Contract No. HY/2011/03
Hong Kong-Zhuhai-Macao Bridge Hong Kong Link
Road
Section between Scenic
Hill and Hong Kong Boundary Crossing Facilities
Quarterly EM&A Report No.
35 (March 2021
to May 2021)
6 July 2021
Revision 1
Main Contractor Designer
Contents
Executive
Summary
1.4 Construction
Works Undertaken During the Reporting Period
2.1 Summary
of EM&A Requirements
3....... Environmental Monitoring
and Audit
3.1 Implementation of Environmental
Measures
3.2 Air Quality Monitoring Results
3.4 Water
Quality Monitoring Results
3.5 Dolphin Monitoring Results
3.6 Mudflat Monitoring Results
3.7 Solid and Liquid Waste Management
Status
3.8 Environmental Licenses and Permits
4....... Environmental Complaint and Non-compliance
4.2 Summary of Environmental Complaint,
Notification of Summons and Successful Prosecution
5....... Comments, Recommendations
and Conclusion
Figures
Figure 1.1 Location of
the Site
Figure 2.1 Environmental
Monitoring Stations
Figure 2.2 Transect
Line Layout in Northwest and Northeast Lantau Survey Areas
Appendices
Appendix A Environmental
Management Structure
Appendix B Construction
Programme
Appendix C Location of
Works Areas
Appendix D Event and
Action Plan
Appendix E Implementation
Schedule of Environmental Mitigation Measures
Appendix F Site Audit
Findings and Corrective Actions
Appendix G Air Quality Monitoring Data and
Graphical Plots
Appendix H Noise Monitoring Data and
Graphical Plots
Appendix L Summary of
Environmental Licenses and Permits
Appendix
M Record of
¡§Notification of Summons and Prosecutions¡¨
Executive Summary
The
Hong Kong-Zhuhai-Macao Bridge (HZMB) Hong Kong Link Road (HKLR) serves to
connect the HZMB Main Bridge at the Hong Kong Special Administrative Region
(HKSAR) Boundary and the HZMB Hong Kong Boundary Crossing Facilities (HKBCF)
located at the north eastern waters of the Hong Kong International Airport
(HKIA).
The
HKLR project has been separated into two contracts. They are Contract No.
HY/2011/03 Hong Kong-Zhuhai-Macao Bridge Hong Kong Link Road-Section between
Scenic Hill and Hong Kong Boundary Crossing Facilities (hereafter referred to
as the Contract) and Contract No. HY/2011/09 Hong Kong-Zhuhai-Macao Bridge Hong
Kong Link Road-Section between HKSAR Boundary and Scenic Hill.
China
State Construction Engineering (Hong Kong) Ltd. was awarded by Highways
Department as the Contractor to undertake the construction works of Contract
No. HY/2011/03. The main works of the Contract include land tunnel at Scenic
Hill, tunnel underneath Airport Road and Airport Express Line, reclamation and
tunnel to the east coast of the Airport Island, at-grade road connecting to the
HKBCF and highway works of the HKBCF within the Airport Island and in the
vicinity of the HKLR reclamation.
The Contract is part of the HKLR Project and HKBCF Project, these
projects are considered to be ¡§Designated Projects¡¨, under Schedule 2 of the
Environmental Impact Assessment (EIA) Ordinance (Cap 499) and EIA Reports
(Register No. AEIAR-144/2009 and AEIAR-145/2009) were prepared for the
Project. The current Environmental
Permit (EP) EP-352/2009/D for HKLR and EP-353/2009/K for HKBCF were issued on 22
December 2014 and 11 April 2016, respectively. These documents are available
through the EIA Ordinance Register. The construction phase of Contract was commenced on 17 October 2012.
BMT
Hong Kong Limited was appointed by the Contractor to implement the
Environmental Monitoring & Audit (EM&A) programme for the Contract in
accordance with the Updated EM&A Manual for HKLR (Version 1.0) and provided
environmental team services to the Contract until 31 July 2020.
This
is the thirty-fifth Quarterly EM&A report for the Contract which summarizes
the monitoring results and audit findings of the EM&A programme during the
reporting period from 1 March to 31 May 2021.
Environmental
Monitoring and Audit Progress
The EM&A programme were undertaken in
accordance with the Updated EM&A Manual for HKLR (Version 1.0). A summary
of the monitoring activities during this reporting period is presented as
below:
Monitoring Activity |
Monitoring Date |
|||
Mar 2021 |
Apr 2021 |
May 2021 |
||
Air
Quality |
1-hr
TSP at AMS5 |
4,
10, 16, 22 and 26 |
1,
7, 13, 19, 23 and 29 |
5,
11, 17, 21 and 26 |
1-hr
TSP at AMS6 |
4,
10, 16, 22 and 26 |
Not
applicable.(see remark 2) |
Not
applicable.(see remark 2) |
|
24-hr
TSP at AMS5
|
3,
9, 15, 19, 25 and 29 |
1,
7, 12, 16, 22 and 28 |
4,
10, 14, 20, 25 and 31 |
|
24-hr
TSP at AMS6 |
3,
9, 15, 19 and 25 (see remark 3) |
Not
applicable.(see remark 2) |
Not
applicable.(see remark 2) |
|
Noise |
4,
10, 16 and 22 |
1,
7, 13, 19 and 29 |
5, 11, 17 and 26 |
|
Water Quality |
Not
applicable.(see remark 1) |
Not
applicable.(see remark 1) |
Not
applicable.(see remark 1) |
|
Chinese
White Dolphin |
Not
applicable.(see remark 1) |
Not
applicable.(see remark 1) |
Not applicable.(see
remark 1) |
|
Mudflat Monitoring (Ecology) |
15, 29, 30 and 31
|
-
|
- |
|
Mudflat Monitoring (Sedimentation rate) |
15 |
- |
- |
|
Site Inspection |
3,
11, 17 and 26 |
1,
7, 15, 21 and 29 |
6,
12, 20 and 28 |
Remarks:
1) Water quality monitoring and dolphin monitoring
were temporarily suspended during the reporting period.
2) The existing air quality monitoring location
AMS6 ¡V Dragonair / CNAC (Group)Building (HKIA) was
handed over to Airport Authority Hong Kong on 31 March 2021. 1-hr and 24-hr TSP
monitoring at AMS6 was temporarily suspended starting from 1 April 2021. A new
alternative air quality monitoring location is still under processing during
the reporting period.
3) Due to motor failure of HVS at monitoring
station AMS6 ¡V Dragonair / CNAC (Group) Building (HKIA),
24-hr TSP monitoring at AMS6 on 29 March 2021 was cancelled.
Breaches of Action and Limit Levels
A
summary of environmental exceedances for this reporting period is as follows:
Environmental Monitoring |
Parameters |
Action Level (AL) |
Limit Level (LL) |
Air Quality |
1-hr
TSP |
0 |
0 |
24-hr
TSP |
0 |
0 |
|
Noise |
Leq (30 min) |
0 |
0 |
Water Quality |
Suspended
solids level (SS) |
Not
applicable. (see remark 1) |
Not
applicable. (see remark 1) |
Turbidity
level |
Not
applicable. (see remark 1) |
Not
applicable. (see remark 1) |
|
Dissolved
oxygen level (DO) |
Not
applicable. (see remark 1) |
Not
applicable. (see remark 1) |
|
Dolphin Monitoring |
Quarterly
Analysis (Mar 2021 to May 2021) |
Not
applicable. (see remark 2) |
Not
applicable. (see remark 2) |
Remarks:
1) Water quality monitoring was temporarily
suspended during the reporting period. Thus, no water quality monitoring
results and exceedances from March 2021 to May 2021 are presented.
2) Dolphin monitoring was temporarily suspended
during the reporting period. Thus, no quarterly analysis of dolphin monitoring
results and exceedances from March 2021 to May 2021are presented.
Implementation of Mitigation Measures
Site
inspections were carried out to monitor the implementation of proper
environmental pollution control and mitigation measures for the Project.
Potential environmental impacts due to the construction activities were
monitored and reviewed.
