effect of tide on water quality of jones creek, darwin ... · fortune, j and mauraud, n.(2015)....

Department of Land Resource Management

http://www.lrm.nt.gov.au

Effect of tide on water quality of Jones Creek, Darwin Harbour.

Report No. 02/2015D

Julia Fortune and Nathalie Mauraud

Aquatic Health UnitDepartment of Land Resource Management

Aquatic Health Unit. Department of Land Resource Management. Palmerston NT 0831.

Website: www.nt.gov.au/lrm/water/aquatic/index.html

Disclaimer: The information contained in this report comprises general statements based on scientific

research and monitoring. The reader is advised that some information may be unavailable, incomplete or

unable to be applied in areas outside the Darwin Harbour region. Information may be superseded by future

scientific studies, new technology and/or industry practices.

Copyright protects this publication. It may be reproduced for study, research or training purposes subject to

the inclusion of an acknowledgement of the source and no commercial use or sale.

This report should be cited as:

Fortune, J and Mauraud, N.(2015). Effect of tide on water quality of Jones Creek, Darwin Harbour. Report

No. 02/2015D. Department of Land Resource Management, Aquatic Health Unit. Palmerston, NT.

© Northern Territory of Australia, 2015 ISBN 978-1-74350-079-8

Acknowledgements:

Thanks to Matthew Majid and Andrew Gould of the Aquatic Health Unit for their technical support during

field campaigns. Simon Townsend (DLRM) provided helpful comments that improved the report.

Contents

1. SUMMARY ...........................................................................................................................i-ii

2. INTRODUCTION ...................................................................................................................... 1

2.1 Objectives ........................................................................................................................... 1

3. METHOD ................................................................................................................................. 3

3.1 Field stations and sampling ............................................................................................. 3

3.2 Field collection .................................................................................................................... 4

3.3 Sample analysis ................................................................................................................. 5

4. RESULTS AND DISCUSSION ................................................................................................. 6

4.1 Discrete sampling ............................................................................................................... 7

4.1.1 Physico-chemical results. ............................................................................................. 7

4.1.2 Nutrient results ............................................................................................................. 9

4.2 Variability of water quality indicators over tidal cycles ....................................................... 10

4.2.1 Temperature .............................................................................................................. 19

4.2.2 pH .............................................................................................................................. 19

4.2.3 Electrical conductivity ................................................................................................. 20

4.2.4 Dissolved oxygen ....................................................................................................... 20

4.2.5 Turbidity ..................................................................................................................... 22

4.2.6 Chlorophyll Fluorescence ........................................................................................... 23

4.3 Hysteresis analysis for indicative neap and spring events. ............................................... 24

4.3.1 Temperature .............................................................................................................. 25

4.3.2 pH ............................................................................................................................... 26

4.3.3 Conductivity ................................................................................................................ 26

4.3.4 Dissolved oxygen ........................................................................................................ 26

4.3.5 Turbidity ...................................................................................................................... 26

4.3.6 Chlorophyll-a ............................................................................................................... 26

4.3.7 Neap hysteresis plots ................................................................................................. 27

4.3.8 Spring hysteresis plots ................................................................................................ 28

4.4 Longitudinal variation of water quality along Jones Creek .................................................... 30

5. CONCLUSION ....................................................................................................................... 32

6. REFERENCES ...................................................................................................................... 33

APPENDIX. ................................................................................................................................. 34

List of Figures

Figure 1. Darwin Harbour region and major estuarine arms entering Darwin Harbour port. Jones

Creek study area within insert. ...................................................................................................... 2

Figure 2. Logger deployment sites along Jones Creek. ................................................................ 4

Figure 3(a). Neap tidal stage for sampling period July 15- 20. ....................................................... 6

Figure 3(b). Spring tidal stage for sampling period August 9-13. ................................................... 6

Figure 4. Typical physico-chemical conditions for neap and spring tide conditions measured in

Jones Creek. ................................................................................................................................. 8

Figure 5. Typical nutrient concentrations measured for neap and spring tide conditions in Jones

Creek. ........................................................................................................................................... 9

Figure 6. Temperature, pH and conductivity measured at site 1 during neap tide ...................... 11

Figure 7. Dissolved oxygen, turbidity and chlorophyll fluorescence measured at site 1 during

neap tide. .................................................................................................................................... 12

Figure 8. Temperature, pH and conductivity measured at site 2 during neap tide. ..................... 13


neap tide. .................................................................................................................................... 14

Figure 10. Temperature, pH and conductivity measured at site 3 during neap tide. ................... 15


neap tide. .................................................................................................................................... 16

Figure 12. Temperature, pH and conductivity measured at site 2 during spring tide. .................. 17

Figure 13. Dissolved oxygen, turbidity and chlorophyll fluorescence measured site 2 during

spring tide. .................................................................................................................................. 18

Figure 14. pH vs depth during neap cycle (July 2013). .............................................................. 20

Figure 15. pH vs depth spring cycle (Aug 2013). ....................................................................... 20

Figure 16. Typical modulation of dissolved oxygen with change in tide and time of day Neap tide

– July.. ........................................................................................................................................ 21

Figure 17. Typical modulation of dissolved oxygen with change in tide and time of day Spring

tide– August.. .............................................................................................................................. 21

Figure 18. Change in turbidity with outgoing tide (Jones Creek July 15, 2013). .......................... 23

Figure 19. Neap tide episode examined for hysteresis analysis (shaded). July 18, 2013. ........... 25

Figure 20. Spring tide episode examined for hysteresis analysis (shaded). August 9, 2013. ....... 25

Figure 21. Neap tide hysteresis plots for site 2. a) pH, b) Temperature, c) Conductivity, d)

Dissolved oxygen, e) Turbidity and f) Chlorophyll. ....................................................................... 27

Figure 22. Spring tide hysteresis plots for site 2. a) pH, b) Temperature, c) Conductivity, d)

Dissolved oxygen, e) Turbidity and f) Chlorophyll. ....................................................................... 28

Figure 23. Median values for transect sites 1 to 3, Jones Creek Neap deployment July 2013. ... 31

a) Temperature, b) pH, c) Conductivity, d) Dissolved oxygen, e) Chlorophyll and f) Turbidity. ..... 31

i

1. SUMMARY

The Aquatic Health Unit of the Department of Land Resource Management (DLRM) continues to

monitor the water quality of Darwin Harbour to assess the health of the estuary and its coastal

waterways and report through the Darwin Harbour Report Cards and other publications. Darwin

Harbour is a dynamic estuarine system influenced by freshwater inflows from major rivers such as

the Elizabeth and Blackmore systems and oceanic processes such as tides. These factors,

among others, lead to variability in routinely measured water quality parameters.

