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ICHTHYS GAS FIELD DEVELOPMENT
DOCUMENT COVER SHEET Total # of Pages (incl. Doc Cover Sheet)
31
Company Document No L384-AW-REP-10047 Revision No 0
Document Title Seagrass Dark Recovery Experiment Report
Contract/Purchase Order No 800429
Equipment Tag No
Contractor Document No EL1112047 Contractors Rev No 0
Contractor shall ensure that documents have been fully checked and approved prior to submittal to INPEX.
Prepared Isabel Jimenez Checked Joanna Lamb Approved Craig Blount
Date 26/03/2013 Date 26/03/2013 Date 26/03/2013
Notes: Contractor Name, Address and Logo
Cardno (NSW/ACT) Pty Ltd
Level 9, 203 Pacific Highway
St Leonards NSW 2065
Cardno WA Pty Ltd
11 Harvest Terrace
West Perth WA 6008
Telephone: 08 9273 3888
Cardno NT Pty Ltd
Level 6 93 Mitchell St
Darwin NT 0800
A 18/03/2013 Issued for review
B 22/03/2013 Issued for review
0 26/03/2013 Issued for use
REV No DATE ISSUE PURPOSE
Revision History to Company’s Document Number
DOCUMENT NUMBER: C067-AG-FRM-0003 REV 2
Seagrass Dark Recovery Experiment
Ichthys Nearshore Environmental Monitoring Program L384-AW-REP-10047
Prepared for INPEX
March 2013
Seagrass Dark Recovery Experiment
Ichthys Nearshore Environmental Monitoring Program L384-AW-REP-10047
Seagrass Dark Recovery Experiment Ichthys Nearshore Environmental Monitoring Program
Prepared for INPEX Cardno ii
Document Information
Prepared for INPEX
Project Name Ichthys Nearshore Environmental Monitoring Program
File Reference L384-AW-REP-10047_0_Seagrass Dark Recovery Experiment Report.docx
Job Reference L384-AW-REP-10047
Date March 2013
Contact Information
Cardno (NSW/ACT) Pty Ltd Cardno (WA) Pty Ltd Cardno (NT) Pty Ltd
Level 9, The Forum 11 Harvest Terrace Level 6, 93 Mitchell Street
203 Pacific Highway West Perth WA 6005 Darwin NT 0800
St Leonards NSW 2065
Telephone: 02 9496 7700 Telephone: 08 9273 3888 Telephone: 08 8942 8200
Facsimile: 02 9499 3902 Facsimile: 08 9486 8664 Facsimile: 08 8942 8211
International: +61 2 9496 7700 International: +61 8 9273 3888 International: +61 8 8942 8211
www.cardno.com.au www.cardno.com.au www.cardno.com.au
Document Control
Version Date Author Author Initials
Reviewer Reviewer Initials
A 18/03/2013
Isabel Jimenez
Andrea Nicastro
Brendan Alderson
IJ
AN
BA
Joanna Lamb
Craig Blount
JL
CB
B 22/03/2013 Isabel Jimenez IJ Joanna Lamb
Craig Blount
JL
CB
0 26/03/2013 Isabel Jimenez IJ Joanna Lamb
Craig Blount
JL
CB
This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement for the performance of the Services. Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document.
Seagrass Dark Recovery Experiment Ichthys Nearshore Environmental Monitoring Program
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Executive Summary
A Seagrass Monitoring Program has been developed to detect potential changes in seagrass health
indicators and infer whether any changes are a result of dredging and/or spoil disposal activities associated
with the Ichthys Project (the Project) in Darwin Harbour. The Dredging and Spoil Disposal Management
Plan – East Arm (DSDMP, INPEX 2012) sets out the framework for the Monitoring Program including a dark
recovery (shading) experiment (the Experiment) to mimic potential effects of dredging and to investigate
what the expected rate of recovery of seagrass may be.
The Experiment involved exposing seagrass plots to continuous darkness (the Shaded treatment) at two
sites (Casuarina and Fannie Bay) for a period of two months (24 September to 24 November 2012), followed
by a three month recovery phase (25 November 2012 to 24 February 2013). For comparison, seagrass was
also monitored in Control (unshaded) plots.
After two months exposure to darkness (Dark phase), quasi-complete mortality was observed in the Shaded
plots at both sites, whereas shoot densities in the Control plots remained similar to initial levels. No recovery
was observed in the Shaded plots in the three months following removal of the shade screens (i.e. in the
Recovery phase). Further, shoot density declined considerably in Control plots and decreased to
approximately 5% of initial levels by the end of the Experiment.
The severe decline of seagrass density in Control plots during the Recovery phase of the Experiment
coincided with widespread natural seasonal declines in seagrass distribution and abundance. Strong
westerly winds and increased wave heights were noted in January 2013 following Tropical Cyclone (TC)
Narelle which formed off the coast of Western Australia, and these were associated with increased sediment
resuspension and turbidity in shallow areas. The subsequent decrease in benthic light availability during this
period most likely accounted for the observed decline within the Control plots and would have prevented
seagrass potentially recovering in Shaded plots. Benthic light availability was reduced to 0% of surface
irradiance for two to three weeks, a level known to impact Halodule and Halophila sp. Hence, the similarity
in the rate and severity of the decline in the Shaded plots (during the Dark phase) to the decline in the
Control plots (during the Recovery phase) illustrated how a simulated dredging impact was comparable to
that of a natural, weather-related impact (i.e. an increase in turbidity during the wet season).
The results of the Experiment are consistent with expected seasonal growth patterns of ephemeral tropical
seagrasses such as Halodule and Halophila spp. (i.e. a wet season die-off). This is further supported by
results from monitoring and mapping surveys undertaken since June 2012 (Cardno 2012c, Cardno 2012d,
Geo Oceans 2013), which indicated that seagrass distribution in Darwin Harbour reached a seasonal peak
towards the end of the dry season (October), after which severe and widespread decline occurred during the
wet season.