Complaint Log
There
was one complaint received in relation to the environmental impacts during this
reporting period.
Notifications of Summons
and Prosecutions
There
were no notifications of summons or prosecutions received during this reporting
period.
Reporting Changes
This
report has been developed in compliance with the reporting requirements for the
subsequent EM&A reports as required by the Updated EM&A Manual for HKLR
(Version 1.0).
The
proposal for the change of Action Level and Limit Level for suspended solid and
turbidity was approved by EPD on 25 March 2013.
The
revised Event and Action Plan for dolphin monitoring was
approved by
EPD on 6 May 2013.
The
original monitoring station at IS(Mf)9 (Coordinate:
813273E, 818850N) was observed inside the perimeter silt curtain of Contract
HY/2010/02 on 1 July 2013, as such the original impact water quality monitoring
location at IS(Mf)9 was temporarily shifted outside
the silt curtain. As advised by the
Contractor of HY/2010/02 in August 2013, the perimeter silt curtain was shifted
to facilitate safe anchorage zone of construction barges/vessels until end of
2013 subject to construction progress.
Therefore, water quality monitoring station IS(Mf)9
was shifted to 813226E and 818708N since 1 July 2013. According to the water quality
monitoring team¡¦s observation on 24 March 2014, the original monitoring
location of IS(Mf)9 was no longer enclosed by the
perimeter silt curtain of Contract HY/2010/02. Thus, the impact water quality
monitoring works at the original monitoring location of IS(Mf)9
has been resumed since 24 March 2014.
Transect
lines 1, 2, 7, 8, 9 and 11 for dolphin monitoring have been revised due to the
obstruction of the permanent structures associated with the construction works
of HKLR and the southern viaduct of TM-CLKL, as well as provision of adequate
buffer distance from the Airport Restricted Areas. The EPD issued a memo and confirmed that
they had no objection on the revised transect lines on 19 August 2015.
The
water quality monitoring stations at IS10 (Coordinate: 812577E, 820670N) and
SR5 (811489E, 820455N) are located inside Hong Kong International Airport
(HKIA) Approach Restricted Areas. The previously granted Vessel's Entry Permit
for accessing stations IS10 and SR5 were expired on 31 December 2016. During
the permit renewing process, the water quality monitoring location was shifted
to IS10(N) (Coordinate: 813060E, 820540N) and SR5(N) (Coordinate: 811430E,
820978N) on 2, 4 and 6 January 2017 temporarily. The permit has been granted by
Marine Department on 6 January 2017. Thus, the impact water quality monitoring
works at original monitoring location of IS10 and SR5 has been resumed since 9
January 2017.
Transect
lines 2, 3, 4, 5, 6 and 7 for dolphin monitoring have been revised and transect
line 24 has been added due to the presence of a work zone to the north of the
airport platform with intense construction activities in association with the
construction of the third runway expansion for the Hong Kong International
Airport. The EPD issued a memo and confirmed that they had no objection on the
revised transect lines on 28 July 2017. The alternative dolphin transect lines
are adopted starting from August¡¦s dolphin monitoring.
A
new water quality monitoring team has been employed for carrying out water
quality monitoring work for the Contract starting from 23 August 2017. Due to
marine work of the Expansion of Hong Kong International Airport into a
Three-Runway System (3RS Project), original locations of water quality
monitoring stations CS2, SR5 and IS10 are enclosed by works boundary of 3RS
Project. Alternative impact water quality monitoring stations, naming as
CS2(A), SR5(N) and IS10(N) was approved on 28 July 2017 and were adopted
starting from 23 August 2017 to replace the original locations of water quality
monitoring for the Contract.
The
role and responsibilities as the ET Leader of the Contract was temporarily
taken up by Mr Willie Wong instead of Ms Claudine Lee from 25 September 2017 to
31 December 2017.
The
topographical condition of the water monitoring stations SR3 (Coordinate:
810525E, 816456N), SR4 (Coordinate: 814760E, 817867N), SR10A (Coordinate:
823741E, 823495N) and SR10B (Coordinate: 823686E, 823213N) cannot be accessed
safely for undertaking water quality monitoring. The water quality monitoring
has been temporarily conducted at alternative stations, namely SR3(N) (Coordinate 810689E, 816591N), SR4(N) (Coordinate:
814705E, 817859N) and SR10A(N) (Coordinate: 823644E, 823484N) since 1 September
2017. The water quality monitoring at station SR10B was temporarily conducted
at Coordinate: 823683E, 823187N on 1, 4, 6, 8 September 2017 and has been
temporarily fine-tuned to alternative station SR10B(N2) (Coordinate: 823689E,
823159N) since 11 September 2017. Proposal for permanently relocating the
aforementioned stations was approved by EPD on 8 January 2018.
The works area WA5
was handed over to other party on 22 June 2013.
According to latest
information received in July 2018, the works area WA7 was handed over to other
party on 28 February 2018 instead of 31 January 2018.
Original WQM stations
IS8 and SR4(N) are located within the active work area of TCNTE project and the
access to the WQM stations IS8 (Coordinate: E814251, N818412) and SR4(N)
(Coordinate: E814705, N817859) are blocked by the silt curtains of the Tung
Chung New Town Extension (TCNTE) project. Alternative monitoring stations
IS8(N) (Coordinate: E814413, N818570) and SR4(N2) (Coordinate: E814688,
N817996) are proposed to replace the original monitoring stations IS8 and
SR4(N). Proposal for permanently relocating the aforementioned stations was
approved by EPD on 20 August 2019. The water quality
monitoring has been conducted at stations IS8(N) and SR4(N2) on 21 August 2019.
There were no marine works conducted by Contract
No. HY/2011/03 since July 2019. A proposal for temporary suspension of marine
related environmental monitoring (water quality monitoring and dolphin
monitoring for the Contract No. HY/2011/03) was justified by the ET leader and
verified by IEC in mid of September 2019 and it was approved by EPD on 24
September 2019. Water quality monitoring and dolphin monitoring for the
Contract will not be conducted starting from 1 October 2019 until marine works
(i.e. toe loading removal works) be resumed. As discussed with Contract No.
HY/2012/08, they will take up the responsibility from Contract No. HY/2011/03
for the dolphin monitoring works starting from 1 October 2019.
According to
information received in January 2020, the works area WA3 and WA4 were handed
over to Highways Department on 23 December 2019 and 14 March 2019 respectively.
The role and responsibilities as the
IEC of the Contract has been taken up by Mr. Manson Yeung instead of Mr. Ray
Yan since 18 May 2020.
Mr. Leslie Leung was
Environmental Team Leader of the Contract for July 2020. The role and
responsibilities as the Environmental Team Leader of the Contract has been
taken up by Ms. Claudine Lee with effective from 1 August 2020.
The existing air quality
monitoring location AMS6 ¡V Dragonair / CNAC (Group)
Building (HKIA) was handed over to Airport Authority Hong Kong on 31 March
2021. 1-hr and 24-hr TSP monitoring at AMS6 was temporarily suspended starting
from 1 April 2021. A new alternative air quality monitoring location is still
under processing.
The role and
responsibilities as the IEC of the Contract has been taken up by Mr Brian Tam
instead of Mr Manson Yeung since 12 April 2021.
