PRIMARY RESEARCH PAPER

Optical properties of a tropical estuary during wet and dryconditions in the Nha Phu estuary, Khanh Hoa Province,south-east Vietnam

Lars Chresten Lund-Hansen • Doan Nhu Hai •

Nguyen Ngoc Lam • Morten Holtegaard Nielsen

Received: 13 October 2009 / Revised: 20 January 2010 / Accepted: 21 January 2010 / Published online: 9 February 2010

� Springer Science+Business Media B.V. 2010

Abstract There has been a strong research focus on

optical properties in temperate estuaries but very

much less in tropical estuaries. These properties

comprise light and beam attenuation dominated by

suspended particulate matter, Chl a, and CDOM.

Spatially and temporally distributed data on optical

properties in a tropical wet and dry estuary are

compared and discussed in relation to those of

temperate estuaries. Sampling in the Nha Phu estuary,

Vietnam, consisted of five stations on a transect from

head to mouth that was sampled four times during dry

conditions and three times during wet conditions

between May 2006 and April 2008. Methods com-

prised CTD, optical measurements, and water sam-

pling for suspended matter, Chl a, and CDOM.

Results showed high light attenuation—Kd(PAR)—in

wet conditions and low in dry. Kd(PAR) was highest

at the estuary head and lower in the outer part. Spatial

and temporal variations in Kd(PAR) were in general

dominated by variations in suspended particulate

matter concentrations in both wet and dry conditions.

Chl a concentrations were low and showed no strong

variations between wet and dry conditions. CDOM

absorption coefficients were higher in wet conditions

with high values at the head and lower in the central

part of the estuary. The depth of the photic zone was

reduced by up to 50% during wet conditions. A

residence time in the estuary of 5–6 days was derived

from the rate of change of Kd(PAR) after a period of

heavy rain and discharge of freshwater into the

estuary. This complied with a residence time of four

and a half days derived from a basic physical relation.

Optical properties were in general comparable to

temperate estuaries in dry conditions although Chl a

concentrations were lower in Nha Phu. A second

distinctive point, as compared to temperate estuaries,

was the episodic character with days of strong rainfall

followed by longer periods of dry weather. All

sampling, both wet and dry, was carried out in the dry

season which implies a less definitive perception of

wet and dry seasons.

Keywords Tropical wet and dry estuary �Diffuse attenuation coefficient Kd(PAR) �Suspended particulate matter � Chl a � CDOM �Vietnam

Handling editor: Luigi Naselli-Flores

L. C. Lund-Hansen (&)

Marine Ecology, Department of Biological Sciences,

Aarhus University, Ole Worms Alle 1, Build. 1134,

8000 Arhus C, Denmark

e-mail: lund-hansen@biology.au.dk

D. N. Hai � N. N. Lam

Institute of Oceanography, Cau da 01, Nha Trang,

Vietnam

M. H. Nielsen

Arctic Technology Center, Department of Civil

Engineering, Technical University of Denmark, Building

204, Kemitorvet, 2800 Kgs. Lyngby, Denmark

123

Hydrobiologia (2010) 644:207–216

DOI 10.1007/s10750-010-0114-1

Introduction

Optical properties in terms of light and light attenu-

ation are, together with nutrients, the main variables

concerning photosynthesis and thus primary produc-

tion in aquatic environments (Kirk, 1994; Falkowski &

Raven, 2007)—a production that is generally high in

the estuarine environments, which are the boundaries

between land and ocean (Jørgensen, 1996; Wolanski,

2007). A great deal of the optical research in estuar-

ies has, however, been focused on mid-latitude and

temperate estuaries (Cloern, 1987; Gallegos et al.,

1990; McMahon et al., 1992; Pfannkuche, 2002;

Lund-Hansen, 2004; Lund-Hansen et al., 2010). Less

attention has been paid to optical properties in tropical

estuaries, with some exceptions (Philips et al., 1995;

Vant, 1990; Christian and Sheng 2003). Tropical

estuaries differ from mid-latitude estuaries in several

ways where the seasonal monsoon-governed wet and

dry periodicity is a pronounced feature (Wolanski,

2007). Precipitation is generally high in the tropics

(Zhang & Wang, 2008), and the wet and dry annual

cycle leads to shifts between periods of extended and

low freshwater flow in tropical estuaries (Wolanski

et al., 1996).

