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Results released on seagrass responses to dredging in northwest Australia

27 February 2017

Halophila ovalis meadow at Thevenard Island (Kath McMahon)

Three new reports have been released on the primary producer responses to dredging.

There is almost no knowledge of how seagrass primary producers in the northwest of Australia will respond to the environmental changes produced by dredging. Consequently, it is difficult to predict and then manage the impacts of dredging on these critical habitats with an acceptable level of certainty. 

Research within the Western Australian Marine Science Institution (WAMSI) Dredging Science Node focuses on two of the most significant stresses produced by dredging: the reduction in light availability to plants; and the smothering of seagrass and algae as suspended sediments settle.

Project leader Kathryn McMahon from Edith Cowan University and team members from The University Australia, Department of Parks and Wildlife WA and The University of Adelaide have looked at three key areas:

  1. Seagrasses of the northwest of Western Australia: biogeography and considerations for dredging-related research;

  2. The current state of knowledge regarding the effects of dredging-related ‘pressure’ on seagrasses using information compiled from unpublished industry data, as well as published reports, articles and books; and

  3. Genetic variability within seagrass of the northwest of Western Australia

The first project identified five dredging related stressors that are likely to directly impact seagrass habitat and prioritised the top three in the following list that are of most interest for impact prediction and management of dredging events:

  • reduced benthic light quantity;

  • burial by sediment;

  • sediment anoxia and increased hydrogen sulfide production;

  • altered benthic light quality (i.e. spectral characteristics); and

  • increased suspended sediment

It identified knowledge gaps in three key areas:

  • How dredging affects environmental conditions that are likely to impact seagrass.

  • Thresholds for dredging-related stressor beyond which seagrasses will be affected; and

  • Monitoring during dredging campaigns.

The second project identified the seagrass species Halophila ovalis, Halodule uninervis and Cymodocea serrulata, as the focus of subsequent research into thresholds and indicators of response to dredging-related pressures.

The third study is the first of its kind to examine the patterns of genetic diversity in seagrasses in the Pilbara region which strongly influences their ability to adapt to, resist or recover from these pressures. 

This study identified  that: 

  1. Most meadows examined  had  relatively  high  clonal  diversity  (i.e.  many  unique  individuals in the meadow), so both sexual reproduction and vegetative growth are important for maintaining these populations; and

  2. There was a reasonably high level of migration of genes over distances of 2−5km, but lower levels over  greater  distances. The  study  also  showed  that  not  all  seagrass  meadows  and  species  in NW Australia  have  a  similar  level  of  genetic  diversity. This information should be incorporated into management decisions as the level of genetic diversity has implications for the ability of populations to resist and recover from disturbance.

The WAMSI Dredging Science Node is one of the largest single issue research programs in Australia meeting the needs of the State Government and industry to improve their understanding of how key primary producers are affected by dredging-related pressures.

The full reports can be found on the WAMSI DSN Primary Producer project page

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

 

Category: 
Dredging Science