Table 1.1 Construction
Activities during Reporting Period
Description of Activities |
Site Area |
E&M
Works |
Airport Road |
Finishing Works for Highway Operation and
Maintenance Area Building |
Portion X |
Finishing Works for Scenic Hill Tunnel
Ventilation Building |
West Portal |
Establishment of Road Signs |
S15 |
Landscape maintenance works |
SHT East Portal |
Trial pits for alternate permanent power
supply to weather station |
East Coast Road |
Table 2.1 Summary
of Impact EM&A Requirements
Environmental
Monitoring |
Description |
Monitoring
Station |
Frequencies |
Remarks |
Air Quality |
1-hr TSP |
AMS 5 & AMS
6 |
At least 3 times every 6 days |
While the
highest dust impact was expected. |
24-hr TSP |
At least once every 6 days |
-- |
||
Noise |
Leq (30mins), |
NMS 5 |
At least once per week |
Daytime on normal weekdays
(0700-1900 hrs). |
Water Quality |
¡P Depth ¡P Temperature
¡P Salinity ¡P Dissolved
Oxygen (DO) ¡P Suspended
Solids (SS) ¡P DO
Saturation ¡P Turbidity ¡P pH |
¡P Impact
Stations: ¡P Control/Far
Field Stations: ¡P Sensitive
Receiver Stations: |
Three times per week
during mid-ebb and mid-flood tides (within ¡Ó 1.75 hour of the predicted time) |
3 (1 m below water surface,
mid-depth and 1 m above sea bed, except where the water depth is less than 6
m, in which case the mid-depth station may be omitted. Should the water depth be less than 3
m, only the mid-depth station will be monitored). |
Dolphin
|
Line-transect Methods |
Northeast Lantau survey
area and Northwest Lantau survey area |
Twice
per month |
-- |
Mudflat |
Horseshoe crabs, seagrass beds, intertidal soft shore communities,
sedimentation rates and water quality |
San Tau and Tung Chung Bay |
Once every 3 months |
-- |
Remarks:
1) Original WQM stations IS8 and SR4(N) are located
within the active work area of TCNTE project and the access to the WQM stations
IS8 (Coordinate: E814251, N818412) and SR4(N) (Coordinate: E814705, N817859)
are blocked by the silt curtains of the Tung Chung New Town Extension (TCNTE)
project. Alternative monitoring stations IS8(N) (Coordinate: E814413, N818570)
and SR4(N2) (Coordinate: E814688, N817996) are proposed to replace the original
monitoring stations IS8 and SR4(N). Proposal for permanently relocating the
aforementioned stations was approved by EPD on 20 August 2019. The water
quality monitoring has been conducted at stations IS8(N) and SR4(N2) on 21
August 2019.
2) The water quality monitoring programme
and dolphin monitoring programme were temporarily suspended during the
reporting period, since no marine works were scheduled or conducted.
Table 2.2 Action and Limit
Levels for 1-hour TSP, 24-hour
TSP and Noise
Environmental Monitoring |
Parameters |
Monitoring Station |
Action Level |
Limit Level |
Air
Quality |
1-hr
TSP |
AMS
5 |
352 µg/m3 |
500 µg/m3 |
AMS
6 |
360 µg/m3 |
|||
24-hr
TSP |
AMS
5 |
164 µg/m3 |
260 µg/m3 |
|
AMS
6 |
173 µg/m3 |
|||
Noise |
Leq (30 min) |
NMS 5 |
When
one documented complaint is received |
75
dB(A) |
Table 2.3 Action
and Limit Levels for Water Quality
Parameter
(unit) |
Water Depth |
Action
Level |
Limit Level |
Dissolved Oxygen (mg/L) |
Surface and Middle |
5.0 |
4.2 except 5 for Fish
Culture Zone |
Bottom |
4.7 |
3.6 |
|
Turbidity (NTU) |
Depth average |
27.5 or 120% of upstream
control station¡¦s turbidity at the same tide of the same day; The action level has been
amended to ¡§27.5 and 120% of upstream control station¡¦s turbidity at the same
tide of the same day¡¨ since 25 March 2013. |
47.0 or 130% of turbidity
at the upstream control station at the same tide of same day; The limit level has been
amended to ¡§47.0 and 130% of turbidity at the upstream control station at the
same tide of same day¡¨ since 25 March 2013. |
Suspended Solid (SS)
(mg/L) |
Depth average |
23.5 or 120% of upstream
control station¡¦s SS at the same tide of the same day; The action level has been
amended to ¡§23.5 and 120% of upstream control station¡¦s SS at the same tide of
the same day¡¨ since 25 March 2013. |
34.4 or 130% of SS at the
upstream control station at the same tide of same day and 10mg/L for Water
Services Department Seawater Intakes; The limit level has been
amended to ¡§34.4 and 130% of SS at the upstream control station at the same
tide of same day and 10mg/L for Water Services Department Seawater Intakes¡¨
since 25 March 2013 |
(1) Depth-averaged
is calculated by taking the arithmetic means of reading of all three depths.
(2) For DO,
non-compliance of the water quality limit occurs when monitoring result is
lower that the limit.
(3) For SS &
turbidity non-compliance of the water quality limits occur when monitoring
result is higher than the limits.
(4) The change to
the Action and limit Levels for Water Quality Monitoring for the EM&A works
was approved by EPD on 25 March 2013. Therefore, the amended Action and Limit
Levels are applied for the water monitoring results obtained on and after 25
March 2013.
Table 2.4 Action
and Limit Level for Dolphin Impact Monitoring
|
North Lantau
Social Cluster |
|
NEL |
NWL |
|
Action Level |
STG < 70% of baseline
& |
STG < 70% of baseline
& |
Limit Level |
STG < 40% of baseline
& |
Remarks:
(1)
STG means quarterly average encounter rate of
number of dolphin sightings.
(2)
ANI means quarterly average encounter rate of
total number of dolphins.
(3)
For North Lantau Social Cluster, AL will be
triggered if either NEL or NWL fall below the criteria; LL will be triggered if
both NEL and NWL fall below the criteria.
Table 2.5 Derived
Value of Action Level (AL) and Limit Level (LL)
|
North Lantau
Social Cluster |
|
NEL |
NWL |
|
Action Level |
STG < 4.2 & ANI
< 15.5 |
STG < 6.9 & ANI
< 31.3 |
Limit Level |
(STG < 2.4 & ANI
< 8.9) and (STG < 3.9 & ANI < 17.9) |
Remarks:
(1)
STG means quarterly average encounter rate of
number of dolphin sightings.
(2)
ANI means quarterly average encounter rate of
total number of dolphins.
(3)
For North Lantau Social Cluster, AL will be
triggered if either NEL or NWL fall below the criteria; LL will be triggered if
both NEL and NWL fall below the criteria.
