The Gulf St Vincent
Seagrasses are the most common plants growing on the seabed of the Gulf St Vincent, where they create a variety of habitats on an otherwise smooth sedimentary bottom. Amphibolis (Wire Weed), Halophila (Paddle Weed), Heterozostera (Gar Weed) and Posidonia (Ribbon Weed) are common.
Figure 2.39 Seagrass loss in Cockburn Sound, 1954-1978
Source: modified from Cambridge & McComb 1984
The shallow low-medium energy waters of the Gulf St Vincent have in the past been an ideal environment for the growth of seagrass, and the production of great volumes of detritus are part of the normal seasonal cycles for these sea- grass meadows. The major growth is in spring and summer; leaves are shed throughout the year, but mainly in autumn. The high productivity of the sea- grasses of the Gulf St Vincent constantly adds leaf detritus to the seafloor. A proportion of the undecomposed detritus is carried away by tides and currents and is distributed by waves along the shore.
When shedding occurs, the seagrass leaves are buoyant at first but then sink to form drifts on the bottom (Kinloch 1999). Much of the resultant detritus remains within the seagrass meadows, where it is slowly broken down by bacteria and consumed by plankton, thus forming a significant basis for fish and crustacean life in the Gulf. About 20% of the detritus, together with seagrass leaf sheaths and rhizomes, is entrained by storms. This moves with wind and tide-induced currents in clumps and layers just above the seabed until it comes to rest at low-energy points such as hollows, where it may be buried with sediment, or it may reach the shore.
Seagrass detritus (together with algae and other matter) in the breakers, on the shoreface, and buried by sand on the beachface is a normal feature of Adelaide's beaches. Because of their high cellulose content, seagrass leaves break down slowly and may remain in the nearshore zone for many months. Wrack on and within the beachface provides an important food source for beach meiofauna, as well as affording some protection to the beach during minor storms. At Adelaide, seagrass detritus accumulates at the low-energy, downdrift northern end of the beach system and also adjacent to and within the three small boat harbours at Glenelg, West Beach, and North Haven. At these locations it is important to clear seagrass detritus promptly: it fouls boat propellers and clogs cooling systems; the smell from the anaerobic decay of detritus buried within the sediment ingress to the harbour leads to complaints from residents and from boat owners in the marina facilities; and when decaying material is dredged and disposed of downdrift in the breaker zone, complaints come from recreational beach users. These problems require frequent monitoring, a rapid response to storm events that move sediment and detritus into the harbour, and good communication between harbour management and the users of the facilities and neighbouring beaches. In this way the function of the harbour in trapping the normal movement of seagrass detritus leads to an expensive complication to maintenance dredging.
The Adelaide Metropolitan coast also provides a detailed picture of the inter-relationship of seagrass loss and its effects on nearshore sand movement. Following the rise of metropolitan wastewater discharge to the Gulf in the 1960s, the loss of seagrass – especially adjacent to sewage outfalls – in the 1970s was spectacular. Subsequently the rate of loss has declined. Comparison of a time sequence of aerial photographs and some field truthing (Hart 1997) suggests that about 7000 ha has been lost in waters off Adelaide; there has been considerable thinning of the beds, and the shoreward edge of the seagrass has in many areas retreated by 500 m. Neverauskas (1987) considered that the loss was due both to increased turbidity and to epiphyte growth in the nutrient- enriched waters.
Detailed annual profiling of the Adelaide beaches and coastal waters to 1 km offshore has been carried out since 1973, primarily to monitor sand movement and the effects of the beach replenishment scheme. This record shows that in the areas of seagrass loss the substratum has been eroded to an average amount of 0.3 m. Loss of this depth of sand over several thousand hectares constitutes a large volume which appears to have moved onshore, accounting for the much higher volume than expected of sand accumulated at the northern (downdrift) end of the metropolitan beach system by 1995 (DENR 1997). In addition to this one-off increment of sand to the beaches (suggesting that the Adelaide beach sand replenishment rates will need to be increased in the future), it is inferred that seagrass loss has led to an increase of wave energy at the shore, with a consequent rise in littoral drift speeds.
At Adelaide, knowledge of both the biological and physical connections within the dynamic Australian coastal zone is clearly vital to sustainable coastal management.
Figure 2.40 Seagrass recession off the metropolitan coast of Adelaide
Source: Hart 1997