By Jo Myers & Mat Vanderklift (CSIRO)
Dredging associated with large port developments can reduce the light required for seagrass photosynthesis and smother their growth. This can be caused by large plumes of sediment which some seagrasses are very sensitive to (along with natural disturbances such as cyclones), while others have the capacity to cope, or to recover quickly.
Relatively little is known about normal patterns in seagrass composition, abundance and distribution in north-western Australia, including whether they have natural cycles in abundance across seasons. Knowing more about seagrass ecosystems is important for designing or interpreting studies that aim to detect potential dredging-related impacts on seagrass, and when making predictions about whether they can recover from disturbances, and if so, how quickly.
Results from research on seagrasses in the Pilbara, funded by the Western Australian Marine Science Institution (WAMSI) Dredging Science Node, has found that the most common species of seagrass in the region have similar life histories (they tend to be relatively good at colonising new space) but tend to have different patterns of spatial and temporal variation in abundance and reproduction. This creates challenges for people who assess environmental approvals, because it means that the temporal dynamics of seagrasses in the Pilbara might be less predictable than those of seagrasses in adjacent regions (the Kimberley and the Gascoyne). Monitoring programs that are established to detect potential changes in abundance will need to ensure that their design accommodates this variability.
WAMSI Scientist monitoring recovery of seagrass from cleared experimental plots
Another key finding from the research, led by CSIRO and Edith Cowan University, was the main mechanisms by which seagrasses recover after disturbance. In tropical regions, such as northwest Australia, small-leaved species of seagrasses are often characterised by natural patterns of loss and recovery that can span months or years. Vegetative growth (extension of rhizomes by remaining plants) accounts for most recovery, though recovery from seeds has also been recorded. Understanding which of these mechanisms dominates at a particular location is important for predicting the potential for seagrasses to recover after loss or reduction in abundance.
A field experiment undertaken at Thevenard Island during 2014 and 2015 involved clearing areas of seagrass and the placement of partial and full barriers around the cleared areas — these barriers were designed to disentangle vegetative growth from regeneration from seeds. The experiment showed that the primary mechanism for recovery of cleared areas was vegetative growth.
Halophila ovalis seagrass. Tropical seagrasses are important habitats for marine turtles including the loggerhead turtle that feeds on fauna associated with seagrass beds such as ascidians, clams, mussels and other invertebrates (Photo: Mat Vanderklift, CSIRO)
The main seagrass species at the study site was Halophila ovalis, which is also the most widespread species in the Pilbara. One key implication of this finding is that recovery from disturbances that remove seagrass from relatively small areas should occur within months, provided that sufficient meadow remains for rhizomes to colonise from. When seagrass loss occurs over a larger area, recovery might rely more heavily on immigration of plant fragments or seeds from distant sites, which will take much longer.
For further information about WAMSI and the Dredge Science Node, including access to the final reports (Theme 5.3 and Theme 5.4) see wamsi.org.au/dredging-science-node
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.