Opportunistic field observations of new seagrass shoots over the shade screens indicate a potential for rapid
recovery should conditions be favourable; however it is unknown whether the shoots grew from seed or
vegetatively from fragments. Seagrass mapping results found a 250% increase in overall seagrass habitat
extent between June and October 2012, including a ten-fold habitat expansion at East Point, further
indicating the potential for rapid growth and recovery from declines potentially associated with dredging. The
Seagrass Monitoring Program will continue to monitor seagrass presence throughout the duration of the
dredging program.
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Glossary
Term or Acromym Definition
Benthic On the seafloor
DSDMP Dredging and Spoil Disposal Management Plan – East Arm
DSV Dive Support Vessel
EIS Environmental Impact Statement
GPS Global Positioning System
HSE Health Safety Environment
Intertidal The portion of shoreline between low and high tide marks, that is intermittently
submerged
LAT Lowest Astronomical Tide
NEMP Nearshore Environmental Monitoring Plan
NTU Nephelometric Turbidity Units
PAR Photosynthetically Available Radiation
Permanova Permutational Analysis of Variance
Photoquadrat Virtual sampling unit of known dimensions within a photograph of the seafloor,
used to quantify seagrass density and percent cover
QA Quality Assurance
SE Standard error of the mean
Subtidal Waters below the low-tide mark
Turbidity An indication of water clarity
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Table of Contents
Executive Summary iii
Glossary iv
1 Introduction 7
1.1 Background 7
1.2 Requirement to Monitor Seagrass in Darwin Harbour 7
1.3 Objectives 7
2 Methodology 8
2.1 Vessels, Safety and Environmental Management 8
2.2 Sampling Design 8
2.2.1 Sites, Timing and Frequency of Surveys 8
2.2.2 Experimental Treatments and Plots 10
2.3 Measurement of Shoot Density 11
2.3.1 Image Analysis 11
2.3.2 Statistical Analysis 12
2.4 Water Quality and Light Availability 13
2.5 Quality Control 14
3 Results 15
3.1 Shoot Density 15
3.2 Water Quality and Light Availability 16
3.3 Quality Control 16
4 Discussion 19
5 Conclusions 21
6 Acknowledgements 22
7 References 23
Tables
Table 2-1 Co-ordinates of dark recovery experiment sites 8
Table 2-2 Survey dates for the Experiment and corresponding activities 8
Table 2-3 Explanation of factors used in the statistical analyses 13
Table 2-4 Terms used in describing the outcomes of the statistical analyses 13
Table 3-1 Mean and standard error (SE) of shoot density (Shoots m-2
) for Halodule and Halophila sp. during the Dark and Recovery phases at A) Casuarina Beach and B) Fannie Bay 15
Table 3-2 Summary of Permanova for seagrass shoot density showing the level of significance. * = P(perm) < 0.05; - = redundant term, ns = not significant. 16
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Figures
Figure 2-1 Location of dark recovery experiment sites 9
Figure 2-2 Example of shade screen plot installed at Casuarina Beach showing the reinforced pegs that anchored the shade screen to the seabed and swim line linking each plot 10
Figure 2-3 Diver taking images of seagrasses within plots at Casuarina Beach (quadrat can be seen on the seabed) 11
Figure 2-4 Quadrat set-up showing the individual sub-quadrats within the larger 1 m2 quadrat (the field of
view of the camera was slightly larger than each sub-quadrat) 12
Figure 3-1 Mean seagrass shoot density (±SE) for Shaded and Control plots during two months of dark exposure (grey shaded area) and three months of recovery (clear area) at Casuarina and Fannie Bay 16
Figure 3-2 Time-series of turbidity, PAR (mol photons m-2
s-1
) and water temperature at the Casuarina Water Quality monitoring site from 24 September 2012 to 15 February 2013 17
Figure 3-3 Time-series of turbidity, PAR (mol photons m-2
s-1
) and water temperature at the Fannie Bay Water Quality monitoring site (Site 01) from 24 September 2012 to 15 February 2013 18
Figure 4-1 Field observation of Halodule sp. growing in sediment layer deposited over a shade screen at Casuarina (23 November 2012). Image scale approximately 9 cm x 12 cm 20
Appendices
Appendix A Results of Statistical Analyses 24
Appendix B Water Quality Summary Data 27
Appendix C Quality Control 29
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1 Introduction
1.1 Background
INPEX is the operator of the Ichthys Gas Field Development Project (the Project). The Project comprises the
development of offshore production facilities at the Ichthys Field in the Browse Basin, some 820 km west-
south-west of Darwin, an 889 km long subsea gas export pipeline (GEP) and an onshore processing facility
and product loading jetty at Blaydin Point on Middle Arm Peninsula in Darwin Harbour. To support the
nearshore infrastructure at Blaydin Point, dredging works will be carried out to extend safe shipping access
from near East Arm Wharf to the new product loading facilities at Blaydin Point, these will be supported by
piles driven into the sediment. A trench will also be dredged to seat and protect the GEP for the Darwin
Harbour portion of its total length. Dredged material will be disposed at the spoil ground which is located
approximately 12 km north-west of Lee Point. A detailed description of the dredging and spoil disposal
methodology is provided in Section 2 of the DSDMP (INPEX 2012).
1.2 Requirement to Monitor Seagrass in Darwin Harbour
Following an Environmental Impact Statement (EIS) (INPEX 2011), the Project was approved subject to
conditions that included monitoring for potential effects of dredging or spoil disposal on local ecosystems
(including seagrasses) and potentially vulnerable populations. Dredging can impact seagrasses directly
through physical removal or smothering and indirectly through the creation of turbid plumes and
sedimentation. As seagrasses are photosynthetic, reduction of light from increased turbidity may affect their
growth and survival. Excessive sedimentation and settlement of suspended material on leaf blades may also
interfere with photosynthesis (McMahon et al. 2011).
