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LONG-TERM SEAGRASS MONITORING IN THE PORT OF THURSDAY ISLAND:

MARCH 2017 Sozou, AM, Wells, JN & Rasheed, MA

Report No. 17/32

2017

LONG-TERM SEAGRASS MONITORING IN THE PORT OF THURSDAY ISLAND:

MARCH 2017

A Report for Far North Queensland Ports Corporation Limited (Ports North)

Report No. 17/32

June 2017

Prepared by Alysha Sozou, Jaclyn Wells & Michael Rasheed

Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER)

James Cook University Townsville

Phone : (07) 4781 4262 Email: [email protected]

Web: www.jcu.edu.au/tropwater/

Information should be cited as: Sozou, A, Wells, JN & Rasheed, M. 2017, ‘Long-term Seagrass Monitoring in the Port of Thursday Island: March 2017. Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) Publication 17/32, James Cook University, Cairns, 38 pp. For further information contact: Seagrass Ecology Group Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) James Cook University [email protected] PO Box 6811 Cairns QLD 4870 This publication has been compiled by the Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER), James Cook University. © James Cook University, 2017. Except as permitted by the Copyright Act 1968, no part of the work may in any form or by any electronic, mechanical, photocopying, recording, or any other means be reproduced, stored in a retrieval system or be broadcast or transmitted without the prior written permission of TropWATER. The information contained herein is subject to change without notice. The copyright owner shall not be liable for technical or other errors or omissions contained herein. The reader/user accepts all risks and responsibility for losses, damages, costs and other consequences resulting directly or indirectly from using this information. Enquiries about reproduction, including downloading or printing the web version, should be directed to [email protected] Acknowledgments: This project was funded by Ports North. We wish to thank TropWATER staff for their time in the field. Thanks also to GBR Helicopters and Wis Wei/McDonald Charters.

Thursday Island long-term seagrass monitoring: March 2017 – TropWATER 17/32 2017

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KEY FINDINGS 1. In March 2017 the overall condition of seagrasses in the Port of

Thursday Island was rated as satisfactory, with total area of seagrass monitoring meadows above the long term average for the program.

2. While this represented a decline in condition since 2016, area, biomass and species composition of monitoring meadows remained within historical ranges, with most meadows achieving good or better for at least two of the three seagrass indicators.

3. For some meadows, the loss in overall condition was actually the result of them expanding into deeper areas, with the newly colonised sections not yet reaching a mature biomass and species composition.

4. Climate was generally favourable for seagrass growth between the 2016 and 2017 surveys, although higher rainfall in the months preceding the 2017 survey may have contributed to some of the observed seagrass changes.

5. There was no indication that observed changes in seagrass were related to port or other anthropogenic activities and in 2017, the satisfactory condition of seagrass meadows suggests seagrasses remain resilient and continue to co-exist well with current human activities.

Overall seagrass condition 2017


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IN BRIEF Seagrasses have been monitored in the Port of Thursday Island biennially since 2002 and annually since 2016. Nine seagrass monitoring meadows representing the range of different seagrass community types found in the Thursday Island region are assessed for changes in area (distribution), biomass (density), and species composition. Changes in the seagrass biomass, area and species composition are used to develop a seagrass condition index (see section 2.3).

Figure 1. Seagrass condition index for the Port of Thursday Island seagrass monitoring meadows in

2017.

In March 2017 the overall condition of seagrass in the Port of Thursday Island was satisfactory (Figure 1). While this was a decline from the previous year’s score, seagrasses remained in a healthy condition with 8 of the 9 meadows having two of the three indicators (biomass, area and species composition) in good or better condition. In addition, the total area of seagrass habitat mapped within the nine monitoring meadows was above average at 138 ± 19 ha, but reduced from the monitoring program peak distribution recorded in 2016 (Figure 2).

The condition of individual meadows varied, with Halodule uninervis inshore Meadow 8 located near Aplin Pass and the subtidal Enhalus acoroides Meadow 6 in better condition than 2016, and one inshore Halodule uninervis meadow remaining in the same condition (Meadow 3). The remaining monitoring meadows declined in overall condition since 2016. The changes do not appear to be related to location, but rather to the dominant species, with H. uninervis meadows faring better overall than E. acoroides meadows (Figure 1) although none of the changes are considered concerning and no meadows were classified as being in poor condition in 2017.

The loss in overall condition for two of the Enhalus acoroides meadows (meadows 2 & 4 between Rebel Marine and the Engineer’s Wharf) was a direct result of these meadows expanding into new deeper areas where they had not been previously recorded. These recently colonised areas had much lower biomass and


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a higher composition of colonising species than the previously established sections of the meadow, which lead to the overall reduction in biomass and species composition scores in 2017.

Climate conditions were generally favourable for seagrass growth during 2016/17 (Figure 3). Air temperature in the months preceding the survey was lower than the corresponding period leading up to the 2016 survey, and did not reach summer peaks recorded in previous years. In combination with below average tidal exposure to air, the risk of desiccation and thermal stress to seagrasses was likely minimal. Monthly solar exposure has been similar for the past three years and indicates a favourable light environment. However, as solar exposure is only a rough proxy it does not necessarily accurately represent the amount or quality of light reaching the seafloor. Increased rainfall in the months directly preceding the 2017 survey may have altered the light environment within the seagrass canopy. A more precise analysis of the light environment and a determination of whether seagrasses are receiving adequate light for growth was not possible as in situ light monitoring is not part of this program at this time.

This survey is the first to shift sampling frequency from every two years to annual monitoring. The changes observed will assist in better understanding the seagrass dynamics and conditions in the Port of Thursday Island and their relationship to annual cycles of climate and other potential drivers of seagrass change.

The Thursday Island seagrass monitoring program forms part of James Cook University’s (JCU) broader seagrass assessment and research program that examines condition of seagrasses in the majority of Queensland commercial ports. At the closest monitoring location to Thursday Island (Weipa, located in the Gulf of Carpentaria (Figure 4)) seagrasses were also in satisfactory condition. This is in contrast to many locations on the east coast of Queensland where seagrasses are recovering from major climate associated impacts. For full details of the Queensland ports seagrass monitoring program see: www.jcu.edu.au/portseagrassqld.

Figure 2. Total area of seagrass within the Thursday Island Monitoring Area from 2002 to 2017 (error bars = “R” reliability estimate). Dashed line indicates 8-year mean of total meadow area.


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Figure 3. Recent climate trends in Thursday Island: changes in climate variables as a proportion of the

long-term average. See Section 3.3 for detailed climate data.


