different transplanting strategies. KEY RESULTS: The limited validation was successful, and reasonable long-term outcomes could be predicted, based only on short-term data. CONCLUSIONS: This approach for modelling seagrass growth and development enables long-term predictions of the outcomes to be made from different strategies for transplanting seagrass, even when empirical long-term data are difficult or impossible to collect. More validation is required to improve confidence in the model's predictions, and inclusion of more mechanism will extend the model's usefulness. Marine restoration represents a novel application of functional-structural plant modelling.

Enhancing growth of mangrove seedlings in the environmentally extreme Arabian Gulf using treated sewage sludge.

The response of mangrove (Avicennia marina) seedlings to treated (wet) sludge from a sewage treatment plant (STP) was tested in a randomized block design experiment at a tree nursery on Mubarraz Island in the Arabian Gulf. The growth response of seedlings to half-strength and full-strength STP sludge was monitored over 103 days and compared with the response to freshwater, seawater and half-strength seawater treatments. Sludge treatments resulted in significantly greater plant growth, leaf number, leaf biomass and root biomass than the other treatments did. The positive effect of STP sludge on seedling growth is attributed to enhanced levels of total nitrogen (8.9 ± 0.1 mg l-1) and total phosphorus (7.8 ± 0.2 mg l-1) in the sludge and its low salinity. These results suggest that sludge from sewage treatment plants may be beneficially used in mangrove nurseries and plantations in this arid region, where soils are nutrient-poor and fresh water is scarce.

Effects of desalination brine and seawater with the same elevated salinity on growth, physiology and seedling development of the seagrass Posidonia australis.

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.

A novel adaptation facilitates seed establishment under marine turbulent flows.

Seeds of Australian species of the seagrass genus Posidonia are covered by a membranous wing that we hypothesize plays a fundamental role in seed establishment in sandy, wave swept marine environments. Dimensions of the seed and membrane were quantified under electron microscopy and micro-CT scans, and used to model rotational, drag and lift forces. Seeds maintain contact with the seabed in the presence of strong turbulence: the larger the wing, the more stable the seed. Wing surface area increases from P. sinuosa < P. australis < P.coriacea correlating with their ability to establish in increasingly energetic environments. This unique seed trait in a marine angiosperm corresponds to adaptive pressures imposed on seagrass species along 7,500 km of Australia's coastline, from open, high energy coasts to calmer environments in bays and estuaries.

Analysis of differences in photosynthetic nitrogen use efficiency of alpine and lowland Poa species.

This study investigates factors determining variation in photosynthetic nitrogen use efficiency (φN) in seven slow- and fast-growing Poa species from altitudinally contrasting sites. The species and their environmental origin were (in order of increasing relative growth rate): two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina) and three temperate lowland perennials (P. pratensis, P. compressa and P. trivialis), as well as one temperate lowland annual (P. annua). Plants were grown hydroponically under identical conditions with free access to nutrients in a growth room. Photosynthesis per unit leaf area measured at growth irradiance (500 µmol m-2 s-1) was slightly higher in the slow-growing alpine species. At saturating light intensities, photosynthesis was considerably higher in the alpine species than in the lowland species. Carboxylation capacity and Rubisco content per unit leaf area were also greater in the alpine species. Despite variation between the species, the in vivo specific activity of Rubisco showed little relationship to relative growth rate or photosynthetic rate. Both at light saturation and at the growth irradiance, φN was lowest in the slow-growing alpine species P. fawcettiae, P. costiniana and P. alpina, and highest in the fast-growing P. compressa and P. annua. The proportion of leaf nitrogen that was allocated to photosynthetic capacity and the in vivo catalytic constant of Rubisco accounted for most of the variation in φN at light saturation. Minor variations in intercellular CO2 partial pressure also contributed to some extent to the variations in φN at light saturation. The low φN values at growth irradiance exhibited by the alpine species were additionally due to a lower percentage utilisation of their high photosynthetic capacity compared to the lowland species.