The DSDMP sets out a monitoring program to examine the potential impact on seagrasses from dredging
and spoil disposal activities associated with the Project in and around Darwin Harbour, including a seagrass
recovery (shading) experiment to mimic potential effects of dredging and to investigate what the expected
rate of recovery of seagrass may be (see Section 8.3.2 of the DSDMP). The Nearshore Environmental
Management Plan (NEMP) establishes the methodology and indicators for the monitoring program and the
Experiment (Cardno 2012a).
1.3 Objectives
The DSDMP sets out the objectives of the Experiment as follows:
> Gain an understanding of the potential for seagrasses in Darwin Harbour (and surrounds) to recover from
dredging-related impacts; and
> Provide supporting data that may be used in the event of a Level 3 trigger exceedance to determine what
level of response is appropriate and practicable.
The methodology was designed to have sufficient replication to determine whether seagrass can recover
from potential dredging related impacts, accounting for any spatial variation in rates of recovery.
This report describes the results of the Experiment that was undertaken between 24 September 2012 and
24 February 2013.
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2 Methodology
2.1 Vessels, Safety and Environmental Management
Field work conducted during the Experiment was carried out from the DSV Josh Sarelle and DSV Bushman
operated by Neptune Diving Services (NDS). All work was completed in accordance with the Project Health
Safety and Environment (HSE) Plan. Diving was conducted using a combination of Self Contained
Underwater Breathing Apparatus (SCUBA) (Australian Diver Accreditation Scheme (ADAS) Level AS
2815.1) and surface supply breathing apparatus (SSBA) (ADAS Level AS 2815.2) in accordance with
Australia/New Zealand Standard Occupational Diving Operations Part 1: Standard Operational Practice
(ASNZS 2299.1:2007). Data were collected by scientific divers and site installation and maintenance was
completed by commercial divers from NDS.
2.2 Sampling Design
2.2.1 Sites, Timing and Frequency of Surveys
The Experiment was undertaken in seagrass beds at Casuarina Beach and Fannie Bay (Table 2-1; Figure
2-1). Consistent with the methodology described in the NEMP, these sites provided information for varying
seagrass density covers as seagrasses at Casuarina Beach tend to be of lower density than those within
Fannie Bay. Both locations lie outside of the Zone of Moderate Impact for turbidity and sedimentation
impacts to seagrass predicted to occur as a result of the Project’s dredging and spoil disposal activities (refer
to Section 6.5 of the DSDMP).
Table 2-1 Co-ordinates of dark recovery experiment sites
Sites Latitude (°S) Longitude (°E)
Casuarina Beach -12.361783° 130.853783°
Fannie Bay -12.431450° 130.830150°
The Experiment involved six surveys undertaken at monthly intervals (Table 2-2), with Survey 1 being the
initial Experiment set up. There were two phases of the Experiment: Dark and Recovery. The ‘Dark’ phase
(two months) simulated conditions of no light (potentially associated with great turbidity). The ‘Recovery’
phase simulated recovery of seagrass from two months of ’dark’ conditions. As stated in the NEMP, plots
were merely inspected for the presence of seagrass after one month Dark exposure (Survey 2), and shoot
density was quantified at the end of the Dark phase and start of the Recovery phase (Survey 3).
Due to poor weather conditions (strong westerly winds and large swell) sampling was unable to be
completed during Survey 5 (i.e. the two month recovery survey).
Table 2-2 Survey dates for the Experiment and corresponding activities
Recovery Experiment Sampling Dates Phase
Survey 1 (Site Set up) 24 to 26 September 2012 Initial set up
Survey 2 24 to 25 October 2012 1 month Dark
Survey 3 20 to 24 November 2012 2 month Dark
Survey 4 20 to 24 December 2012 1 month Recovery
Survey 5 21 to 24 January 2013 2 month Recovery
Survey 6 22 to 24 February 2013 3 month Recovery
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Figure 2-1 Location of dark recovery experiment sites
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2.2.2 Experimental Treatments and Plots
At each site (Casuarina Beach and Fannie Bay) divers installed five replicate 1 m2 plots on the seabed in two
experimental treatments (‘Shaded’ and ‘Control’) for a total of 20 plots. At each site, plots were placed
approximately 3 m apart and were connected with a swim line to assist the divers in relocation for
subsequent surveys. Each plot was marked out with 6 mm silver rope, pegged down onto the seabed with
steel reinforced pegs and numbered tags were installed on each plot (Figure 2-2). The allocation of plots to
treatments was done alternately to ensure that all plots from a particular treatment were not grouped
together.
Plots in the ‘Shaded’ treatment were covered by a shade screen for the first two months of the Experiment
(Dark phase: Surveys 1 to 3). The shade screen consisted of shade mesh (which was rated to exclude 95%
of light) attached to a reinforced steel reo-bar frame, approximately 1 m2 in size. The frame was secured to
the seabed at each corner and along the edges with steel reinforced pegs. Control plots remained
uncovered for the duration of the Experiment. Two additional shaded indicator plots (i.e. with shade screens
installed) were installed at each site to verify the loss of seagrass leaves and rhizomes throughout the Dark
phase of the Experiment. Seagrass in these indicator plots was monitored without disturbing the actual
Experimental plots prior to the start of the Recovery phase (i.e. when shade screens were removed). To
prevent surrounding seagrasses translocating nutrients into the treatment plots, all plots were ‘gardened’
around the perimeter of the frames at each survey.
Although the aboveground biomass of seagrass within the shaded indicator plots was greatly reduced
following the first month of the Experiment, it was decided that the shade screens be left in place for an
additional month to ensure all sub-surface seagrass biomass (i.e. rhizomes) underneath the shade screen
had disappeared. During Survey 3, the indicator plots exhibited the complete loss of aboveground biomass
and no presence of live rhizome material was apparent; the screens were then removed for the recovery
phase (Surveys 4 to 6).