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Table of Contents

KEY FINDINGS ........................................................................................................................................... i

IN BRIEF ................................................................................................................................................... ii

1 INTRODUCTION ................................................................................................................................... 1

1.1 Queensland Ports Seagrass Monitoring Program ................................................................................ 1 1.2 Thursday Island Seagrass Monitoring Program ...................................................................................... 1

2 METHODS ........................................................................................................................................... 4

2.1 Sampling approach and methods ......................................................................................................... 4 2.2 Habitat mapping and Geographic Information System ........................................................................ 5 2.3 Seagrass meadow condition index ......................................................................................................... 7

2.3.1 Baseline Conditions ............................................................................................................... 7 2.3.2 Meadow Classification .......................................................................................................... 8 2.3.3 Threshold Definition ............................................................................................................. 8 2.3.4 Grade and Score Calculations ............................................................................................. 10 2.3.5 Score Aggregation ............................................................................................................... 11

3 RESULTS ............................................................................................................................................ 12

3.1 Seagrasses in Thursday Island ............................................................................................................ 12 3.2 Seagrass Condition for Monitoring Meadows .................................................................................... 13

3.2.1 Inshore Halodule uninervis dominated meadows (Meadows 1, 3, 5, 8) ............................ 13 3.2.2 Enhalus acoroides dominated meadows (Meadows 2, 4, 6, 26, 27) .................................. 14

3.3 Thursday Island climate patterns ....................................................................................................... 26 3.3.1 Rainfall ................................................................................................................................ 26 3.3.2 Air Temperature .................................................................................................................. 27 3.3.4 Tidal Exposure of Seagrass Meadows ................................................................................ 28 3.3.3 Daily Global Solar Exposure ............................................................................................... 29

4 DISCUSSION ...................................................................................................................................... 30

4.1 Seagrass distribution, abundance and resilience ............................................................................... 30 4.2 Implications for management ............................................................................................................ 31

5 REFERENCES ...................................................................................................................................... 32

6 APPENDICES ...................................................................................................................................... 34

Appendix 1 .................................................................................................................................................. 34 Appendix 2 .................................................................................................................................................. 35 Appendix 3a ................................................................................................................................................ 37 Appendix 3b ................................................................................................................................................ 38


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1 INTRODUCTION Seagrasses provide a range of critically important and economically valuable ecosystem functions and services including nutrient cycling and particle trapping that improves water quality, coastal protection, support of fisheries production and the capture and storage of carbon (Costanza et al. 1997, Hemminga and Duarte 2000, Orth et al. 2006, Barbier et al. 2011). Seagrass meadows show measurable responses to changes in water quality, making them ideal candidates for monitoring the long-term health of marine environments (Dennison et al. 1993, Abal and Dennison 1996, Orth et al. 2006). 1.1 Queensland Ports Seagrass Monitoring Program

A long-term seagrass monitoring and assessment program has been established in the majority of Queensland commercial ports. The program was developed by the Seagrass Ecology Group at James Cook University’s Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) in partnership with the various Queensland port authorities. While each location is funded separately, a common methodology and rationale is used providing a network of seagrass monitoring locations throughout Queensland (Figure 4). A strategic long-term assessment and monitoring program for seagrasses provides port managers and regulators with the key information to ensure that seagrasses and ports can co-exist. It is useful information for planning and implementing port development and maintenance programs so they have a minimal impact on seagrass. The program also provides an ongoing assessment of many of the most threatened seagrass communities in the state. The program not only delivers key information for the management of port activities to minimise impacts on seagrasses but has also resulted in significant advances in the science and knowledge of tropical seagrass ecology. It has been instrumental in developing tools, indicators and thresholds for the protection and management of seagrasses and an understanding of the drivers of tropical seagrass change. It provides local information for individual ports as well as feeding into regional assessments of the status of seagrasses. For more information on the program and reports from the other monitoring locations see www.jcu.edu.au/portseagrassqld.

1.2 Thursday Island Seagrass Monitoring Program

Torres Strait Islanders have a deep social and spiritual connection with the marine environment and have lived a subsistence-fisher lifestyle for hundreds of years. Due to the high reliance on fishing in the Thursday Island area, habitats that support commercial and traditional fisheries, such as seagrasses, are of critical importance to the region. A fine-scale baseline survey of seagrass habitat conducted at the port in March 2002 identified seagrass as the dominant benthic habitat with over 1500 ha of seagrass habitat mapped in the survey area (Rasheed et al. 2003). This has important implications for future port and coastal developments that may impact upon these extensive meadows. Nine seagrass meadows were selected from the baseline survey for long-term monitoring (Figure 5).

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Legend

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Figure 4. Location of Queensland Port seagrass assessment sites.


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Ports North is responsible for managing and monitoring Thursday Island’s port environment. Ports North has recognised that seagrasses form a key ecological habitat in the Thursday Island region and commissioned James Cook University’s TropWATER to establish a long-term seagrass monitoring program for the Port of Thursday Island in 2002. The goal of the program is to assess the health of seagrass meadows in the Thursday Island’s port environment. Ports North uses the results from the program to evaluate the health of the port marine environment and help identify possible detrimental effects of port operations on seagrass meadows. The program also provides an assessment of climatic influences on seagrass meadows, and may act as a reference tool for other organisations involved in management of community use of the inshore area.

This report presents results of the March 2017 seagrass monitoring survey. The objectives of the program were to:

1. Map the distribution of nine seagrass monitoring meadows identified within the port limits; 2. Monitor seagrass distribution, density (biomass) and species composition within the monitoring

meadows; 3. Assess changes in seagrass meadows and compare results with previous Thursday Island monitoring

surveys; 4. Incorporate the results into the Geographic Information System (GIS) database for the Port of

Thursday Island.


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Figure 5. Port of Thursday Island seagrass monitoring meadows and habitat characterisation sites 2017.

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Source: Sozou, AM, Wells, JN, Rasheed MA (2017). 'Long-term seagrass monitoring in the Port of Thursday Island, March 2017'. JCU Publication, Centre for Tropical Water & Aquatic Ecosystem Research Publication, Cairns.Aerial photograph: Beach Protection Authority.

