By Jack Coates-Marnane.
Moreton Bay is on the shores of Brisbane. A major challenge is facing the ecosystems and economies Moreton Bay supports – the bay is getting shallower.
Moreton Bay and its habitats, including seagrass beds, coral communities, support fisheries and is an important cultural asset for Australians. With protection from the open ocean Moreton Bay also serves as an important harbour for international trade and shipping through the port of Brisbane, built at the mouth of the Brisbane River.
An increase in fine sediment supply to Moreton Bay following land clearing has been implicated in the gradual degradation of habitats of Moreton Bay including; a reduction in seagrass distribution, the local extinction of subtidal oyster reefs, and a decline in coral community health. However, relatively little is known about the magnitude, timing and process that controlled initial sediment erosion in the catchment and its distribution and accumulation in Moreton Bay.
We used deep sediment cores from the central Moreton Bay region to reconstruct rates of sedimentation over the last few thousand years. We found that there has been a 3-9 fold increase in the rate of fine sediment accumulation in the last 60 to 100 years compared to long term 1000 to 3500 year average. Furthermore, infilling has been largely focused in remnant channels of western Moreton Bay and now these channels have almost been completely filled.
The timing of the increase in rate of sediment accumulation coincides with destabilization of river channels within the catchment of Moreton Bay, following European settlement in the region in the 1840s and the expansion of agricultural practices (see Kemp et al., 2015). Although this process of infilling occurs naturally it has been accelerated recently due to an increase the supply of fine sediments from eroding river channels.
Further, the distribution of sub-tidal fine sediments has expanded significantly compared to when it was first mapped in the 1970s. Parts of western Moreton Bay which were once clean quartz sands have now been replaced by fine muds. As the bay gradually becomes shallower, fine sediments that get delivered to the bay are more likely to remain in the water column – increasing turbidity. The significance of this is that poorer water quality conditions are likely to be observed in locations that were previously unaffected.
These findings also have important implications for the operation of Queensland’s largest port (The Port of Brisbane). Following the 2013 flood, the deposition of sediments within shipping channels delayed oil tankers docking within the port for up to 5 days.
The port relies on the ongoing maintenance of deep channels for shipping navigation. As the natural storage capacity of the basin decrease any new sediment delivered to the Bay has a greater likelihood of being deposited in artificial shipping channels rather than being stored in the natural remnant channels. This means that greater effort will be required to maintain the channels.
What we are observing today in Moreton Bay is the result of a long legacy of catchment land-clearing and agricultural development. This research also shows that natural harbours such as Moreton Bay naturally infill.
Much of east coast Queensland lacks any deep water ports that are found in other Australian states (e.g. Sydney Harbour). Any future ports developments on the Queensland east coast first need to consider the longevity of such infrastructure and the ecological impact of the inevitable increase in dredging effort that will be required to maintain them.
For South East Queensland, the preservation of remnant vegetation in the catchment should be a priority focus for local governments interested in water security, port operations, and in the maintenance of the ecological value of Moreton Bay for the future. From 2013 to 2014 296,000 hectares of bushland was cleared – three times as much as in 2008 and 2009.
The resurgence in clearing of native vegetation came largely as a result of policy changes introduced by the Newman government in 2012. The land clearing reform Bill that is designed to re-instate previous levels of protection for native woodlands is currently before the QLD Parliament. If parliament chooses to protect native woodlands once again, it will be a step in the right direction for caring for Moreton Bay.
Coates-Marnane J., Olley J., Burton J., Sharma A. 2016. Catchment clearing accelerates the infilling of a shallow sub-tropical bay in east coast Australia. Estuar. Coast. Shelf Sci., 174, 27-40. doi:10.1016/j.ecss.2016.03.006