case study 18.3  Seagrasses: successful marine macrophytes

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A.J. McComb and W.C. Dennison

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Figure 1 Distribution of seagrasses along the Australian coastline, distinguishing temperate form tropical (grazed and ungrazed) species. More specific ecosystems are identified such as the estuarine seagrasses of south-eastern Australia. Relative sized of the main genera are depicted, ranging from the small-statured, grazed species to the tall temperate seagrass genera of southern Australia. (Courtesy W.C. Dennison)

Australia has a high diversity of seagrasses in its coastal waters, with 38 species out of a worldwide total of 66 species (Larkum et al. l989). Most intertidal and subtidal habitats contain at least one seagrass species. Broadly speaking, sea-grasses can be categorised as tropical (grazed and ungrazed) and temperate (Figure 1).

Seagrasses of tropical Australia

Large herbivores such as dugongs (Dugong dugong) and green sea turtles (Chelonia mysas) graze on some tropical seagrass meadows heavily, feeding on leaves and/or rhizomes. Repeated grazing promotes seagrass communities dominated by small, fast-growing genera with high reproductive potential for rapid recolonisation (e.g. Halophila, Halodule). Such seagrass communities are common in the tropics; herbivores are only excluded from intertidal areas, turbid water or areas without access to deep water. In those tropical waters without herbivores, larger, slow-growing genera dominate (e.g. Enhalus, Thalassia, Cymodocea, Syringodium).

Australia’s tropical waters fall into three major zones: the northwest coast, Gulf of Carpenteria and Great Barrier Reef. The northwest coast has large tides and turbid waters, and thus seagrass communities are sparse and generally restricted to intertidal pools or lagoons. The Gulf of Carpenteria has extensive seagrass beds, particularly on the western side, which are regularly affected by cyclones. The Great Barrier Reef has extensive deepwater seagrass beds dominated by Halophila spp. growing between reefs. There are also scattered intertidal and shallow subtidal seagrasses along the coast or on reef flats. Extensive seagrass meadows are also found in Shark Bay on the west coast and Hervey Bay, Queensland; both bays are transitional between tropical and temperate waters.

Seagrasses of temperate Australia

Temperate seagrasses in Australia are rarely affected by grazing. Large, robust and relatively slow-growing genera occur around southern, temperate Australia, most notably in the genera Posidonia and Amphibolis (Figure 1). These genera, as well as another common temperate seagrass, Zostera, support large epiphyte communities that contribute to productivity of southern seagrass communities. Detritus to support epiphytes comes from slow decomposition of very fibrous leaves typical of Posidonia, Amphibolis and Zostera. Temperate regions extend across the southwest, south and southeast coasts of Australia. The southwest coast, extending from Shark Bay to the Great Australian Bight, has a series of offshore limestone reefs which provide protected waters ideal for seagrass meadows. This region is especially species rich and is considered a centre for relatively recent speciation in some genera. Several species of Posidonia and two endemic species of Amphibolis have centres of diversity in this southwest region. Seagrasses along the south coast of mainland Australia and Tasmania are restricted to bays and waters protected by headlands from Southern Ocean storms. For example, Spencer Gulf in South Australia is a vast protected embayment and contains extensive seagrass meadows. Southeast Australia has a series of estuaries with seagrasses growing in the saline waters vulnerable to human impacts.

Seagrass productivity

Seagrasses have low rates of photosynthesis per unit of leaf material but have dense leaf canopies, making them highly productive ecosystems. Seagrass meadows generate biomass about three times faster than an average crop system, placing them alongside tropical and temperate forests as the most productive ecosystems known (Whittaker 1975). High productivity is achieved in seagrass meadows through mechanisms such as those described in Section 18.2.2 — for example CO2 recycling, nutrient capture from suspended detritus particles and reduced self-shading in dense canopies as a result of water turbulence. Seagrass meadows and fast-growing forests have as much as 20 m2 of leaf surface to each square metre of seabed or earth, contrasting with agricultural crops where leaf area indices fall in the range 1–10 m2 m–2.

Rapid leaf turnover and propagation of new individuals both contribute to high productivity of seagrass communities, particularly the fastest-growing, small-statured seagrass species. Individual plants produce a new leaf about every 7 d, followed by elongation of the leaf at a rate of 2–5 cm d–1. Hence productivity of seagrass meadows, converted to daily carbon increment, reaches 4 g carbon m–2 d–1. By turning over leaves rapidly, seagrasses avoid excessive epiphyte loads that would otherwise restrict light harvesting. As old leaves decay, inorganic nutrients are efficiently reabsorbed to sustain new growth. Reduced carbon from detritus and organic matter excreted from photosynthesising leaves and anaerobic roots stimulate recycling by providing substrates for microbes in sediments.

Reproduction

Reproductive capacity, identified as a feature of the success of seagrasses, is achieved through a suite of vegetative and sexual mechanisms. Asexual (vegetative) reproduction gives rise to new clonal individuals through rhizome growth, akin to that in many wetland species. Once new shoots (ramets) initiated at nodes on a rhizome become photosynthetically autonomous, the rhizome decays leaving a new individual to extend the colony.

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Figure 2. Pollen release under water from male flowers of Halophila capricornia, showing the pollen assemblage that rises from self-association of individual pollen grains. (Photograph courtesy Seagrass Ecology Group, Northern Fisheries Centre, Queensland Department of Primary Industries)

However, genetic analysis shows that seagrass colonies are not entirely clonal, suggesting that a degree of sexual re-production occurs. Indeed very small flowers can be found with some difficulty on seagrasses, often dioecious (separate male and female flowers) and therefore heavily outcrossing. Pollen is a threadlike structure about 2 mm long (Figure 2) which adheres to a water-insoluble matrix on the receptive female stigma to achieve fertilisation. The mechanisms by which this thread of pollen reaches a flower constitute exquisite adaptations to the marine environment. Three modes of transport have been reported.

First is surface water pollination which occurs within a few hours during the year’s lowest tide. Buoyant pollen is released, floats to the water surface and forms an interconnected raft which attaches to any female stigma at the surface. The second mechanism of fertilisation entails pollen threads associating at the surface of the sediment in a strand up to a metre long; if this strand encounters a stigma, fertilisation can take place. A third mechanism (hydrophilous pollination) involves release of pollen into the water surrounding seagrass plants and occasional, random fertilisation when pollen drifts onto flowers. The chances of hydrophilous pollination are therefore low.

Sexual reproduction combines with dispersal of seagrass seeds to produce genetic diversity. Seeds are carried in water currents, float through buoyancy conferred by attached bubbles and pass through the gut of grazing animals. Seeds can then germinate in a new colony or lie dormant, providing a seed bank for later recruitment. In this way, seagrasses have devel-oped a robust reproductive strategy ensuring that new individuals with some degree of genetic diversity are perpetually being added to a seagrass community.

References

Larkum, A.W.D., McComb, A.J. and Shepherd S.A. (eds) (1989). Biology of the Seagrasses. A Treatise on the Biology of Seagrasses with Special Reference to the Australian Region, Elsevier: Amsterdam

Whittaker, R.H. (1975). Communities and Ecosystems, 2nd edn, Macmillan: New York

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