FISHERMAN ISLANDS AND WHYTE ISLAND MANGROVE HEALTH ASSESSMENT 2004

September 2004

FRC Environmental

FRC Environmental was commissioned by the Port of Brisbane Corporation to assess the health of mangrove communities at Fisherman Islands and Whyte Island.  Previous studies commissioned by the Port of Brisbane (in 1999/2000 and in 2002) had found there were large areas of dead mangroves, and mangroves in poor condition in these two areas.  This study was designed to:
· assess the current condition of mangrove communities;
· assess the extent and nature of any historical and on-going environmental impacts acting on the mangrove community; and
· to discuss ways the health of mangroves in these areas could be improved.
 
First of all we examined previous maps of the area and aerial photographs and determined the areas we needed to investigate the most.  These were predominantly areas that were not in good condition, and the boundaries between different categories.  In April 2004, over a total of 7 days, we assessed the condition or health of the mangroves.  Three areas were surveyed:
the mangroves near the coal loader at Fisherman Islands;
the large area of mangroves adjacent to the PBC offices on the eastern side of Fisherman Islands; and
the mangroves on the eastern side of Whyte Island.
 
At each survey point we recorded our position and the health and type of mangrove community, using the same classification system as used in previous surveys.  We also recorded the amount and type of macroalgae in the area and the number of seedlings, and also made notes regarding the degree of disturbance, factors that may be influencing the health of the mangroves and the presence of macrofauna.

Sites for photographic monitoring established in previous surveys were re-photographed in this survey.

Mangrove community structure and health were then digitally mapped onto rectified aerial photographs, and the results compared to those of previous years.

As expected, the composition (i.e. the height, density and type of mangroves in the forest) are similar to when monitoring began in 1999.  At Whyte Island, although there have been some changes, overall the health of mangroves is similar to when monitoring began.  At Whyte Island the health of some mangroves has declined, however there are also areas where mangroves appear to be re-establishing.

However, at Fisherman Islands there have been some relatively major changes to mangrove health since monitoring commenced in 1999.  The area of mangroves in good health has decreased, with the boundary between good and fair health moving seaward.  Despite this, there was also some evidence of regrowth in previously dead areas, and there were also improvements in the health of mangroves in some areas.

The pneumatophores, or air breathing roots, in the more seaward, healthy mangrove forests, were covered in epiphytic algae.  In contrast, in ponded areas and in the more landward, poorer health mangroves there were mats of algae typical of nutrient rich waters.  This trend is likely to be related to the higher degree of tidal inundation / flushing within the seaward areas.  In general, in the healthy mangrove communities there were also more snails and crabs than in areas of fair or poor health.  In dead areas and in areas of poor health there were almost no animals.  There were also more mangrove seedlings in the areas of good health.  Mangroves throughout the study area, and particularly the mangroves in the southern section of Whyte Island, had been attacked by caterpillars.  New growth in particular had been attacked.

The dieback, and the declines in mangrove health, recorded at Fisherman Islands and Whyte Island are likely to be the result of the complex interaction of a number of factors, including:

· loss due to reclamation;
 
· realignment of shorelines due to changing wave / current climates in the area.  This may be due to for example, increased boat wash; channel dredging; or changes in water movements due to re-alignment of the foreshore;
 
· movement of sediment within the wetland area, with both sediment washing in, and the redistribution of sediment within the wetlands;

· restricted tidal exchange, including ponding of water;
 
· nutrient (often indicated by macroalgal growth) or toxicant excess;
 
· herbivore / insect attack;
 
· seagrass or algal wrack accumulation;

· storm damage; and

· long term or cyclic impacts such as climate or sea level change.

The ponding of water is likely to be the principle factor impacting the mangroves at Fisherman Islands and Whyte Islands.

The ponding of water is related to a number of factors which have blocked natural drainage lines, including:

previous unconfined deposition of dredge spoil material in the area;
 
the growth of thick algal mats within the ponded areas;
 
changes to the wave climate resulting in changes to sediment deposition and the creation of sand bunds; and

the construction of the Port Road, and long term climatic changes.
 
Overall, in this survey, there appears to have been a decrease in the area of ponded water, and along with this a slight improvement in mangrove health.  A detailed study of the historical distribution of saltmarsh and mangroves in this area, combined with examination of climate change over this period may also help in determining current changes in mangrove health.  High nutrient levels in the ponded water are likely to be exacerbating the situation, and are likely to be a result of high nutrient levels in general in the waters of Western Moreton Bay, and also the result of point source discharges such as effluent from Wynnum Waste Water Treatment Plant.  We recommend further studies to establish the source of nutrient loads, particularly in the Whyte Island mangroves.
 
Other factors such as high toxicant concentrations, high rates of herbivory and smothering by sand or seagrass wrack are all likely to be having relatively minor impacts on mangrove health.  However, these factors are likely to place additional stress on those mangroves already in poor health, decreasing their chances of recovery.

Remediation works (if performed) should focus on, but not be limited to, controlling algal growth and reinstating drainage of the ponded areas.  However, works such as these will require careful planning and environmental management; will require numerous permits; and are likely to be expensive.