The purposes of the present study are to assess the

variation in optical properties in a tropical estuary in

both wet and dry conditions. Wet conditions are here

taken as several days of heavy rain and high fresh-

water discharge into the estuary, whereas dry condi-

tions are at least 4–5 days of dry weather (see later).

Emphasis is focused on the diffuse attenuation

coefficient—Kd(PAR)—in relation to optical constit-

uents such as: suspended particulate matter (SPM),

Chl a, and dissolved organic matter (CDOM).

Kd(PAR) expresses the vertical attenuation in the

water of the photosynthetically available radiation,

which is the visible part of the spectrum (400–

700 nm) utilized for photosynthesis (Kirk, 1994). The

questions addressed are accordingly: (1) Are there

differences in optical properties between wet and dry

conditions? (2) What are the differences? (3) Are

different optical constituents governing Kd(PAR) in

wet and dry conditions? (4) What is the typical range

of optical properties between wet and dry conditions?

(5) What is the time-scale for the transition from wet

to dry conditions regarding the optical properties?

Materials and methods

The Nha Phu estuary is located just north of the equator

in south-east Vietnam (128 22.80N; 1098 13.70E). The

estuary has a rectangular shape with a distance of about

20 km from the river outlet to the head of the estuary,

and a width of 5–6 km (Fig. 1). The total volume is

0.66 km3 at mean tide, with a surface area of 105 km2,

and about half (49 km2) of the area is shallow water

(\2 m) in the inner part. Depths increase gently from

around 2 m at station 4 to about 45 m at station 1a

(Fig. 1). The drainage area of the two rivers enter-

ing the estuary is about 1,200 km2 with a high

(*1,300 mm) precipitation in the rainy season (Sep-

tember–December) and a lower (*340 mm) in the dry

season. A numerical model describing current condi-

tions in some of the estuaries and bays near Nha Trang

has recently been published (Barthel et al., 2009). The

discharge in the major river entering the Nha Phu

estuary varies between a maximum of 14.6 m3 s-1

(rainy season), and a minimum of 1.8 m3 s-1 (dry

season) during an average year as measured near Ninh

Hoa (Fig. 1). The tide in the Nha Phu estuary is

dominant diurnal with a tidal range of 1–2 m depend-

ing on phase. Samplings were carried out between May

2006 and April 2008 as follows: 2 in 2006, 2 in 2007,

and 3 in 2008 with 4 dry and 3 wet samplings (see

‘‘Discussion’’ section). Precipitation was measured at

Nha Trang Airport (Fig. 1) on a 24-h basis.

The SBE 19 Plus Seabird CTD (www.seabird.com)

was used to measure salinity and temperature. PAR

data were obtained with a LICOR sensor (www.

licor.com) (Li-192) placed on top of a stainless steel

frame. A Wetlabs C-star transmissometer (www.

wetlabs.com) was mounted on the frame in May 2007

for measuring light transmission at an operating

wavelength of 660 nm (red light) and a path length of

25 cm. All data were stored on a Campbell CR10X

data logger placed in a steel chasing. The beam

attenuation coefficient related to particles (cp)

was derived as: cp – cw = ln(Tr)/r where Tr is the

transmittance, r the path length, and cw the light

attenuation coefficient in pure water (Smith & Baker,

1981). Absorption by CDOM was assumed to be

negligible at 660 nm (Jerlov, 1976). The diffuse

attenuation coefficient Kd(PAR) was derived through

linear regression of the log-transformed data. The

208 Hydrobiologia (2010) 644:207–216

123

linear regression model described the data very well

(r2 * 0.97) in accordance with: Iz = I0 e-Kd(PAR)*z

where Iz is the irradiance (lE m-2 s-1) at the depth

(m) z, I0 the irradiance just below the surface, and

Kd(PAR) the diffuse attenuation coefficient (Kirk,

1994). The photic depth is defined as the depth in the

water column with 1% of the 100% surface PAR and

where primary production is possible (Kirk, 1994).