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2 METHODS 2.1 Sampling approach and methods

The Port of Thursday Island seagrass monitoring survey was conducted March 7th-8th 2017. The nine monitoring meadows selected from the baseline survey were representative of the range of seagrass meadows and communities identified, and were located in areas potentially influenced by port operations and coastal developments. A complete background site description and detailed methods of the monitoring program is available in Rasheed et al. (2003). A combination of helicopter aerial assessment and boat-based camera surveys was used for the seagrass survey (Figure 6). Above-ground seagrass biomass was measured using a visual estimates of biomass technique as described by Mellors (1991) and Kirkman (1978). The method involves an observer ranking above-ground seagrass biomass within three randomly placed 0.25 m2 quadrats at each site. Three separate biomass ranges were used: low biomass, high biomass, and an Enhalus acoroides range. Measurements for each observer are later calibrated to previously obtained biomass values from seagrass harvested from quadrats and dried in the lab to determine mean above-ground biomass in grams dry weight per square metre (g DW m-2) at each site. The relative proportion of each seagrass species within each survey quadrat was also recorded.

Results from previous baseline surveys suggested the analysis of biomass for meadows where the large growing species E. acoroides was present but not dominant required a different method compared to meadows where E. acoroides was dominant (Roelofs et al. 2003). The dry weight biomass for E. acoroides is many orders of magnitude higher than other tropical seagrass species and dominates the average biomass of a meadow where it is present. Therefore, isolated E. acoroides plants occurring within the H. uninervis dominated meadows (Meadows 1, 3, 5 and 8) were excluded from biomass comparisons in order to track the dynamics of these morphologically distinct species.

B

Figure 6. Seagrass methods using (A) helicopter aerial surveillance, and (B, C) boat-based CCTV surveillance.

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C


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2.2 Habitat mapping and Geographic Information System

Spatial data from the March 2017 survey were entered into the Port of Thursday Island Geographic Information System (GIS). Three seagrass GIS layers were created in ArcGIS® - site information, seagrass meadow characteristics and seagrass landscape category.

• Site information- data containing seagrass percent cover and above-ground biomass (for each species), depth below mean sea level (dbMSL), sediment type, latitude and longitude, sampling method and comments.

• Seagrass meadow characteristics- area data for seagrass meadows with summary information on meadow characteristics. Seagrass meadows were assigned a meadow identification number which was used to compare individual meadows among annual monitoring surveys. Identification numbers for core monitoring meadows were also used to reference meadows throughout the results section. Seagrass community types were determined according to species composition from nomenclature developed for seagrass meadows of Queensland (Table 1). Abundance categories (light, moderate, dense) were assigned to community types according to above-ground biomass of the dominant species (Table 2).

• Seagrass landscape category- area data showing the seagrass landscape category determined for each meadow (Figure 7).

Table 1. Nomenclature for Queensland seagrass community types.

Table 2. Density categories and mean above-ground biomass ranges for each species used in

determining seagrass community type in the Port of Thursday Island.

Density

Mean-above ground biomass (g DW m-2)

H. uninervis

(narrow)

H. ovalis

H. decipiens

H. uninervis (wide)

C. serrulata/rotundata

S. isoetifolium

T. hemprichi

H. spinulosa

Z. muelleri E. acoroides

T. ciliatum

Light < 1 < 1 < 5 < 15 < 20 < 40

Moderate 1 - 4 1 - 5 5 - 25 15 - 35 20 - 60 40 - 100

Dense > 4 > 5 > 25 > 35 > 60 > 100

Community type Species composition

Species A Species A is 90-100% of composition Species A with Species B Species A is 60-90% of composition

Species A with Species B/Species C Species A is 50% of composition Species A/Species B Species A is 40-60% of composition


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Figure 7. Landscape categories for seagrass meadows in Queensland Seagrass meadow boundaries were determined from a combination of techniques. Exposed inshore boundaries were mapped directly from helicopter and guided by recent satellite imagery of the region (Source: ESRI; Google Earth). Subtidal boundaries were interpreted from a combination of subtidal survey sites and the distance between sites, field notes, depth contours and recent satellite imagery. Each seagrass meadow was assigned a mapping precision estimate (±m) based on the mapping method used for that meadow (Table 3). Mapping precision estimates ranged from 1–5 m for intertidal seagrass meadows to 10–50 m for intertidal to subtidal meadows. The mapping precision estimate was used to calculate a range of meadow area for each meadow, and was expressed as a meadow reliability estimate (R) in hectares. The reliability estimate for subtidal habitat is based on the distance between sites with and without seagrass when determining the habitat boundary. Table 3. Mapping precision and methods for seagrass meadows in the Port of Thursday Island 2017.

Mapping precision

Mapping method

1-5 m

Meadow boundaries mapped in detail by GPS from helicopter; Intertidal meadows completely exposed or visible at low tide; Relatively high density of mapping and survey sites; Recent aerial photography aided in mapping.

10-50 m

Meadow boundaries determined from helicopter and camera/grab surveys; Inshore boundaries mapped from helicopter; Offshore boundaries interpreted from survey sites and aerial photography; Relatively high density of mapping and survey sites.

Isolated seagrass patches The majority of area within the meadows consisted of un-vegetated sediment interspersed with isolated patches of seagrass. Aggregated seagrass patches Meadows are comprised of numerous seagrass patches but still feature substantial gaps of un-vegetated sediment within the meadow boundaries. Continuous seagrass cover The majority of area within the meadows comprised of continuous seagrass cover interspersed with a few gaps of un-vegetated sediment.


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2.3 Seagrass meadow condition index

A condition index was developed for the Thursday Island seagrass monitoring meadows based on changes in mean above-ground biomass, total meadow area and species composition relative to a baseline. Seagrass condition for each indicator in Thursday Island was scored from 0 to 1 and assigned one of five grades: A (very good), B (good), C (satisfactory), D (poor) and E (very poor). The flow chart in Figure 8 summarises the methods used to calculate seagrass condition.

Figure 8. Flow chart to develop Thursday Island grades and scores. 2.3.1 Baseline Conditions Baseline conditions for seagrass biomass, meadow area and species composition were established from sampling data calculated from 2002-2016 (8 sampling events for Meadows 1-6, 8 and 27, and 9 sampling events for Meadow 26). This baseline was set based on results of the Gladstone Harbour 2014 pilot report card (Bryant et al. 2014). Where possible, a long-term average of 10 sampling years of data is considered a more accurate representation of baseline conditions as this incorporates a range of environmental conditions over a longer time period including El Niño and La Niña. Once the monitoring program has collected over 10 years of data, the 10 year long-term average will be used in future assessments and reassessed each decade.


