A healthy bed of dense, green seagrass, busy with animals and fish swimming above, is an incredible sight.
Seagrasses are flowering plants that grow underwater in coastal marine and estuarine environments. They are generally only found in shallow, well-lit water in the shelter of bays and inlets, and often form dense meadows which resemble the lush, green grasslands that grow on land. On a warm, sunny day in clear water you can actually see the bubbles of oxygen rising up out of the seagrass leaves. One square metre of seagrass can generate up to 10 litres of oxygen per day via photosynthesis.
Seagrass communities are important in maintaining ecological processes, including primary productivity, nutrient cycling, food web pathways and provision of habitat. They supply gases and nutrients that are an essential part of marine food webs, hold sediments together and provide habitats for adult and young fish and other animals. They act as nursery grounds for the young of a range species, many of which are commercially important, including snapper, bream and garfish, western crayfish and shrimps.
There are 60 seagrass species in the world, and 16 of them are found only in temperate Australian waters. In Victoria, the main types of seagrass are the eelgrasses (Heterozostera tasmanica, Heterozostera nigricaulis and Zostera muelleri), Sea Nymph (Amphibolis antarctica), Paddleweed (Halophila australis) and Strapweed (Posidonia australis). Eelgrasses and Sea Nymph are the most common.
Caption: Seagrass stems grow into the sediment. Known as rhizomes, these underground stems and the seagrass’ roots help stabilise sandy seafloors 📷 Nicole Mertens
Seagrasses have terrestrial (land based) ancestors and have only returned to the sea in recent evolutionary history. Like flowering plants on land, they produce flowers and seeds, but these are small and difficult to see in many species. Most seagrass species produce flowers in autumn or summer. Pollen from the flowers is released into the water and carried to flowers on other plants where pollination occurs. Seeds or fruits are produced which then develop into a new plant. In the warmer months seagrasses propagate by sending out rhizomes (underground stems) which give rise to new shoots and more rhizomes. A single seed can therefore give rise to a large area of genetically uniform shoots connected by a network of rhizomes.
The eelgrass Zostera muelleri grows on sheltered intertidal mudflats. The two Heterozostera eelgrasses grow on subtidal sediments: H. nigricaulis mostly in bays and estuaries, and H. tasmanica in sheltered coastal waters. Sea Nymph grows on moderately exposed sand and sand covered reefs, and is not known to occur east of Wilsons Promontory. Paddleweed is most common on sheltered sands and Strapweed is only found on sheltered subtidal sediments in Corner Inlet and Nooramunga, where it forms extensive beds. Estuary grass or Tassel Weed (Ruppia megacarpa) is related to true seagrass and often considered similar habitat. Tassel Weed occurs in shallow estuaries, coastal lagoons and salt lakes.
Seagrasses are anchored in silty or sandy substrates by an extensive underground network rhizomes and roots that stabilise the sediment. Leaves or blades extending up from the base act as a baffle, slowing the surrounding water flow. This allows sediment to accumulate around them and helps prevent erosion. Seagrass roots absorb nutrients, but unlike land plants they do not take up water. To cope with living in oxygen-poor mud, seagrasses have evolved air canals that carry oxygen from the leaves to the buried rhizomes and roots. Nutrients and gases are also absorbed across the leaf surface.
As well as stabilising the sediment, they form a surface where algae and sessile (non-moving) invertebrates such as stalked sea squirts (ascidians), sponges, anemones and large razor clams can attach themselves. and a sanctuary for mobile invertebrates. Larger predators include octopus, squid, fish, seabirds, and crabs.Seagrass beds are primary habitat for many syngnathids, including pipefishes, seahorses and seadragons, which are species of conservation significance.