The depth was derived from: Iz = I0 e–Kd(PAR)*z for a

known Kd(PAR) where I0 = 100 at the surface and

Iz = 1 at the depth z.

Chl a, SPM concentrations, and CDOM absorption

coefficients were all determined for water samples

collected at 1 m depth using a Niskin water sampler

(5 l). Water samples were stored cold and dark on

board before samples were processed in the laboratory

upon arrival back from each cruise. For Chl a

concentrations, an exact volume (0.5–1 l) of water

was filtered through GF75 (Advantec) filters (0.7 lm),

using a vacuum of maximum 30 kPa (0.3 bar). The

filters were packed individually in tinfoil bags and

stored in a household freezer. 1–2 days later the filters

were stored in 5 ml 96% ethanol at 5�C, for a

minimum of six and a maximum of 20 h, for pigment

extraction from the retained phytoplankton. Samples

were centrifuged and the absorbance of the superna-

tant was measured at 665 and 750 nm with a spectro-

photometer (Thermo Spectronic HELIOS k) following

Strickland & Parson (1972). Suspended particulate

matter concentrations (SPM) were determined by

filtration of an exact volume (0.5–1 l) of water through

pre-combusted (4008C, 6 h) GF/F Whatman filters

(0.7 lm). Distilled water was used to remove any salt

residues from the filters after filtration. The filters were

kept dark and cold until arrival in the laboratory where

they were dried (608C, 16 h) and weighed. Afterwards,

filters were burned (5008C, 16 h), cooled, and weighed

to derive organic and inorganic content (Bowers &

Mitchelson-Jacobs, 1996).

Water samples for CDOM absorption coefficients

were pre-filtered upon arrival from the cruise using

GF75 (Advantec) filters (0.7 lm). The samples were

stored cold and dark in amber-colored 250 ml glass

containers. Samples were once more filtered using

0.2 lm membrane filters before spectrophotometer

(Thermo Spectronic HELIOS k) analyses. Samples

were analyzed about 2 weeks after completing the

cruise. The absorption coefficient at 440 nm was

taken as proportional to CDOM concentrations (Kirk,

1994) by:

Gio

5

Nha Trang Bay

20

Ninh Hoa

10

5 km

Cai River

Airport

Fig. 1 The Nha Phu

Estuary with stations

Hydrobiologia (2010) 644:207–216 209

123

g440 ¼ 2:303 A440 � A750ð Þ=L ð1Þ

where A440 and A750 are the absorbencies at 440 and

750 nm, L is the cuvette path length (0.1 m) and

2.303 converts the base of the logarithm from 10 to e.

A Levelogger 3.1.1 (www.solinst.com) was placed

under water (*2.5 m) at a pier (128 21.40N; 109812.20E) on the southern shore of the Nha Phu in

January 2008 and recorded time-series of water level

(pressure) at 5 min intervals.

Multiple correlation analysis of optical properties

did not show reasonable results, whereby the focus

was to search the dataset for correlations between

optical properties.

Results

Samples of water and CTD casts were acquired

during periods of 4 dry and 3 wet conditions between

May 2006 and April 2008. Average Kd(PAR), SPM,

Chl a, CDOM, and salinity is shown for the 4 wet and

the 3 dry conditions at each station, and as the

distance from the head of the estuary (0 km)

(Fig. 2A–E). The Kd(PAR) is clearly higher during

wet conditions at nearly all stations, and Kd(PAR)

decreases gradually with distance from the head of

the estuary in both wet and dry conditions (Fig. 2A).