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Baseline conditions for species composition were determined based on the percent contribution of each species to mean meadow biomass of the baseline years. The meadow was classified as either single species dominated (one species comprising ≥80% of baseline species), or mixed species (all species comprise ≤80% of baseline species composition). Similar to seagrass biomass and area, the species composition baseline was calculated from 2002-2016 (only in the years where species were present; depending on meadow at Thursday Island). The baseline calculation was based only on the percent composition of what was considered to be the stable state species. Where a meadow baseline contained an approximately equal split in two dominant species (i.e. both species accounted for 40-60% of the baseline), the baseline was set according to the percent composition of the more persistent/stable species of the two (see Section 2.3.4 and Figure 9 for further description). 2.3.2 Meadow Classification A meadow classification system was developed for the three condition indicators (biomass, area, species composition) in recognition that for some seagrass meadows these measures are historically stable, while in other meadows they are relatively variable. The coefficient of variation (CV) for each baseline for each meadow was used to determine historical variability. Meadow biomass, area and species composition was classified as either stable or variable (Table 4). Two further classifications for meadow area were added in the 2016 reporting year: highly stable and highly variable, in recognition that some meadows are very stable while others have a naturally extreme level of variation (Table 4). The CV was calculated by dividing the standard deviation of the baseline years by the baseline for each of condition indicator. Table 4. Coefficient of variation (CV) thresholds used to classify historical stability or variability of

meadow biomass, area and species composition.

Indicator Class

Highly stable Stable Variable Highly variable Biomass - CV < 40% CV > 40% -

Area < 10% CV > 10, < 40% CV > 40, <80% CV > 80%

Species composition - CV < 40% CV > 40% -

2.3.3 Threshold Definition Seagrass condition was assigned one of five grades (very good, good, satisfactory, poor, very poor). Threshold levels for each grade were set relative to the baseline and were selected based on meadow class. This approach accounted for historical variability within the monitoring meadows and expert knowledge of the different meadow types and assemblages in the region (Table 5).


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Table 5. Threshold levels for grading seagrass indicators for various meadow classes. Upwards/ downwards arrows are included where a change in condition has occurred in any of the three condition indicators (biomass, area, species composition) from the previous year.

Seagrass condition indicators/

Meadow class

Seagrass grade

A Very good

B Good

C Satisfactory

D Poor

E Very Poor

Biom

ass

Stable >20% above 20% above -20% below 20-50% below 50-80% below >80% below

Variable >40% above 40% above -40% below 40-70% below 70-90% below >90% below

Area

Highly stable >5% above 5% above -10% below 10-20% below 20-40% below >40% below

Stable >10% above 10% above -10% below 10-30% below 30-50% below >50% below

Variable >20% above 20% above -20% below 20-50% below 50-80% below >80% below

Highly variable > 40% above 40% above - 40% below 40-70% below 70-90% below >90% below

Spec

ies

com

posi

tion

Stable and variable;

Single species dominated

>0% above 0-20% below 20-50% below 50-80% below >80% below

Stable; Mixed species >20% above 20% above -

20% below 20-50% below 50-80% below >80% below

Variable; Mixed species >20% above 20% above-

40% below 40-70% below 70-90% below >90% below

Increase above threshold from previous year

Decrease below threshold from previous year


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2.3.4 Grade and Score Calculations

A score system (0-1) was applied to each grade to allow numerical comparisons of seagrass condition among meadows within a location, and among all the locations monitored by TropWATER (Table 6; see (Carter et al. 2015) for a detailed description). Score calculations for each meadow’s condition required calculating the biomass, area and species composition for that year (described in Section 2.3.1), allocating a grade for each indicator by comparing 2017 values against meadow-specific thresholds for each grade, then scaling biomass, area and species composition values against the prescribed score range for that grade. Scaling was required because the score range in each grade was not equal (Table 6). Within each meadow, the upper limit for the very good grade (score = 1) for species composition was set as 100% (as a species could never account for >100% of species composition). For biomass and area the upper limit was set as the maximum mean plus standard error (SE; i.e. the top of the error bar) value for a given year, compared among years during the baseline period. For meadows on Thursday Island this upper limit will be recalculated each year until the 10 year baseline period is complete. In previous report cards the upper limit was based on the mean + SE of any survey year, meaning biomass and area values in the very good range potentially would require constant recalculation; defining the upper limit using baseline years is a new approach in 2016 that “locks in” the upper value. Table 6. The score range for each grade used for TropWATER seagrass report cards.

Grade Description Score Range

Lower bound Upper bound

A Very good >0.85 1.00

B Good >0.65 <0.85

C Satisfactory >0.50 <0.65

D Poor >0.25 <0.50

E Very poor 0.00 <0.25

Where species composition was determined to be anything less than in “perfect” condition (i.e. a score <1), a decision tree was used to determine whether equivalent and/or more persistent species were driving this grade/score (Figure 9). If this was the case then the species composition score and grade for that year was recalculated including those species. Concern regarding any decline in the stable state species should be reserved for those meadows where the directional change from the stable state species is of concern (Figure 9). This would occur when the stable state species is replaced by species considered to be earlier colonisers. Such a shift indicates a decline in meadow stability (e.g. a shift from E. acoroides to H. uninervis). An alternate scenario can occur where the stable state species is replaced by what is considered an equivalent species (e.g. shifts between C. rotundata and C. serrulata), or replaced by a species indicative of an improvement in meadow stability (e.g. a shift from H. decipiens to H. uninervis or any other species). The directional change assessment was based largely on dominant traits of colonising, opportunistic and persistent seagrass genera described by Kilminster et al. (2015). Adjustments to the Kilminster model included: (1) positioning S. isoetifolium further towards the colonising species end of the list, as successional studies following disturbance demonstrate this is an early coloniser in Queensland seagrass meadows (Rasheed 2004); and (2) separating and ordering the Halophila genera by species. Shifts between Halophila species are ecologically relevant; for example, a shift from H. ovalis to H. decipiens, the most marginal species found in


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Gladstone Harbour, may indicate declines in water quality and available light for seagrass growth as H decipiens has a lower light requirement (Collier et al. 2016) (Figure 9).

Figure 9. Decision tree and directional change assessment for grading and scoring seagrass species

composition.

2.3.5 Score Aggregation

Each overall meadow grade/score was defined as the lowest grade/score of the three condition indicators within that meadow. The lowest score, rather than the mean of the three indicator scores, was applied in recognition that a poor grade for any one of the three described a seagrass meadow in poor condition. Maintenance of each of these three fundamental characteristics of a seagrass meadow is required to describe a healthy meadow. This method allowed the most conservative estimate of meadow condition to be made (Bryant et al. 2014). Thursday Island grades/scores were determined by averaging the overall meadow scores for each monitoring meadow within the port, and assigning the corresponding grade to that score (Figure 8; Table 6). Meadows were not subjected to a weighting system at this stage of the analysis. The classification process (outlined in Section 2.3.2) at the meadow analysis stage applied smaller and therefore more sensitive thresholds for meadows considered stable, and less sensitive thresholds for variable meadows. The classification process served therefore as a proxy weighting system where any condition decline in the (often) larger, stable meadows was more likely to trigger a reduction in the meadow grade compared with the more variable, ephemeral meadows. Port grades are therefore more sensitive to changes in stable than variable meadows.

