Seagrass

Seagrasses are an important part of many coastal systems, but are also under threat in many areas around the world as a result of habitat loss and reductions in water quality.

Seagrasses are closely related to land plants and are typically restricted to shallow coastal waters  where there is

• ample light to support growth
• sandy/muddy bottoms where the roots can take in sufficient nutrients without being up-rooted and washed away by waves and currents.

For food, seagrasses obtain most of their nutrients from the sediments they grow in and have extensive root systems that help reduce erosion and stabilise coastal sediments (which improves water quality).

Meadow-forming seagrasses in Port Phillip Bay and Western Port are dominated by two species:

• long eelgrass
  • almost exclusively subtidal (always submerged regardless of the tides)
  • growing to a maximum depth of eight metres within Port Phillip Bay.
• short eelgrass
  • tends to occur in intertidal and very shallow subtidal habitats,

There is a smaller quantity of a third seagrass, wire weed or sea nymph, which is restricted to the more exposed regions of the bays.

The seagrasses around the bays are from the eelgrass species and they act as key ecosystem ‘engineers’ that:

• stabilise sediments and improve water quality
• reduce coastal erosion
• provide nursery habitats and refuge for key fish species, and
• support algae and invertebrates that provide food for other species.

When the stabilising roots of seagrass are lost, the result is increased turbidity and reduced water clarity. Increased turbidity then kills more seagrass and this process continues in a vicious cycle.

The seagrass meadows in Port Phillip Bay can be divided into three broad categories: 

• persistent,
• ephemeral (nutrient-limited), and
• ephemeral (light-limited).

Persistent seagrass meadows are relatively stable over time, whereas ephemeral meadows (the seagrass moves around the bays) are much more variable and have shown major increases and declines over the last half century.

Persistent meadows

Persistent meadows tend to be found in locations protected from high wave exposure (for example, Corio Bay and Swan Bay). These seagrasses live in fine, muddy sediments, and most of their nutrients come from fixed-nitrogen and the internal recycling of detritus. They are mostly independent of external changes.

Ephemeral (nutrient-limited) meadows

In contrast, ephemeral seagrass meadows live in more exposed parts of the bay, particularly the Bellarine Bank and the southern areas of the bay, and have shown major increases and declines since the middle of last century. These seagrass meadows are nutrient-limited, and major losses of seagrass occurred in these areas during the Millennium drought when catchment inputs of nutrients were low. The areas were also subject to major changes in wind and waves.

Ephemeral (light-limited) meadows

The third category of seagrass habitat occurs along the north-west coast of the bay where nutrients are derived from the Western Treatment Plant. The combination of fine sediments and wave exposure means that turbidity is often relatively high, which limits seagrass growth.

During the last major drought (1997–2009) Port Phillip Bay lost considerable areas of seagrass, including over 90% of seagrass extent along the Bellarine Bank.

The distribution of seagrass in Port Phillip Bay is influenced by two physical processes:

• wave exposure, and
• light in relation to depth.

Wave exposure excludes seagrasses from colonising wave-exposed coastlines.

The maximum depth for long eelgrass in Port Phillip Bay, at about 8 m, broadly corresponds with the minimum light requirements which are equivalent to 10–20%of the light at the surface of the water.

In Port Phillip Bay, the largest expanses of seagrass are located primarily in the western parts of the bay in shallow waters protected from prevailing westerly winds and waves including:

• Swan Bay
• Corio Bay
• parts of the southern shore of the Geelong Arm, and
• the western shore of the Bellarine Peninsula.

There is little seagrass found on the eastern shore of Port Phillip Bay as the coastline is more exposed to prevailing winds and waves. Seagrass meadows in Port Phillip Bay get nutrients from a number of sources, but principally from:

• the catchments (riverine inputs)
• treated sewage (Western Treatment Plant)
• seasonal influxes from Bass Strait, and
• atmospheric sources, primarily nitrogen. 

Compared with Port Phillip Bay, Western Port seagrass is a far more dominant habitat and there are more extensive intertidal areas. Four species of seagrass are present in the bay:

• Wire weed or sea nymph occurs in the oceanic Western Entrance segment.
• Short eelgrass occurs on the intertidal mudflats.
• Long eelgrass occurs mainly in the shallow subtidal areas and on the lower intertidal mudflats.
• The small seagrass paddleweed also occurs, but its distribution is patchy and, compared with eelgrass, is usually in deeper, darker water.

The large-scale loss of seagrass area in Western Port during the 1970s and 1980s has been attributed to a range of possible causes including physical smothering of leaves by sediment in shallow water and a consequent reduction in light reaching the seagrass leaves.

The main driving factor affecting seagrass recovery in Western Port is water quality, particularly dynamic factors such as suspended sediments that reduce light which reaches the seagrass. Nutrient levels and availability are also important.

Seagrass meadows support:

• larvae and breeding
• unique species like pipefish and seadragons, and
• are of particular importance for King George whiting.

Eelgrass habitat was the most critical for fish biodiversity in Western Port because of its expansive cover and unique role for larval settlement and development in shallow areas. Although alternative habitats provide potential refuge for older juveniles and adults of some fish species maintaining fish biodiversity in Western Port relies heavily on eelgrass.

During flood events, seagrasses can tolerate brief and/or episodic reductions in water clarity, but suffer where available light is reduced for sustained periods of time.  

What we need to know

Seagrass in Western Port is widespread but lacks long-term data.

Uncertainties surround the cause of the loss of seagrass in Western Port in the 1970s. It is unknown what role, if any, nutrients performed in the loss of seagrass or in its recovery. Generally, the role of nutrients as a driver of seagrass growth in Western Port requires further study

Research in Western Port is currently underway to understand the factors controlling seagrass distribution, options for recovery and the links between seagrass and nutrient cycling on tidal flats.