There are no differences between wet and dry

Kd(PAR) at stations 1a–b outside the estuary

(Fig. 1). The high station 4 Kd(PAR) in dry condi-

tions is probably related to wave-induced resuspen-

sion in more shallow (\2 m) water, governed by a

strong landward afternoon sea breeze typical of the

dry conditions. The maximum Kd(PAR) of 2.76 m-1

is obtained during dry conditions. The high Kd(PAR)

standard deviation at station 4, as compared to the

other stations, reflects this variation of Kd(PAR)

governed by both the resuspension as well as inflow

to the Nha Phu of turbid freshwater (see Discussion).

There is a clear difference between average dry

(6.3 g m-3) and average wet (27.5 g m-3) SPM

concentrations at all stations but with no clear spatial

variation (Fig. 2B). The variation in SPM concentra-

tions during wet conditions are high at station 4, as for

Kd(PAR). Note that SPM dry conditions were only

obtained during one sampling. However, the average

of 6.3 g m-3 corroborates with the average SPM of

5.6 g m-3 obtained at dry conditions in March 2009

(Hai, unpublished data). The higher SPM concentra-

tions at station 1a are related to outflow of Cai River

water, which flows northwards passing the mouth of

the estuary (see Discussion). The average organic

matter content of the SPM is comparatively high (32.3 ±

11.6%) in dry as compared to wet conditions

(21.9 ± 2.5%). Wet conditions are characterized by

0

0.5

1

1.5

2

2.5

3

Kd (

PAR

)(m

-1)

0

10

20

30

40

50

SPM

(g

m-3

)0

0.51

1.5

2

2.53

3.5

Chl

-a (

mg

m-3)

0

0.050.1

0.150.2

0.250.3

0.35

CD

OM

(m

-1)

272829303132333435

0 5 10 15 20 25 30 35

Salin

ity (

PSU

)

Km

4

3 2b2a

1b 1aA

B

C

D

E

Fig. 2 A Diffuse attenuation coefficient Kd(PAR) (m-1) at

stations with standard deviation for wet (––) and dry (––)

periods, B suspended particulate matter (SPM) (g m-3), C Chl

a concentrations (mg Chl a m-3), D CDOM absorption

coefficients (m-1), and E salinity(PSU)

210 Hydrobiologia (2010) 644:207–216

123

heavy rainfall and freshwater discharge into the

estuary, whereby the lower organic content reflects

that the SPM was recently derived from the catchment

area. SPM sampling at the Gio River, a minor

tributary to the Nha Phu (Fig. 1), shows a measured

SPM variation between 43.8 and 190.2 g m-3 in wet

conditions (Table 1).

There are no clear differences in Chl a concen-

trations between wet and dry conditions except for

one high wet condition value (2.56 mg Chl a m-3)

at station 4 (Fig. 2C). The 2.56 mg Chl a m-3

concentration was reached on May 18 2007, and

might be related to inflow of nutrients during the

May 10–14 period of heavy rain (see Discussion).

Present low average Chl a concentrations (*0.5 mg

Chl a m-3) corroborate with data from a 2-year-long

harmful algae study in Nha Phu, which showed

similar low concentrations during both wet and

dry conditions (Hai, unpublished data). Higher

(5–10 mg Chl a m-3) Chl a concentrations have

been measured close to the head of the estuary (Lan

Huong, pers. comm.).

CDOM absorption coefficients (m-1) are just a

little higher during wet conditions at inner stations

(Fig. 2D). The coefficient decreases gradually toward

the mouth of the estuary during both wet and dry

conditions. The raised figure at station 1b relates to

water from the Cai River, which discharges into the

Nha Trang Bay and flows northwards, as with SPM.

Note that CDOM dry absorption coefficients were

only obtained during one sampling in April 2007 but

absorption coefficients are very similar to the dry

condition values obtained in March 2009 (Hai,

unpublished data). There is a clear negative (r2 =

0.74) correlation between salinity and CDOM absorp-

tion coefficient: CDOM = –0.046x ? 1.6 as com-

piled for 2 wet and 1 dry conditions (Fig. 3). The

CDOM absorption coefficient (1.6 m-1) at zero

salinity above compares with the CDOM absorption

coefficient of 1.96 m-1 actually measured at Gio

River in January 2008 (Table 1).

