Port of Weipa
Long-term seagrass monitoring
September 2003.

Anthony Roelofs, Michael Rasheed, Ross Thomas
Queensland Department of Primary Industries and Fisheries

Seagrass area and density (biomass) surveyed in the Port of Weipa IMA in September 2003 was within the levels of variation recorded for seagrass during the previous three years of surveys. The only increase in seagrass in September 2003 was from two small new Enhalus acoroides meadows in Mission River.

The total area of the core monitoring meadows (meadows A1 to A7) in September 2003 was similar to September 2002, however there was a reduction in areas in meadow A5 where Halodule uninervis (narrow form) was dominant. This species had also shown declines during the last two years of surveying throughout the Port of Weipa area, most notably in Pine River Bay. The possible causes of these declines, including thermal and other climatic stresses, are being investigated during this long-term monitoring survey period. This 2003 survey has initiated thermal stress investigations by deploying temperature loggers within the intertidal zone of some seagrass meadows at Weipa. The intertidal zone is where most seagrass declines at Weipa have occurred since 2002. Temperature loggers will provide a record of thermal changes associated with the daily tidal cycle. The data will be compared with known thermal ranges for the suite of seagrass species found in Weipa to determine whether high temperature has been the cause of the observed degradation of intertidal seagrasses.

The average biomass of core monitoring meadows (A2, A3, A5, A6 & A7) was statistically similar to the September 2002 survey suggesting no major changes in seagrass health has occurred over the past year. However, the results do indicate a negative trend in seagrass biomass for meadows A2, A3 and A5 from 2000 to 2003. This trend will be of concern if it continues. The time series of data for Weipa seagrass biomass is only relatively short and the reliability of this trend is uncertain given the lack of long-term data. Short-term changes are common for these types of seagrass meadows and there have been examples of loss and recovery within five to ten year timeframes for similar seagrass meadow types in tropical Queensland (see Rasheed and Thomas 2003 - Karumba; Thomas and Rasheed 2003 - Mourilyan Harbour). Long-term monitoring will improve our understanding of tropical seagrass variability. A further two years of monitoring seagrasses at Weipa will add to our database of change in seagrasses found within tropical ports and enable us to separate out short-term changes from long-term trends.

We observed further evidence of ‘browning’ or ‘burning’ of Enhalus plants in the Embley River causing the plants to appear stunted where the leaf blades had rotted away during the September 2003 survey. This phenomenon was first observed in April 2002 but was not evident in September 2002. The burning was attributed to periods of high solar irradiance during low tide emersion of the Enhalus plants, although this has not been substantiated. Temperature logger data collected for 2003/2004 will assist with understanding the thermal stresses these plants are subjected to during tidal exposure periods.

The September 2003 seed sampling survey provided no evidence of a viable seed bank within the Pine River Bay sand and mud banks where Halodule uninervis (narrow) and Halophila ovalis meadows had disappeared in 2002. This suggests that if/when seagrass recovery occurs, it is likely to come from a source other than dormant seeds. There were a few very isolated individual Halodule uninervis (narrow) plants found in a poor state of health. If environmental conditions become more conducive to growth, these plants could reproduce vegetatively and begin to recolonise the Pine River Bay sand banks. With the absence of a viable seed store to provide new growth however, recovery of these areas is likely to be slow.

The most likely causes of seagrass loss in Weipa and other Queensland locations (Rasheed and Thomas 2003; Roelofs et al. 2003; Thomas and Rasheed 2003) in 2002 appears related to climate conditions affecting the growth of seagrasses. Low rainfall and a reduction in associated runoff, high temperatures and greater exposure to more intense solar irradiation are all possible causes of the declines observed in 2002 and 2003 (see Roelofs et al. 2003). There was no readily discernable anthropogenic or port related cause to the observed losses of seagrass in 2002 or to the lack of recovery that had occurred by the 2003 monitoring survey. It is likely that natural phenomena such as climate are driving the observed change.

FUTURE MONITORING

Seagrass distribution and density will again be monitored in 2004 and 2005. The 2005 survey will be expanded to include the entire Port of Weipa area to compare with previous surveys. A brief summary/progress report on the seagrass distribution and abundance in relation to previous surveys will be included with the maps following each survey. After the 2005 survey and the completion of three years annual long term monitoring, a more detailed Ecoports monograph will be produced bringing together results of the program to date. Timing of outputs will be as follows:

• A verbal report on the condition of seagrass meadows given to PCQ within one week of the surveys being completed.
• An updated GIS layer of seagrass distribution for inclusion in the Weipa GIS and hard copies of seagrass maps (with brief report) for distribution submitted within four months following the 2004 monitoring event.
• A more detailed Ecoports monograph will be produced following the 2005 survey with the draft report submitted within six months of the survey and a final monograph within one month of review comments.
  

A Seagrass-Watch program has been started with the Napranum community. Results from the program will assist the community’s understanding of seagrasses and will assist development of dugong and sea turtle management plans. Results will also assist the results of the QDPI/PCQ seagrass monitoring program.

For a copy of the complete report contact Dr Michael Rasheed on michael.rasheed@dpi.qld.gov.au