Assessment of the Health, Viability and Sustainability of Mangrove Communities at Fisherman Islands

May 2000

Prepared by: WBM Oceanics Australia

The Port of Brisbane Corporation is responsible for the management of the Port of Brisbane, a ship loading and cargo facility located at the mouth of the Brisbane River. Part of the management responsibilities of the Corporation is to ensure that the activities of the port do not unduly impact on the adjacent wetland areas at the Fisherman Islands.

These wetlands have contained areas of dead mangroves for some time. In the absence of adequate baseline and/or monitoring data, the PBC has insufficient information to determine the causes for the mangrove death and/or identify opportunities for management practices to address this issue.

WBM Oceanics was commissioned to undertake a survey of the mangroves of the Fisherman Islands area and map the distribution of mangrove communities and current health status. The objectives of the study were to determine the current status of the resource and identify the potential impacting processes leading to mangrove death.

Surveys of the study area were conducted in November 1999 to January 2000. This involved the traversal of the site and recording detailed information such as mangrove species, canopy height, community structure, mangrove health, macroalgae and macrofauna abundance (relative to a number of criteria).

Detailed review of low level aerial photography of the Port area from 1972 to 1999 was also undertaken to map changes in both land use and mangrove distribution.

The results of the study indicate that a large proportion of the mangroves within the Fisherman Islands area were of poor health and that recovery/regrowth in these areas was limited. It should be noted that adjacent areas (such as Whyte Island and Luggage Point) are also suffering similar mangroves losses, and the degradation process appears to be occurring on a regional basis. The author understands that the QDPI are currently undertaking investigations to determine the distribution of effected mangroves within western Moreton Bay.

The reasons for mangrove decline in the study area are not clear, however three impacting processes are suspected. These are:

• land reclamation practices undertaken in the 1980’s as part of the expansion of the Port facility. These practices, which differ from current reclamation techniques, have resulted in the direct loss of mangroves and large scale changes to drainage patterns. This has resulted in the ponding of waters around mangroves which prevents the re-establishment of mangroves and provides an on-going impacting process in adjoining areas.
• excessive algal growth, possibly related to elevated nutrients in adjacent waters, including the lower reaches of the Brisbane River, from sewerage discharges (Wynnum and to a lesser extent Luggage Point treatment plants) and inputs from the Brisbane River catchment. The macroalgae may directly effect mangroves by smothering mangrove roots systems, reducing their efficiency, or reducing the recruitment of seedlings. Furthermore, the growths of the macroalgae may from small “bunds”, leading to ponding waters and resulting in mangrove stress/death. Ponded waters also provide areas of open water which are suitable for the continual growth of macroalgae.
• development of sand bunds related to increased wave energy at some sections of frontal mangroves. Dredging activities have removed seaward shallow areas which previously dissipated wave energy. Some areas now contain sand bunds at the crest of the seaward fringe that has the effect of either directly smothering mangroves and/or blocking flow paths and ponding waters within mangrove areas. This process is most evident along the Boat Passage, but is also occurring within the Brisbane River south of the Bulk Coal Facility.

It is likely that the above processes interact to provide the current range of impacts. For example sand bunds cause the ponding of waters, resulting in the loss of mangroves and the development of open water, which then leads to the growth of macroalgae. These processes may further blocks flow paths and elevates the ponded water levels, increasing the area of impact.

Whilst some management options are available to address the impacting processes within the study area, most of the impacting processes are at a much larger scale than the study area. A, broadscale approach from a variety of organisations responsible for management of waterways, and nutrient inputs from the Brisbane River catchment, may be required to adequately address the issue.