Salinity is clearly lower at station 4 and a little

lower at the outer stations in wet conditions (Fig. 2E).

The dilution of the salt water is most pronounced at

station 4 at the freshwater source near the head with a

slight increase in salinity toward the mouth of the

estuary and the open ocean. The minimum wet

condition salinity is 28.2 PSU in January 2008 as

compared to an average dry condition salinity of

32.3 PSU (Fig. 2E). The reduction in salinity of

4.1 PSU between wet and dry conditions shows that

station 4 is only partly influenced by the freshwater

entering the estuary in wet conditions. The distance

between station 4 and the head is about 6 km (Fig. 1).

Table 2 shows the average wet/dry condition ratio

for Kd(PAR), SPM, Chl a, and CDOM. It is seen that

Kd(PAR) is 2.1 times higher, SPM is 4.7 times higher,

and Chl a and CDOM ratios are 1.4 and 1.5 times

higher in wet conditions on average. The table shows

also the total number of observations (n) of a

specified variable in wet and dry conditions. There

are, for instance, 10 observations of Kd(PAR) in wet

conditions and 27 in dry. Table 3 shows observations

of Kd(PAR), SPM, Chl a, CDOM, salinity, and photic

depths (Z0) for very typical wet (20 January, 2008)

and very typical dry (27 April 2008) conditions at

stations 4, 3, 2b, and 2a.

The depth of the photic zone varies between

24.3 m in dry conditions and 6.7 m in wet at station

Table 1 SPM concentration (g m-3) and CDOM absorption coefficient (m-1) in the river at the Gio station about 4 km up-river

from the estuary head in Nha Phu at two different wet periods in May 2007 and January 2008

Date SPM Org. mat. CDOM

16 May 2007 43.8 ± 0.9 (n = 2) 20.3 0.66 ± 0.01 (n = 2)

24 January 2008 190.2 ± 15.2 (n = 2) 8.7 1.96 (n = 1)

Org. mat. organic matter in percent of SPM. Figures with average and standard deviation

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

27 28 29 30 31 32 33 34

Salinity (PSU)

CD

OM

(m

-1)

Fig. 3 The relation between salinity (PSU) and CDOM

absorption coefficients (m-1)

Hydrobiologia (2010) 644:207–216 211

123

2b. The actual depth is only 14 m at this station

(Table 3) and a change from 14 to 6.7 m in photic

depth equals a reduction of nearly 50% between dry

and wet conditions. The depth of the photic zone is

proportional to primary production following a

general (Cloern, 1991) and substantiated (Lund-

Hansen & Sørensen, 2009) relation for primary

production. This strongly indicates that primary

production might be reduced in Nha Phu during wet

conditions with high SPM concentrations and

reduced photic depth, although there are at present

no data on primary production to support this

statement.

The PAR attenuation in terms of a partitioning for

each of the optical constituents at wet and dry con-

ditions is evaluated through Kd(PAR) = Kw(PAR) ?

KCDOM(PAR) ? KChl a(PAR) ? KSPM(PAR), as atten-

uation coefficients are additive (Kirk, 1994). Kw(PAR)

is the PAR absorption by the water and Kw(PAR) =

0.027 m-1 (Smith & Baker, 1981), and KChl a(PAR) is

the PAR absorption by Chl a and KChl a(PAR) =

0.029(Chl a), taken from Lund-Hansen (2004)

where (Chl a) is the measured Chl a concentration.

KCDOM(PAR) was obtained from Pfannkuche (2002) as

KCDOM(PAR) = 0.221(CDOM440) where (CDOM440) is

the CDOM absorption coefficient determined at

440 nm. The term—KSPM(PAR)—in the equation

may vary with grain size, the refractive index of the

grains, and organic matter content of the suspended

flocs (Kirk, 1994) whereby KSPM(PAR) = Kd(-

PAR) - (Kw(PAR) ? KCDOM(PAR) ? KChl a(PAR))

following Philips et al. (1995) and Christian & Sheng

(2003). The partitioning is compiled in terms of

percentages, i.e., what percentage of the PAR is

attenuated by water, CDOM, Chl a, and SPM,

respectively? Percentages are compiled for station 4

as the inner station and for stations 3–2b–2a as the

outer stations, respectively. Results show that PAR

attenuation at station 4 is governed by SPM during

both dry (79%) and wet conditions (88%) (Fig. 4).