Tidal stage has been identified as an important factor to consider when collecting samples for

water quality, however little is known about the role of tides and their influence on water quality,

particularly tropical macrotidal systems.

Given the dynamic nature of the estuary, changes in water quality with factors of variation such as

tide, time and location are significant. To better understand this source of variation a field survey

in Jones Creek, a small tidal creek of Darwin Harbour was undertaken. During the survey, water

quality in time series was recorded over neap and spring conditions to help elucidate this effect.

Water quality parameters examined were temperature, pH, electrical conductivity (EC), turbidity,

dissolved oxygen (DO), chlorophyll-fluorescence and photosynthetic available radiation (PAR).

The key outcomes of the study are tabulated below (Table 1).

In summary, tides can have a strong influence on some water quality parameters such as EC,

turbidity, pH and DO. Increased current velocity and sediment resuspension during ebb and flood

tides contributes to observed patterns. While tides appear to influence some parameters, others

such as temperature and DO were influenced to a greater degree by solar radiation and appear to

vary as a function of the time of day in addition to tides. Longitudinal gradients were also notable

with dissolved oxygen, pH, chlorophyll-a, turbidity and conductivity indicating clear spatial

variation in the tidal creek.

ii

Table 1. Synopsis of observed effects of neap and spring tidal cycles on water quality indicators measured

in Jones Creek.

Indicator Neap Spring

Electrical conductivity Clear tidal pattern Strong tidal influence

Temperature

Increases to reach peak at the slack

of the tide. Generally higher

temperatures.

Increases to reach peak during slack

tide. Greater variation between flood

and ebb tides.

Turbidity Clear temporal pattern with tide

Strong tidal influence

High values at high tide

Low values at low tide

pH Clear tidal influence



Clear tidal influence



Dissolved oxygen

Tidal influence

lowest value at low tide

Highest value at high tide

Decrease at slack tide

Tidal influence

lowest value at low tide

Highest value at high tide


Chlorophyll-a Temporal pattern


Temporal pattern


Highest values at low tide

1

2. INTRODUCTION

Darwin Harbour is a large macro tidal estuary of the wet-dry tropics of the Northern Territory. The

estuary is a dynamic system flanked by extensive mangrove forests covering 20,400 hectares of

the region and represents around 5% of the NT’s entire mangrove area (Brocklehurst and

Edmeades 1996). The catchment covers an area of 3200 km2 (water and terrestrial surfaces)

where the river systems of the Elizabeth and Blackmore River estuary drain into the main port of

Darwin Harbour (Fig. 1). The region is also subject to increasing development with the population

centres of Darwin and Palmerston comprising approximately 120,000 people and an expanding

industry base.

Water quality monitoring has continued in the estuary since 1987 with water quality report cards

produced annually since 2009. Tidally induced fluctuations observed in water quality and the

seasonal extremes of wet and dry seasons often determine the state of water quality in Darwin

Harbour. An emphasis on better understanding these factors as determinants of variation

continues to underlie current monitoring effort in the region.

Tides have been identified as an important factor to consider when collecting water samples and

recent standardised sampling for tide undertaken by the Aquatic Health Unit suggests that water

quality is relatively stable during neap tides (Padovan, 2005; DLRM 2013; Mauraud, 2013;

Fortune, 2015). Tides can significantly influence the circulation of sediments within an estuary

and the degree of mixing. These advective forces can therefore drive variation in a range of water

quality parameters such as salinity, turbidity, chlorophyll-a, dissolved oxygen and pH.

Jones Creek is a small tidal creek located near Channel Island in the Middle Arm of Darwin

Harbour (Fig. 1 and Fig. 2). The navigable length of the creek is approximately 3.75km with a

terrestrial and tidal catchment area that covers 9.25 km2 consisting of large tracts of fringing

mangrove that are inundated at high tide. The level of human disturbance within the catchment is

minimal. This tropical tidal creek was chosen because of the limited disturbance and accessibility

to enable evaluation of water quality with tidal modulation over neap and spring conditions.

2.1 Objectives

The main objective of this report is to: (a) Identify the potential impact of tide on water quality

indicators measured in a tidal creek of Darwin Harbour during neap and spring tidal events using

water quality loggers and (b) examine any longitudinal gradients during neap tide.

2

Figure 1. Darwin Harbour region and major estuarine arms entering Darwin Harbour port. Jones Creek study area within insert.

3

3. METHOD

Seabird loggers were deployed at 3 sites along Jones Creek (Fig. 2; Table 1). The distance

between deployment sites was roughly 780m extending from the mouth of the creek at site 1 to

the upper reaches at site 3. Each sonde was attached to a cage with heavy weights, two anchors

and tied to a buoy. At the end of the deployment, the sondes were collected and data downloaded

from the units. Logger units were elevated 80cm from the seabed by a support frame.

3.1 Field stations and sampling

Survey time-series measurements were conducted during the dry season to minimise the

confounding effects of seasonality and collect water quality data over varying tidal regimes.

Three Seabird sondes (SBE16) were deployed in Jones Creek between July 15th and July 19th

during a neap tide event and one Seabird sonde was deployed between August 9th and August

13th over a spring tide. All sondes logged the following parameters for the duration of their

deployment: water temperature, depth, dissolved oxygen, chlorophyll fluorescence, turbidity, PAR

(photosynthetically available radiation), pH and electrical conductivity.

The July deployment occurred over a neap tide (tide range 3m at Darwin Harbour gauge at

Stokes Hill Wharf) with the August deployment (tide range 7 m) undertaken during spring tide

cycle conditions.

The Seabird sondes recorded data every 15 minutes for the deployment period in July and every

10 minutes for the deployment in August. Times for deployment, retrieval and location are

presented in Table 2.

