Data

WAMSI 2 - Dredging Node - Project 5.4 - Recovery mechanisms

Australian Ocean Data Network
CSIRO O&A, Information & Data Centre (Point of Contact) Vanderklift, Mat (Point of Contact)
Viewed: [[ro.stat.viewed]] Cited: [[ro.stat.cited]] Accessed: [[ro.stat.accessed]]
ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rfr_id=info%3Asid%2FANDS&rft_id=https://marlin.csiro.au/geonetwork/srv/eng/catalog.search#/metadata/2299d13f-4c1d-77d8-e053-08114f8cf79f&rft.title=WAMSI 2 - Dredging Node - Project 5.4 - Recovery mechanisms&rft.identifier=Anzlic Identifier: ANZCW0306014630&rft.publisher=Australian Ocean Data Network&rft.description=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.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)&rft.creator=Anonymous&rft.date=2015&rft.coverage=northlimit=-21.6; southlimit=-22.3; westlimit=114.1; eastLimit=114.4&rft.coverage=northlimit=-21.6; southlimit=-22.3; westlimit=114.1; eastLimit=114.4&rft_subject=oceans&rft_subject=CTDs (Conductivity-Temperature-Depth Profilers)&rft_subject=Water Column Samplers&rft_subject=Cameras - Still - Underwater&rft_subject=Countries | Australia&rft_subject=Marine Features (Australia) | Exmouth Gulf, WA&rft_subject=Primary producer response to dredging related pressures&rft_subject=Western Australian Marine Science Institute&rft_subject=Practical salinity of the water body&rft_subject=Temperature of the water body&rft_subject=Pressure (measured variable) in the water body exerted by overlying sea water and any medium above it&rft_subject=Pressure (measured variable) in the water body exerted by overlying sea water only&rft_subject=CTD&rft_subject=research vessel&rft.type=dataset&rft.language=English Access the data

Contact Information



Brief description

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.

Lineage

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)

Notes

Credit
CSIRO, Edith Cowan University, Western Australian Science Institution
Credit
Mathew Vanderklift
Credit
Douglas Bearham
Credit
Mick Haywood
Credit
Paul Lavery
Credit
Roisin McCallum
Credit
James McLaughlin
Credit
Kathryn McMahon
Credit
Nick Mortimer

Data time period: 2014-11-23 to 2015-06-30

114.4,-21.6 114.4,-22.3 114.1,-22.3 114.1,-21.6 114.4,-21.6

114.25,-21.95

text: northlimit=-21.6; southlimit=-22.3; westlimit=114.1; eastLimit=114.4

Other Information
WAMSI 2 DSN 5.4 Recovery Mechanisms Final Report

uri : https://publications.csiro.au/rpr/pub

URI : https://publications.csiro.au/rpr/pub?list=SEA&pid=csiro:EP163957

WAMSI 2 - Dredging Node - Project 5.4 - Recovery mechanisms - Data Link (Data Link (CSIRO DAP))

uri : https://data.csiro.au/dap/landingpage

URI : https://data.csiro.au/dap/landingpage?pid=csiro:17650

Identifiers
  • global : 2299d13f-4c1d-77d8-e053-08114f8cf79f
  • Local : Marlin Record Number: 14630
  • Local : Anzlic Identifier: ANZCW0306014630