WAMSI 2 Dredging Science Node: Theme 5 Project 5.4 Recovery mechanisms [dataset]

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School of Natural Sciences


The objective of Project 5.4 was to determine whether recovery of seagrass in the Pilbara following disturbance is by sexual (recruitment from seeds) or asexual (vegetative regrowth from rhizome extension) means, and the relative importance of each, thereby determining the capacity, timeframes and mechanisms of recovery from light and sediment deposition effects. This was achieved through an experiment in which plots cleared of seagrass were enclosed by a barrier or left open, and changes in cover were compared to unmanipulated control (and procedural control) plots. The experiment was done at two sites, a 2-m deep (the 'shallow' site) and a 6-m deep (the 'deep' site). The experiment was intended to run for 6 months, but after 5 months disturbance caused by Tropical Cyclone (TC) Olwyn reduced seagrass cover at both sites, and removed the experimental apparatus at the shallow site. Nevertheless, results yielded by surveys prior to TC Olwyn unambiguously showed strong evidence for recovery through vegetative regrowth (full recovery in cleared plots with no barriers, plus a pattern of increasing cover from the edges of the plots) and no evidence for recovery through recruitment from seeds (no seagrass was ever recorded in any cleared plot with a barrier). The main species of seagrass present was Halophila ovalis, which is widespread throughout northwestern Australia. The transferability of inferences from this study to other places in the Pilbara is hampered by the substantial variation in abundance and species composition from place to place - different species might have different mechanisms of recovery. The nature of the experimental disturbance (complete removal of all seagrass, including roots and rhizomes) is a reasonable facsimile of a severe dredging-induced disturbance, but the spatial extent of the experimental clearances (~0.5 m2) is orders of magnitude smaller than the spatial extents of dredging-induced mortality induced elsewhere: it is plausible that recovery of patches within meadows is more easily achieved through rhizome extension than recovery that encompasses spatial extents of hectares.

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At each of the two sites at Thevenard Island (21.5 S, 115.0 N: one at 2 m and the other at 6 m depth), 12 circular plots (diameter 75 cm) were completely cleared of all seagrass by hand. Six of the cleared plots were surrounded by a dark plastic border sunk into the sediment to a depth of 6 cm to prevent rhizome extension from surrounding meadow, and six cleared plots were left without a border to allow rhizome extension. Six uncleared plots were surrounded by a plastic border with gaps to create a procedural control (to verify whether the presence of the border itself influenced the ability of seagrass to grow within the plot), and six additional uncleared plots were marked as unmanipulated controls. The experiment was established on the 23rd and 24th of November 2014, and recovery was monitored at 10, 38, 45 and 98 days afterwards. Final measurements were to be taken in June 2015 at approximately 190 days from the establishment of the experiment, but in March 2015 Tropical Cyclone Olwyn (Category 3) passed within 40 km of Thevenard Island, and when the experiment was visited on 11 June 2015, none of the experimental apparatus (borders, tags, star pickets) remained at the 2-m ('shallow') site. The plots were intact at the 6-m ('deep') site, but visibility was extremely poor, and although photographs were taken they could not be analysed with confidence. On each monitoring date, photographs were taken of each plot. Divers also produced diagrams of any seagrass growth in cleared plots. Each photograph was analysed using TransectMeasure (® SEAGIS) software. A random array of 200 dots was placed over the image of the plot, and for each dot an operator (R. McCallum) identified the substrate immediate underneath as seagrass species, sediment, epibenthos or macroalgae under the dot. To further distinguish whether any regrowth was occurring from the edges of the plots (implying vegetative recovery) the dots were assigned into 'edge' (outer 20% of the plot) and 'centre' (inner 80% of the plot)

Location Details

Max Latitude: 21°23′60″ S,

Min Latitude: 21°30′0″ S

Max Longitude: 115°5′60″ E

Min Longitude: 114°54′0″ E

Coordinate System: WGS84

Data URLs

Access record on CSIRO here

Research Activity Title

WAMSI 2 Dredging Science Node: Theme 5 Project 5.4 Recovery mechanisms

Research Activity Description

WAMSI Dredging Science Node - Theme 5 Primary producers

Defining thresholds and indicators of response by corals, seagrasses and filter-feeders to dredging-related pressures.

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.


Mat Vanderklift: Mat.Vanderklift@csiro.au


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