Table 2. Times and location of the recordings

Site Seabird

sonde Latitude Longitude Date Start Date Finish

site 1 SB1 -12.5499 130.87901 15/07/2013 12:15 19/07/2013 9:45

site 2 SB2 -12.554 130.88544 15/07/2013 13:00 19/07/2013 10:15

site 3 SB3 -12.5582 130.89099 15/07/2013 14:00 19/07/2013 10:45

site 2 SB2 -12.554 130.88544 9/08/2013 9:30 13/08/2013 8:50

At each site, during deployment and recovery of the Seabird loggers, surface water

(approximately 0.25 m depth) was measured for pH, dissolved oxygen (% saturation),

Darwin

Harbour

4

conductivity and temperature using a Quanta multi-parameter probe. Turbidity was measured with

a Hach turbidity meter.

Conducting these measurements allowed the assessment of the accuracy of the SBE16 data.

Adjustment was made for dissolved oxygen which where erroneously low due to insufficient

flushing time (fractions of second) within the logger. On average a 1.9 mg/L difference (range -

1.4 – 4.4 mg/L) was found between logger and discrete measurements for all instruments and on

both sample periods. Table A1 (Appendix) outlines the difference between logger and discrete

measurements taken with a Quanta instrument. Corrected values (concentration and %

saturation) are presented in this report (see Table A1 Appendix). The dissolved data should only

be considered in terms of showing trends rather than the actual values presented.

Figure 2. Logger deployment sites along Jones Creek.

3.2 Field collection

Discrete water quality data were collected from surface waters (0.25m depth) and then analysed

in the laboratory for nutrients and chlorophyll-a.

5

3.3 Sample analysis

Surface samples were collected in plastic bottles and stored on ice in the field prior to laboratory

analysis. Samples were analysed for total nitrogen (TN), nitrite (NO2--N), nitrate (NO3

--N),

ammonia-N, total phosphorus (TP), and filterable reactive phosphorus (FRP). Nutrient samples

for nitrite, nitrate and FRP were filtered through 0.45m filters in the field. All samples were

collected, transported and stored using recommended sampling and preservation protocols and

chain of custody documentation.

The chemical and nutrient analyses were carried out by Charles Darwin University (Chlorophyll-

a)and Northern Territory Environmental Laboratories (All nutrients). All nutrient samples were

determined using APHA standard methods.

Table 3. Laboratory methods for water quality.

Measurement Method APHA (1998) number

Nitrate Automated cadmium reduction 4500-NO3 I

Nitrite Automated cadmium reduction 4500-NO3 I

Ammonia Automated phenate method 4500-NH3 G

Total nitrogen Persulphate method 4500-N C

Total Filterable N Persulphate method (filtered sample) 4500-N C

Filterable reactive P Flow injection analysis for orthophosphate 4500-P F

Total phosphorus Persulphate digestion followed by automated

ascorbic acid method. 4500-P H

Total Filterable P Persulphate digestion followed by automated

ascorbic acid method (filtered sample). 4500-P H

Chlorophyll a Fluorometry (3) or spectrophotometry (H2) 10200 H (APHA 2005)

APHA (1998) unless otherwise stated. APHA (2005) for chlorophyll-a; method depends on concentration.

3.3 Statistical analysis

Data for each parameter were plotted with depth as a function of time. A number of discrete

samples were collected during deployment and retrieval for neap and spring tides. Data collected

for deployment sites are presented as mean concentration with standard error with the exception

of pH where medians are provided. Graphs presenting each parameter measured versus depth

were undertaken using SigmaPlot V12.5. Pearson product moment correlations (Pearson’s r)

were performed to measure correlation between tide height and water quality variables with a

6

significance level of 0.05 applied. Characteristics of hysteresis plots were used to evaluate any

temporal variation relative to the contribution of depth in association with discrete neap and spring

events over the logger deployment period.

3.4 Tidal stage for sampling periods

Tidal stage data was collected from a tidal gauge located at the East Arm Port facility on Darwin

Harbour. The stage height for deployment periods indicate the change in neap and spring tidal

stage range (Fig. 3a and 3b).

Figure 3(a). Neap tidal stage for sampling period July 15 - 20, 2013.

Figure 3(b). Spring tidal stage for sampling period August 9-13, 2013.

7

4. RESULTS AND DISCUSSION

4.1 Discrete sampling

Sampling was undertaken during the deployment and retrieval of loggers at each site (1-3) during

neap conditions and at site 2 during the spring tide event. Physical parameters of temperature,

pH, conductivity, salinity, dissolved oxygen and turbidity were performed in addition to sampling

for chlorophyll-a (Fig. 4) and nutrients (Fig. 5).

Results provide some context for typical water quality conditions associated with neap and spring

conditions at the Jones Creek sites.

4.1.1 Physico-chemical results.

Sites and tide

Site 1-Neap Site 2-Neap Site 3-Neap Site 2-Spring

Tem

pera

ture

(0C

)

0

5

10

15

20

25

30

Sites and tide


Conductiv

ity (

S/c

m)

35000

40000

45000

50000

55000

Sites and tide


Sa

linity

(p

pt)

0

10

20

30

40

Sites and tide


Dis

solv

ed O

xygen (

%sat)

80

85

90

95

100

105

8

Sites and tide


Turb

idity

(N

TU

)

0

1

2

3

4

5

Sites and tide


Ch

loro

ph

yll-

a (

g/L

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Sites and tide


pH

6.0

6.5

7.0

7.5

8.0

8.5

Figure 4. Typical physico-chemical conditions for neap and spring tide conditions measured in Jones

Creek.

Water temperatures for both tidal conditions were similar for the July and August period. Whilst

higher pH was observed with spring tides in contrast to neap conditions at all 3 sites examined.

This is likely to be related to the higher marine inflows bringing slightly more alkaline waters into

the tidal creek.

Conductivity and salinity was higher with spring tide conditions in comparison to neap tidal

incursion and similarly with patterns in pH can be explained by stronger inundation of marine

waters. Dissolved oxygen was observed to be slightly lower with spring tide deployment and

retrieval periods in comparison with neap conditions.

Turbidity was marginally higher during spring tide with the likelihood of suspended sediments

conveyed more readily by advective forces on the spring tide. Chlorophyll-a concentration was

low for both tidal regimes however slightly lower during spring tide conditions.