Table 3.1 Summary
of 1-hour TSP Monitoring Results Obtained During the Reporting Period
Reporting Period |
Monitoring Station |
Average (mg/m3) |
Range (mg/m3) |
Action Level (mg/m3) |
Limit Level (mg/m3) |
Mar 2021 |
AMS5 |
148 |
88 ¡V 229 |
352 |
500 |
AMS6 |
126 |
62- 233 |
360 |
||
Apr 2021 |
AMS5 |
67 |
27 ¡V 146 |
352 |
|
AMS6 |
|
|
360 |
||
May 2021 |
AMS5 |
64 |
8 ¡V 232 |
352 |
|
AMS6 |
|
|
360 |
Table 3.2 Summary
of 24-hour TSP Monitoring Results Obtained During the Reporting Period
Reporting Period |
Monitoring Station |
Average (mg/m3) |
Range (mg/m3) |
Action Level (mg/m3) |
Limit Level (mg/m3) |
Mar 2021 |
AMS5 |
61 |
37 ¡V 93 |
164 |
260 |
AMS6 |
77 |
47 - 112 |
173 |
||
Apr 2021 |
AMS5 |
45 |
26 - 76 |
164 |
|
AMS6 |
|
|
173 |
||
May 2021 |
AMS5 |
28 |
22 - 34 |
164 |
|
AMS6 |
|
|
173 |
Table 3.3 Summary of Construction Noise Monitoring
Results Obtained During the Reporting Period
Reporting period |
Monitoring Station |
Average Leq (30 mins), dB(A)* |
Range of Leq (30 mins), dB(A)* |
Action Level |
Limit Level Leq (30 mins), dB(A) |
Mar 2021 |
NMS5 |
57 |
56 - 58 |
When one documented complaint is received |
75 |
Apr 2021 |
59 |
55 - 62 |
|||
May 2021 |
59 |
57 - 63 |
Table 3.9 Measured
Mudflat Surface Level Results
Baseline Monitoring |
Impact Monitoring |
|||||
Monitoring Station |
Easting |
Northing (m) |
Surface Level |
Easting |
Northing (m) |
Surface Level (mPD) |
S1 |
810291.160 |
816678.727 |
0.950 |
810291.177 |
816678.710 |
1.153 |
S2 |
810958.272 |
815831.531 |
0.864 |
810958.270 |
815831.535 |
0.957 |
S3 |
810716.585 |
815953.308 |
1.341 |
810716.586 |
815953.308 |
1.395 |
S4 |
811221.433 |
816151.381 |
0.931 |
811221.427 |
816151.391 |
1.087 |
Table 3.10 Comparison
of Measurement
Comparison of measurement |
Remarks and
Recommendation |
|||
Monitoring Station |
Easting |
Northing (m) |
Surface Level |
|
S1 |
0.017 |
-0.017 |
0.203 |
Level
continuously increased |
S2 |
-0.002 |
0.004 |
0.093 |
Level continuously increased |
S3 |
0.001 |
0.000 |
0.054 |
Level continuously increased |
S4 |
-0.006 |
0.010 |
0.156 |
Level continuously increased |
Table 3.11 Impact
Water Quality Monitoring Results (Depth Average)
Date |
Mid Ebb Tide |
Mid Flood Tide |
||||
DO (mg/L) |
Turbidity (NTU) |
SS (mg/L) |
DO (mg/L) |
Turbidity (NTU) |
SS (mg/L) |
|
02-Mar-2021 |
6.7 |
4.6 |
5.1 |
6.7 |
4.5 |
4.5 |
04-Mar-2021 |
6.3 |
4.8 |
5.7 |
6.4 |
5.5 |
8.1 |
06-Mar-2021 |
6.5 |
2.2 |
2.8 |
6.5 |
2.3 |
1.6 |
09-Mar-2021 |
6.7 |
3.2 |
7.3 |
6.7 |
2.8 |
6.0 |
11-Mar-2021 |
6.4 |
5.5 |
4.5 |
6.3 |
5.5 |
4.9 |
13-Mar-2021 |
6.6 |
3.8 |
3.7 |
6.5 |
3.3 |
5.7 |
16-Mar-2021 |
6.3 |
4.7 |
4.3 |
6.3 |
4.4 |
4.7 |
18-Mar-2021 |
8.3 |
3.7 |
3.3 |
8.4 |
3.4 |
3.6 |
20-Mar-2021 |
8.4 |
2.2 |
2.8 |
8.4 |
1.8 |
2.1 |
23-Mar-2021 |
6.3 |
3.3 |
1.0 |
6.2 |
3.8 |
0.9 |
25-Mar-2021 |
5.9 |
2.6 |
1.2 |
6.1 |
2.8 |
1.1 |
27-Mar-2021 |
6.1 |
3.9 |
2.6 |
6.3 |
4.2 |
3.1 |
30-Mar-2021 |
6.8 |
3.5 |
3.4 |
6.3 |
3.4 |
4.6 |
Average |
6.7 |
3.7 |
3.7 |
6.7 |
3.7 |
3.9 |
Mudflat Ecology Monitoring
3.6.6 In order to collect baseline information of mudflats in the study site,
the study site was divided into three sampling zones (labeled as TC1, TC2, TC3)
in Tung Chung Bay and one zone in San Tau (labeled as ST) (Figure 2.1 of Appendix O). The horizontal shoreline of sampling zones
TC1, TC2, TC3 and ST were about 250 m, 300 m, 300 m and 250 m, respectively (Figure 2.2 of Appendix O). Survey of horseshoe crabs, seagrass beds and
intertidal communities were conducted in every sampling zone. The present
survey was conducted in March 2021 (totally 4 sampling days on 15th, 29th, 30th and 31st
March 2021).
3.6.8
Active search method was adopted for horseshoe crab monitoring by two
experienced surveyors in every sampling zone. During the search period, any
accessible and potential area would be investigated for any horseshoe crab
individuals within 2-3 hour of low tide period (tidal level below 1.2 m above
Chart Datum (C.D.)). Once a horseshoe crab individual was found, the species
was identified referencing to Li (2008). The prosomal width, inhabiting
substratum and respective GPS coordinate were recorded. A photographic record
was taken for future investigation. Any grouping behavior of individuals, if
found, was recorded. The horseshoe crab surveys were conducted on 15th (for ST),
29th (for TC1),30th (for TC2) and 31st
(for TC3) March 2021, which were cool and sunny days.
3.6.10
Active search method was adopted for seagrass bed monitoring by two
experienced surveyors in every sampling zone. During the search period, any
accessible and potential area would be investigated for any seagrass beds
within 2-3 hours of low tide period. Once seagrass bed was found, the species,
estimated area, estimated coverage percentage and respective GPS coordinates
were recorded. The seagrass beds surveys were conducted on 15th (for ST),
29th (for TC1), 30th (for TC2) and 31st
(for TC3) March 2021,
which were cool and sunny days.
3.6.11
The intertidal soft shore community surveys were conducted in low tide
period on 15th (for ST), 29th (for TC1), 30th (for TC2) and 31st
(for TC3) March 2021. In every sampling zone, three 100m horizontal transect lines were laid
at high tidal level (H: 2.0m above C.D.), mid tidal level (M: 1.5m above C.D.)
and low tidal level (L: 1.0m above C.D.). Along every horizontal transect line;
ten random quadrats (0.5 m x 0.5m) were placed.
3.6.12 Inside a quadrat, any visible epifauna was collected and was in-situ
identified to the lowest practical taxonomical resolution. Whenever possible a
hand core sample (10 cm internal diameter ´ 20 cm depth) of
sediments was collected in the quadrat. The core sample was gently washed
through a sieve of mesh size 2.0 mm in-situ. Any visible infauna was collected
and identified. Finally, the top 5 cm surface sediment was dug for visible
infauna in the quadrat regardless of hand core sample was taken.
3.6.13 All collected fauna were released after recording except some tiny individuals
that were too small to be identified on site. These tiny individuals were taken
to laboratory for identification under dissecting microscope.
H¡¦= -£U ( Ni / N ) ln ( Ni / N ) (Shannon and Weaver, 1963)
J = H¡¦ / ln S, (Pielou, 1966)
3.6.16
In total of 22 and 18 individuals of Carcinoscorpius rotundicauda and Tachypleus tridentatus were
found in present survey. The recorded individuals were mainly
distributed along the shoreline in ST and TC3. All of them were observed on
similar substratum (fine sand or soft mud, slightly submerged). Photo records
of the observed horseshoe crab are shown in Figure 3.1 of Appendix O and the present survey result regarding horseshoe crab are presented in Table 3.1 of Appendix O. The complete survey records are presented
in Annex II of Appendix O.
3.6.17
For Carcinoscorpius rotundicauda, more individuals (16 ind.) were found in ST with average body size 43.30 mm (prosomal width ranged 32.07 ¡V 57.15 mm). In TC3, 6 individuals with average body size 46.24 mm (prosomal width ranged 35.66 ¡V 54.47 mm) were found in present survey. The search record in ST (2.67 ind. hr-1. Person-1) and TC3 (1.00 ind. hr-1. Person-1) were very low. No Carcinoscorpius rotundicauda was recorded in TC1 and TC2 in present survey.
3.6.18
For Tachypleus
tridentatus, 10 individuals with
average body size 45.90mm (prosomal width ranged 33.21 ¡V 60.37mm) were found in ST. In
TC3, 8 individuals with average body size 46.14mm (prosomal width ranged 37.96 ¡V 54.67 mm) were found in present survey. The search records in ST (1.67 ind. hr-1. Person-1) and TC3 (1.33 ind. hr-1. Person-1) were very
low. No Tachypleus
tridentatus was found in TC1 and TC2 in present survey.
3.6.19
No mating pair or large individual (≥100mm) was found
in present survey.
