Port of Weipa long‐term seagrass monitoring: August 2012

Carter, A.B., McKenna, S.A., and Rasheed, M.A. (2013) Port of Weipa long‐term seagrass monitoring: August 2012. Report. TropWATER, James Cook University, Cairns, QLD, Australia.

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This report details the results of the Port of Weipa seagrass monitoring program including the most recent survey in August 2012. Seagrasses in the Port of Weipa were in a reasonable condition in 2012, with the biomass of monitoring meadows remaining similar to or increasing from the previous year. However there were some reductions in area of meadows and there is still an overall declining trend in seagrass biomass for some meadows over the 13 years of monitoring, indicating seagrasses may remain vulnerable to further natural or anthropogenic stress. Importantly, however, the key meadow on the western bank of the Embley River that is closest to maintenance dredging activity has shown a significant increase in biomass and is likely to be resilient to planned maintenance dredging activities in 2013.

Changes in seagrass biomass between 2011 and 2012 varied for the core monitoring meadows. Biomass increased significantly in the Enhalus acoroides dominated meadow on the western bank of the Embley River (A2), the largest monitoring meadow, but did not change in the smaller E. acoroides dominated and Halodule uninervis dominated meadows. Biomass in A2 in 2012 was similar to other high biomass years such as 2002 and 2010, and coincided with the re‐appearance of moderate/dense biomass hotspots similar to those recorded in 2010. Biomass in two of the smaller monitoring meadows (A3 and A6) more than doubled between 2011 and 2012 although the highly variable nature of these meadows meant that this change was outside the ability of statistical analysis to detect change.

Biomass for the core monitoring meadows has varied considerably between years, with an overall trend of decline in three (A2, A6, A7) of the five meadows in the 13 years of monitoring. These declines are likely associated with natural shifts in tidal exposure and changes in light and temperature associated with local climate conditions. These changes were 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.

In 2012, meadow area around the major port operations (IMA) was below the thirteen‐year average of 1035 ± 66 ha. Total meadow area within the IMA reduced by 5.8% between August 2011 and 2012 reversing a 3 year trend of increasing area. While the declines were recorded for all of the meadows, all but meadow A2 remained within the range of previously recorded values.

The 12 months prior to the 2012 survey were characterised by above‐average rainfall and below‐average tidal exposure. Preliminary results from light and temperature loggers deployed since September 2010 indicate a strong influence of tidal cycles on photosynthetically active radiation (PAR) and water temperature, with the highest PAR recorded during midday low tides, particularly in the shallower intertidal meadows. The PAR data set obtained so far also indicates PAR is reduced between October and March each year, coinciding with increased rainfall. 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 changes.

Seagrasses have been resilient to the impacts associated with regular port maintenance dredging during the life of the monitoring program. However, the continued low biomass of some of the meadows in Weipa may leave them vulnerable to additional stresses including those associated with dredging. Ongoing monitoring will provide the information necessary to inform the management of maintenance and capital dredging programs in Weipa to ensure the protection of these seagrasses. It forms an integral component of the Dredge Technical Advisory Consultative Committee’s assessment of potential dredge mitigation strategies that may need to be applied to continue to protect seagrasses within the port. Results from 2012 monitoring show the large meadow on the western bank of the Embley River (A2) that is closest to the majority of maintenance dredging in the port was in a relatively robust condition. The relatively high biomass of this meadow indicates that it should remain resilient to planned maintenance dredging activities in 2013, provided the duration of dredging remains relatively short and that there are no major loses associated with climate or other impacts leading up to the 2013 dredging campaign.

Item ID: 39670
Item Type: Report (Report)
Keywords: seagrass habitats, port management, recovery, resilience, dredging, marine monitoring, environmental monitoring
Additional Information:

Access: This report is openly accessible from the link to TropWATER's Technical Reports web page above.

Copyright: Please refer to the copyright statement in the report.

Funders: North Queensland Bulk Ports Corporation (NQBP)
Date Deposited: 25 Aug 2015 05:59
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
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