9

4.1.2 Nutrient results

Sites and tide


Nit

rite

NO

2-N

(g

/L)

0

2

4

6

8

Sites and tide


Nit

rate

NO

3-N

(g

/L)

0

2

4

6

8

10

12

14

16

18

Sites and tide


Fil

tera

ble

Re

acti

ve

Ph

osp

ho

rus

(g

/L)

0

2

4

6

8

10

Sites and tide


Tota

l Pho

sph

oru

s (

g/L

)

0

5

10

15

20

25

Sites and tide


To

tal

Nit

rog

en

(g

/L)

60

80

100

120

140

160

180

200

220

Sites and tide


Am

mo

nia

NH

3-N

(g

/L)

0

2

4

6

8

Figure 5. Typical nutrient concentrations measured for neap and spring tide conditions in Jones Creek.

Nutrient concentrations across both tidal regimes were notably low however soluble fraction

nutrients of nitrate and filterable reactive phosphorus were higher during spring tide conditions.

Nitrite was measurably low during the spring tide (<1 g/L) in contrast to neap conditions with up

to 6.2 g/L measured. Likewise total phosphorus, nitrogen and ammonia concentrations were

lower during spring conditions. FRP constituted a reasonably large proportion of the total

10

phosphorus pool over the spring tide. Discrete sampling results, albeit limited, were similar to

previous studies (Butler and Padovan, 2005).

4.2 Variability of water quality indicators over tidal cycles

This section presents time-series data for water quality indicators measured in Jones creek for

both neap and spring cycles (Fig. 6 -13). Correlations with depth are presented in tables 4 and 5

and further analysis and discussion of variability is presented in hysteresis analysis (Sec 4.3). The

median, minimum and maximum for site deployments are presented in the Appendix (Table A2

and Table A3).

11

Figure 6. Temperature, pH, conductivity and PAR measured at site 1 during neap tide

12

Figure 7. Dissolved oxygen (corrected), turbidity, chlorophyll fluorescence and PAR measured at site 1

during neap tide.

13

Figure 8. Temperature, pH, conductivity and PAR measured at site 2 during neap tide.

14


during neap tide.

15

Figure 10. Temperature, pH, conductivity and PAR measured at site 3 during neap tide.

16


during neap tide.

17

Figure 12. Temperature, pH, conductivity and PAR measured at site 2 during spring tide.

18


during spring tide.

19

Table 4. Range of water quality indicators at site 2 during neap and spring tide.

Water quality indicators range

Location Temperature

(°C ) pH

EC (mS/cm )

Turbidity (NTU)

DO (%saturation)

CF (µg/L )

Site 2 - neap tide 25.4-26.2 7.9-8.0 53.2-54.1 2.8-7.2 58.9-98.4 0.6-3.3

Site 2 - spring tide* 24.3-25.8 7.9-8.1 53.0-54.6 3.4-11.8 53.1-103.3 0.6-7.1

(* Only one SBE16 instrument was deployed for the spring tide event).

4.2.1 Temperature

Variation in temperature indicates clear temporal patterns. Variation between tidal conditions was

minor with temperature ranging between 25.1 and 26.2°C at neap tide (1.2 °C variation) and 24.3

and 25.8°C over the spring tide (1.5 °C variation).

Neap tide temperature tended to increase during day time to reach a peak toward the evening

and at low tide. Decreasing trends then persist through the evening with reduced or no sunlight.

Similarly spring tide temperature tended to follow comparative patterns.

Temperatures recorded during neap tide are slightly higher than temperatures recorded for spring

tide however this is not significant and likely to be associated with longer residence times and

reduced mixing associated with neap tidal movement. Overall temperatures measured during the

deployments indicate a similar pattern of variation during both tidal settings. Temperature varied

over both tidal cycles and with elevations measured more often with the slack of the outgoing tide.

Temperature maxima during spring tide events typically occurred at the slack of ebb tide (Fig.12).

Temperature is also influenced by the time of the with sunlight maxima peaking around midday

and light intensity modulating with diurnal change.

4.2.2 pH

pH clearly modulated with tidal cycle indicating clear variation with the neap and spring tidal

events examined. pH varied between 7.3 and 8.0 during neap tide and between 7.9 and 8.1

during spring tide (Fig 14 and 15).

On the incoming tide pH increases and on outgoing tides pH decreases albeit within a very

narrow range of 0.2 for each site. Correlation coefficients for both neap and spring tides indicated

strong positive relationship with tide. This relationship appeared to be more significant during

spring conditions (r =0.946).

Changes in pH are similar for both tidal conditions, however temporal variations are more regular

with spring tide and indicate strong positive correlations with depth (Table 5). pH varies with tidal

cycle for neap and spring tide conditions. Highest values are measured at high tide and lowest

values at low tide (Fig. 14 and 15).

20

Depth (m)

3 4 5 6 7 8

pH

7.86

7.88

7.90

7.92

7.94

7.96

7.98

8.00

8.02

Depth (m)

2 3 4 5 6 7 8 9

pH

7.86

7.88

7.90

7.92

7.94

7.96

7.98

8.00

8.02

8.04

8.06

8.08

Figure 14. pH vs depth during neap cycle (July 2013). Figure 15. pH vs depth spring cycle (Aug 2013).

4.2.3 Electrical conductivity

Changes in electrical conductivity (EC) show a clear tidal pattern. EC varied between 53.1 and

54.5 mS/cm during neap tide and between 53.0 and 54.6 mS/cm over spring tide conditions.

Variation in EC is observed during both tidal cycle conditions. At neap tide, changes in EC are

reasonably consistent with the modulation of each tide.

At spring tide, variations of EC appear to more strongly follow tidal cycle variations (r = -0.73). EC

was observed to increase with outgoing tides with peaks at top of the tide. Highest values of EC

are recorded at the top of the outgoing tide. Modulations were more variable with spring tides

with both neap and spring cycles largely remaining representative of marine influence. Variation

of EC tended to differ in the extent of change between neap and spring tidal events. At neap tide,

EC appears to follow tidal modulation increasing with incoming flood tide. During spring tide

incursion EC appears to more strongly follow the tide (Table 5). EC was observed to peak

around the slack of the flood tide at each outgoing movement with neap and spring tides.

Hysteresis analysis (Sec 4.3) further explores these patterns.

