Nutrient Response to Sewage Abatement in Hong Kong

Prof. Paul J. Harrison

Atmospheric, Marine & Coastal Environment Program Institute for the Environment HK University of Science & Technology Outline

• Background of natural factors influencing water quality

• Anthropogenic inputs – Pearl River discharge - Sewage discharge

• Meeting the challenges - sewage treatment for Victoria harbor Why Is Hong Kong Not As Bad As Expected? • Physical Factors – monsoon winds - river discharge and rainfall - wind and tidal mixing • Chemical and Biological factors

•These factors govern the ecosystem sensitivity to various anthropogenic inputs.

• Need to understand how the ecosystem functions. Dead Zone Northern Gulf of Mexico: a large area of hypoxia “dead zone” (20,000 km2)

(<2 O2 mg/L)

Science 281, (1998) South China Sea

• Nutrient-poor sea

Pearl • Hong Kong western River and southern waters Estuary excessive nutrients

a e S

a n i h C

h t u o S Fine resolution land-use 1994 vs 2003

1994 USGS1993 2003 Landuse2003 2003 PRD Land-use map

Red = Urban area

From HKSAR Planning Department Pearl River Estuary & Hong Kong Wide and Shallow Estuary

Diverse – estuarine to oceanic – Complex topographically – enclosed bays and islands Water area = 10 times land area

50 km 10 Water Quality Zones in Hong Kong Shallow (10 m) with some deeper 25 m channels Tidal Mixing

Important in mixing O2 into bottom waters and nutrients in the water column. Neap tide Spring Tide

Tidal Height During Neap and Spring Tides Wind Mixing

• Wind mixing can help reduce the

occurrence of hypoxia (mixes O2 from the air into deep waters

• Reduces the formation algal blooms by dilution. AFCD Phytoplankton Monitoring Stations EPD’s Water Quality Monitoring Stations Increase in TIN and NO3 from 1986-2001

1986 2001 Excessive Nutrients Can Cause

Low O2 in Bottom water

NutrientsÆ algae Æ plankton Æ fish Sink

Bacterial decomposition Æ low O2 in bottom water Surface Algal Bloom Red Tides in Hong Kong during 1983-98 (Yin et al. 2002) $ $$$$$$ $ $ Zone M $$$ $$$$$$$ $$$ $ $ $ (126) $$$ $ N $ $ $ New Territories $ $ $ $ $$$$$$$ $ $$$ $ $ $$ $$$$$$$$$$$$$$ $ $ $ $$$$$$$$$$$$ $$$$$$$$$$ $ $$$$$$$$$$ $ $ Zone W $ $$ Zone T $$$$$$$$$$$$$$ $ (29) $ $ $ $ $ $ (288) $ $$$$$$$ $$ $ $ $ $ $ $$ $ $$$$ $ $ $$$ $$ $$$ $ $ $$$ $$$$$$$ $ $ $$ $ $ $ $$$$ $ $$ $ $ $$$ l. Is $ $ $$ $ Zone P au $ HK Isl $ $ nt $$ $ (122) La $$$ $ $ $$$ $ $$$ $ $$$$ $ $$$$ $$$ $$$$$$ $ $ $ $ $$$$ $ $ $ Zone V $ $ (21) Zone L Zone H (36) (108) 10 0 10 Kilometers High Number of Red Tides in Eastern Waters

•Most red tides occur in spring when river discharge is low •Occur in Mirs Bay – far from the Pearl River estuary. High Biomass Vs Toxic Red Tides

• High biomass algal blooms which

cause low O2 bottom waters seems to be a more serious concern than toxic red tide blooms. High Biomass Blooms e.g. Chain-forming Diatoms

Thalassiosira sp. Chaetoceros debilis Nutrient Sources • Sewage – continuous all year – excess phosphorus sewage treatment Æ ecosystem response (improving) BUT – chl biomass increase in southern waters?

• Pearl River discharge – summer (rain), excess nitrogen and transported offshore - phosphorus potentially controls algal biomass in summer in waters near Pearl River.

