Seagrass — The secret meadow under the sea

Seagrass meadows are one of the most critical natural resources in Western Port and form the beautiful secret garden under the sea.

Seagrass in Western Port is an important marine habitat that provides food and shelter for many creatures. Small animals, including the juvenile stages of many fish species, are able to shelter among the seagrass protected from predators. These areas also provide food for multiple species of fish, crustaceans and birds.

Seagrass, like other plants, are photosynthetic organisms that rely on good light and water quality to survive. When these conditions are met, they are able thrive in the shallow inter-tidal mudflats and adjacent subtidal areas of Western Port.  Seagrass reproduces both by seed and beneath the mud and sand via rhizomes, stabilising near shore habitat, regulating coastal erosion, improving water quality and trapping carbon.

Seagrass cover has declined in many places across the world since the 1960’s. Meadows in Western Port declined by an alarming 70% during the 1970’s and 80’s, reducing the original cover of 250km2 in the mid-1970s to 76km2 only a few years later. It is unclear what caused the rapid decline in seagrass cover, however, it may be linked to multiple factors, including the release of significant sediment and other pollutants into the bay as a result of land clearing, intensification of agriculture, coastal erosion and the management of drains that were dug to progressively convert the former Koo Wee Rup swamp land into productive agricultural land from the late 1800s-early 1900s. The Koo Wee Rup swamp would have played an important role in naturally retaining and filtering river water from a large part of the catchment prior to it entering the norther sections of the bay where much of the seagrass was lost.

Seagrass and fish
Seagrass in Western Port is important habitat for fish and plays a critical role in stabilising sediments and helping to reduce coastal erosion.

Western Port is recognised as a Ramsar wetland of international importance for migratory birds and a UNESCO Biosphere Reserve. So, the observed decline in the 70’s and 80’s marked a significant threat to the multiple ecosystems that are reliant on seagrass.

Melbourne Water has collaborated with Deakin University to undertake projects aimed at understanding the status and potential for regeneration of seagrass in Western Port. These projects align with research priorities that help support the Healthy Waterways Strategy and they involve a team of research students, academics, and research partners, including PhD students Oliver Dalby (Dalby et al 2022; 2023), Yi Mei Tan (Tan et al 2020; 2023), and more recently Lucy Coals, Laney Callahan, Abigail Wookey and Elizabeth Keehner.

Oliver focused on assessing the status, change, and suitability of seagrass habitats using remote sensing imagery collected since the 70’s validated across previous studies and against contemporary, on-ground observations. His research revealed a recovery in seagrass extent since the 1980s, finding the total seagrass cover in 2019 to be 222 km2. These changes in seagrass cover likely reflect improvements in catchment management and water quality that have improved over time as well as the progressive clockwise flushing of fine sediments out of the bay. 

Seagrass recovery per decade
Seagrass recovery per decade between 1974-2019 (red is seagrass loss, light green is seagrass gain).

While it is encouraging to observe the natural regeneration of seagrass since the 80’s it is important to explore whether deliberate restoration activities could be developed to help in the efforts to reduce the overall global decline.  As part of Oliver’s research, he also developed seagrass habitat suitability models to predict locations within Western Port that are most likely to support seagrass now (and into the future), but where seagrass recovery has not yet occurred. These locations then became candidate locations for seagrass restoration activities.

Yi Mei Tan explored a range of restoration protocols for Zostera muelleri,the seagrass species that now dominates the inter-tidal mudflats of Western Port. Her work revealed that efforts to restore intertidal zones, where impacts from waves, tides and wind are most significant, is a challenging endeavour. However, she has determined that collecting and propagating seagrass or by planting seeds directly is likely to be the best way forward compared with a variety of transplant methods that rely on removing seagrass from healthy meadows. Some locations may require a combination of several methods for best overall outcome.  Further work by Lucy, Lany, Abigail and Elizabeth is looking at improving nursery propagation techniques and trialling a range of seedling and direct seeding planting methods in the field to increase seagrass survival.

Seagrass seedling transplant methods (clockwise from top left)—Discs, Frames, Nails, Plugs
Seagrass seedling transplant methods (clockwise from top left)—Discs, Frames, Nails, Plugs

Links to the Healthy Waterways Strategy - Westernport Catchment

The protection and restoration of seagrass in Western Port aligns with multiple performance objectives in the Healthy Waterways Strategy that aim to improve water quality by reducing sediment and other pollutants entering waterways and ending up in Western Port. These include activities such as controlling sediment from construction, improving run off from rural land, harvesting and infiltrating stormwater and establishing and maintaining riparian vegetation. Protecting and restoring seagrass meadows in Western Port supports environmental values such as fish and birds as well as key recreational activities such as fishing.