3 RESULTS 3.1 Seagrasses in Thursday Island

A total of 266 habitat characterisation sites were surveyed in the Port of Thursday Island monitoring meadows in March 2017. (Figure 5). The total area of seagrass habitat mapped within the nine monitoring meadows was 138 ± 19 ha (Figure 2). Nine seagrass species (from three families) were identified during the survey (Table 7). For a complete list of seagrass species found within the port limits see Rasheed et al. (2003). Table 7. Seagrass species present at the Port of Thursday Island in 2017.

3.2 Seagrass Condition for Monitoring Meadows

The overall condition of seagrasses in the Port of Thursday Island in March 2017 was satisfactory (Table 8). Six of the nine monitoring meadows reduced from very good or good to satisfactory due to reductions in area or changes to species compositions and biomass (Table 8; Figures 12-20). Table 8. Grades and scores for seagrass indicators (biomass, area and species composition) for the

Port of Thursday Island 2017.

Meadow Biomass Area Species Composition Overall Meadow Score

1 0.831 0.608 0.990 0.608

2 0.679 1.000 0.627 0.627

3 0.864 0.666 0.980 0.666

4 0.576 1.000 0.619 0.576

5 1.000 0.550 0.994 0.550

6 0.664 0.841 0.736 0.664

8 0.875 0.862 0.977 0.862

26 0.659 0.730 0.535 0.535

27 0.696 0.627 0.741 0.627

Overall Score for the Port of Thursday Island

0.635

3.2.1 Inshore Halodule uninervis dominated meadows (Meadows 1, 3, 5, 8) All intertidal meadows consisted of aggregated patches of dense Halodule uninervis community types (Figures 12, 14, 16 and 18). Meadow 3 remained in good condition and Meadow 8 on the Northern side of Thursday Island near Aplin Pass, improved to be classified as very good. Reductions in meadow area caused the downgrading of condition scores for Meadows 1 and 5. Above-ground biomass and species composition were above the long-term average in all H. uninervis dominated meadows and classed as either good or very good condition (Figures 12, 14, 16 and 18). Meadow 1: Overall meadow condition declined from good to satisfactory between 2016 and 2017 due to a reduction in meadow area to 2.32 ± 0.73 ha, falling below the long-term average (Table 8; Figure 12). Mean above-ground biomass remained well above the long-term average at 7.08 ± 1.15 g DW m-2. Species composition remained very good.

Meadow 3: All three indicators remained in good (area) or very good condition (biomass and species composition) (Table 8) resulting in an overall score of good. Above-ground biomass was 6.04 ± 1.27 g DW m-2 and consisted of aggregated patches of H. uninervis with an area of 0.29 ± 0.22 ha (Figure 14).

Meadow 5: Overall meadow condition declined from very good to satisfactory due to a reduction in meadow area from 4.14 ± 1.02 ha in 2016 to 2.6 ± 0.72 ha in 2017 (Table 8; Figure 16). Biomass and species composition were

both in very good condition, with an average biomass of 13.65 ± 1.52 g DW m-2, the highest biomass recorded since monitoring began (Figure 16).

Meadow 8: Overall meadow condition improved from good to very good due to average biomass surpassing the long-term average to 12.43 ± 1.48 g DW m-2 (Table 8; Figure 18). Meadow area and species composition remained in very good condition, with the meadow covering 14.57 ± 2.79 ha. Syringodium isoetifolium was recorded in 2014 and 2016, but was again absent in this survey. Meadow 8 was the only meadow to score an overall very good condition in the 2017 survey. 3.2.2 Enhalus acoroides dominated meadows (Meadows 2, 4, 6, 26, 27) All Enhalus acoroides dominated meadows had continuous cover (Meadows 2, 4, 6; Figures 13, 15, and 17) or aggregated patches (Meadows 26 and 27; Figures 19-20) of light E. acoroides community types. All three subtidal meadows on the southern side of Thursday Island (Meadows 2, 4 and 6) increased in area due to expansion along the deeper margins of the meadow (Figure 10). Meadows were categorised by high species diversity with up to eight species of seagrass recorded in each meadow. Species composition of monitoring meadows remained largely unchanged prior to 2017, however the dominance of E. acoroides declined in all five monitoring meadows in 2017 and likely drove reductions in average above-ground biomass below meadow long-term averages (Figures 13, 15, 17, 19-20; Appendix 2-3a). Madge Reef between Thursday Island and Horn Island has undergone changes in topography during the 14 years of the monitoring program. It was noted in 2010 that sediment accretion at the southern end of Madge Reef (Meadow 26) likely led to an increased bank height. Between 2012 and 2014 the footprint of mangroves in this area grew substantially, with only minor increases in area since 2014 (Figure 5). Meadow 2: The area and biomass indicators remained in very good or good condition in 2017, with meadow area reaching the greatest distribution recorded since monitoring began with 11.38 ± 4.13 ha; however, a decline in the dominance of E. acoroides from 90% to 65% of the species composition led to the overall meadow classification of satisfactory (Table 8; Figure 13).

Meadow 4: Meadow area also reached the highest distribution recorded since monitoring began with 2.01 ± 1.61 ha; however, both the biomass and species composition indicators were in satisfactory condition with biomass decreasing by over 45% to 23.44 ± 2.09 g DW m-2 (Table 8; Figure 15).

Meadow 6: Overall meadow condition improved from satisfactory to good with the expansion of meadow area above the long-term average to 13.90 ± 5.77 ha (Table 8; Figure 17). Trends in biomass and species composition followed those observed in Meadows 2 and 4.

Meadow 26: Both biomass and area were in good condition. A reduction in the dominance of E. acoroides by over 45% shifted the overall meadow score from good in 2016 to satisfactory in 2017 (Figure 19; Table 8; Appendix 2).

Meadow 27: Overall meadow condition declined from good to satisfactory between 2016 and 2017 due to meadow area falling below the long-term average to 6.18 ± 0.68 ha (Table 8; Figure 20). Area and species composition remained in good condition, though also fell below long-term averages (Figure 20; Appendix 2).