Figure 2-2 Example of shade screen plot installed at Casuarina Beach showing the reinforced pegs that anchored the shade screen to the seabed and swim line linking each plot
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2.3 Measurement of Shoot Density
2.3.1 Image Analysis
In Survey 1, immediately prior to the installation of the shade screens, plots were surveyed to assess initial
seagrass conditions and to estimate seagrass shoot density. Sampling was undertaken using photography.
A 1 m2 quadrat was placed over the plot and images were taken using a Canon G12 digital camera with
underwater housing mounted to a camera frame (Figure 2-3).
Figure 2-3 Diver taking images of seagrasses within plots at Casuarina Beach (quadrat can be seen on the seabed)
To ensure sufficient image quality for analysis, the 1 m2 quadrats were divided into eight smaller adjoining
frames (sub-quadrats) each covering 0.075 m2 (Figure 2-4). The frame held the camera approximately
43 cm from the seabed, ensuring that each sub-quadrat was captured in the camera’s field of view and
resulting in eight images of each plot. The total area covered by the eight sub-quadrats (i.e. the area
surveyed within the larger 1 m2 quadrat) was 0.6 m
2. Initially, still images were captured by taking
photographs of each sub-quadrat; however, this was often difficult due to the presence of wave surges
(especially at Casuarina Beach), which at times prevented the diver from taking suitable quality still images.
This problem was overcome by taking video footage and extracting still images of each sub-quadrat from the
video footage prior to processing. In the event that water clarity was poor and no suitable images of plots
could be captured, in situ diver counts of seagrass shoots in each sub-quadrat were also conducted to
ensure that data was collected for that particular plot/site.
Each image was colour corrected prior to processing to enhance image quality and allow for a more accurate
estimation of seagrass shoot density within each sub-quadrat.
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The number of shoots for each seagrass species within eight sub-quadrats within plots was counted and
recorded for all replicates within both treatments and then added together to provide a count for the entire
quadrat. Shoot counts for individual species were converted to a density measurement by dividing the
recorded shoot count for each replicate plot by 0.6 m2 (i.e. the survey area of the larger quadrat).
Figure 2-4 Quadrat set-up showing the individual sub-quadrats within the larger 1 m2 quadrat (the
field of view of the camera was slightly larger than each sub-quadrat)
2.3.2 Statistical Analysis
Statistical analyses were undertaken on total seagrass shoot density.
Repeated Measures Analysis of Variance was undertaken using PERMANOVA+ software in PRIMER v6 to
examine the rate of seagrass recovery from shading-induced disturbance. The analyses focused on testing
the null hypothesis that there were no differences in seagrass shoot density among surveys, treatments or
sites and, in effect, examining the rate of recovery of seagrass from a disturbance where light had been
reduced. Factors in the analysis were:
> Survey (fixed, orthogonal) – 4 levels (Surveys 1, 3, 4 and 6);
> Site (random, orthogonal) – 2 levels (Fannie Bay and Casuarina Beach);
> Treatment (fixed, orthogonal) – 2 levels (Shaded and Control); and
> Plots (nested within Site and Treatment, random) – repeated measures term.
Statistical analyses were based on dissimilarity matrices created using Euclidian distance measures.
Results of the statistical analyses (i.e. rejection of null hypotheses) were interpreted through a series of
statistically significant main factors and interactions (Table 2-3 and Table 2-4). Where significant
interactions or main factor effects were detected, post hoc permutational t-tests using PERMANOVA+
software were carried out to identify the levels of factors in which differences occurred. No multiple test
corrections were applied to t-test results, consistent with a conservative statistical approach and in line with
the Precautionary Principle.
Sub-quadrat 1
Sub-quadrat 2
Sub-quadrat 3
Sub-quadrat 4
Sub-quadrat 5
Sub-quadrat 6
Sub-quadrat 7
Sub-quadrat 8
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Table 2-3 Explanation of factors used in the statistical analyses
Component of Variation Interpretation
Survey Indicates a significant difference between Surveys (Survey 1 to Survey 6)
independent of Site and Treatment.
Site Indicates larger-scale significant variability between Sites (Fannie Bay and
Casuarina Beach) independent of Survey and Treatment.
Treatment Indicates a significant difference between Treatments (Shaded and Control)
independent of Surveys and Sites.
Plot (Site x Treatment) Indicates smaller-scale significant variability among Plots within Sites and
Treatments independent of Survey.
Survey x Site Indicates differences between Surveys are dependent on the Site and vice
versa.
Survey x Treatment
Indicates differences between Surveys are dependent on the Treatment and
vice versa. Indicative of a shading effect and recovery of seagrasses
consistent for both Sites.
Site x Treatment Indicates the variability among Sites is dependent on the Treatment and vice
versa.
Survey x Site x Treatment
Indicates differences among Surveys are dependent on both Sites and
Treatment and vice versa. Indicative of a shading effect and recovery of
seagrasses, although dependant on the Site.
Residual
This term is a measure of the variation in the data not explained by the
variation attributed to the main factors in the experimental model (i.e. Survey,
Sites, Treatment, Plots and their associated interactions).
Table 2-4 Terms used in describing the outcomes of the statistical analyses
Outcome (code) Interpretation
Redundant term (-) A term becomes redundant if a lower order interaction including that term is
significant.
Non-significant (ns)
Non-significant describes the convention by which a statistical comparison is
deemed not to be an actual effect (i.e. accept the null hypothesis that there is
no effect). Here the cut-off point was set at P > 0.05.
Significant (asterisks)
The statistical comparison indicating the presence of an actual effect. These
signify the probability (P) of an effect being considered to actually occur.
Here, * = P < 0.05, ** = P < 0.01, *** = P < 0.001. These indicate that the
likelihood of an effect occurring by chance alone (and therefore not explained
by the factor being considered) would be 5 in 100, 1 in 100, or 1 in 1000
respectively. By convention, significant terms are indicated in tables in bold
typeface.