HK waters are very productive Sewage Outfalls in 2001 Transect from Western Waters Through Victoria Harbor

Legend Water sampling stations selected N Boundary of Special Administrative Region

MM2 MM3 Shenzhen MM4 MM1

MM7 MM5 MM17

DM2 DM1

TM8 DM3 TM5 MM6 TM7 DM4 TM3 TM6 MM16 TM4

DM5 New TM2 Territories

PM1PM4 PM2 NM5 VM14 PM5 Estuary PM3 MM15 NM2 PM6 NM3 NM1 WM4 PM7 PM9 Kowloon VM12 NM6 VM15 WM3 PM8 PM10N SM10 VM7 VM4 VM2 VM8 VM6 VM1 VM5 JM3 MM14 JM4 NM8 WM2 EM1 Hong Kong Island Lantau Island SM11 SM9 EM2

WM1

SM3 EM3 SM13 SM2 SM4 SM7 SM12 SM1 MM13 SM5 MM8 Oceanic SM6

SM20 SM17 SM18 SM19

0510km Figure showing EPD routine water sampling stations Vertical Mixing in Victoria Harbor 35 30 Jan 25 Jul 20 Surfac 15 eBottom 10 5 NH is a good marker 35 4 30 Feb Aug 25 20 for sewage input in 15 10 5 Victoria Harbor. 35 30 Mar Sep 25 20 15 10

M) 5 μ

( 35 4 30 Apr Oct

NH 25 20 15 10 5 35 30 May Nov 25 20 15 10 5 35 30 Jun Dec 25 20 15 10 5 0 N6 2 W4 3 V8 6 2 E1 3MM8 N6 2 W4 3 V8 6 2 E1 3MM8 From: Yin & Harrison 2005 Stations Mar Poll Bull

Average NH4 during 1990-2000 for each station across the transect. 3 Jan Jul 2

● Surface 1

3 ∆ Bottom Feb Aug 2 Phosphate

1 • Low in dry season 3 Mar Sep 2 • Little change in the 1 estuarine-influenced 3 (µM) 4 Apr Oct

PO 2 waters during wet 1 season 3 May Nov 2 • Higher in Vic. Harbor

3 Jun Dec 2

0 N6 2 W4 2 V8 6 2 E1 3 MM8 N6 2 W4 2 V8 6 2 E1 3 MM8

Stations From: Yin & Harrison 2005 Mar Poll Bull 100 80 Jan Jul ● Surface 60 40 20 ∆ Bottom 100 Total Inorganic Nitrogen 80 Feb Aug 60 40 20 • Low in dry season 100 80 Mar Sep 60 • High in western waters 40 20 during wet season. 100 80 Oct

TIN (µM) Apr 60 40 20 100 80 May Nov 60 40 20 100 80 Jun Dec 60 High N from 40 Pearl River 20 0 N6 2 W4 2 V8 6 2 E1 3 MM8 N6 2 W4 2 V8 6 2 E1 3 MM8 Stations From: Yin & Harrison 2005 Mar Poll Bull Sewage Outfalls in 2001 Now70% 70% of the of sewage sewage receives is treated at primary treatment at SCI Stonecutters Island due to HATS • Unique opportunity + NH4 TransectTransect 1:1: WestWest ÆÆ EastEast

2001 2001 2001 25 20 15 (uM) 4 10

NH S4 S5 S6 5 Victoria Harbor Victoria Harbor Victoria Harbor 0 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05

Year

25 25 Western waters Eastern waters 20 20 S1 2001 S8 15 15

(uM) 2001 4 (uM) 10

4 10 NH

NH 5 5 0 0 89 92 98 01 89 92 98 01 1986 1995 04-05 1986 1995 04-05 Year Year - PO4 TransectTransect 1:1: WestWest ÆÆ EastEast

2.0

1.5

(uM) 1.0 4

PO S6 0.5 S4 S5 S6 Victoria Harbor Victoria Harbor Victoria Harbor 0 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05 Year

2.0 2.0 Western waters Eastern waters 1.5 1.5 S1 S8 (uM) (uM) 1.0 1.0 4 4 PO PO 0.5 0.5

0 0 89 92 98 01 1986 89 92 1995 98 01 04-05 1986 1995 04-05 Year Year Chl a TransectTransect 1:1: WestWest ÆÆ EastEast

) S4 S5 S6 -1 8 Victoria Harbor Victoria Harbor Victoria Harbor 6

Chla (ug L 2

0 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05 86 89 92 95 98 01 04-05 Year

10 10 Western waters Eastern waters ) 8

) 8 -1 S1 -1 S8 6 6 S8

4 4

2 2 Chla (ug L Chla (ug L 0 0 89 92 98 01 89 92 98 01 1986 1995 04-05 1986 1995 04-05 Year Year Chl a Chl in Southern Waters

10 ) Southern waters -1 8 S9 6 4 2 Chla (ug L 0 86 89 92 95 98 01 04-05 Year 10

) Southern waters

-1 8 S10 6 4 2 Chla (ug L 0 86 89 92 95 98 01 04-05

Year DO (mg / L) 3 5 7 9 1986 Figure showing EPDroutine water stations sampling 12 samplingWater stations selected S1 Legend 992 89 Year Lantau Island 1995 801 98 S1 04-05 05 Shenzhen S9 68 29 80 04-05 01 98 95 92 89 86 S10 S2 S12 S3 S2 10km Territories New New S11 S4 Hong Kong S5 Kowloon S6 68 29 80 04-05 01 98 95 92 89 86 S7 S3 S8 68 29 80 04-05 01 98 95 92 89 86 S4 N