Figure 10. Changes in biomass and area (Meadows 1, 2, 3, 4,5, 6, 8) in the Port of Thursday Island (2006-2017).

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Figure 12. Changes in biomass, area and species composition for the Halodule uninervis dominated monitoring Meadow 1 at Thursday Island from 2002 to 2017 (biomass error bars = SE; area error bars = “R” reliability estimate).

Figure 13. Changes in biomass, area and species composition for the Enhalus acoroides dominated

monitoring Meadow 2 at Thursday Island from 2002 to 2017 (biomass error bars = SE; area error bars = “R” reliability estimate).

Figure 14. Changes in biomass, area and species composition for the Halodule uninervis dominated


3.3 Thursday Island climate patterns

3.3.1 Rainfall Total annual rainfall in the Thursday Island area in 2016/2017 was above the long-term average (since 1995) following the lowest recorded value seen since seagrass monitoring began in 2002 (Figure 21). The survey month (March 2017) had below average total monthly rainfall and was similar to March 2016, but the three months preceding survey had above average total monthly rainfall (Figure 22).

Figure 21. Total annual rainfall (mm) recorded at Horn Island, 2000/2001 – 2016/2017. Twelve-month

year (2016/2017) is 12 months prior to survey. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au.

Figure 22. Total monthly rainfall (mm) recorded at Horn Island, January 2012 – March 2017. Annual

monitoring survey months coloured red. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au.

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3.3.2 Air Temperature Maximum daily air temperature recorded in the Thursday Island region in 2016/2017 (31.0°C) was above the long-term annual average (since 1995) of 30.4°C (Figure 23). Maximum air temperature in the three months prior to the monitoring survey was just below (February) or about equal to the long-term average (December, Janurary) (Figure 24). The monitoring survey month maximum temperature (31.1°C) was also above the long-term average (30.5°C) (Figure 24).

Figure 23. Maximum daily air temperature (annual average, °C) recorded at Horn Island, 2000/2001 –

2015/2016. Twelve month year (2015/2016) is 12 months prior to survey. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au.

Figure 24. Maximum daily air temperature (monthly average, °C) recorded at Horn Island, January

2012 – March 2017. Annual monitoring survey months coloured red. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au.

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3.3.4 Tidal Exposure of Seagrass Meadows Annual daytime tidal exposure of intertidal meadows was below the long-term average in 2016/2017, with the lowest total exposure hours seen since 2013/2014 (Figure 25). Annual total hours exposed has been below average since 2010/11 with the exception of the previous monitoring year, 2015/16. In the 12 months preceding the 2017 survey, meadows exposure was below average for all months but April and May 2016 (Figure 26). Total hours the meadow exposed during the survey month (March 2017) was similar to the long-term average.

Figure 25. Annual daytime tidal exposure (total hours)* of seagrass meadows at the Port of Thursday

Island, 2000/2001 – 2016/2017. Twelve-month year is 12 months prior to survey. Source: Maritime Safety Queensland, 2016. * Assumes intertidal banks expose at a tide height of 0.8m above Lowest Astronomical Tide.

Figure 26. Monthly daytime tidal exposure (total hours) at the Port of Thursday Island, January 2012 –

March 2017. Source: Maritime Safety Queensland, 2017. * Assumes intertidal banks expose at a tide height of 0.8m above Lowest Astronomical Tide.

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3.3.3 Daily Global Solar Exposure Daily global solar exposure (GSE) is a measure of the total amount of solar energy (Megajoules per square metre, MJ m-2) falling on a horizontal surface in one day. Values are generally highest in clear sun conditions during spring/summer and lowest during autumn/winter. Solar exposure in the Thursday Island region in the year leading up to the March 2017 survey (21.0 MJ m-2) was above the annual long-term average (20.8 MJ m-

2) (Figure 27). In the 12 months prior to the 2017 and 2016 surveys, GSE values for the majority of months were consistently similar to, if not above, the long-term monthly averages, while the March 2014 survey had below average values in the six months immediately preceding the survey (Figure 28).

Figure 27. Daily global solar exposure (annual mean, MJ m-2) recorded at Horn Island, 2000/2001 –

2016/2017. Twelve-month year (2016/2017) is 12 months prior to survey. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au

Figure 28. Daily global solar exposure (monthly mean, MJ m-2) recorded at Horn Island, January 2012 – March 2017. Source: Bureau of Meteorology, Station 027058, available at: www.bom.gov.au

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4 DISCUSSION 4.1 Seagrass distribution, abundance and resilience

Seagrasses in the Port of Thursday Island were in satisfactory condition in March 2017. While this was a decline from the previous year, seagrasses remained in a healthy condition with 8 of the 9 meadows having at least two of the three indicators (biomass, area and species composition) in good or better condition. In addition, the total area of seagrass habitat mapped within the nine monitoring meadows remained above the long-term average indicating that seagrasses remained in a resilient state in 2017. Individual meadow condition changes appear to be dependent upon the seagrass community type rather than proximity to port infrastructure or human activities. Biomass and species condition scores were good or very good in Halodule uninervis meadows, with meadow area driving overall meadow condition scores. For subtidal Enhalus acoroides meadows, area increased as meadows expanded further offshore than previously recorded. However, biomass and species composition condition generally declined as a result. None of these changes resulted in any seagrass indicator rated as being in poor condition, with 2017 values remaining within the range previously recorded in the monitoring program. Analysis of the climate data for Thursday Island and the surrounding area in the twelve months prior to March 2017 showed environmental conditions were generally favourable for seagrass growth. Distribution and abundance of seagrasses are controlled by a range of environmental conditions such as temperature, air exposure, nutrients and light (Erftemeijer and Herman 1994, Longstaff and Dennison 1999, Carter et al. 2014b), with the responses of seagrasses to changes in these conditions dependant on species-specific tolerances (Collier and Waycott 2009). Exposure to desiccation through temperature extremes, extended tidal exposure and high UV can lead to physiological stress to seagrass leaf structure and photosystems and cause declines (Stapel 1997, Bjork et al. 1999, Rasheed and Unsworth 2009, Chartrand et al. 2012). Such impacts to seagrasses have previously been recorded nearby in Weipa (Unsworth et al. 2012) and have been linked to declines at Thursday Island (Carter et al. 2014a). However, in the 12 months leading up to the 2017 seagrass survey, the amount of day time tidal exposure was substantially lower than the long term average and although air temperature was higher than average it had reduced considerably from the previous year and was unlikely to be problomatic for local seagrasses.

