Port of Weipa long term seagrass monitoring: 2003-2005

Seagrass meadows in the Port of Weipa have changed substantially during the six years of seagrass monitoring. Most of the changes appeared to be in response to regional and local climate and the physical and physiological characteristics of the seagrass meadows, rather than human activities that have occurred in the port. In general terms intertidal seagrass meadows throughout the port area had become more patchy (greater areas of open unvegetated substrate) with substantial reductions in biomass (density) for Enhalus acoroides dominated seagrass meadows. In addition complete Halodule/Halophila meadows were lost in the Pine River Bay area in 2002 and have yet to recover.\ud \ud The most likely cause of changes to intertidal Enhalus meadows was desiccation of Enhalus plants when they were exposed at low tide (i.e. "burning"). Burning was likely to be caused by a combination of high air temperatures, low rainfall and higher solar radiation intensities. Climate data suggests there has been a trend for less rainfall and higher solar radiation over the last five years. Also maximum temperatures measured from intertidal temperature loggers during 2004/2005 have reached 41.5°C which is close to the limit where seagrass cell systems have been shown to be irreparably damaged.\ud \ud Differing responses of subtidal meadows and intertidal meadows of different species such as Halodule and Halophila were likely to be due to the differing susceptibility to desiccation between species and locations. Physical differences in growth form allowed Halophila and Halodule to lie flat on the moist sediment surface during low tide exposure and deeper subtidal Enhalus meadows were protected from low tide exposure.\ud \ud There had still been no recovery of Halodule and Halophila meadows in the Pine River Bay area since their loss in 2002. Seed sampling conducted in 2003 indicated that there were no seed reserves in the sediment which suggested that recruitment and recovery may be slow due to a limited local supply of propagules. Similar meadows in the southern Gulf of Carpentaria that had a seed bank had recovered from the 2002 losses in 2005 and 2006. There was some evidence of an increase in biomass for the remaining Halodule meadows in the Embley River. This may indicate a return to more favourable environmental conditions for Halodule growth.\ud \ud The declines in the Embley River Enhalus meadows and Pine River Bay Halodule/Halophila meadows were likely to have some local fisheries implications. Analysis of fisheries commercial catch data for barramundi, mud crab and grey mackerel have shown declines that corresponded with the recent reductions in seagrass cover and biomass for Weipa.\ud \ud The monitoring program indicates that despite some seagrass declines, the Weipa marine environment is relatively healthy with observed changes likely to be associated with regional and local climatic factors and the nature of the seagrass meadows rather than anthropogenic or port related impacts. The recent decline in the Evans Landing meadow is a concern however, and will be monitored closely in the upcoming 2006 survey. We have established a good understanding of the range of natural changes in seagrass meadows in Weipa through a period of "normal" and "drought" conditions. Through our state wide seagrass monitoring network we have been able to put these changes in a regional perspective and separate local versus regional drivers of seagrass change. This background has placed us in a good position to detect any anthropogenic causes of change to seagrasses beyond this natural background. Future monitoring will continue to enhance this ability and provide port and fisheries management with information on the status of the marine environment and fish habitats within the Weipa area and an early warning of changes to marine environmental health caused by port or other human activities