Fruits of the Sea: Seeds of Coastal Seagrasses

Do marine plants respond to natural disturbances in the same way as their terrestrial relatives - by triggering dormant seed growth, sprouting new shoots and releasing off-spring to be carried away by the elements?

A research project into the recovery of tropical seagrass meadows from natural or human impacts has challenged views that rapid regrowth occurs mainly by seeds present in mud or transported to disturbed areas by sea currents.

Dr Graeme Inglis

The project, undertaken by CRC Reef scientist Dr Graeme Inglis together with research students from James Cook University, found regrowth from seedlings is unlikely in heavily disturbed intertidal areas when most of the surrounding parent plants have been wiped out.

Seagrass meadows, widely valued as important habitats for many prawn and fish species and the main source of food for dugong and green turtles, are often threatened by coastal development and river catchment run-off.

In tropical regions, vast meadows can be destroyed by catastrophic natural events such as cyclones and floods. In 1992, floodwaters from Cyclone Fran wiped out more than 1000 square kilometers of seagrasses in Hervey Bay with the subsequent loss of many dugongs.

"But it appears that at least some tropical seagrasses are quite resilient to these disturbances and, given the right conditions, can recover to their previous abundance within a matter of years following a cyclone," Dr Inglis said.

The study identified these conditions and found that some species have adapted unusual reproductive techniques that enables them to build up large stores of seeds beneath existing meadows.

"Like their dry-land counterparts, seagrasses produce flowers and seeds. This means there are at least two ways in which they can re-establish themselves after a disturbance: through lateral growth by surviving plants or through the germination and establishment of seeds. A third method, where drifting pieces of plants take hold again and regrow, is also possible, although as yet unproven," Dr Inglis said.

During the reproductive season male flowers release long thread-like pollen which is carried by water currents before becoming entangled on the female plant. Fruits and seeds are produced following pollination of the female flower.

The different species of seagrasses produce a variety of seeds and fruits. Some are fleshy and germinate soon after they are release. Others have a hard seed coat and remain dormant in mud for years. One species, commonly found in the Great Barrier Reef region, Halodule uninervis, falls into the dormant category and has an unusual way of releasing its seeds directly into the sediment where they are unable to be dispersed by water currents.

While past studies have suggested that severely damaged seagrass beds simply recover as their seeds grow into establishing plants, Dr Inglis says this type of regrowth is limited to areas already well colonized. "Few seeds are found outside existing meadows and only a small proportion, less than one percent, of seeds germinate in any year. So far, seedlings, or young sprouts, have been recovered only from existing seagrass beds."

By monitoring the seed banks over a two year period, and undertaking experiments where different sized patches of seagrasses were cleared, Dr Inglis was able to show that seed reserves - holding as many as 10,000 seeds per square metre - rapidly deplete once existing plants are removed. Less than two percent of seeds remained in the mud within one month of the disturbance. "It appears that the chances of establishment from seedlings are very low in disturbed intertidal habitats when the parent plants are no longer there," says Dr Inglis.

Small-scale clearance of patches of Halodule uninervis by Dr Inglis in established meadows near Townsville, and by CRC research student Michael Rasheed at Green Island, have shown consistent rapid regrowth. Recovery has occurred principally by horizontal growth of the surrounding meadow with relatively little contribution from seed reserves in the sediment.

Looking further into the causes of seed mortality and germination, Dr Inglis and his team ran another experiment by burying seeds at different depths in the mud and keeping out predators, such as crabs and prawns, using small cages.

Surprisingly, they found that crabs and prawns are not major predators of Halodule uninervis seeds and may, in fact, help bury the seeds through their burrowing actions. Deeper buried seeds were three times more likely to germinate than those placed only a few centimeters into the mud.

So the integrity of seagrass beds is the key factor for germinating Halodule uninervis seeds. Established seagrass meadows modify and enrich their surrounding environment. Their leaves slow water currents near the sea floor and their roots prevent erosion of the mud in which they grow. Decay of the plants also means more organic material in sediments for seagrasses to grow and the ability of releasing seeds directly into mud appears to give them a better chance of survival.

Dr Inglis and his team are continuing with their work to determine how an area regenerates after a cyclone wipes out all the seagrasses and their seeds, and whether cyclones themselves are responsible for dispersing seeds to new places.

By Don Alcock