Seagrass Dark Recovery Experiment

Transcription

1 Seagrass Dark Recovery Experiment Ichthys Nearshore Environmental Monitoring Program L384-AW-REP L384-AW-REP Prepared for INPEX March 2013

2 Document Information Prepared for INPEX Project Name File Reference L384-AW-REP-10047_0_Seagrass Dark Recovery Experiment Report.docx Job Reference L384-AW-REP 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: Telephone: Telephone: Facsimile: Facsimile: Facsimile: International: International: International: 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. Prepared for INPEX Cardno ii

3 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. Prepared for INPEX Cardno iii

4 Glossary Term or Acromym Definition Benthic DSDMP DSV EIS GPS HSE Intertidal LAT NEMP NTU PAR Permanova Photoquadrat QA SE Subtidal Turbidity On the seafloor Dredging and Spoil Disposal Management Plan East Arm Dive Support Vessel Environmental Impact Statement Global Positioning System Health Safety Environment The portion of shoreline between low and high tide marks, that is intermittently submerged Lowest Astronomical Tide Nearshore Environmental Monitoring Plan Nephelometric Turbidity Units Photosynthetically Available Radiation Permutational Analysis of Variance Virtual sampling unit of known dimensions within a photograph of the seafloor, used to quantify seagrass density and percent cover Quality Assurance Standard error of the mean Waters below the low-tide mark An indication of water clarity Prepared for INPEX Cardno iv

5 Table of Contents Executive Summary iii Glossary 1 Introduction Background Requirement to Monitor Seagrass in Darwin Harbour Objectives 7 2 Methodology Vessels, Safety and Environmental Management Sampling Design Sites, Timing and Frequency of Surveys Experimental Treatments and Plots Measurement of Shoot Density Image Analysis Statistical Analysis Water Quality and Light Availability Quality Control 14 3 Results Shoot Density Water Quality and Light Availability Quality Control 16 4 Discussion 19 5 Conclusions 21 6 Acknowledgements 22 7 References 23 iv 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 Prepared for INPEX Cardno v

6 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 m 2 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 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 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 Prepared for INPEX Cardno vi

7 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 westsouth-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 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 Prepared for INPEX Cardno 7

8 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 ) and surface supply breathing apparatus (SSBA) (ADAS Level AS ) in accordance with Australia/New Zealand Standard Occupational Diving Operations Part 1: Standard Operational Practice (ASNZS :2007). Data were collected by scientific divers and site installation and maintenance was completed by commercial divers from NDS. 2.2 Sampling Design 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 Fannie Bay 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 month Dark Survey 3 20 to 24 November month Dark Survey 4 20 to 24 December month Recovery Survey 5 21 to 24 January month Recovery Survey 6 22 to 24 February month Recovery Prepared for INPEX Cardno 8

9 Figure 2-1 Location of dark recovery experiment sites Prepared for INPEX Cardno 9

10 2.2.2 Experimental Treatments and Plots At each site (Casuarina Beach and Fannie Bay) divers installed five replicate 1 m 2 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 m 2 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 Prepared for INPEX Cardno 10

11 2.3 Measurement of Shoot Density 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 m 2 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 m 2 quadrats were divided into eight smaller adjoining frames (sub-quadrats) each covering m 2 (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 m 2 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. Prepared for INPEX Cardno 11

12 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 m 2 (i.e. the survey area of the larger quadrat). Sub-quadrat 1 Sub-quadrat 5 Sub-quadrat 2 Sub-quadrat 6 Sub-quadrat 3 Sub-quadrat 7 Sub-quadrat 4 Sub-quadrat 8 Figure 2-4 Quadrat set-up showing the individual sub-quadrats within the larger 1 m 2 quadrat (the field of view of the camera was slightly larger than each sub-quadrat) 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. Prepared for INPEX Cardno 12

13 Table 2-3 Explanation of factors used in the statistical analyses Component of Variation Interpretation Survey Site Treatment Plot (Site x Treatment) Survey x Site Survey x Treatment Site x Treatment Survey x Site x Treatment Residual Indicates a significant difference between Surveys (Survey 1 to Survey 6) independent of Site and Treatment. Indicates larger-scale significant variability between Sites (Fannie Bay and Casuarina Beach) independent of Survey and Treatment. Indicates a significant difference between Treatments (Shaded and Control) independent of Surveys and Sites. Indicates smaller-scale significant variability among Plots within Sites and Treatments independent of Survey. Indicates differences between Surveys are dependent on the Site and vice versa. 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. Indicates the variability among Sites is dependent on the Treatment and vice versa. 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. 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 (-) Non-significant (ns) Significant (asterisks) A term becomes redundant if a lower order interaction including that term is significant. 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 > 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 < 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 ( o C), 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. Prepared for INPEX Cardno 13

14 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. Prepared for INPEX Cardno 14

15 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 Months Dark November Month Recovery December Month Recovery February 2013 Halodule sp. Control 414 ± ± ± ± 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 ± ± ± ± 13 Shaded 479 ± ± 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 Prepared for INPEX Cardno 15