3.6.20
In the survey of March 2015, there was one important
finding that a mating pair of Carcinoscorpius
rotundicauda was found in ST (prosomal width:
male 155.1mm, female 138.2mm). It indicated the importance of ST as a breeding
ground of horseshoe crab. In June 2017, mating pairs of Carcinoscorpius
rotundicauda were found in TC2 (male 175.27 mm,
female 143.51 mm) and TC3 (male 182.08 mm, female 145.63 mm) (Figure 3.2 of Appendix O). In December 2017
and June 2018, one mating pair was of Carcinoscorpius
rotundicauda was found in TC3 (December 2017:
male 127.80 mm, female 144.61 mm; June 2018: male 139 mm, female 149 mm). In
June 2019, 2 mating pairs
of Tachypleus tridentatus with
large body sizes (male 150mm and Female 200mm; Male 180mm and Female 220mm) was
found in TC3. Another mating pair of Tachypleus tridentatus was found in ST (male 140mm and Female
180mm). In March 2020, a pair of Tachypleus tridentatus with large body sizes (male 123mm and
Female 137mm was recorded in TC1. Figure
3.2 of Appendix O shows the photographic records of
mating pairs found. The recorded mating pairs were found nearly burrowing in
soft mud at low tidal level (0.5-1.0 m above C.D.). The smaller male was
holding the opisthosoma (abdomen carapace) of larger female from behind. A
mating pair was found in TC1 in March 2020, it indicated that breeding of
horseshoe crab could be possible along the coast of Tung Chung Wan rather than
ST only, as long as suitable substratum was available. Based on the frequency
of encounter, the shoreline between TC3 and ST should be more suitable mating
ground. Moreover suitable breeding period was believed in wet season (March ¡V
September) because tiny individuals (i.e. newly hatched) were usually recorded
in June and September every year (Figure 3.3 of Appendix O). No mating pair was found in March
2021 (present survey).
3.6.21
No large individuals (prosomal
width >100mm) of Carcinoscorpius rotundicauda and Tachypleus tridentatus was
recorded in March 2021 (present survey). In December 2018,
one large individual of Carcinoscorpius rotundicauda was found in TC3 (prosomal width 148.9
mm). In March 2019, 3 large individuals (prosomal width ranged 220 ¡V 310mm) of Carcinoscorpius
rotundicauda were observed in TC2. In June 2019,
there were 3 and 7 large individuals of Tachypleus
tridentatus were recorded in ST (prosomal width
ranged 140 ¡V 180mm) and TC3 (prosomal width ranged 150 ¡V 220mm), respectively. In March 2020, a mating pair of Tachypleus tridentatus was
recorded in TC1 with prosomal width 123 mm and 137mm. Based on their sizes, it indicated that individuals
of prosomal width larger than 100 mm would progress its nursery stage from
intertidal habitat to sub-tidal habitat of Tung Chung Wan. The photo records of the large
horseshoe crab are shown in Figure 3.4 of Appendix O. These large individuals might move onto intertidal
shore occasionally during high tide for foraging and breeding. Because they
should be inhabiting sub-tidal habitat most of the time. Their records were
excluded from the data analysis to avoid mixing up with juvenile population
living on intertidal habitat.
3.6.22
No marked individual of horseshoe
crab was recorded in March 2021
(present survey). Some marked individuals were found in the previous surveys of
September 2013, March 2014 and September 2014. All of them were released
through a conservation programme in charged by Prof.
Paul Shin (Department of Biology and Chemistry, The City University of Hong
Kong (CityU)). It was a re-introduction trial of
artificial bred horseshoe crab juvenile at selected sites. So that the
horseshoe crabs population might be restored in the natural habitat. Through a
personal conversation with Prof. Shin, about 100 individuals were released in
the sampling zone ST on 20 June 2013. All of them were marked with color tape
and internal chip detected by specific chip sensor. There should be second
round of release between June and September 2014 since new marked individuals
were found in the survey of September 2014.
3.6.23
The artificial bred individuals, if found, would be
excluded from the results of present monitoring programme
in order to reflect the changes of natural population. However, the mark on
their prosoma might have been detached during moulting
after a certain period of release. The artificially released individuals were
no longer distinguishable from the natural population without the specific chip
sensor. The survey data collected would possibly cover both natural population
and artificially bred individuals.
Population difference among the sampling zones
3.6.24 Figure
3.5 and 3.6 of Appendix O show
the changes of number of individuals, mean prosomal width and search record of
horseshoe crabs Carcinoscorpius rotundicauda
and Tachypleus tridentatus in respectively in each sampling zone
throughout the monitoring period.
3.6.25 To consider the
entire monitoring period for TC3 and ST, medium to high search records (i.e.
number of individuals) of both species (Carcinoscorpius rotundicauda and Tachypleus tridentatus) were usually found in wet
season (June and September). The search record of ST was higher from September
2012 to June 2014 while it was replaced by TC3 from September 2014 to June
2015. The search records were similar between two sampling zones from September
2015 to June 2016. In September 2016, the search record of Carcinoscorpius rotundicauda
in ST was much higher than TC3. From March to June 2017, the search records of
both species were similar again between two sampling zones. It showed a natural
variation of horseshoe crab population in these two zones due to weather
condition and tidal effect. No obvious difference of horseshoe crab population
was noted between TC3 and ST. In September 2017, the search records of both
horseshoe crab species decreased except the Carcinoscorpius rotundicauda
in TC3. The survey results were different from previous findings that there
were usually higher search records in September. One possible reason was that
the serial cyclone hit decreased horseshoe crab activity (totally 4 cyclone
records between June and September 2017, to be discussed in 'Seagrass survey'
section). From December 2017 to September 2018, the search records of both
species increased again to low-moderate level in ST and TC3. From December 2018 to September 2019, the search records of Carcinoscorpius rotundicauda change from very low to low while the change
of Tachypleus tridentatus was similar during this period. Relatively higher
population fluctuation of Carcinoscorpius rotundicauda was observed in TC3. From March 2020 to September 2020, the search
records of both species, Carcinoscorpius rotundicauda and Tachypleus
tridentatus, were increased to moderate level in
ST. However, the search records of both species, Carcinoscorpius rotundicauda and Tachypleus
tridentatus, were decreased from very
low to none in TC3 in this period. From December 2020 to March 2021 (present
survey), the search records of both species, Carcinoscorpius rotundicauda and Tachypleus
tridentatus, were increased to low-moderate level in both ST and TC3. It is similar to the previous findings of March. It shows
another growing phenomenon of horseshoe crabs and it may due to the weather variation of starting of wet season.
3.6.26 For TC1, the
search record was at low to moderate level throughout the monitoring period.
The change of Carcinoscorpius rotundicauda
was relatively more variable than that of Tachypleus tridentatus. Relatively, the search
record was very low in TC2. There were occasional records of 1 to 4 individuals
between March and September throughout the monitoring period. The maximum
record was 6 individuals only in June 2016.
Seasonal variation
of horseshoe crab population
3.6.29 Throughout the monitoring period, the search records of horseshoe crabs
were fluctuated and at moderate ¡V very low level in June (Figures 3.5 and 3.6 of Appendix O). Low ¡V Very low search record was found in June 2013, totally 82
individuals of Tachypleus tridentatus
and 0 ind. of Carcinoscorpius rotundicauda were
found in TC1, TC3 and ST. Compare with the search record of June 2013, the
numbers of Tachypleus tridentatus
were gradually decreased in June 2014 and 2015 (55 ind. in 2014 and 18 ind. in
2015); the number of Carcinoscorpius
rotundicauda raise to 88 and 66 individuals. in
June 2014 and 2015 respectively. In June 2016, the search record increased
about 3 times compare with June 2015. In total, 182 individuals of Carcinoscorpius rotundicauda
and 47 individuals. of Tachypleus tridentatus
were noted, respectively. Then, the search record was similar to June 2016. The
number of recorded Carcinoscorpius rotundicauda (133
ind.) slightly dropped in June 2017. However, that of Tachypleus tridentatus rapidly increased (125
ind.). In June 2018, the search record was low to moderate while the numbers of
Tachypleus tridentatus
dropped sharply (39 ind.). In June 2019, 10 individuals of Tachypleus tridentatus were
observed in TC3 and ST. All of them, however, were large
individuals (prosomal width >100mm), their records are excluded from the
data analysis to avoid mixing up with the juvenile population living on
intertidal habitat. Until September 2020, the number of Carcinoscorpius rotundicauda and Tachypleus tridentatus gradually increased to 39
ind. and 28 ind., respectively. In December 2020, the number of Carcinoscorpius
rotundicauda and Tachypleus
tridentatus greatly decreased to 3 ind. and 7
ind., respectively. In March 2021 (present survey), the number of Carcinoscorpius rotundicauda and and Tachypleus tridentatus gradually
increased to 22 ind. And 18 ind., respectively. Throughout
the monitoring period, similar distribution of horseshoe crabs population were
found.