In tropical Queensland seagrass meadows are particularly sensitive to impacts associated with high rainfall and flooding, which can reduce the light available for seagrass growth. Severe rainfall and associated flooding has been linked to recent declines in seagrass biomass and area recorded along the east coast of Queensland (McKenna et al. 2015, Davies et al. 2014, Jarvis et al. 2014, York et al. 2014). Meadows in the Thursday Island region however are not so heavily influenced by river flow and flooding (Carter et al. 2014b) and have escaped many of the recent impacts experienced along the east coast of Queensland. Rainfall in the Thursday Island area leading up to the 2017 survey while nearly double that experienced in the preceding twelve months was only marginally above the long term average and was unlikely to have had a major impact on seagrasses.

Monthly solar exposure has been similar for the past three years, and indicates a favourable light environment for seagrass growth has been maintained. However, this measure does not take into account the effects of the water column on light reaching seagrasses so is an imprecise proxy for benthic light. A more precise analysis of the light environment and a determination of whether seagrasses are receiving adequate light for growth is not possible as in situ light monitoring is not part of this program at this time. The reduction in overall condition of seagrass in the Port of Thursday Island between 2016 and 2017 appears to be associated with the expansion of the subtidal Enhalus acoroides meadows into deeper areas and the continued modification of the topography of intertidal mud banks of Madge Reef rather than related to climate impacts. The loss in overall condition for two of the Enhalus acoroides meadows (meadows 2 & 4 between Rebel Marine and the Engineer’s Wharf) was a direct result of these meadows expanding into new

deeper areas where they had not been previously recorded. These recently colonised areas had much lower biomass and a higher composition of colonising species than the previously established sections of the meadow, which led to the overall reduction in biomass and species composition scores in 2017. The deeper penetration of the high light requiring species Enhalus acoroides is actually a good indicator of generally improved water quality conditions and aligns with the available climate data.

Madge Reef between Thursday Island and Horn Island has undergone changes in topography during the 14 years of the monitoring program. It was noted in 2010 that sediment accretion at the southern end of Madge Reef (Meadow 26) likely led to an increased bank height. Between 2012 and 2014 the footprint of mangroves in this area grew substantially, with continued minor increases in area since 2014. It is possible that these changes leading to a shallowing of sections of the bank have made the area being less suitable for Enhalus acoroides, a species that is particularly vulnerable to tidal exposure related stresses (Unsworth et al. 2012) and may explain the reduced composition of this species since 2010. 4.2 Implications for management

The results of seagrass monitoring in 2017 show seagrass habitats generally remained in a healthy condition in the Port of Thursday Island. Condition of individual meadows varied, but does not appear to be related to the proximity of the meadow to port infrastructure or other human activities. The expansion of meadows directly adjacent to port facilities into deeper waters is a positive indication that seagrass habitats continue to coexist with current port and human activities around Thursday Island. Due to the extensive distribution of seagrasses surrounding Thursday and Horn Island, future infrastructure developments will continue to require careful design and management to ensure minimal impacts on seagrass.

The monitoring program has demonstrated that seagrass distribution, biomass and species composition can vary over time, and provides a guide to the capacity of resilience of these seagrass meadows to future natural or anthropogenic impacts. The most recent results indicate that seagrasses remain resilient, however if further declines occur over the next year this could change. The shift to annual monitoring will further aid in interpreting the causes of seagrass change as common impacts to seagrass health (e.g. cyclones, flood events, seasonal climate cycles) occur on shorter time-scales and will provide a greater understanding of the local seagrass health and dynamics within the Port of Thursday Island.

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Carter, A., H. Taylor, S. McKenna, P. York, and M. Rasheed. 2014b. The effects of climate on seagrasses in the Torres Strai, 2011-2014. Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER), James Cook University, Cairns.

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Collier, C. J., K. Chartrand, C. Honchin, A. Fletcher, and M. Rasheed. 2016. Light thresholds for seagrasses of the GBR: a synthesis and guiding document. Including knowledge gaps and future priorities. Report to the National Environmental Science Programme, Cairns.

Collier, C. J., and M. Waycott. 2009. Drivers of change to seagrass distributions and communities on the Great Barries Reef: Literature review and gaps analysis. Reef and Rainforest Research Centre Limited, Cairns.

Costanza, R., R. d'Arge, R. De Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R. O'neill, and J. Paruelo. 1997. The value of the world's ecosystem services and natural capital. Nature 387:253-260.

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Kirkman, H. 1978. Decline of seagrass in northern areas of Moreton Bay, Queensland. Aquatic Botany 5:63-76.

Longstaff, B. J., and W. C. Dennison. 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.

McKenna, S., Jarvis, J., Sankey, T., Reason, C., Coles, R. and Rasheed, M. 2015. Declines of seagrasses in a tropical harbour, North Queensland, Australia, are not the result of a single event. Journal of biosciences, 40: 389-398.

Mellors, J. E. 1991. An evaluation of a rapid visual technique for estimating seagrass biomass. Aquatic Botany 42:67-73.

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6 APPENDICES Appendix 1

An example of calculating a meadow score for biomass in satisfactory condition.

1. Determine the grade for the 2017 (current) biomass value (i.e. satisfactory).

2. Calculate the difference in biomass (Bdiff) between the 2017 biomass value (B2017) and the area value of the lower threshold boundary for the satisfactory grade (Bsatisfactory):

B = B − B

Where Bsatisfactory or any other threshold boundary will differ for each condition indicator depending on the baseline value, meadow class (highly stable [area only], stable, variable, highly variable [area only]), and whether the meadow is dominated by a single species or mixed species.

3. Calculate the range for biomass values (Brange) in that grade: B = B − B

Where Bsatisfactory is the upper threshold boundary for the satisfactory grade. Note: For species composition, the upper limit for the very good grade is set as 100%. For area and biomass, the upper limit for the very good grade is set as the maximum value of the mean plus the standard error (i.e. the top of the error bar) for a given year during the baseline period for that indicator and meadow.

4. Calculate the proportion of the satisfactory grade (Bprop) that B2015 takes up: B = BB

5. Determine the biomass score for 2017 (Score2017) by scaling Bprop against the score range (SR) for the

satisfactory grade (SRsatisfactory), i.e. 0.15 units: Score = LB + B × SR Where LBsatisfactory is the defined lower bound (LB) score threshold for the satisfactory grade, i.e. 0.50 units.

Appendix 2

Species composition of monitoring meadows in the Port of Thursday Island; 2002–2017.

