Monitoring of seagrass meadows in Cairns Harbour and Trinity Inlet: December 2002.

Campbell SJ, Rasheed MA, Thomas R.
Queensland Department of Primary Industries and Fisheries.

This report provides results of the first of planned annual long-term seagrass monitoring surveys for Cairns Harbour and Trinity Inlet conducted in December 2002/January 2003. The monitoring program was developed following a baseline survey in December 2001 and examines selected representative seagrass meadows in Cairns Harbour and Trinity Inlet. Total area of each monitoring meadow, species composition and estimates of seagrass above ground biomass were measured for comparison with the baseline survey.

Overall seagrass area and biomass for subtidal monitoring meadows remained stable since the baseline survey. Area of intertidal monitoring meadows was also similar to the baseline survey but there had been significant declines in biomass (meadow density) since December 2001. At intertidal meadows situated on the Esplanade a significant reduction in seagrass biomass and depth penetration occurred. These changes were characterised by a 48% reduction in above ground biomass in intertidal Zostera capricorni and a reduction in the depth to which Zostera capricorni grew from 1.2 m to 0.9 m below mean sea level. These changes were even more pronounced at the Ellie Point section of this meadow where there was also a 45% reduction in total seagrass area. Subtidal monitoring meadows at Bessie Point had a three-fold increase in the above ground biomass of Halodule uninervis between 2001 and 2002. The biomass of monitoring meadows in Trinity Inlet remained stable for the small intertidal Zostera capricorni monitoring meadow but declined by 94% for the subtidal Halophila ovalis monitoring meadow.

The causes of seagrass biomass changes between 2001 and 2002 were most likely associated with atypical climate conditions in 2002. Available climatic data showed that 2002 was characterised by elevated temperatures and low rainfall. The reduction in intertidal seagrass biomass at the Esplanade meadows may be associated with stress on seagrass physiology from elevated temperatures and desiccation from high solar radiation. Low nutrient inputs to this area may also explain the observed biomass declines. No reduction in biomass was found for intertidal Zostera capricorni in Trinity Inlet suggesting that other factors may also be contributing to seagrass biomass decline at the Esplanade. Esplanade seagrasses are exposed to prevailing southeasterly winds and high turbidity and the existing low light conditions coupled with high temperatures may have lead to seagrass decline.

Anthropogenic disturbance from dredging activities within Cairns harbour during 2002 cannot be discounted, but as dredging practices in the area have not changed significantly in recent years it is unlikely to explain the observed declines. Increases in seagrass biomass that occurred in subtidal monitoring meadows at Bessie Point were most likely due to increased light availability associated with low rainfall and runoff. The Bessie Point meadow was unlikely to be affected by elevated temperatures, as this area was not exposed to shallow pools of high temperature water or exposed to air and desiccation.

The continuing decline of seagrass in the Ellie Point area is a concern and emphasises the importance of continuing an annual seagrass monitoring program. The continuing program will provide an ongoing assessment of the 'environmental health' of marine environments in Cairns Harbour and Trinity Inlet and further develop our understanding of the relationships between climatic changes, anthropogenic disturbance and seagrass abundance. This survey represents the beginning of the monitoring program and as such the program is still defining the range of natural changes that may be expected within Cairns Harbour and Trinity Inlet. Continued monitoring through a number of years will provide a better context for interpreting observed changes in seagrass and assessing potential human induced impacts.

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