Port of Weipa long-term seagrass monitoring: 2000-2011
Other Publication ResearchOnline@JCUAbstract
This report details the results of the August 2011 monitoring program in the Port of Weipa. Seagrasses in the Port of Weipa remained in a reasonable but vulnerable condition. Several meadows have shown a long-term decline in biomass since monitoring began in 2000. These declines are likely associated with natural shifts in tidal exposure and changes in light and temperature associated with local climate conditions. The long-term nature of these declines may have left some meadows in a vulnerable state with a low resilience to further natural or anthropogenic impacts. In 2011, meadow area within the Intensive Monitoring Area (IMA) around the major port operations was approximately equal to the twelve-year average of 1039 ± 67 ha following three consecutive years of modest increases in meadow area of ~ 20 ha per year. Seagrass biomass within four of the five core monitoring meadows declined between 2010 and 2011 and biomass values were statistically similar to other low biomass years over the 12 years of monitoring. The most significant biomass decline was recorded in the large intertidal Enhalus acoroides dominated A2 meadow opposite Lorim Point, where the 2011 biomass value of 6.13 ± 0.8 g DW m⁻² was less than half of the biomass reported in 2010. In contrast, biomass in the Halodule dominated intertidal A5 meadow on the eastern bank of the Embley River more than doubled between 2010 and 2011. Changes in biomass for these meadows over the course of the monitoring program are significantly correlated with the amount of daytime tidal exposure in the month prior to the survey as well as the amount of solar radiation in the twelve months prior to monitoring. Preliminary results from light and temperature loggers deployed in September 2010 indicate a negative correlation between photosynthetically active radiation (PAR) with rainfall and meadow depth, and high within- and between-meadow variation in average and peak water temperatures and PAR. Continued collection of fine-scale light and temperature data within the monitoring meadows will enhance the ability of the program to pinpoint some of the causes of seagrass declines. Seagrass distribution and species composition were also mapped within the broader Weipa port limits as occurs every three years. Total seagrass meadow area was 3996 ± 231 ha, its highest level since 2001 and an increase of 21% since 2008 monitoring. Aggregated patches of seagrass continued to be the dominant landscape category and described 55% of meadows. Species composition in many of the individual meadows had changed since 2008, which is characteristic of the dynamic nature of seagrass meadows. Seagrasses appear to have been resilient to the impacts associated with regular port maintenance dredging during the life of the current monitoring program. However, Fisheries North remain concerned that the continued low biomass of some of the meadows in Weipa leaves them vulnerable to additional stresses including those associated with dredging. Seagrass monitoring will continue to provide information necessary to inform the management of maintenance and capital dredging programs in Weipa, and forms an integral component of the Dredge Technical Review Panel's assessment of potential dredge mitigation strategies that may need to be applied to continue to protect seagrasses within the Port.
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36
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Department of Employment, Economic Development and Innovation (DEEDI)
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Cairns, QLD, Australia
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