4.2.4 Dissolved oxygen

Changes in dissolved oxygen indicate a temporal pattern with DO varying with tides during both

tidal cycle conditions.

DO measured increases and decreases around each high and low tide in addition to reflecting

diurnal patterns. During both tidal conditions and at every change of tide (high and low) DO

levels tended to decrease. Butler and Padovan (2005) also found similar complex patterns for DO

suggesting a number of factors may play a role in determining variability.

Changes in dissolved oxygen indicate a temporal pattern and variations of DO are observed

within the tidal cycle for both neap and spring tides. Highest values were measured at high tide

21

and lowest values at low tide. However a decrease in dissolved oxygen is observed at each

change of tide (slack of the tide) and typically occurs when mixing is reduced during the change

of the incoming and outgoing tides. A more detailed examination of a neap and spring event also

indicated clear diurnal changes with time of day another determinant in the variation for dissolved

oxygen (Fig. 16 and 17). Dissolved oxygen indicated responses associated with solar radiation

inputs with variation (diurnal peaks and troughs) a function of the time of day and dampened by

tidal movement, particularly at the top of each tide.

Time

13

:00

:00

13

:15

:00

13

:30

:00

13

:45

:00

14

:00

:00

14

:15

:00

14

:30

:00

14

:45

:00

15

:00

:00

15

:15

:00

15

:30

:00

15

:45

:00

16

:00

:00

DO

(%

Satu

ration)

60

65

70

75

80

85

90

95

Depth

(m

)

3

4

5

6

7

8

Figure 16. Typical modulation of dissolved oxygen with change in tide and time of day Neap tide – July.

( Dissolved oxygen) and ( - - - depth).

Time

09

:22

:00

09

:32

:00

09

:42

:00

09

:51

:00

10

:01

:00

10

:11

:00

10

:21

:00

10

:31

:00

10

:41

:00

10

:51

:00

11

:00

:00

11

:10

:00

11

:20

:00

11

:30

:00

DO

(%

sa

tura

tio

n)

50

55

60

65

70

75

80

85

90

Depth

(m

)

1

3

5

7

9

0

2

4

6

8

10

Figure 17. Typical modulation of dissolved oxygen with change in tide and time of day Spring tide–

August. ( Dissolved oxygen) and ( - - - depth).

22

4.2.5 Turbidity

Variation in turbidity appears to be influenced by tidal modulation with neap and spring tides

generating turbidity flux along the tidal creek.

At neap tide, changes in turbidity do not appear to significantly change with tide and ranged from

2.2 to 9.1 NTU. However highest values are recorded at low tides and lowest values are recorded

at high tide. During neap conditions turbidity was observed to slightly increase with an outgoing

tide (Fig.18).

During spring tide conditions turbidity variation indicated strong tidal influence. Turbidity increases

on incoming tides and decreases on outgoing tides and varies between 3.4 and 11.8 NTU slightly

higher than for neap conditions. Highest values of turbidity are recorded around high tide and

lowest values around low tide. However it was observed to decrease at the top of the high tide

during slack conditions.

Tidal modulation influences turbidity which also appeared to differ in the degree of variation

between spring and neap tidal conditions. Spring tides induced more turbid conditions with

highest values at high tide and lowest values at low tide. During neap tidal excursions turbidity

does not appear to be as strongly driven by the tide however during spring events a reasonable

positive correlation was observed for turbidity (r =0.683). The constrained range of values

observed during neap conditions is likely due to dampened tidal velocities limiting mixing and

resuspension. During both tidal conditions changes in turbidity also appear to relate to diurnal

variations however this may be more aligned with Chla-Fl elevations. Pico-plankton which form a

large component of the algal composition (pers. comm. D.Purcell, Australian Institute of Marine

Science) could contribute to turbidity in Darwin Harbour.

Turbidity indicated a clear tidal pattern differing between spring and neap tide conditions with a

maxima typically observed during spring events. Highest values were recorded at the slack end of

an outgoing tide and lowest values at high tide or slack of the tide while current speeds are

minimal. The variation of turbidity is greater during spring tide when tidal velocities are stronger.

This phenomenon has been observed in previous studies (Padovan, 1997; Wilson et al. 2004).

During these cycles tidal mixing and resuspension of sediments in the water column is more

pronounced.

23

Time

05:00:00 09:00:00 13:00:00 17:00:00

Tur

bid

ity (

NT

U)

3

4

5

6

7

8

De

pth

(m

)

3

4

5

6

7

8

Outgoing tide

Figure 18. Change in turbidity with outgoing tide (Jones Creek July 15, 2013).

4.2.6 Chlorophyll Fluorescence

Changes in chlorophyll fluorescence (CF) indicate a temporal pattern with variations observed

within neap and spring tidal conditions (Table 5). CF fluctuated between 0.6 and 3.3 µg/L during

the neap tide and between 0.6 and 1.9 µg/L during spring tide conditions. During both tidal cycles

highest values of CF were recorded during low tides and lowest values during high tides.

CF was also observed to change with diurnal variation, increasing during day light hours and then

declining during the evening. Changes in chlorophyll fluorescence indicate a strong temporal and

tidally driven pattern and variations occur over neap and spring tidal cycles.

24

Table 5. Correlation coefficients (r) for water quality parameters measured versus depth during neap tides. Shaded cells indicate those where good correlation and estimates of r were significant (p <0.05).

Site Parameter r p

SB1 Temp -0.108 0.03

pH 0.875 9.68x10

-120

EC -0.694 3.39x10

-55

PAR/Irradiance -0.157 0.002

DO %sat 0.130 0.015

Turbidity -0.446 1.0x10

-19

Chla-Fl -0.333 3.96x10-11

SB2 Temp 0.089 0.085

pH 0.874 1.413x10

-118

EC -0.559 3.53x10

-32

PAR/Irradiance 0.0799 0.123

DO %sat 0.228 0.0000857

Turbidity -0.543 4.95x10

-30

Chla-Fl -0.392 4.29x10-11

SB3 Temp -0.148 0.0041

pH 0.837 8.2x10-99

EC -0.634 2.9x10-43

PAR/Irradiance -0.0274 0.598

DO %sat 0.0266 0.609

Turbidity -0.726 3.8x10-62

Chla-Fl -0.538 2.46x10-29

ns = not significant

Table 6. Correlation coefficients (r) for water quality parameters measured versus depth during a spring tidal regime. Shaded cells indicate those where good correlation estimates of r were significant (p<0.05).