DO (mg /L) 3 5 7 9 3 5 7 9 3 5 7 9 1986 Bottom DO Bottom DO 68 29 80 04-05 01 98 95 92 89 86 68 29 80 04-05 01 98 95 92 89 86 68 29 80 04-05 01 98 95 92 89 86 992 89 S8 S6 S7 S5 1995 Year 801 98 04-05 Bottom DO

Legend 12 Water sampling stations selected Shenzhen 9 S9 7

5 New Territories DO (mg / L) DO (mg 3 N 1986 89 92 1995 98 01 04-05 Year S1 S2 Kowloon S3 S4 S9 S6 S5 9 Lantau S10 Hong Kong Island S11 S7 7 S10

S12 S8 5 0510km DO (mg / L) DO (mg 3 Figure showing EPD routine water sampling stations 1986 89 92 1995 98 01 04-05 Year 9 9 S11 S12 7 7

5 5 DO (mg / L) DO (mg / L) 3 3 1986 89 92 1995 98 01 04-05 1986 1995 04-05 Year Year CrossCross--borderborder InfluenceInfluence

(Watershed(Watershed contribution)contribution)

PearlPearl RiverRiver dischargedischarge

RainfallRainfall Guangdong Province & Pearl River Region (satellite image courtesy of Pearl River Water Resources Commission, 1998) HK Nutrient Source 1: Pearl River Estuary

Rainfall in HK & Pearl River Discharge during 1990-98 Summer 600 25000 500 Monthly 20000 Pearl 400 Average 15000 River Rainfall 300 Discharge (mm) 10000 (m3s-1) P 200 e 100 5000 a 0 r 0 l 1 2 3 4 5 6 7 8 9101112 R Month - Kedong Yin, 2003 iv e r E s t u a Hong Kong r y SW winds

Summer Scaleis a Critical Period 50 km Nutrient Sources 1) Anthropogenic Sources • Sewage N:P = 8:1 • Rain N:P = 50:1 Excess nitrogen • River N:P > 100:1 in Summer

2) Natural Sources (N:P <16:1) from coast - South China Sea

Algae need N:P = 16:1 NO3 at Humen (a river mouth of the Pearl River)

160 140 120 100 µM 100 M) μ

( 80 3 60 NO 40 20 At River Mouth 0 45678910111212345 2002 2003 Monthly Average of total inorganic nitrogen during 1991-2000 (Yin 2002)

0 20 40 Kilom N Pearl River

Shenzhen, Guangdong

i n New Territories

y a l. n s u I g HK Isl. nta La # Macau

Western waters Southern Water Mirs Bay

100 100 100

I 50 50 50

T Surface 0 Bottom 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112 Month Monthly Average of phosphate during 1991-2000 (Yin 2002)

0 20 40 Kilom N Pearl River

Shenzhen, Guangdong

i n New Territories

y a l. n s u I g HK Isl. nta La # Macau

s Western waters Southern Water Mirs Bay

r 3 3 3

o h 2 2 2

p s 1 1 1

P 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112 Month Monthly Average of Molar N:P ratio during 1991-2000 (Yin 2002)

0 20 40 Kilom N Pearl River

Shenzhen, Guangdong

i n New Territories

y a l. n Is g u HK Isl. nta La # Macau

i Mirs Bay t Western waters Southern Water

192 192 64 Surface

/ Bottom 48 128 128

r 64 64

a 16

o 0 24681012 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112

M Month Unused Nitrogen in Summer in Western and Southern waters

Potential P limitation in summer near estuary

Nitrogen Unused N (exported offshore)? Phosphorus Add more P

Concentrations N used up and more algal biomass is Time produced Ecosystem Responses to Input of Nutrients

Riverine, Atmospheric Input Phosphorus control

Algal Biomass Increase Excess Nitrogen

Resuspension Sinking

Benthic-Pelagic exchange O2 Decrease Only Episodic Low O2 Events – Late Summer

• Good Physics – wind and tidal mixing and tidal currents dilute the algal biomass. - Shallow water depth

• Good Chemistry - low phosphorus compared to some rivers Summary

¾ Excess nitrogen in summer in western and southern waters. ¾ P is potentially the most limiting nutrient since N/P ratios are high in the estuarine influenced western waters in summer. ¾ At the present, removal of P would more likely control algal biomass production. ¾ Improving conditions in Victoria Harbor due to sewage abatement.