Marion L. Cambridge

The loss of seagrasses in Cockburn Sound, Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development

The areas of seagrass meadows in Cockburn Sound, a marine embayment in Western Australia, were estimated from historical aerial photographs supplemented by ground surveys, studies on meadows in adjoining areas, and coring for rhizome remains. Ten species of seagrasses with different habitat tolerances are recorded for the area, with Posidonia sinuosa Cambridge et Kuo forming the most extensive meadows. It is estimated that from 1954 to 1978 the meadow area was reduced from some 4200 to 900 ha. Based on measurements of aboveground productivity at several sites, this represents a reduction of leaf detritus production from 23 000 to 4000 t (dry wt.) y−1. The major loss of seagrass occurred during a period of industrial development on the shore, and the discharge of effluents rich in plant nutrients.

The loss of seagrass in Cockburn Sound, western Australia. II: Possible causes of seagrass decline

This paper examines possible reasons for the extensive loss of seagrass in Cockburn Sound following industrial development. Transplanted seedlings survived poorly in Cockburn Sound compared with an adjoining embayment. Altered temperature, salinity, sedimentation and water movement do not explain the death of seagrass over wide areas, and there is no evidence for a role of pathogens. Oil refinery effluent reduced seagrass growth in aquaria at concentrations similar to those at the point of discharge, but could not account for the widespread deterioration observed in the field. Severe grazing by sea urchins was observed on meadows already under stress and does not appear to be a primary cause of decline; caged, transplanted seedlings also deteriorated. Increased light attenuation by phytoplankton blooms may have affected the ddepth to which seagrasses could survive, but would have had little significant effect in shallow water; marked phytoplankton blooms were recorded only after extensive seagrass decline had taken place. Light reduction by enhanced growth of epiphytes and loose-lying blankets of filamentous algae in nutrient enriched waters is suggested as the most likely cause of decline. Heavy epiphyte fouling was consistently observed on seagrasses in deteriorating meadows, as well as on declining, transplanted seedlings, and is known to significantly impair photosynthesis in other systems. Extensive seagrass decline coincided with the discharge of effluents rich in plant nutrients.

The Central Role of Dispersal in the Maintenance and Persistence of Seagrass Populations

Global seagrass losses parallel significant declines observed in corals and mangroves over the past 50 years. These combined declines have resulted in accelerated global losses to ecosystem services in coastal waters. Seagrass meadows can be extensive (hundreds of square kilometers) and long-lived (thousands of years), with the meadows persisting predominantly through vegetative (clonal) growth. They also invest a large amount of energy in sexual reproduction. In this article, we explore the role that sexual reproduction, pollen, and seed dispersal play in maintaining species distributions, genetic diversity, and connectivity among seagrass populations. We also address the relationship between long-distance dispersal, genetic connectivity, and the maintenance of genetic diversity that may enhance resilience to stresses associated with seagrass loss. Our reevaluation of seagrass dispersal and recruitment has altered our perception of the importance of long-distance dispersal and has revealed extensive dispersal at scales much larger than was previously thought possible.

Changes in seagrass coverage in Cockburn Sound, Western Australia between 1967 and 1999

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.

ANNUAL PRIMARY PRODUCTION AND NUTRIENT DYNAMICS OF THE SEAGRASSES POSIDONIA SINUOSA AND POSIDONIA AUSTRALIS IN SOUTH-WESTERN AUSTRALIA

Abstract 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.

Global analysis of seagrass restoration: the importance of large-scale planting

In coastal and estuarine systems, foundation species like seagrasses, mangroves, saltmarshes or corals provide important ecosystem services. Seagrasses are globally declining and their reintroduction has been shown to restore ecosystem functions. However, seagrass restoration is often challenging, given the dynamic and stressful environment that seagrasses often grow in. From our world-wide meta-analysis of seagrass restoration trials (1786 trials), we describe general features and best practice for seagrass restoration. We confirm that removal of threats is important prior to replanting. Reduced water quality (mainly eutrophication), and construction activities led to poorer restoration success than, for instance, dredging, local direct impact and natural causes. Proximity to and recovery of donor beds were positively correlated with trial performance. Planting techniques can influence restoration success. The meta-analysis shows that both trial survival and seagrass population growth rate in trials that survived are positively affected by the number of plants or seeds initially transplanted. This relationship between restoration scale and restoration success was not related to trial characteristics of the initial restoration. The majority of the seagrass restoration trials have been very small, which may explain the low overall trial survival rate (i.e. estimated 37%). Successful regrowth of the foundation seagrass species appears to require crossing a minimum threshold of reintroduced individuals. Our study provides the first global field evidence for the requirement of a critical mass for recovery, which may also hold for other foundation species showing strong positive feedback to a dynamic environment.Synthesis and applications. For effective restoration of seagrass foundation species in its typically dynamic, stressful environment, introduction of large numbers is seen to be beneficial and probably serves two purposes. First, a large-scale planting increases trial survival - large numbers ensure the spread of risks, which is needed to overcome high natural variability. Secondly, a large-scale trial increases population growth rate by enhancing self-sustaining feedback, which is generally found in foundation species in stressful environments such as seagrass beds. Thus, by careful site selection and applying appropriate techniques, spreading of risks and enhancing self-sustaining feedback in concert increase success of seagrass restoration.

Seagrasses of south-western Australia with special reference to the ecology of Posidonia australis Hook f. in a polluted environment

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.

TWO NEW SPECIES OF SEAGRASSES FROM AUSTRALIA, POSIDONIA SINUOSA AND P. ANGUSTIFOLIA (POSIDONIACEAE)

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.

Morphology, anatomy and histochemistry of the Australian seagrasses of the genus Posidonia könig (posidoniaceae). II. Rhizome and root of Posidonia australis Hook. f

Abstract Rhizome branching of Posidonia australis Hook. f. occurs at irregular intervals in leaf axils, with one or no branch per axil. The cell walls of the rhizome epidermis and hypodermis are slightly thickened and lignified. Cortical cells have many starch grains. Fibre strands and vascular bundles are scattered among the cortical tissues. The cell walls of rhizome fibre cells consist mainly of cellulose and hemicellulose with a little lignin but the middle lamellae are heavily lignified. The fibres of both leaf sheath and rhizome resist decay and build up banks beneath the living seagrass. The rhizome stele has a central xylem surrounded by the phloem bundles which are finally surrounded by the endodermis. At the node region, one or a pair of roots may be produced. The thickened hypodermal walls of the root have a suberin lamella. The radial walls of the root endodermis are slightly thickened and have a Casparian strip. Both hypodermis and endodermis may restrict exchange of the water and solutes. A vascular system with weakly lignified tracheids is present in all roots.

The nitrogen and phosphorus nutrition of developing plants of two seagrasses, Posidonia australis and Posidonia sinuosa

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 plants N and P was associated with dead and morinund tissue. Leaf bases lost 84% of their N and 95% of their P during senescence.