Site Parameter r p

SB2 Temp 0.241 0.00000049

pH 0.946 3.07x10

-282

EC -0.743 5.51x10

-102

PAR/Irradiance -0.522 2.0x10

-41

DO %sat 0.450 5.9x10

-30

Turbidity 0.683 3.67x10

-80

Chla-Fl -0.476 8.05x10-34

4.3 Hysteresis analysis for indicative neap and spring events.

Specific events for neap and spring tide were examined for each water quality parameter against

depth yielding 12 hysteresis plots (Fig. 21-22). The parameters examined included water

temperature, pH, dissolved oxygen, Chlorophyll-a (FLU), conductivity and turbidity. PAR was

omitted given the dependence on sunlight rather than tidal influence.

25

The neap and spring tide events examined (Fig. 19 and 20) represent a 3 m and a 5.5 m tidal

movement respectively for an outgoing (ebb) and incoming (flood) tide.

Figure 19. Neap tide episode examined for hysteresis analysis (shaded). July 18, 2013.

Figure 20. Spring tide episode examined for hysteresis analysis (shaded). August 9, 2013.

The majority of events produced hysteresis loops in a circular (or similar) pattern, highlighting

complex behaviours of solutes and tide at different portions of the ebb and flood tide. Generally

the type of hysteresis loop generated by the individual events correlated in a predicable manner.

4.3.1 Temperature

Temperatures during neap conditions revealed a clockwise pattern with outgoing ebb tide

indicating higher water temperatures during neap conditions (Fig. 21a). This pattern was the

reverse during spring tides where temperature on the incoming flood tide was higher (Fig. 22a).

Differences between temperatures on ebb and flood tides were more marked during spring tide

modulation.

26

4.3.2 pH

The neap pH hysteresis plot indicated a clockwise loop with higher pH on incoming waters and

lower outgoing conditions (Fig. 21b). An initial peak on the ebb tide may correspond with the

typical top of the tide prior to outgoing waters with an outgoing movement. pH variation during

the spring tide was slightly larger with higher pH on the incoming flood tide a pattern consistent

with incoming marine waters (Fig. 22b).

4.3.3 Conductivity

Similarly variation for conductivity was greater during spring tide conditions than that for neap

conditions (Fig. 22c). The spring tide hysteresis plot indicated a clockwise pattern with

conductivity higher on the flood tide consistent with higher salinity waters entering the tidal creek

from the broader harbour. The neap plot implied a counter-clockwise pattern with less variation

on the ebb and flood tides (Fig. 21c).

4.3.4 Dissolved oxygen

Dissolved oxygen was more closely aligned on the ebb and flood tides during neap conditions

(Fig. 21d). A similar anti-clockwise pattern was observed for the spring hysteresis plot (Fig. 22d)

with dissolved oxygen decreasing on the outgoing ebb tide and steadily increasing on the flood

tide. Differences in dissolved oxygen were greater during spring conditions between the ebb and

flood event examined.

4.3.5 Turbidity

Turbidity during both events was low maintaining values <11 NTU over the episode examined.

The hysteresis plot for spring tide revealed greater variation in turbidity between flood and ebb

tide with outgoing ebb tide indicating the highest values decreasing with the outgoing movement

(Fig. 22e). Neap hysteresis analysis for turbidity was more complex with a number of minor

spikes albeit remaining below 6 NTU for the period observed. Flood and ebb conditions were

more closely aligned (Fig. 21e).

4.3.6 Chlorophyll-a

Chlorophyll-a remained below 1.5 g/L during the event examined. Neap hysteresis indicated an

anti-clockwise pattern with higher concentrations observed on the ebb outgoing tide (Fig. 21f).

The flood tide indicated a pattern of increasing concentration with the rising tide, a clear

hysteresis loop was observed. The spring tide hysteresis pattern was similar however appeared

to reflect a counter clockwise trend. Variance was notably different between ebb and flood

episodes (Fig. 22f).

27

4.3.7 Neap hysteresis plots

Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

pH

7.90

7.92

7.94

7.96

7.98

8.00

8.02

a) Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

Te

mp

era

ture

(0C

)

25.5

25.6

25.7

25.8

25.9

26.0

b)

Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

Conductivi

ty (

mS

/cm

)

53.0

53.2

53.4

53.6

53.8

c) Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

Dis

solv

ed

Oxyg

en

(%

sa

t)

55

60

65

70

75

80

d)

Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

Turb

idity

(N

TU

)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

e)

Depth (m)

4.5 5.0 5.5 6.0 6.5 7.0

Chlo

rophy

ll (F

LU

)

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

f)

Ebb tide (outgoing)

Flood tide (incoming)

Figure 21. Neap tide hysteresis plots for site 2. a) pH, b) Temperature, c) Conductivity, d) Dissolved

oxygen, e) Turbidity and f) Chlorophyll.

28

4.3.8 Spring hysteresis plots

Depth (m)

2 3 4 5 6 7 8 9

pH

7.88

7.90

7.92

7.94

7.96

7.98

8.00

8.02

8.04

8.06

8.08

a) Depth (m)

2 3 4 5 6 7 8 9

Tem

pera

ture

(0C

)

24.2

24.4

24.6

24.8

25.0

25.2

25.4

25.6

25.8

b)

Depth (m)

2 3 4 5 6 7 8 9

Conductiv

ity

(m

S/c

m)

53.0

53.2

53.4

53.6

53.8

c) Depth (m)

2 3 4 5 6 7 8 9

Dis

solv

ed O

xy

gen (

% S

at)

60

70

80

90

100

110

d)

Depth (m)

2 3 4 5 6 7 8 9

Turb

idity

(N

TU

)

2

4

6

8

10

12

e) Depth (m)

2 3 4 5 6 7 8 9

Chlo

rophy

ll (F

LU

)

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

f)

Ebb tide (outgoing)

Flood tide (incoming)

Figure 22. Spring tide hysteresis plots for site 2. a) pH, b) Temperature, c) Conductivity, d) Dissolved

oxygen, e) Turbidity and f) Chlorophyll.

29

Table 7. Synopsis of hysteresis pattern for parameters examined.