2.4 Water Quality and Light Availability
Time series measurements of temperature (oC), turbidity (NTU) and underwater light (PAR, (mol photons
m-2
s-1
) taken at 15 minute intervals were used to assist the interpretation of temporal changes in seagrass
density in the Control plots throughout the Experiment and in the Shaded plots during the Recovery Phase.
Data were recorded at Water Quality Monitoring sites at Fannie Bay and Casuarina (Figure 2-1) for the
duration of the Experiment (Appendix B). The monitoring stations, established as part of the Water Quality
and Subtidal Sedimentation Monitoring Program (Cardno 2012b), were installed at approximately -3m LAT
offshore from the Experimental sites and at a height of approximately 1.5 m above the seabed. Telemetered
data were used for the period 13 to 24 February 2013 at Casuarina as the logged data had not been
recovered from the loggers at the time of reporting.
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2.5 Quality Control
The Quality Control processes followed in the field and in the office by all Project personnel (i.e. field and
office staff) in order to complete the scope of work to a consistent and high quality are described in detail in
the Method Statement and in the Work Instructions (Cardno 2012c). Results of Quality Control procedures
are given in Appendix C.
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3 Results
3.1 Shoot Density
Most of the seagrass recorded in plots was Halodule sp. (Table 3-1). Halophila sp. was recorded at both
sites at the start of the Experiment but only at low densities and it accounted for less than 5% of total
seagrass density at that time. Halophila sp. had disappeared from both Control and Shaded plots by the end
of the Dark phase; therefore, results described hereafter pertain mostly to Halodule sp. Changes in total
seagrass (both species combined) shoot density in Shaded and Control plots during the Dark and the
Recovery phases are illustrated in Figure 3-1.
Significant differences in seagrass shoot density between Control and Shaded treatments depended on the
Survey and Site (p<0.05, Table 3-2). Two months after the start of the Experiment, the Shaded plots were
almost completely unvegetated (Figure 3-1 and Table 3-1) and differed significantly from the Control plots,
where seagrass shoot density remained similar to initial levels (pairwise comparisons, Appendix A).
Seagrass in the Shaded plots showed no sign of recovery in the three months following removal of the shade
screens. During this time, seagrass density in the Control plots declined steadily and reached values of
approximately 3 to 5% of initial levels by the end of the Experiment. The timing of the decline in the Control
plots differed slightly among the two sites, occurring in the first month of the recovery phase at Fannie Bay
and within the last two months at Casuarina Beach (Figure 3-1).
Table 3-1 Mean and standard error (SE) of shoot density (Shoots m-2
) for Halodule and Halophila sp. during the Dark and Recovery phases at A) Casuarina Beach and B) Fannie Bay
A. Casuarina Beach
Species Treatment
Start
September 2012
2 Months Dark
November 2012
1 Month Recovery
December 2013
3 Month Recovery
February 2013
Halodule sp. Control 414 ± 105
272 ± 33
374 ± 50
12 ± 4
Shaded 573 ± 84
2 ± 2
6 ± 2
0 ± 0
Halophila sp. Control 10 ± 7
0 ± 0
0 ± 0
0 ± 0
Shaded 29 ± 15 0 ± 0 0 ± 0 0 ± 0
B. Fannie Bay
Species Treatment Start 2 Months Dark
1 Month Recovery
3 Month Recovery
Halodule sp. Control 649 ± 182
581 ± 203
154 ± 44
30 ± 13
Shaded 479 ± 117
0 ± 0
0 ± 0
0 ± 0
Halophila sp. Control 4 ± 4
0 ± 0
0 ± 0
0 ± 0
Shaded 1 ± 1 0 ± 0 0 ± 0 0 ± 0
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Figure 3-1 Mean seagrass shoot density (±SE) for Shaded and Control plots during two months of dark exposure (grey shaded area) and three months of recovery (clear area) at Casuarina and Fannie Bay
Table 3-2 Summary of Permanova for seagrass shoot density showing the level of significance. * = P(perm) < 0.05; - = redundant term, ns = not significant.
Source of Variation Seagrass Shoot Density
Survey −
Site −
Treatment −
Survey x Site ns
Survey x Treatment ns
Site x Treatment ns
Plot (Site x Treatment) ns
Survey x Site x Treatment *
3.2 Water Quality and Light Availability
Time series of turbidity (NTU), light (PAR) and water temperature measured at Water Quality monitoring
stations at Casuarina Beach and Fannie Bay are shown in Figure 3-2 and Figure 3-3, respectively.
Turbidity levels before January 2013 were generally below 10 NTU at both sites. This increased
considerably from 10 January 2013, coinciding with strong westerly winds and increased wave heights
associated with TC Narelle. As a result, PAR was greatly reduced for two to three weeks in late January
2013, approximately two months into the recovery phase.
3.3 Quality Control
The Quality Control results for data checking are presented in Appendix C.
0
200
400
600
800
1000
24-Sep-12 24-Oct-12 24-Nov-12 24-Dec-12 24-Jan-13 23-Feb-13
Sh
oo
ts m
-2
Casuarina Control
Casuarina Shaded
Fannie Bay Control
Fannie Bay Shaded
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Figure 3-2 Time-series of turbidity, PAR (mol photons m-2
s-1
) and water temperature at the Casuarina Water Quality monitoring site from 24 September 2012 to 15 February 2013
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Figure 3-3 Time-series of turbidity, PAR (mol photons m-2
s-1
) and water temperature at the Fannie Bay Water Quality monitoring site (Site 01) from 24 September 2012 to 15 February 2013
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4 Discussion
The aim of the Experiment was to investigate the rate of recovery of seagrass in Darwin Harbour by
simulating losses potentially related to dredging-induced increases in turbidity. The Experiment involved
exposing seagrass plots to continuous darkness (the Shaded treatment) at two sites (Casuarina and Fannie
Bay) for a period of two months (The Dark phase - 24 September to 24 November 2012), followed by a three
month period of monitoring (the Recovery phase - 25 November 2012 to 24 February 2013).
