Hinchinbrook Island mangroves
In Queensland there are approximately 4600 km2 of mangroves (Faulkner 1994). No matter where mangroves are located in the temperate or tropical zones, they tend to occupy sheltered intertidal zones that are characterised by low gradients, low wave energy, and usually muddy shorelines, although mangroves can also occupy sandy or rocky locations.
One of the world's highest zones of mangrove productivity and species diversity (with an estimated 27-30 species) is located around Hinchinbrook Island, Queensland (see figure 2.36) (AIMS 1998, Bunt & Bunt 1999). Within this area, mangroves cover an area of 120 square kilometres, and the mangroves are generally tall (up to 40 metres), with a closed canopy, although height tends to vary depending on proximity to the shoreline (Bunt 1982, Kelleher et al. 2000). On the seaward edge of the mangroves, communities are relatively simple, while greater diversity becomes evident in more sheltered zones and at the landward zone of the mangrove community (Environment Australia, n.d.).
The largest areas of mangroves are located along the Hinchinbrook Channel (a flooded valley between the island and a low coastal plain on the mainland with shifting sand dunes), and in Missionary Bay at the northern end of the island, which is located within a flooded valley sheltered by sand dunes (AIMS 1998, Bunt & Bunt 1999, Environment Australia n.d.). The location of these latter mangroves is an example of a 'drowned bedrock valley' coastal setting as described by Thom (1984). An existing and soft substrate must exist for mangrove colonisation (usually from silt deposition from rivers, or from seabed sand). Once established, mangroves enhance sedimentation accumulation as they trap sediments and organic material.
The mangroves in the Hinchinbrook area (and in general) are adapted to highly dynamic environments which are intermittently inundated by saline tides and occasionally by freshwater from land during heavy rainfall and low tides (Lear & Turner 1977). In the Hinchinbrook region, the greatest influence on sediment movement and the establishment, distribution and composition of mangroves is tidal action, rather than freshwater run-off and groundwater (Boto & Wellington 1988). It is usually only during periods of heavy monsoonal rains that the mangroves are influenced by freshwater, but even with heavy rains the salinity is still high in the Missionary Bay mangrove system
Figure 2.36 Mangrove distribution Hinchinbrook Island
Source: modified from Queensland Government 2001
(Boto & Wellington 1988). However, in the Hinchinbrook Channel area, freshwater catchments discharge into each side of the channel (Environment Australia n.d.).
Mangroves can either be stable or unstable; boundaries may retreat inland, or extend seaward by a process of progradation and sediment entrapment and accumulation. This in turn is dependent on local sea levels, where, for example, a lowering of sea level or land uplift results in mangrove colonisation of new intertidal areas, with higher sea levels (or land subsidence) leading to inland retreat. This is also influenced by rainfall, and in the Hinchinbrook area there is evidence of landward migration of mangroves and the replacement of saltpan areas, because of increasing rainfall and reductions in salinity in landward zones (Saintilan & Williams 1999, p. 120).
The roles of the Hinchinbrook (and other) mangroves are quite diverse. They trap sediments and organic material, in part, via specialised and exposed root systems (pneumatophores) which reduce wave energy and allow the mangroves to breathe during flooding (AIMS 1998, Duke et al. 1998). In trapping sediments and nutrients, the mangroves prevent coastal erosion and provide a buffer zone between land and sea against agricultural run-off and other pollution. The mangroves also protect the coasts from cyclones and floods, they act as a sink for atmospheric carbon, provide an important habitat for fauna (including fish, dugongs and numerous bird species), and provide an important link in the coastal food chain, where detritus from the mangroves is consumed by bacteria and higher organisms (Faulkner November 1994, McCormick 1994, AIMS 1998, Ewel et al. 1998). Marine vegetation including mangroves, seagrass, algae and microalgae 'are now recognised as being the primary source of productivity in detritus based marine food chains' (PPK 1992, p. 62). Moreover, 75% of commercial fish and Crustacea in Queensland are directly dependent on mangrove ecosystems (ΈΡΑ 2000).
Tropical mangroves are highly productive, and organic matter and nutrients are transported between land and sea by complex hydrodynamics. Water circulation and the transport of nutrients and sediments differs within the mangrove creeks (strong tidal flows which flush detritus out to sea) and the forested areas, causing friction, slow water movement, and water entrapment, and resulting in waterlogged and anaerobic soils (Robertson & Alongi 1999, Bridgewater & Cresswell 1999). Through this water movement and stagnation, inorganic matter and nutrients are imported (e.g. via rainfall, riverflows, run-off, chemical release, and human influences) and utilised by the mangroves, while organic matter is believed to be exported and provides a basis for the estuary food chains (e.g. via tidal transport and soil leaching by freshwater). The exact contribution of nutrients from the mangroves is unclear, given that AIMS (1998) found that only a small amount of nutrients is released offshore from the Missionary Bay area each year. However, mixing of mangrove coastal and offshore waters does occur through a 'coastal boundary layer' which provides a buffer between mangrove coastal and offshore waters (Wolanski et al. 1997).
Because of the importance of the Hinchinbrook Island mangroves, much of the area is protected within a national park, a fish habitat reserve which was declared in 1983 (in the southern part of the Channel), a state marine park, and a portion of the Great Barrier Reef World Heritage Area (Environment Australia 1997). Hinchinbrook Island was also declared a 'feature protection area' in 1932 (Environment Australia 1997). Land-uses within the area include restricted tourism and recreation, aquaculture, agriculture (primarily sugar cane) and urban development, but aside from indirect impacts from agricultural run-off and coastal development, the mangroves of this area have had little disturbance by human activities (Environment Australia n.d., Bunt & Bunt 1999).
Some mangroves were, however, cleared in the early 1970s, and a marina- resort development proposed in the late 1980s at Oyster Point resulted in the clearance of some mangroves. The main concerns associated with the clearance was bank destabilisation, coastal erosion, and the smothering of important sea grasses in the region, in addition to the increased impacts associated with tourism (Hansard 1999). Although this development was halted in 1994 by the Commonwealth government because of the impact on the mangroves, it was resumed in 1996 (Hansard 1999). The impacts of this development on the mangroves and their role in coastal stabilisation are as yet unclear.