Presentation by Ken T. M. Wong, University of Hong Kong (HKU), Hong Kong, at the Delft3D - User Days (Day 3: Water quality and ecology), during Delft Software Days - Edition 2017. Wednesday, 1 November 2017, Delft.

1. Tracking of red tide movement
in Hong Kong waters
Dr. Ken T.M. Wong
Department of Civil Engineering, The University of Hong Kong
International Association for Hydro-Environment Engineering and
Research - Hong Kong Chapter

2. Red tide - red discoloration of the sea by micro-organisms
(mainly micro-algae)
Algal Bloom - rapid growth/ germination of phytoplankton
(micro-algae) to concentration as high as 100,000 cells per ml
Red Tide and Algal Bloom
Harmful Algal Bloom (HAB)
• Oxygen depletion
• Cause shellfish to contain toxins
• Cause mass mortality of fish,
invertebrates etc.
• Cause fish to contain toxins
• Cause skin or respiratory irritations
{phyto – of a plant; relating to plant
plankton – the small and microscopic organisms drifting or floating in the sea or
fresh water, consisting chiefly of diatoms, protozoans…} (Oxford dictionary)

3. (From Agriculture, Fisheries and Conservation Department HKSAR)
Number of Red Tide Incidents
in HK from 1975 to 2016

4. Diel Vertical Migration
Night TimeDay Time
O2
O2
O2
Importance of Vertical Processes in Red Tides
Photosynthesis
DO ↑8-10 mg/l
i) Dense algal bloom in water
O2
O2
Respiration
DO↓＋stagnant
water → 1-2mg/l
nutrientNutrient (N, P, Si)
O2
Turbulence

5. 0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12Day
Massremained
NEST
Random Walk
Double exponential curve fit
Tf ≈ 10 days
Tracer mass in the Fish Culture Zone
Numerical tracer experiment for determining the tidal
flushing time in a semi-enclosed bay (Yung Shu Au)
1 2
1
fT
k k
 
 
  tktk
o
ee
M
M 21
1 
 
Tracer mass
Flushing time
Mariculture
zone

6. When do algal blooms occur?
• In weakly-flushed semi-enclosed coastal
waters, the optimal algal growth rate (~ 1
per day) is much greater than the
loss/flushing rate
• Hence under favorable light and nutrient
conditions, an algal bloom will be able to
take off
• However, this is negated by the vertical
mixing out of the euphotic zone, sinking,
and other losses (e.g.flushing, respiration)
Based on field observations and theoretical modeling, we
propose the first quantitative model for forecast algal blooms
start
Diffusivity < Critical
Turbulence?
blooms
likely
Nutrient >
threshold?
blooms
unlikely
Y
N
N
Y

7. Probability of
red tide occurrence
in coming week
High
Medium
Low
Red Tide Risk Map (2009-3-26)
Red tide occurrence
in past week
Eastern
Waters
Southeastern
Waters
Southern
Waters
Western Waters
Tolo Harbour
Northeastern
Waters
Clear Water Bay Gonyaulax
Polygramma
2009-03-25 – 2009-03-26
Silver Mine Bay Beach
Karenia Digitata
2009-03-23 – 2009-03-28
Fish culture zone
Red tide risk forecast map

8. Worse Red Tide in Hong Kong’s history in 1998
-Caused by Karenia digitatum (new harmful species)
- Transported from NE to Southern Hong Kong
waters
- Lead to the closure of 7 beaches
- Impacted 16 out of the 26 maricultural zones in
Hong Kong
- Wiped out over 80% of the standing fish stock
- Estimated mariculture loss over HK$315 Million
(US$ 40M)
(Newspaper cuttings
from SCMP and
Mingpao Daily, 1998)
“I woke up in the morning and
tones of fish were dead”
“It came so quickly we had no
warning”

9. Typical Hydrodynamic Flow Pattern in Hong Kong
Dry Season Wet Season

10. Hindcasting of red tide movement

11. Hydrodynamic transport
1. The year of 1997-1998 was the strongest El Niño year of the century (anomalous
weather conditions)
2. The massive red tide observed in southern waters and East Lamma Channel from
April 9-16 may have originated in Nan Au, northeast Mirs Bay, or Tap Mun area.
3. In a typical dry season, a red tide initiated in north Mirs Bay, Tolo Harbour, or Daya
Bay would not normally be transported into the East Lamma Channel.
4. The change in wind direction under diurnal tide conditions in early April greatly
enhanced the transport of blooms in Mirs Bay into southern waters and East
Lamma Channel
5. Hydrodynamic mass transport could play a central role in the occurrence and
spatial coverage of the red tides in Hong Kong waters.

12. Sok Kwu Wan
Lo Tik Wan
Ma Wan
Cheung Sha Wan
Mar 23
Mar 29
Apr 04
Legend
Silver Mine Bay
Forecast for Red Tide Track in March 2009
香港近海水質預報及管理系統
A Water Quality Forecast and
Management System for Hong Kong

13. (2009-11-13)
Red tide occurrence in the past week
Probability of red tide occurrence in the
coming week
High LowMedium
Forecast for Red Tide Track in November 2009

14. Temporary Fish Raft Relocation（魚排臨時遷移）
‘No go” zones
Fish Raft Move Away
from Culture Zone

15. Massive Red Tide in Southern Waters 15-27August 2011
(causative species – Cochlodinium spp.)

16. Public Concern and Questions Raised
- Where was the origin of the red tide? Did it come from the mainland or did
it originate locally?
- What are the major factor(s) and mechanism(s) triggering this red tide?
- Is there any similarity between previous red tides of the same species?
- Is there any similarity between the 1998 red tide and this one?
- What is the likelihood of future outbreaks of this species in the region?
- What additional studies are needed to explore this phenomenon?

17. Weather Anomaly
- 2011 was the hottest August in Hong
Kong’s record
- Total rainfall was only 35% of the
normal amount
- Weak southerly to southeasterly
prevailing wind
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
Su Mo Tu We Th Fr Sa
10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
NE
TOL
S
SE
E
W

18. Hydrodynamic Transport and Possible Origin

19. Hydrodynamic Transport and Possible Origin

20. Hydrodynamic Transport and Possible Origin

21. Massive Red Tide in August 2011
- While the red tide was first observed at Nei Ling Ding Island and Wan Shan
Archipelago, only the western part of Hong Kong is affected by the red tide
originate from Nei Ling Ding Island
- Red tide in the centre part of Hong Kong is more likely formed locally
(Cochlodinium spp. is well known of cyst formation)
- The very hot weather, low rainfall and weak southeastly could have produced a
favourable condition for the red tide formation
- Similar to 1998, weather anomaly could have played an important role in the
formation and transport of the massive red tide

22. Emergency and Natural Disasters

23. Summary
• Red tides are highly dynamic and complex phenomona
governed by physical-biological interactions
• With the assistance of hydrodynamic modelling, we can obtain
better understanding on the driving mechanism of red tides
and be more predictive on the fate and transport of red tide
incidents
• The predictive model would be useful for emergency response
and disaster management