In the Dark phase of the Experiment, seagrass shoot density declined to zero after two months of continued
darkness, with no significant change at the Control plots. However, there was virtually no recovery of
seagrass in the Shaded plots in the following three months of the Recovery phase.
As seagrass shoot density in the Control plots also gradually declined to zero in the Recovery phase of the
Experiment, it is likely that seagrass at the sites was:
> in a natural phase of decline; or
> affected by a natural disturbance.
Possible reasons for natural declines in seagrass distribution and abundance can be inferred from data
collected in the Recovery Phase on turbidity, benthic light availability and weather conditions. Turbidity
increased substantially in January 2013 associated with TC Narelle and this resulted in complete light
attenuation at both Casuarina and Fannie Bay for approximately three weeks. Analogous shading
experiments have demonstrated that, should extreme turbidity levels reduce light below the requirements of
seagrass for more than approximately three weeks, deleterious effects on seagrass condition may occur.
Results of shading studies in Gladstone Harbour (Chartrand et al. 2010) indicate that changes in morphology
can be seen from two weeks (Halophila spp.) to one month (Halodule spp.) following exposure to low light.
Different species respond differently to declines in benthic light availability as a result of increased turbidity
and sedimentation (McMahon et al. 2011, Duarte et al. 2006, Erftemeijer and Lewis 2006, Larkum et al.
2006, Vermaat et al. 1997). Halophila spp. have been found to tolerate light levels as low as 3 to 8 % of
surface irradiance (SI), compared to 5 to 30 % SI for Halodule spp. (Erftemeijer et al. 2006).
In shading experiments in the Gulf of Carpentaria (Longstaff and Dennison 1999), plant death in Halophila
ovalis occurred after 38 days exposure to darkness, while Halodule pinifolia survived 100 days. Therefore,
the reduction in benthic light availability during the Recovery phase (to 0% SI for approximately three weeks)
would be expected to impact on seagrass growth within the Control plots, and would most certainly have
prevented recovery in the treatment plots. Hence, the similarity in the rate and severity of the decline in the
Shaded plots (during the Dark phase) to the decline in the Control plots (during the Recovery phase)
illustrated how a simulated dredging impact was comparable to that of a natural, weather related impact
(i.e. an increase in turbidity during the wet season).
Light and turbidity measurements from the Water Quality monitoring stations provided contextual information
on temporal changes in light availability near the experimental sites. It should be noted that the sites were
located within intertidal seagrass beds further inshore from the monitoring stations. Wave action at these
shallower sites may cause further sediment resuspension, which could result in greater inshore turbidity.
The differing rates of seagrass decline observed at Casuarina Beach and Fannie Bay may have been
associated with localised differences in turbidity, potentially not evident at the offshore stations. Other
factors that may have impacted on the condition of seagrass during the Recovery phase included physical
impact from wave action and sedimentation from wind and wave driven resuspension associated with strong
westerly winds, as recorded during that period (Cardno 2013).
Results from monitoring and mapping surveys undertaken since June 2012 (Cardno 2012c, Cardno 2012d,
Geo Oceans 2013) indicate that a seasonal peak in seagrass distribution in Darwin Harbour is reached
towards the end of the dry season in October/November. In particular, results from towed video mapping
surveys completed in February 2013 indicate that seagrass distribution and abundance across the foreshore
of Darwin Harbour has declined by approximately 75% since October 2012 (Geo Oceans 2013). Results are
consistent with expected seasonal growth patterns of ephemeral tropical seagrasses such as Halodule and
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Halophila spp. (Coles et al. 2011), likely characterised by wet season die-offs followed by recovery in the dry
season.
Some evidence of recovery occurred at the end of the Dark phase (Survey 3), where divers observed shoots
of Halodule sp. growing in an estimated 2 to 3 cm of sediment deposited on shade screens (Figure 4-1).
Upon removal of the shade screens for the start of the recovery phase, no below ground biomass was
visible, indicating that growth had most likely occurred in the overlaying sediment rather than from surviving
seagrass below. Considering that the sediment layer would have deposited over a period of a month since
the previous survey, the observed new growth would have occurred within a timeframe of one week to one
month. While it is unknown whether the shoots grew from seed or vegetatively from fragments, these
observations indicate a potential for rapid growth and recovery should conditions be favourable. Seagrass
mapping results found a 250% increase in overall seagrass habitat extent between June and October 2012,
including a ten-fold habitat expansion at East Point, further indicating rapid growth and recovery potential
during the dry season.
Figure 4-1 Field observation of Halodule sp. growing in sediment layer deposited over a shade screen at Casuarina (23 November 2012). Image scale approximately 9 cm x 12 cm
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5 Conclusions
No seagrass recovery was observed at the conclusion of the Recovery Experiment undertaken between
September 2012 and February 2013. The Recovery phase of the Experiment coincided with a natural
weather-related reduction of light to the seabed, which most likely accounted for declines in seagrass density
in Control plots and the absence of recovery in the Shaded plots. The period of elevated turbidity and
reduced benthic light associated with the passage of TC Narelle off the northwest coast of Australia was
comparable both in intensity and duration with potential dredging impacts mimicked in the Experiment.
Recent habitat mapping results indicate that rapid seagrass growth and habitat expansion occurs in the dry
season between June and October, after which severe and widespread declines occur during the wet
season. This is consistent with expected seasonal growth patterns of ephemeral tropical seagrasses such
as Halodule and Halophila spp. This, together with opportunistic field observations of new seagrass shoots
over the shade screens, indicates a potential for rapid recovery should conditions be favourable.
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6 Acknowledgements
This report was written by Isabel Jimenez, Andrea Nicastro, and Brendan Alderson; Yesmin Chikhani
assisted with table production. Fieldwork was carried out by Brendan Alderson, Hamish Maitland, Kane
Organ, and Daniel Pygas. Image analysis was carried out by Yesmin Chikhani and Blaise Bratter. The
report was reviewed by Joanna Lamb and Craig Blount.