16 Shoots m Casuarina Control Casuarina Shaded Fannie Bay Control Fannie Bay Shaded 0 24-Sep Oct Nov Dec Jan Feb-13 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. Prepared for INPEX Cardno 16

17 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 Prepared for INPEX Cardno 17

18 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 Prepared for INPEX Cardno 18

19 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 Prepared for INPEX Cardno 19

20 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 Prepared for INPEX Cardno 20

21 5 Conclusions No seagrass recovery was observed at the conclusion of the Recovery Experiment undertaken between September 2012 and February 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. Prepared for INPEX Cardno 21

22 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. Prepared for INPEX Cardno 22

23 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. Report for INPEX, Cardno Ecology Lab Pty Ltd, Sydney. Cardno (2012d). Bimonthly Seagrass Monitoring Report- Dredging Report 1 -. Report for INPEX, Cardno Ecology Lab Pty Ltd, Sydney. Cardno (2013). Fortnightly Water Quality Report - Weeks 20/21: 7 to 20 January 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 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 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 Erftmeijer, P.L.A and Lewis, R.R.R (2006). Environmental impacts of dredging on seagrasses: A review. Marine Pollution Bulletin 52: Geo Oceans (2013). Seagrass Habitat Monitoring Survey 3 February 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: 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: 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) Prepared for INPEX Cardno 23

24 Ichthys Nearshore Environmental Monitoring Program APPENDIX A RESULTS OF STATISTICAL ANALYSES Prepared for INPEX Cardno 24

25 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 Si Tr SuxSi SuxTr SixTr Pl(SIxTr) SuxSixTr Res Total Pairwise Comparison Term 'SuxLoxTr' for pairs of levels of factor 'Treatment' Groups t P(perm) Unique perms P(MC) Control, Shaded Within level 'RE01' of factor 'Survey' Within level 'Casuarina' of factor 'Site' Control, Shaded Within level 'RE01' of factor 'Survey' Within level 'Fannie Bay' of factor 'Site' Control, Shaded Within level 'RE03' of factor 'Survey' Within level 'Casuarina' of factor 'Site' Control, Shaded Within level 'RE03' of factor 'Survey' Within level 'Fannie Bay' of factor 'Site' Control, Shaded Within level 'RE04' of factor 'Survey' Within level 'Casuarina' of factor 'Site' Control, Shaded Within level 'RE04' of factor 'Survey' Within level 'Fannie Bay' of factor 'Site' Control, Shaded Within level 'RE06' of factor 'Survey' Within level 'Casuarina' of factor 'Site' Control, Shaded Within level 'RE06' of factor 'Survey' Within level 'Fannie Bay' of factor 'Site' Prepared for INPEX Cardno 25

26 Pairwise Comparison Term 'SuxSixTr' for pairs of levels of factor 'Survey' Groups t P(perm) Unique perms P(MC) RE01, RE RE01, RE RE01, RE RE03, RE RE03, RE RE04, RE Within level 'Casuarina' of factor 'Site' Within level 'Control' of factor 'Treatment' RE01, RE RE01, RE RE01, RE RE03, RE RE03, RE RE04, RE Within level 'Casuarina' of factor 'Site' Within level 'Shaded' of factor 'Treatment' RE01, RE RE01, RE RE01, RE RE03, RE RE03, RE RE04, RE Within level 'Fannie Bay' of factor 'Site' Within level 'Control' of factor 'Treatment' RE01, RE RE01, RE RE01, RE 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' Prepared for INPEX Cardno 26

27 Ichthys Nearshore Environmental Monitoring Program APPENDIX B WATER QUALITY SUMMARY DATA Prepared for INPEX Cardno 27

28 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 /09/12 24/02/13 Casuarina /09/12 24/02/13 B. Temperature Mean Min 5pct 10pct 20pct 50pct 80pct 90pct 95pct Max From To Fannie Bay /09/12 24/02/13 Casuarina /09/12 24/02/13 C. PAR Mean Min 5pct 10pct 20pct 50pct 80pct 90pct 95pct Max From To Fannie Bay /09/12 24/02/13 Casuarina /09/12 24/02/13 Prepared for INPEX Cardno 28

29 Ichthys Nearshore Environmental Monitoring Program APPENDIX C QUALITY CONTROL Prepared for INPEX Cardno 29

30 Site 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 Survey Plot No Shoot Density (Shoots m -2 ) QA Shoot Density (Shoots m -2 ) Difference (Shoots m- 2) Relative Error (% Original Count) Fannie Bay % Correction % Replaced with video counts % % % % % % % % Replaced with video counts Casuarina % % % % % Replaced with video counts % Replaced with video counts % Replaced with video counts % % Replaced with video counts % % % % Replaced with video counts % Replaced with video counts % % Replaced with video counts % Replaced with video counts % % % % % % Replaced with video counts % % % Replaced with video counts % % Replaced with video counts % % % Prepared for INPEX Cardno 30