Parameter Tide event Rotational

direction Maxima

Temperature (0C)

Neap

Spring

Anti-clockwise

Clockwise

Ebb

Flood

pH Neap

Spring

Clockwise

Clockwise

Top Ebb

Top Flood

Conductivity (mS/cm)

Neap

Spring

Anti-clockwise

Clockwise

Top Ebb

Flood

Dissolved oxygen (%Sat)

Neap

Spring

Anti-clockwise

Anti-clockwise

Top Ebb

Top Ebb

Turbidity (NTU)

Neap

Spring

Anti-clockwise

Anti-clockwise

Top Ebb/Flood

Top Ebb

Chlorophyll-a (FLU)

Neap

Spring

Anti-clockwise

Clockwise

Top Ebb

Slack/between Ebb-Flood

Some indicators, such as pH, electrical conductivity and turbidity, are strongly influenced by tidal

conditions. Other indicators (temperature, chlorophyll fluorescence, DO) vary over tidal cycles

but also appear to be influenced by other factors. Solar radiation (as PAR) in particular plays a

role in variation of temperature and dissolved oxygen in line with diurnal variations.

During neap tide conditions, high tides are associated with a decrease of dissolved oxygen and

chlorophyll fluorescence, peaks of turbidity, highest values of pH and lowest values of

conductivity. In comparison neap tides or more so the turn of the tide between flood and ebb

episodes are associated with lower dissolved oxygen, pH, high electrical conductivity, turbidity

and chlorophyll fluorescence values.

Spring tidal conditions at high tides are associated with highest values of pH and lowest values

electrical conductivity, lower dissolved oxygen and chlorophyll fluorescence and high values of

turbidity. Spring tides typically induce strong tidal currents which produce larger fluctuations in

water quality and greater variation.

30

4.4 Longitudinal variation of water quality along Jones Creek

Longitudinal variation was examined for neap deployments where loggers were located along

Jones creek from July 15 to 19, 2013. Loggers were deployed at the creek mouth (Site 1) and

extended to site 3, the upper most site (Fig. 2).

Variation around neap tidal conditions for each indicator are presented below (Table 8) and

Figure 23 where medians for each indicator are given. Most water quality indicators measured

appear to be relatively constant with limited range between sites along the creek.

Changes of temperature are very low between sites and within sites with temperatures tending to

increase with distance downstream. pH was slightly higher in the mid-uppermost sites in

comparison to the mouth site. This lower pH range at the creek mouth was not expected and

may have been due to sensor failure. Discrete measurements for pH (Fig. 4) were similar along

the creek albeit limited. During the dry season the Jones Creek system experiences no source of

catchment inflows and in conjunction with residence times and continuous ebb and flood tides

could result in the persistence of marine waters. However the creek mouth is typically subject to

strong marine influence where higher pH would be expected. Conductivity, turbidity and

chlorophyll-a indicated similar trends with high medians at the upper most site. The proximity to

intertidal and mangrove sediments resuspended with tide may contribute to this spatial pattern in

turbidity (Fortune, 2015).

Median DO levels increase with distance from the upper most site consistent with a longitudinal

gradient. The observed DO gradient is similar to those observed in other tidal creeks of Darwin

Harbour (Butler and Padovan 2005; Fortune, 2015).

Although Chla (FLU) levels recorded are appreciably low during neap conditions a slight increase

in the upper reaches was observed (r =-0.538) with a median of 1.1 g/L (Fig. 23).

Changes appear to be reasonably limited for some parameters but an overall longitudinal gradient

along the creek was observed. It is likely that the distance between sites along the small tidal

creek contribute to the lack of strong concentration gradients.

Table 8. Water quality range parameter at the three sites during neap tide.

Water quality indicators range

Location Temperature

(°C ) pH

EC (mS/cm )

Turbidity (NTU)

DO (%saturation)

CF (µg/L )

Site 1 (creek mouth) 25.5-26.2 7.3-7.5* 53.1-54.1 2.2-7.1 58.0-103.3 0.6-1.9

Site 2 (middle) 25.4-26.2 7.9-8.0 53.2-54.1 2.8-7.2 58.9-98.4 0.6-3.3

Site 3 (upper) 25.1-26.2 7.8-7.9 53.2-54.6 2.6-9.1 56.9-95.0 0.6-2.9

*Subject to error given unexpected limited range over deployment.

31

a)

b)

c) d)

e)

f)

Figure 23. Median values for transect sites 1 to 3, Jones Creek Neap deployment July 2013.

a) Temperature, b) pH, c) Conductivity, d) Dissolved oxygen, e) Chlorophyll and f) Turbidity.

A previous study of the Jones Creek system (Butler and Padovan, 2005) found the similar spatial

patterns in water quality. Extensive logging periods and broader spatial sampling design provided

improved sensitivity for the assessment of longitudinal trends. It is recommended that future

sampling adopt such an approach to better capture neap and spring tide conditions. Ideally sites

should be spaced at appropriate distances in order to detect any discernible gradient. However

this can be sometimes constrained by suitable access.

25.74

25.75

25.76

25.77

25.78

25.79

25.8

25.81

Site 1 (mouth) Site 2 (mid) Site 3 (Upper)

Tem

pe

ratu

re (

oC

)

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

8


pH

53.58

53.6

53.62

53.64

53.66

53.68

53.7

53.72

53.74


Co

nd

ucti

vit

y (

mS

/cm

)

68

70

72

74

76

78

80

82

84

86


Dis

so

lved

Oxyg

en

(%

sat)

1.04

1.05

1.06

1.07

1.08

1.09

1.1

1.11


Ch

loro

ph

yll

(F

LU

)

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4

4.1


Tu

rbid

ity (

NT

U)

32

5. CONCLUSION

Water quality is influenced by tidal cycle however while tides may influence some parameters others

such as water temperature and DO are also influenced solar radiation input and therefore vary as a

function of the time of day or a combination of factors.

Given the influence of tide on water quality indicators measured, a monitoring program should take

into account the variability due to tidal cycles. In order to limit this variability, water quality

monitoring programs undertaken by the Aquatic Health Unit occurs at the same tidal cycle (neap

tide) and during a three hour window around high tide, though practical considerations such as

undertaking these during business hours limit monitoring opportunities.