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7 References
Cardno (2012a). Ichthys Project – Nearshore Environmental Monitoring Plan. Report for INPEX, Cardno (NSW/ACT) Pty Ltd, Sydney.
Cardno (2012b). Bimontly Water Quality & Subtidal Sedimentation Report: Dredging Report 1 - Ichthys Nearshore Environmental Monitoring Program. Report for INPEX, Cardno (NSW/ACT) Pty Ltd, Sydney.
Cardno (2012c). Seagrass Monitoring Program Baseline Report – Ichthys Nearshore Environmental Monitoring Program. Report for INPEX, Cardno Ecology Lab Pty Ltd, Sydney.
Cardno (2012d). Bimonthly Seagrass Monitoring Report- Dredging Report 1 - Ichthys Nearshore Environmental Monitoring Program. Report for INPEX, Cardno Ecology Lab Pty Ltd, Sydney.
Cardno (2013). Fortnightly Water Quality Report - Weeks 20/21: 7 to 20 January 2013 - Ichthys Nearshore Environmental Monitoring Program. Report for INPEX, Cardno (NSW/ACT) Pty Ltd, Sydney.
Chartrand, K.M, McKenna, S.A, Petrou, K, Jimenez-Denness, I, Franklin, J, Sankey, T.L, Hedge, S.A, Rasheed, M.A and Ralph, P.J (2010). Port Curtis Benthic Primary Producer Habitat Assessment and Health Studies Update: Interim Report December 2010. DEEDI Publication. Fisheries Queensland, Cairns.
Coles, R and Mackenzie, L (2004). Trigger points and achieving targets for managers. Paper presented at workshop session on management issues during the ISBW-6 workshop, Seagrass 2004 Conference, Townsville, 24 September – 1 October 2004.
Coles, R., Grech, A., Rasheed, M., McKenzie, L., Unsworth, R., & Short, F. (2011). Seagrass ecology and threats in the tropical Indo-Pacific bioregion.
Duarte, C.M, Fourqurean, J.W, Krause-Jensen, D and Olesen, B (2006). Dynamics of seagrass stability and change, in: Larkum, A.W.D. et al. (Ed.) (2006). Seagrasses: biology, ecology and conservation. pp. 271-294
Erftmeijer, P.L.A and Lewis, R.R.R (2006). Environmental impacts of dredging on seagrasses: A review. Marine Pollution Bulletin 52: 1553-1572.
Geo Oceans (2013). Seagrass Habitat Monitoring Survey 3 February 2013 - Ichthys Nearshore Environmental Monitoring Program. Draft Technical Report for Cardno Ecology Lab on behalf of INPEX.
INPEX (2011). Ichthys Gas Field Development Project, Supplement to the Draft Environmental Impact Statement.
INPEX (2012). Dredging and Spoil Disposal Management Plan – East Arm.
Larkum, W.D, Orth, R and Duarte, C.M (2006). Seagrasses: Biology, Ecology and Conservation. Springer, Dordrecht, The Netherlands.
Longstaff, B.J and Denison, W.C (1999). Seagrass survival during pulsed turbidity events: the effects of light deprivation on the seagrasses Halodule pinifolia and Halophila ovalis. Aquatic Botany 65: 105-121.
McMahon, K, Lavery, P.S and Mulligan, M (2011). Recovery from the impact of light reduction on the seagrass Amphibolis griffithii, insights for dredging management. Marine Pollution Bulletin 62: 270-283.
Vermaat, J.E, Agawin, N.S.R, Fortes, M.D and Uri, J.S (1997). The capacity of seagrasses to survive increased turbidity and siltation: the significance of growth form and light use. Ambio 25 (2) 499-504.
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Ichthys Nearshore Environmental Monitoring Program
APPENDIX A RESULTS OF STATISTICAL ANALYSES
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Appendix A-1 Results of PERMANOVA testing for differences in total seagrass shoot density. Significant (P(perm) < 0.05) terms in bold. Monte Carlo (MC) simulation was used to calculate P values where unique permutations < 100
Source df SS MS Pseudo-F P(perm) Unique perms P(MC)
Su 3 3072400 1024100 16.803 0.0412 840 0.0205
Si 1 13347 13347 0.26299 0.613 9810 0.6215
Tr 1 620110 620110 9.0599 0.3345 6 0.2078
SuxSi 3 182850 60951 2.0199 0.123 9947 0.1217
SuxTr 3 627520 209170 2.3676 0.2532 9964 0.2433
SixTr 1 68445 68445 1.3486 0.2741 9816 0.2533
Pl(SIxTr) 16 812030 50752 1.6819 0.0866 9931 0.0847
SuxSixTr 3 265050 88350 2.9278 0.0436 9952 0.0435
Res 48 1448400 30176
Total 79 7110200
Pairwise Comparison
Term 'SuxLoxTr' for pairs of levels of factor 'Treatment'
Groups t P(perm) Unique perms P(MC)
Control, Shaded 1.3036 0.2001 114 0.2291
Within level 'RE01' of factor 'Survey'
Within level 'Casuarina' of factor 'Site'
Control, Shaded 0.80 0.4469 113 0.4411
Within level 'RE01' of factor 'Survey'
Within level 'Fannie Bay' of factor 'Site'
Control, Shaded 8.04 0.01 53 0.0003
Within level 'RE03' of factor 'Survey'
Within level 'Casuarina' of factor 'Site'
Control, Shaded 2.87 0.0081 16 0.0216
Within level 'RE03' of factor 'Survey'