Small plants and animals live on stems or leaves and amongst the rhizomes. These are known as epiphytes (plants) and epifauna (animals) and include bacteria, micro-algae, small macro-algae and encrusting animals such as sponges and sea mosses (bryozoans). Delicate filter-feeding pearl oysters and solitary anemones attach themselves to leaves. Small crustaceans live in seagrass meadows and include species of seed shrimps, sea fleas, sea lice, small pebble crabs and pea crabs. Segmented worms (polychaetes) crawl over seagrass leaves in search of food, or else fix themselves in permanent tubes. Sea snails of different kinds and soft-bodied sea hares are also common. Many-armed sea stars, prickly sea urchins and sea cucumbers are abundant.
Seagrasses are important nurseries for many ecologically, commercially and recreationally important fishes, including King George Whiting (Sillaginodes punctata), Southern Sea Garfish (Hyporhamphus melanochir) and Black Bream (Acanthopagrus butcheri). They are also important breeding and feeding grounds for large numbers of fish and invertebrate species and play an important role in nutrient cycling and the food web of inshore coastal areas.
Seagrasses actively produce small amounts of nutrients required by other coastal plants and animals. Dead seagrass is also an important source of nutrients for many animals like bacteria, marine worms and crustaceans. Large amounts of decaying seagrass provide nutrients that are recycled through the food chain. Dead and broken leaves washed up on the shore provide a valuable food source and habitat for small crustaceans such as sandhoppers and various micro-organisms. Live seagrass itself is a minor source of food for animals that live in seagrass communities but forms a major part of the diet of some animals such as black swans.
Seagrass meadows are sometimes visible from piers and jetties in the shallow, sheltered waters of bays and inlets. Snorkelling over these meadows is possible in some locations and is a great way to experience seagrass communities.
Caption: Seagrasses are highly productive systems, but they are susceptible to many threats and can take a long time to recover 📷 Nicole Mertens
Seagrass meadows are regarded as important indicators of the health of the marine environment. Unfortunately, there has been an extensive decline in seagrasses around the coastline of temperate Australia in the last 20 years. In some areas, seagrasses have completely disappeared. Small scale disturbance is caused by boat propellers and anchors (which frequently rip out clumps of seagrass), dredging, spoil dumping, feeding by plant-eating animals like swans, and natural events such as storms. Chemical pollutants cause more widespread damage, killing plants or interfering with their capacity to grow and reproduce. Increased sea temperatures from climate change and an increase in harmful UV rays due to depletion of the ozone layer, which cause seagrass sun burn and death, are also threats to seagrass communities.
Seagrasses are very sensitive to human disturbance caused by sewage outfalls, heavy metal pollution, agricultural runoff (including pesticides) and other activities. Like all plants, seagrasses need sunlight to photosynthesise and survive. Seagrasses naturally have a number of other organisms (epiphytes and epifauna) living on their leaves. Increased nutrient levels in the surrounding water can cause epiphytic algae to bloom and smother seagrass blades, preventing enough sunlight from reaching the leaves. Aside from nutrient pollution, suspended sediments (e.g. topsoil/runoff from forestry and agricultural activities, construction, or mining; as well as suspension of sand during dredging) can reduce the light available for seagrass to grow.
Seagrass species vary in their susceptibility to disturbance- e.g. 90% of Heterozostera eelgrasses have disappeared from Western Port Bay while the cover of Sea Nymph hasn’t changed much. A common characteristic of all seagrass species is that they are very slow to recover from any sort of disturbance, and some species may never return to their original state. The extent of seagrass recovery depends on the resilience of the species and the scale and nature of the disturbance. While smaller areas can be recolonised, larger areas prove more difficult. Rapid colonisers such as Eelgrass are also far more likely to recover than slow colonising Strapweed. The parks at Westernport and Corner Inlet protect seagrass meadows. Corner Inlet is the stronghold of Strapweed.
Scientists have employed aerial and satellite images which give an accurate picture of the extent of seagrass and show whether seagrass beds are changing. While it is possible to track the loss of seagrass cover, halting and reversing the decline has proved far more difficult. Attempts to replant damaged seagrass beds in various parts of the world have met with little success. While Strapweed and Paddleweed are catagorised as rare in Victoria, there are few laws or projects in Victoria aimed specifically at protecting seagrasses.