3.6.30 The search record of horseshoe crab declined
obviously in all sampling zones during dry season especially December (Figures 3.5 and 3.6 of Appendix O) throughout the monitoring period. Very low
¡V low search record was found in December from 2012 to 2015 (0-4 ind. of Carcinoscorpius rotundicauda
and 0-12 ind. of Tachypleus tridentatus). The horseshoe crabs were inactive and
burrowed in the sediments during cold weather (<15 ºC). Similar results of
low search record in dry season were reported in a previous territory-wide
survey of horseshoe crab. For example, the search records in Tung Chung Wan
were 0.17 ind. hr-1person-1 and 0.00 ind. hr-1
person-1 in wet season and dry season respectively (details see Li,
2008). Compare with the search record of December from 2012 to 2015, which of
December 2016 were much higher relatively. There were totally 70 individuals of
Carcinoscorpius rotundicauda
and 24 individuals of Tachypleus tridentatus in TC3 and ST. Since the survey was carried
in earlier December with warm and sunny weather (~22 ºC during dawn according
to Hong Kong Observatory database, Chek Lap Kok station on 5 December 2016), the horseshoe crab was
more active (i.e. move onto intertidal shore during high tide for foraging and
breeding) and easier to be found. In contrast, there was no search record in
TC1 and TC2 because the survey was conducted in mid-December with colder and
cloudy weather (~20 ºC during dawn on 19 December). The horseshoe crab
activity would decrease gradually with the colder climate. In December of 2017,
2018 and 2019, very low search records were found again as mentioned above.
3.6.31 From September 2012 to December 2013, Carcinoscorpius rotundicauda
was less common species relative to Tachypleus tridentatus. Only 4 individuals were ever recorded in
ST in December 2012. This species had ever been believed of very low density in
ST hence the encounter rate was very low. In March 2014, it was found in all
sampling zones with higher abundance in ST. Based on its average size (mean
prosomal width 39.28 mm - 49.81 mm), it indicated that breeding and spawning of
this species had occurred about 3 years ago along the coastline of Tung Chung
Wan. However, these individuals were still small while their walking trails
were inconspicuous. Hence there was no search record in previous sampling
months. Since March 2014, more individuals were recorded due to larger size and
higher activity (i.e. more conspicuous walking trail).
3.6.32 For Tachypleus tridentatus,
sharp increase of number of individuals was recorded in ST during the wet
season of 2013 (from March to September). According to a personal conversation
with Prof. Shin (CityU), his monitoring team had
recorded similar increase of horseshoe crab population during wet season. It
was believed that the suitable ambient temperature increased its
conspicuousness. However similar pattern was not recorded in the following wet
seasons. The number of individuals increased in March and June 2014 and
followed by a rapid decline in September 2014. Then the number of individuals
fluctuated slightly in TC3 and ST until March 2017. Apart from natural
mortality, migration from nursery soft shore to subtidal habitat was another
possible cause. Since the mean prosomal width of Tachypleus tridentatus continued to grow and
reached about 50 mm since March 2014. Then it varied slightly between 35 - 65
mm from September 2014 to March 2017. Most of the individuals might have
reached a suitable size (e.g. prosomal width 50 - 60 mm) strong enough to
forage in sub-tidal habitat. In June 2017, the number of individuals increased
sharply again in TC3 and ST. Although mating pair of Tachypleus tridentatus was not found in previous
surveys, there should be new round of spawning in the wet season of 2016. The
individuals might have grown to a more conspicuous size in 2017 accounting for
higher search record. In September 2017, moderate numbers of individual were
found in TC3 and ST indicating a stable population size. From September 2018 to
March 2020, the population size was low while natural mortality was the
possible cause. From June 2020 to September 2020, the
population size of Tachypleus tridentatus increased to moderate level in ST while
the mean proposal width of them continued to grow and reach about 55mm. From
December 2020 to March 2021 (present survey), the population size of Tachypleus tridentatus slightly increased in ST and the mean
proposal width of them increased to about 46mm. It
indicated that a stable growth of juveniles after the spawning season.
3.6.33 Recently, Carcinoscorpius rotundicauda
was a more common horseshoe crab species in Tung Chung Wan. It was recorded in
the four sampling zones while the majority of population located in TC3 and ST.
Due to potential breeding last year, the number of Tachypleus tridentatus became increased ST. Since
TC3 and ST were regarded as important nursery ground for both horseshoe crab
species, box plots of prosomal width of two horseshoe crab species were
constructed to investigate the changes of population in details.
Box plot of horseshoe crab populations in TC3
3.6.34
Figure 3.7 of Appendix O shows the changes of prosomal width of Carcinoscorpius rotundicauda
and Tachypleus tridentatus
in TC3. As mentioned above, Carcinoscorpius
rotundicauda was rarely found between September
2012 and December 2013 hence the data were lacking. In March 2014, the major
size (50% of individual records between upper (top box) and lower quartile (bottom
box)) ranged 40 ¡V 60 mm while only few individuals were found. From
March 2014 to September 2018, the median prosomal width (middle line of whole
box) and major size (whole box) decreased after March of every year. It was due
to more small individuals found in June indicating new rounds of spawning.
Also, there were slight increasing trends of body size from June to March of
next year since 2015. It indicated a stable growth of individuals. Focused on
larger juveniles (upper whisker), the size range was quite variable (prosomal
width 60 ¡V 90 mm) along the sampling
months. Juveniles reaching this size might gradually migrate to sub-tidal
habitats.
3.6.35 For Tachypleus
tridentatus, the major size ranged 20-50 mm while
the number of individuals fluctuated from September 2012 to June 2014. Then a
slight but consistent growing trend was observed from September 2014 to June
2015. The prosomal width increased from 25 ¡V 35 mm to 35 ¡V 65 mm. As mentioned, the large individuals might
have reached a suitable size for migrating from the nursery soft shore to
subtidal habitat. It accounted for the declined population in TC3. From March
to September 2016, slight increasing trend of major size was noticed again.
From December 2016 to June 2017, similar increasing trend of major size was
noted with much higher number of individuals. It reflected new round of
spawning. In September 2017, the major size decreased while the trend was
different from previous two years. Such decline might be the cause of serial cyclone
hit between June and September 2017 (to be discussed in the 'Seagrass survey'
section). From December 2017 to September 2018, increasing trend was noted
again. It indicated a stable growth of individuals. From September 2018 to that
of next year, the average prosomal widths were decreased from 60mm to 36mm. It
indicated new rounds of spawning occurred during September to November 2018. In
December 2019, an individual with larger body size (prosomal width 65mm) was
found in TC3 which reflected the stable growth of individuals. In March 2020, the average
prosomal width (middle line of the whole box) of Tachypleus tridentatus in TC3 was 33.97 mm which is
smaller than that in December 2019. It was in normal fluctuation. From June
2020 to December 2020, no horseshoe crab was recorded in TC3. In March 2021
(present survey), 8 individuals of Tachypleus tridentatus with body size (prosomal width 46.1mm) was
found in TC3 which reflected the stable growth of individuals. Across the whole
monitoring period, the larger juveniles (upper whisker) usually reached 60 ¡V 80 mm in prosomal width, even 90 mm
occasionally. The juveniles reaching this size might gradually migrate to
sub-tidal habitats.
