Port Adelaide River estuary – Barker Inlet
In contrast to Queensland, a much smaller area is occupied by mangroves in South Australia – approximately 230 km2 (Banham 1992). The diversity of mangrove species in temperate zones is also relatively limited, and only one species of mangrove has been documented in South Australia: Avicennia marina, which is known there as the Grey Mangrove (Banham 1992, PPK 1992). This species, which is the most adaptable to conditions of higher salinity within temperate mangroves, is scattered along the coast within low energy and warmer waters in Gulf St Vincent and Spencer Gulf. Like the tropical mangroves, most mangroves grow on mudflats, but in some areas of higher wave energy they have been known to grow on sandy substrates (e.g. near Torrens Island), which emphasises the significance of marine sediment sources (Banham 1992).
The mangroves of Barker Inlet in the Port River (figure 2.37) have received the most extensive attention in mangrove research within South Australia, although this has primarily been descriptive in nature. Approximately 14% of South Australia's mangroves occur within the Barker Inlet area, which is dominated by mudflats and mangrove forests with tidal creeks and channels. Within this area, the mangroves are established on the Holocene 'St Kilda formation' (soft and highly organic marine sediments), which was created 6000 years ago when sea level reached about its current height.
Temperate mangroves, such as those in South Australia, are influenced more by the marine environment than by the land environment. This is a similar scenario to the Hinchinbrook mangroves in Missionary Bay, although tropical mangroves are generally influenced more by land environments and freshwater run-off. However, freshwater also drains naturally into the Port River estuary, allowing a mix of fresh and saline waters, and rainfall influences the mangroves during winter months, bringing with them terrestrial sediments. The tides in the Barker inlet may swamp mangroves for periods of 24 hours over many hectares, and this standing water increases sediment-water interactions and results in dieback and recolonisation (PPK 1992). Like the mangroves in Queensland, soils are saline, anoxic, highly organic and poorly drained, and are also encompassed by mats of cyanobacteria (blue-green algae), which help to bind the substratum (Banham 1992).
Parts of the Barker Inlet mangroves are also similar to the Hinchinbrook mangroves in that they are dense and have a closed canopy, but the height of the
Figure 2.37 Mangroves of Barker Inlet, Port River, Adelaide, South Australia
Source: modified from Belperio 1995
mangroves in South Australia contrasts strongly with those in the Hinchinbrook area. In the Barker Inlet, the taller mangroves are on the seaward side (maximum height of 6 metres, compared to 40 metres in Queensland), with a transition to mangroves of 3 – 4 metres inland and 1- 2 metres at the landward edge of the mangroves (characterised more by open woodland). Immediately adjacent to the mangroves on the landward side are significant samphire mudflats which buffer the land and mangroves, while on the seaward side, seagrasses dominate. Such samphire areas or saltmarshes are more prevalent in temperate regions, and it has been suggested that they provide a limited source of nutrients to the mangroves (Banham 1992).
Despite the lesser complexity and diversity of the mangroves in Barker Inlet, they fulfill roles that are similar to those in the Hinchinbrook area, including:
• maintaining bank stability and preventing erosion
• acting as a pollution and nutrient trap/ponding/recycling area (which protects the Inlet from stormwater discharge in the area)
• protecting local industries against damage from wave energy (e.g. salt evaporation ponds)
• acting as important nursery and breeding ground for aquatic species
• acting as important habitat/breeding area for birds and other fauna
• providing educational, research, recreational and scenic values (e.g. an extensive boardwalk has been established in the St Kilda mangroves) (Banham 1992, PPK 1992).
Moreover, 'mangrove and seagrass areas of Barker Inlet are considered to be one of the most important nursery areas for a number of commercially and recreationally important fish and crustacean species in the Gulf' (Janes 1984/1985 in PPK 1992, p. 69).
However, the full productivity of the mangroves, and the nutrient recycling and transportation processes, are still not completely understood (Banham 1992). It has been suggested that the mangroves are more important for providing structural supports for sediments, rather than as transporters of nutrients into the surrounding environs; that is, the nutrient cycle operates within a closed system (Banham 1992). This is similar to findings for the Hinchinbrook mangroves noted by AIMS (1998), with limited transportation of nutrients outside of the mangrove system. However, further research is necessary to determine the degree of nutrient export, and it was noted by Ferguson (1986) that the mangroves were still important for the provision of organic matter as the basis of the estuary food chain.
Like some areas of the Hinchinbrook mangroves, the mangroves in parts of the Barker Inlet are migrating inland, partly because of local sea level rise associated with land subsidence (Kucan 1979, Banham 1992, PPK 1992). In the Swan Alley Creek mangroves of the Inlet, there has been an estimated landward increase of 865 hectares, while further up the South Australian coast there has been a seaward increase (or progradation) of approximately 365 hectares (PPK 1992). Saintilan and Williams (1999) noted that the inland migration of mangroves occurred at a rate of about 17 metres per year in the Gulf St Vincent between 1949 and 1979. The lack of progradation in the St Kilda area has been attributed to changing sedimentation processes, land subsidence, and increases in the growth of Cabbage Weed (Ulva sp.), which is associated with increased nutrient levels in the area arising from human activity (PPK 1992). This alga tends to smother both new mangrove seedlings and the mangrove's pneumatophores. However, Coleman (1998) found that there had been some progradation (approximately 25-30 m from 1979 to 1993) in the North Arm Creek area of Barker Inlet.
Unlike the Hinchinbrook mangroves, which have remained relatively pristine, the mangroves in the Barker Inlet area have been significantly affected by human activity since European settlement, primarily because the mangroves coincide with human settlement areas. Impacts that disrupt the natural processes within the mangroves include, for instance, oil spills, mangrove and samphire clearance, the construction of port facilities, the creation of local industries (e.g. extensive salt evaporation ponds, sewage treatment works and discharge of nutrients into the estuary, power stations and thermal discharge), stormwater drains and water pollution, the construction of causeways which interrupt tidal flows and cause mangrove dieback, the establishment of boat clubs and associated traffic (resulting in erosion of shorelines), and the creation of waste or landfill areas and flood mitigation ponds. The construction of levees for flood mitigation (these levees mark the landward boundary of the mangroves) have also had a significant impact by preventing inland migration of much of the mangroves, inhibiting the natural drainage of freshwater into the estuary, and reducing terrestrial sediment loads.
Due to the significance of the Barker Inlet as a fisheries habitat and nursery, the Barker Inlet-St Kilda area was declared an aquatic reserve (2055 hectares) under the Fisheries Act 1971 to protect the fisheries habitat, and a draft management plan for the Port Adelaide-St Kilda reserve was prepared to protect the mangroves (Ferguson 1986). Torrens Island, which is in the inlet and also contains mangroves, was declared a conservation park in 1963 under the National Parks and Wildlife Act 1972.