Similarly consistency with the time of day for sampling should be considered when undertaking

monitoring given the diurnal patterns for some parameters particularly dissolved oxygen.

In order to better understand the temporal variability of Darwin Harbour, the deployment of logger

sensors (either on a periodic or permanent basis) at designated monitoring locations is

recommended.

Knowledge of the effects of tide on water quality parameters is required when interpreting results

over time within the context of guideline compliance. For example, sampling over a tide cycle has

demonstrated that in some cases the value of an indicator can fluctuate above and below guideline

objectives as a function of tidal stage. Deciphering between the contribution of natural variation with

tide and anthropogenic impacts is an important attribute in monitoring programs. Hence where

water quality objectives or guidelines may be exceeded, knowledge of the potential influence of tidal

stage at sampling will assist in interpreting results.

33

6. REFERENCES

Brocklehurst, P and Edmeades, B. (1996). The Mangrove Associations of Darwin Harbour.

Technical Report No. R96/7. Resource Capability Assessment Branch, Department of Lands,

Planning and Environment, Northern Territory Government.

Department of Land Resource Management (2013). Darwin Harbour Region Report Cards.

Aquatic Health Unit, Department of Land Resource Management. Darwin, Australia.

Fortune, J. (2015). Spatial variability of Darwin Harbour water quality during dry season

neap tides of 2012 and 2013. Report No.16/2015D. Aquatic Health Unit. Department of

Land Resource Management. Palmerston, NT.

Mauraud, N. (2013). Darwin Harbour Water Quality: Supplement to the 2013 Darwin Harbour Region Report Card. Report 12/2013D. Aquatic Health Unit, Water Resources Division. Department of Land Resource Management. Miller, R. L., Bradford, W. L., and Peters, N. E. (1988). Specific Conductance: Theoretical

Considerations and Application to Analytical Quality Control. In U.S. Geological Survey

Water-Supply Paper.

Padovan, A. (1997). The Water Quality of Darwin Harbour –October 1990 to November

1991. Report No. 34/1997D. Water Quality Branch. Water Resources Division. Dept of

Lands Planning and Environment. Palmerston, NT.

Butler, J and Padovan, A. (2005). The water quality of Jones creek, a tidal creek in Darwin

Harbour. Report No. 14/2005D. Water Monitoring Branch, Natural Resource Management

Division. Department of Natural Resources, Environment and the Arts.

Wilson, D., Padovan, A and Townsend, S. (2004). The water quality of spring and neap tidal

cycles in the middle arm of Darwin Harbour. Department Infrastructure, Planning and

Environment, Darwin.

Photograph: P. Cowan

34

APPENDIX Table A1: Comparison of dissolved oxygen data.

Dissolved oxygen comparison of data DO DO % Difference Difference

Site name Date Time Instrument mg/L % % mg/L

Jones Creek 1 15.07.13 1200 Quanta 6.85 102.67 24.09 1.60

15.07.13 12:00 Seabird 7100 5.26 78.58

Jones Creek 2 15.07.13 1245 Quanta 6.77 101.07 4.34 -1.39

15.07.13 12:45 Seabird 7101 8.16 96.73


15.07.13 13:45 Seabird 7102 4.60 68.91


19.07.13 9:45 Seabird 7100 2.06 30.75


19.07.13 10:15 Seabird 7101 3.20 47.89


19.07.13 10:45 Seabird 7102 3.01 44.83


09.08.13 9:10 Seabird 7101 5.15 76.07

Jones Creek 2 13.08.13 930 Quanta 5.93 90.87 15.87 -0.32

13.08.13 9:30 Seabird 7101 6.24 75.00

Median 27.25 1.84

Average

32.54 1.94

Min

4.34 -1.39

Max 65.08 4.48

35

Date

16/07/13 18/07/13 20/07/13 15/07/13 17/07/13 19/07/13

Dis

so

lve

d o

xyg

en

(m

g/L

)

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Site 1 (Mouth)

Figure A1. Corrected dissolved oxygen for site 1 (Mouth) neap tide.

Date

16/07/2013 18/07/2013 20/07/2013 15/07/2013 17/07/2013 19/07/2013

Dis

so

lve

d o

xyg

en

(m

g/L

)

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Site 2 (Mid)

Figure A2. Corrected dissolved oxygen for site 2 (Mid) neap tide.

Date

16/07/13 18/07/13 20/07/13 15/07/13 17/07/13 19/07/13

Dis

so

lve

d o

xyg

en

(m

g/L

)

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Site 3 (Upper)

Figure A3. Corrected dissolved oxygen for site 3 (Upper) neap tide.

Date

Dis

so

lve

d o

xyg

en

(m

g/L

)

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

10/08/13 11/08/13 12/08/13 13/08/1309/08/13 14/08/13

Site 2 (Mid)

Figure A4. Corrected dissolved oxygen for site 2 (Mid) spring tide.

36

Table A2: Median, minimum and maximum for neap tide conditions

Parameter Site Median Min Max

Temperature Site 1 25.8 25.48 26.21

Site 2 25.8 25.41 26.21

Site 3 25.76 25.13 26.21

Conductivity Site 1 53.64 53.14 54.1

Site 2 53.65 53.18 54.12

Site 3 53.73 53.2 54.46

pH Site 1 7.4 7.33 7.45

Site 2 7.94 7.88 8.09

Site 3 7.88 7.79 7.97

DO (%sat) Site 1 83.96 58.01 103.3

Site 2 76.33 58.87 98.37

Site 3 73.91 56.94 95.03

Chla (FLU) Site 1 1.06 0.64 1.9

Site 2 1.09 0.61 1.73

Site 3 1.1 0.61 2.89

Turbidity (NTU) Site 1 3.4 2.19 7.09

Site 2 4 2.8 7.1

Site 3 3.9 2.6 9.1

37

Table A3: Median, minimum and maximum for spring tide conditions

Parameter Site Median Min Max

Temperature Site 2 25.15 24.33 25.76

Conductivity Site 2 53.64 53.09 54.57

pH Site 2 7.98 7.87 8.05

DO (%sat) Site 2 69.26 53.08 101.6

Chla (FLU) Site 2 1.08 0.76 1.88

Turbidity (NTU) Site 2 5.6 3.4 11.8

effect of tide on water quality of jones creek, darwin ... · fortune, j and mauraud, n.(2015)....

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