Within level 'Fannie Bay' of factor 'Site'
Control, Shaded 7.33 0.0087 72 0.0004
Within level 'RE04' of factor 'Survey'
Within level 'Casuarina' of factor 'Site'
Control, Shaded 3.50 0.0088 16 0.0085
Within level 'RE04' of factor 'Survey'
Within level 'Fannie Bay' of factor 'Site'
Control, Shaded 3.0697 0.0073 10 0.0159
Within level 'RE06' of factor 'Survey'
Within level 'Casuarina' of factor 'Site'
Control, Shaded 2.2953 0.1719 3 0.0498
Within level 'RE06' of factor 'Survey'
Within level 'Fannie Bay' of factor 'Site'
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Pairwise Comparison
Term 'SuxSixTr' for pairs of levels of factor 'Survey'
Groups t P(perm) Unique perms P(MC)
RE01, RE03 1.80 0.1611 7117 0.1541
RE01, RE04 0.41 0.6989 7092 0.7072
RE01, RE06 4.0111 0.0078 5491 0.0154
RE03, RE04 1.4254 0.2482 5302 0.2268
RE03, RE06 7.0475 0.0076 5384 0.002
RE04, RE06 7.2394 0.0019 5414 0.0023
Within level 'Casuarina' of factor 'Site'
Within level 'Control' of factor 'Treatment'
RE01, RE03 6.5163 0.0028 2239 0.0036
RE01, RE04 6.4514 0.0054 5012 0.0052
RE01, RE06 6.5166 0.0087 154 0.0045
RE03, RE04 1.1083 0.333 71 0.3304
RE03, RE06 1.4289 0.4379 3 0.2315
RE04, RE06 2.2953 0.0434 20 0.083
Within level 'Casuarina' of factor 'Site'
Within level 'Shaded' of factor 'Treatment'
RE01, RE03 0.32384 0.7662 7180 0.7661
RE01, RE04 2.7417 0.0569 7293 0.053
RE01, RE06 3.3268 0.0255 3431 0.0299
RE03, RE04 2.6658 0.0372 7193 0.0566
RE03, RE06 2.8556 0.0018 3534 0.0408
RE04, RE06 3.7375 0.0073 3453 0.0207
Within level 'Fannie Bay' of factor 'Site'
Within level 'Control' of factor 'Treatment'
RE01, RE03 4.1065 0.0088 155 0.0148
RE01, RE04 4.1065 0.006 155 0.0141
RE01, RE06 4.1065 0.0081 155 0.0162
RE03, RE04 Denominator is 0
RE03, RE06 Denominator is 0
RE04, RE06 Denominator is 0
Within level 'Fannie Bay' of factor 'Site'
Within level 'Shaded' of factor 'Treatment'
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Ichthys Nearshore Environmental Monitoring Program
APPENDIX B WATER QUALITY SUMMARY DATA
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Appendix B-1 Summary of daily average turbidity (NTU), water temperature (°C) and PAR (mol photons m
-2 s
-1) (mean; min; maximum and percentile of occurrence) at Fannie Bay and Casuarina
monitoring stations from 24 September 2012 to 24 February 2013
A. Turbidity Mean Min 5pct 10pct 20pct 50pct 80pct 90pct 95pct Max From To
Fannie Bay 9.1 0.9 1.6 2 2.7 5.6 13.4 20.7 29.4 55.5 24/09/12 24/02/13
Casuarina 7.4 0.7 1.2 1.3 1.6 2.4 5.6 23.2 39.1 71.9 24/09/12 24/02/13
B. Temperature Mean Min 5pct 10pct 20pct 50pct 80pct 90pct 95pct Max From To
Fannie Bay 30.9 28.8 29.1 29.5 30.1 31 31.8 32 32.1 32.2 24/09/12 24/02/13
Casuarina 30.9 28.7 29.2 29.6 30.1 31 31.8 32.1 32.2 32.3 24/09/12 24/02/13
C. PAR Mean Min 5pct 10pct 20pct 50pct 80pct 90pct 95pct Max From To
Fannie Bay 84.5 3.2 13.7 25.4 36.9 79 126.3 154.9 173.4 221.1 24/09/12 24/02/13
Casuarina 98.1 1.9 5.7 33 57.4 96 143.4 170.7 182.6 232.9 24/09/12 24/02/13
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Ichthys Nearshore Environmental Monitoring Program
APPENDIX C QUALITY CONTROL
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Appendix C-1 Quality Control results for data checking of seagrass shoot density determined from diver counts and still images, against video footage of experimental plots at Casuarina and Fannie Bay
Site Survey Plot
No
Shoot Density
(Shoots m-2
)
QA Shoot Density
(Shoots m-2
)
Difference (Shoots m-
2)
Relative Error
(% Original Count) Correction
Fannie Bay 3 1 0 0 0 0%
3 2 52 40 -12 -23% Replaced with video counts
3 4 0 0 0 0%
3 6 0 4 4 0%
3 8 0 0 0 0%
3 10 0 0 0 0%
3 11 0 0 0 0%
3 13 25 20 -5 -20%
3 15 0 0 0 0%
3 17 18 25 7 39% Replaced with video counts
Casuarina 1 82 86 98 12 14%
1 84 76 80 4 5%
1 88 186 190 4 2%
1 90 99 96 -3 -3%
3 76 8 5 -3 -38% Replaced with video counts
3 78 188 277 89 47% Replaced with video counts
3 80 6 3 -3 -50% Replaced with video counts
3 82 190 184 -6 -3%
3 84 1 0 -1 -100% Replaced with video counts
3 86 167 186 19 11%
3 88 2 2 0 0%
3 90 217 236 19 9%
3 92 2 0 -2 -100% Replaced with video counts
3 94 70 111 41 59% Replaced with video counts
4 76 0 0 0 0%
4 80 1 0 -1 -100% Replaced with video counts
4 82 47 58 11 23% Replaced with video counts
4 86 5 6 1 20%
4 88 0 0 0 0%
4 90 43 50 7 16%
4 92 0 0 0 0%
6 76 0 0 0 0%
6 78 6 10 4 67% Replaced with video counts
6 80 0 2 2 0%
6 82 2 2 0 0%
6 84 2 0 -2 -100% Replaced with video counts
6 86 10 11 1 10%
6 88 1 0 -1 -100% Replaced with video counts
6 90 1 1 0 0%
6 92 0 0 0 0%
6 94 12 10 -2 -17%