Appendix 3a

Mean above-ground seagrass biomass (g DW m-2) + standard error and number of biomass sampling sites (in brackets) for each core monitoring meadow within the Port of Thursday Island, 2002–2017.

Monitoring Meadow

Mean Biomass ± SE (g DW m-2) (no. of sites)

March 2002 March 2004 March 2005 March 2006 March 2008

February 2010

February 2012

February 2014 March 2016 March 2017

1 Intertidal Halodule

dominated

0.27 ± 0.13 (10)

3.69 ± 0.80 (28) 4.26 ± 1.05

(23) 4.15 ± 0.43

(22) 4.17 ± 1.11

(27) 8.35 ± 1.53

(17) 8.77 ± 1.13

(25) 9.15 ± 1.64

(25) 7.08 ± 1.15

(26)

2 Subtidal Enhalus

dominated

43.26 ± 6.25 (12)

75.38 ± 6.85 (14) 38.16 ± 4.04

(20) 23.40 ± 1.95

(19) 27.73 ± 1.56

(35) 72.41 ± 4.63

(25) 41.46 ± 1.90

(34) 51.53 ± 2.85

(34) 33.40 ± 1.22

(37)


dominated

0.75 ± 0.07 (3)

2.48 ± 1.23 (7) 1.02 ± 0.40

(8) 3.24 ± 0.69

(9) 2.13 ± 0.75

(12) 3.62 ± 0.95

(5) 8.83 ± 0.88

(5) 9.42 ± 1.89

(9) 6.04 ± 1.27

(8)

4 Subtidal Enhalus

dominated

32.80 ± 8.49 (14)

56.19 ± 13.10 (6) 28.92 ± 5.71

(5) 17.30 ± 4.56

(5) 19.27 ± 2.52

(17) 46.07 ± 8.46

(17) 42.70 ± 3.81

(14) 44.66 ± 5.54

(12) 23.44 ± 2.09

(18)


dominated

3.41 ± 1.31 (8)

7.91 ± 1.23 (26) 5.73 ± 0.88

(25) 4.71 ± 0.62

(26) 7.17 ± 2.25

(18) 10.94 ± 1.49

(21) 7.47 ± 0.98

(24) 9.18 ± 1.42

(20) 13.65 ± 1.52

(20)

6 Subtidal Enhalus

dominated

55.71 ± 8.91 (15)

48.22 ± 8.54 (18) 25.52 ± 4.14

(22) 26.34 ± 3.76

(24) 26.70 ± 2.77

(50) 59.74 ± 5.72

(27) 47.03 ± 6.29

(34) 56.74 ± 2.94

(43) 35.81 ± 1.35

(48)


dominated

0.36 ± 0.25 (5)

7.37 ± 1.31 (31) 10.48 ± 2.18

(31) 4.46 ± 0.39

(32) 11.67 ± 2.95

(23) 16.04 ± 1.92

(31) 8.23 ± 1.49

(48) 6.17 ± 0.67

(55) 12.43 ± 1.48

(33)

26 Subtidal Enhalus

dominated

68.81 ± 9.83 (18)

48.78 ± 5.37 (31)

24.08 ± 3.03 (25)

41.89 ± 3.54 (32)

22.01 ± 1.97 (33)

34.24 ± 3.86 (33)

78.47 ± 8.11 (26)

47.84 ± 3.96 (33)

49.01 ± 3.19 (40)

29.33 +/- 1.53 (38)

27 Subtidal Enhalus

dominated 175.94 (1) 47.57 ± 10.55

(13) 24.36 ± 5.71

(8) 32.38 ± 6.44

(10) 16.72 ± 3.45

(10) 23.45 ± 5.02

(25) 70.20 ± 11.85

(20) 43.85 ± 7.08

(21) 43.28 ± 6.60

(16) 29.57 +/- 2.98

(15)

Appendix 3b

Total meadow area + R (ha) for each core monitoring meadow within the Port of Thursday Island, 2002 – 2017.

Monitoring Meadow

Total meadow area + R (ha)

March 2002 March 2004 March 2006 March 2008 February 2010 February 2012 February 2014 March 2016 March 2017


dominated 2.30 ± 0.80 2.50 ± 0.90 2.20 ± 0.80 3.75 ± 0.19 2.47 ± 0.74 3.25 ± 0.77 2.85 ± 0.77 2.71 ± 0.79 2.32 ± 0.73

2 Subtidal Enhalus

dominated 7.70 ± 2.30 7.80 ± 1.60 7.80 ± 1.60 8.63 ± 0.86 8.91 ± 1.47 8.65 ± 1.59 7.65 ± 1.53 9.05 ± 1.55 11.38 ± 4.13


dominated 0.10 ± 0.05 0.20 ± 0.10 0.30 ± 0.20 0.78 ± 0.04 0.26 ± 0.19 0.40 ± 0.20 0.38 ± 0.21 0.32 ± 0.22 0.29 ± 0.17

4 Subtidal Enhalus

dominated 1.30 ± 0.60 1.00 ± 0.50 0.80 ± 0.50 1.11 ± 0.11 0.79 ± 0.45 0.94 ± 0.49 0.68 ± 0.48 0.89 ± 0.49 2.01 ± 1.61


dominated 2.10 ± 0.80 1.90 ± 0.80 2.00 ± 0.90 5.26 ± 0.26 3.17 ± 0.90 3.64 ± 0.97 4.54 ± 1.09 4.14 ± 1.02 2.56 ± 0.72

6 Subtidal Enhalus

dominated 13.20 ± 2.60 12.40 ± 2.40 12.70 ± 2.50 16.22 ± 1.62 13.18 ± 2.51 12.68 ± 2.16 10.15 ± 2.08 11.33 ± 2.14 13.90 ± 5.77


dominated 12.30 ± 2.00 10.40 ± 2.20 12.20 ± 1.80 8.88 ± 0.44 13.44 ± 2.74 14.29 ± 2.74 16.02 ± 2.85 15.64 ± 2.82 14.57 ± 2.79

26 Subtidal Enhalus

dominated 94.50 ± 1.50 87.70 ± 3.50 89.00 ± 3.10 83.52 ± 4.18 89.24 ± 3.19 86.26 ± 3.11 81.30 ± 3.23 94.75 ± 3.42 84.77 ± 2.88

27 Subtidal Enhalus

dominated 6.10 ± 0.70 7.00 ± 0.90 5.80 ± 0.70 5.88 ± 0.29 8.22 ± 0.86 7.15 ± 0.85 9.58 ± 0.94 7.14 ± 0.79 6.18 ± 0.69

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