Ecology

Seagrass meadows are very productive, support complex food webs and are valued as a habitat and refuge for a number of organisms.

Seagrasses have evolved adaptations to survive in marine environments including salt tolerance and resistance to the energy of waves (rhizomes and roots firmly anchor seagrasses to the sediments and flexible blades offer little resistance to water movement.

Bacteria and fungi are responsible for the decomposition of dead seagrass blades. Very small animalscolonize the dead seagrass blades, feeding on the bacteria and fungi as well as on the dissolved organic matter released from the decomposing blades. This dissolved organic matter also support phytoplankton and zooplankton which in turn provide prey for organisms further up the food web.

Air canals in the leaves carry oxygen down to the rhizomes and roots. The leaf contains a layer of green pigment (chlorophyll) which captures light to provide energy through photosynthesis for their growth.

Seagrass flowers are pollinated underwater by pollen which is carried in water currents. Flowers may be monoecious (male and female elements together) or dioecious (separate male and female flowers). Seeds are produced after flowering and may float considerable distances before they sink and germinate.

Most seagrass pollen is denser than water and will sink slowly after being released from the male flower. To increase dispersal some seagrass use currents or release pollen from floral spikes high above the sea floor.

Seagrasses can also grow by asexual (or vegetative) reproduction. New ‘plants’ arise without flowering or setting seed. Seagrasses grow vegetatively by extending and branching their rhizomes in the same way that grass in a lawn grows. This allows significant areas of seagrass meadow to form from only a few shoots. In this way, seagrasses can recover after being ‘cut’ by grazers such as dugongs or disturbed by storms.

Seagrass meadows contain bottom dwelling seaweeds (macroalgae) attached to sediments, rocky outcroppings, and the seagrasses themselves. Calcareous algae live among the seagrasses, producing calcium carbonate which eventually becomes incorporated into the surrounding sediments. Drift algae form large unattached masses along the sea bottom and drift about with any water movement.