Box plot of horseshoe crab populations in ST
3.6.36
Figure 3.8 of Appendix O shows the changes of prosomal width of Carcinoscorpius rotundicauda
and Tachypleus tridentatus
in ST. As mentioned above, Carcinoscorpius rotundicauda was rarely found between September 2012
and December 2013 hence the data were lacking. From March 2014 to September
2018, the size of major population decreased and more small individuals (i.e.
lower whisker) were recorded after June of every year. It indicated new round
of spawning. Also, there were similar increasing trends of body size from
September to June of next year between 2014 and 2017. It indicated a stable
growth of individuals. The larger juveniles (i.e. upper whisker usually ranged 60 ¡V 80 mm in prosomal width except
one individual (prosomal width 107.04 mm) found in March 2017. It reflected juveniles
reaching this size would gradually migrate to sub-tidal habitats.
3.6.37
For Tachypleus tridentatus, a consistent growing trend
was observed for the major population from December 2012 to December 2014
regardless of change of search record. The prosomal width increased from 15-30
mm to 60 ¡V 70 mm. As mentioned, the large juveniles might have reached a suitable size
for migrating from the nursery soft shore to subtidal habitat. From March to
September 2015, the size of major population decreased slightly to a prosomal
width 40 ¡V 60 mm. At the same time, the number of individuals decreased gradually.
It further indicated some of large juveniles might have migrated to sub-tidal
habitat, leaving the smaller individuals on shore. There was an overall growth
trend. In December 2015, two big individuals (prosomal width 89.27 mm and 98.89
mm) were recorded only while it could not represent the major population. In
March 2016, the number of individual was very few in ST that no box plot could
be produced. In June 2016, the prosomal width of major population ranged 50 ¡V 70 mm. But it
dropped clearly to 30 ¡V 40 mm in September 2016 followed by an increase to 40 ¡V 50 mm in
December 2016, 40 ¡V 70 mm in March 2017 and 50 ¡V 60mm in June 2017. Based on overall higher number of small individuals
from June 2016 to September 2017, it indicated another round of spawning. From September
2017 to June 2018, the major size range increased slightly from 40 ¡V 50 mm to 45 ¡V 60 mm
indicating a continuous growth. In September 2018, decrease of major size was
noted again that might reflect new round of spawning. Throughout the monitoring
period, the larger juveniles ranged 60 ¡V 80 mm in prosomal width.
Juveniles reaching this size would gradually migrate to sub-tidal habitats.
3.6.38 As a summary
for horseshoe crab populations in TC3 and ST, there were spawning of Carcinoscorpius rotundicauda
from 2014 to 2018 while the spawning time should be in spring. The population
size was consistent in these two sampling zones. For Tachypleus tridentatus, small individuals were
rarely found in both zones from 2014 to 2015. It was believed no occurrence of
successful spawning. The existing individuals (that recorded since 2012) grew
to a mature size and migrated to sub-tidal habitat. Hence the number of
individuals decreased gradually. From 2016 to 2018, new rounds of spawning were
recorded in ST while the population size increased to a moderate level.
3.6.39 In March 2019 to June 2019, no horseshoe crab juveniles (prosomal width <100mm) were
recorded in TC3 and ST. All recorded
horseshoe crabs were large individuals (prosomal width >100mm) or mating
pairs which were all excluded from the data analysis. From September 2019 to
September 2020, the population size of both horseshoe crab species in ST
gradually increased to moderate
level while their body sizes were mostly in small to medium range (~23 ¡V 55mm). It indicated the natural stable growth
of the horseshoe crab juveniles. In December 2020, the population size of both
horseshoe crab species in ST dropped to
low level while their body sizes were mostly in small to medium range
(~28 ¡V 56mm). It
showed the natural mortality and seasonal variation of horseshoe crab. In March
2021 (present survey), the population size of both horseshoe crab species in ST
increased again to low-moderate
level while their body sizes were mostly in small to medium range (~32 ¡V 60mm). It indicated the natural stable growth
of the horseshoe crab juveniles.
Impact
of the HKLR project
3.6.40
It was
the 34th survey of the EM&A programme
during construction period. Based on the monitoring results, no detectable
impact on horseshoe crab was revealed due to HKLR project. The population
change was mainly determined by seasonal variation, no abnormal phenomenon of
horseshoe crab individual, such as large number of dead individuals on the
shore) had been reported.
Seagrass Beds
3.6.41 Only seagrass
species Halophila ovalis was found in present survey, which was found in TC3 and ST. In ST, there were
seven small sized and one large
sized of seagrass beds found at tidal
zone 1.5m above C.D nearby mangroves plantation. The larger strand had area ~700m2 in high vegetation
coverage (80 ¡V 90%). At close vicinity, seven small sized (~0.25m2 -160m2) of Halophila ovalis beds were observed at tidal zone 1.5m above C.D.
Six out of seven small sized of Halophila ovalis beds were in moderate vegetation coverage
(40-60%) while the ~50m2 and ~700m2 of Halophila ovalis beds
were in high vegetation coverage (80 ¡V 100%) respectively. In TC3, ten
small patches of Halophila ovalis were found at tidal
zone 1.5 ¡V 2.0m above
C.D. These seagrass patch had area 0.25m2 ¡V 54m2. Some of them were in low to moderate vegetation coverage
(20 ¡V 50%) and some of them were in moderate to high vegetation coverage (60 ¡V
100%). Another seagrass species Zostera japonica was not found in present survey. Table 3.2 of Appendix O summarizes the results of present seagrass beds survey and the
photograph records of the seagrass are shown on Figure 3.9 of Appendix O. The complete record throughout the monitoring period is presented in Annex III of Appendix O.
3.6.42 Since the
commencement of the EM&A monitoring programme,
two species of seagrass Halophila ovalis
and Zostera japonica were recorded in
TC3 and ST (Figure 3.10 of Appendix O). In general, Halophila ovalis was
occasionally found in TC3 in few, small to medium patches. But it was commonly
found in ST in medium to large seagrass bed. Moreover it had sometimes grown
extensively and had covered significant mudflat area at 0.5 ¡V 2.0 m above C.D. between TC3 and ST. Another seagrass species Zostera japonica was found in ST only. It was relatively lower in vegetation area and co-existed with Halophila ovalis nearby the
mangrove strand at 2.0 m above C.D.
3.6.43 According to the previous results, majority
of seagrass bed was confined in ST, the temporal change of both seagrass
species were investigated in details:
Temporal variation of seagrass beds
3.6.44 Figure 3.11 of Appendix O shows the
changes of estimated total area of seagrass beds in ST along the sampling
months. For Zostera japonica, it was not recorded in the 1st and 2nd
surveys of monitoring programme. Seasonal recruitment
of few, small patches (total seagrass area: 10 m2) was found in Mach
2013 that grew within the large patch of seagrass Halophila ovalis. Then, the
patch size increased and merged gradually with the warmer climate from March to
June 2013 (15 m2). However, the patch size decreased and remained
similar from September 2013 (4 m2) to March 2014 (3 m2).
In June 2014, the patch size increased obviously again (41 m2) with
warmer climate followed by a decrease between September 2014 (2 m2)
and December 2014 (5 m2). From March to June 2015, the patch size
increased sharply again (90 m2). It might be due to the
disappearance of the originally dominant seagrass Halophila ovalis
resulting in less competition for substratum and nutrients. From September 2015
to June 2016, it was found coexisting with seagrass Halophila ovalis
with steady increasing patch size (from 44 m2 to 115 m2)
and variable coverage. In September 2016, the patch size decreased again to (38
m2) followed by an increase to a horizontal strand (105.4 m2)
in June 2017. And it did no longer co-exist with Halophila ovalis.
Between September 2014 and June 2017, an increasing trend was noticed from September
to June of next year followed by a rapid decline in September of next year. It
was possibly the causes of heat stress, typhoon and stronger grazing pressure
during wet season. However, such increasing trend was not found from September
2017 to March 2021 (present survey) while no patch of Zostera japonica
was found.
3.6.45 For Halophila ovalis, it was recorded as 3 ¡V 4 medium to large patches (area 18.9 - 251.7 m2; vegetation coverage 50 - 80%) beside the mangrove vegetation at tidal
level 2 m above C.D. in September 2012. The total seagrass bed area grew
steadily from 332.3 m2 in September 2012 to 727.4 m2 in
December 2013. Flowers were observed in the largest patch during its flowering
period. In March 2014, 31 small to medium patches were newly recorded (variable
area 1 ¡V 72 m2 per patch, vegetation
coverage 40-80% per patch) in lower tidal zone between 1.0 and 1.5 m above C.D.
The total seagrass area increased further to 1350 m2. In June 2014,
these small and medium patches grew and extended to each other. These patches
were no longer distinguishable and were covering a significant mudflat area of
ST. It was generally grouped into 4 large patches (1116 - 2443 m2)
of seagrass beds characterized of patchy distribution, variable vegetable
coverage (40 - 80%) and smaller leaves. The total seagrass bed area increased
sharply to 7629 m2. In September 2014, the total seagrass area
declined sharply to 1111m2. There were only 3 - 4 small to large
patches (6 - 253 m2) at high tidal level and 1 large patch at low tidal
level (786 m2). Typhoon or strong water current was a possible cause
(Fong, 1998). In September 2014, there were two tropical cyclone records in
Hong Kong (7th-8th September: no cyclone name, maximum
signal number 1; 14th - 17th September: Kalmaegi, maximum signal number 8SE) before the seagrass
survey dated 21st September 2014. The strong water current caused by
the cyclone, Kalmaegi especially, might have given
damage to the seagrass beds. In addition, natural heat stress and grazing force
were other possible causes reducing seagrass beds area. Besides, very small
patches of Halophila ovalis could be found in other mud flat area in
addition to the recorded patches. But it was hardly distinguished due to very
low coverage (10 - 20%) and small leaves.
3.6.46 In December 2014, all the seagrass patches of Halophila ovalis disappeared in ST. Figure 3.12 of Appendix O shows the difference of the original seagrass beds area nearby the
mangrove vegetation at high tidal level between June 2014 and December
2014.Such rapid loss would not be seasonal phenomenon because the seagrass beds
at higher tidal level (2.0 m above C.D.) were present and normal in December
2012 and 2013. According to Fong (1998), similar incident had occurred in ST in
the past. The original seagrass area had declined significantly during the
commencement of the construction and reclamation works for the international
airport at Chek Lap Kok in
1992. The seagrass almost disappeared in 1995 and recovered gradually after the
completion of reclamation works. Moreover, incident of rapid loss of seagrass
area was also recorded in another intertidal mudflat in Lai Chi Wo in 1998 with
unknown reason. Hence, Halophila ovalis
was regarded as a short-lived and r-strategy seagrass that could colonize areas
in short period but disappears quickly under unfavorable conditions (Fong,
1998).
Unfavourable
conditions to seagrass Halophila ovalis
3.6.47 Typhoon or
strong water current was suggested as one unfavourable condition to Halophila ovalis (Fong, 1998). As
mentioned above, there were two tropical cyclone records in Hong Kong in
September 2014. The strong water current caused by the cyclones might have
given damage to the seagrass beds.
3.6.48 Prolonged light deprivation due to turbid
water would be another unfavorable condition. Previous studies reported that Halophila
ovalis had little tolerance to light deprivation. During experimental
darkness, seagrass biomass declined rapidly after 3 - 6 days and seagrass died
completely after 30 days. The rapid death might be due to shortage of available
carbohydrate under limited photosynthesis or accumulation of phytotoxic end
products of anaerobic respiration (details see Longstaff et al., 1999).
Hence the seagrass bed of this species was susceptible to temporary light
deprivation events such as flooding river runoff (Longstaff and Dennison,
1999).
3.6.49
In
order to investigate any deterioration of water quality (e.g. more turbid) in
ST, the water quality measurement results at two closest monitoring stations
SR3 and IS5 of the EM&A programme were obtained
from the water quality monitoring team. Based on the results from June to
December 2014, the overall water quality was in normal fluctuation except there
was one exceedance of suspended solids (SS) at both stations in September. On
10th September 2014, the SS concentrations measured during mid-ebb
tide at stations SR3 (27.5 mg/L) and IS5 (34.5 mg/L) exceeded the Action Level
(≤23.5 mg/L and 120% of upstream control station¡¦s reading) and Limit Level
(≤34.4 mg/L and 130% of upstream control station¡¦s reading) respectively. The
turbidity readings at SR3 and IS5 reached 24.8-25.3 NTU and 22.3-22.5 NTU
respectively. The temporary turbid water should not be caused by the runoff
from upstream rivers. Because there was no rain or slight rain from 1st
to 10th September 2014 (daily total rainfall at the Hong Kong
International Airport: 0-2.1 mm; extracted from the climatological data of Hong
Kong Observatory). The effect of upstream runoff on water quality should be
neglectable in that period. Moreover, the exceedance of water quality was
considered unlikely to be related to the contract works of HKLR according to
the ¡¥Notifications of Environmental Quality Limits Exceedances¡¦ provided by the
respective environmental team. The respective construction of seawall and stone
column works, which possibly caused turbid water, was carried out within silt
curtain as recommended in the EIA report. Moreover, there was no leakage of turbid
water, abnormity or malpractice recorded during water sampling. In general, the
exceedance of suspended solids concentration was considered to be attributed to
other external factors, rather than the contract works.
3.6.50
Based
on the weather condition and water quality results in ST, the co-occurrence of
cyclone hit and turbid waters in September 2014 might have combined the adverse
effects on Halophila ovalis that
leaded to disappearance of this short-lived and r-strategy seagrass species.
Fortunately, Halophila ovalis was a
fast-growing species (Vermaat et al., 1995). Previous studies showed that the seagrass bed could
be recovered to the original sizes in 2 months through vegetative propagation
after experimental clearance (Supanwanid, 1996).
Moreover it was reported to recover rapidly in less than 20 days after dugong
herbivory (Nakaoka and Aioi,
1999).As mentioned, the disappeared seagrass in ST in 1995 could recover
gradually after the completion of reclamation works for international airport
(Fong, 1998).The seagrass beds of Halophila
ovalis might recolonize in the mudflat of ST through seed reproduction as
long as there was no unfavourable condition in the
coming months.
Recolonization
of seagrass beds
3.6.51
Figure 3.12 of Appendix O shows the
recolonization of seagrass bed in ST from December 2014 to June 2017. From
March to June 2015, 2 - 3 small patches of Halophila
ovalis were newly found co-inhabiting with another seagrass species Zostera japonica. But the total patch
area of Halophila ovalis was still
very low compare with previous records. The recolonization rate was low while
cold weather and insufficient sunlight were possible factors between December
2014 and March 2015. Moreover, it would need to compete with seagrass Zostera japonica for substratum and
nutrient, because Zostera japonica had
extended and covered the original seagrass bed of Halophila ovalis at certain degree. From June 2015 to March 2016,
the total seagrass area of Halophila
ovalis had increased rapidly from 6.8 m2 to 230.63 m2.
It had recolonized its original patch locations and covered its competitor Zostera japonica. In June 2016, the
total seagrass area increased sharply to 4707.3m2. Similar to the
previous records of March to June 2014, the original patch area of Halophila ovalis increased further to a
horizontally long strand. Another large seagrass beds colonized the lower tidal
zone (1.0 - 1.5 m above C.D.). In September 2016, this patch extended much and
covered significant soft mud area of ST, resulting in sharp increase of total
area (24245 m2). It indicated the second extensive colonization of
this r-selected seagrass. In December 2016, this extensive seagrass patch
decreased in size and had separated into few, undistinguishable patches. Moreover,
the horizontal strand nearby the mangrove vegetation decreased in size. The
total seagrass bed decreased to 12550 m2. From March to June 2017,
the seagrass bed area remained generally stable (12438 - 17046.5 m2)
but the vegetation coverage fluctuated (20 - 50% in March 2017 to 80-100% in
June 2017). The whole recolonization process took about 2.5 years.
Second
disappearance of seagrass bed
3.6.52 In September 2017, the whole seagrass bed of Halophila ovalis disappeared again along the shore of TC3 and ST (Figure 3.12 of Appendix O