This is also the case (63%) at stations 3–2b–2a during

wet conditions, whereas Chl a accounts for 54% of

the PAR attenuation during dry conditions. The PAR

attenuations by water and CDOM were of less

significance at all stations in both wet and dry

conditions.

The beam attenuation coefficient expresses the

attenuation by absorption and scattering of a light

(660 nm) beam in the water over a constant distance

(Kirk, 1994) (see ‘‘Materials and methods’’ section).

Results from a wet conditions sampling shows a high

(4.64 m-1) beam attenuation coefficient at the inner

station 4 and low (0.43–0.67 m-1) values at the outer

stations 3 to 1b (Fig. 5). These values are the average

beam attenuation coefficients in the surface (0–3 m)

waters. The raised coefficients in the bottom waters at

stations 4 and 3 are probably due to resuspension by

tidal currents. The present beam attenuation coeffi-

cients were obtained on May 18 2007 following a

Table 2 Wet/dry ratio for diffuse attenuation coefficient

Kd(PAR) (m-1), SPM (g m-3), Chl a (mg m-3), and CDOM

absorption coefficient (m-1)

Wet/dry ratio Wet (n) Dry (n)

Kd(PAR) 2.07 ± 1.02 10 27

SPM 4.68 ± 1.52 15 5

Chl a 1.40 ± 1.39 10 13

CDOM 1.53 ± 0.57 15 5

Average ratio for wet and dry conditions with standard

deviations. n = total number of measurements of a specified

variable at wet or dry conditions

Table 3 Optical properties and salinity in typical wet (January 20, 2008) and dry (April 27, 2008) conditions

Parameter Station 4 (2 m) Station 3 (8 m) Station 2b (14 m) Station 2a (20 m)

Kd(PAR) 1.63–1.35 0.63–0.40 0.62–0.19 0.38–0.19

SPM 17.7–9.60 24.6–4.65 26.0–4.75 23.0–5.15

CDOM 0.28–0.21 0.15–0.1 0.07–0.06 0.06–0.05

Chl a 0.82–0.45 0.22–0.55 0.17–0.59 0.22–0.45

Salinity 31.1–31.7 33.4–32.6 33.4–33.2 32.4–33.2

Z0 2.8–3.4 6.8–11.5 6.7–24.3 12.1–24.2

The first number in each column is wet condition and the second dry. Units are: Kd(PAR), diffuse attenuation coefficient (m-1), SPM,

suspended particulate matter (g m-3), CDOM, absorption coefficient (m-1), Chl a (mg Chl a m-3), salinity (PSU), and photic depth

Z0 (m)

212 Hydrobiologia (2010) 644:207–216

123

period of heavy rainfall (142 mm) during May

10–14. A distinct 2–3 m thick surface layer of higher

beam attenuation coefficients is clearly seen (Fig. 4).

A high (43.8 mg m-3) SPM concentration was

obtained on May 16 at the Gio River (Table 1) just

following the 142-mm of heavy rainfall during May

10–14. Water sampling and CTD casts were obtained

in the estuary on May 14, 18, and 20 but Kd(PAR)

was only obtained on May20. There was no rainfall

during May 14–20, and May 20 was considered as

dry conditions. The high (r2 = 0.73) correlation

between salinity (x) and Kd(PAR) on May 20:

Kd(PAR) = –0.62x ? 20.4 was applied to derive

Kd(PAR) at the stations on May 14 and 18 with the

purpose of estimating a typical time-scale for changes

from wet to dry conditions. Results show that the

time-scale for a change from wet to dry conditions is

5–6 days, at an average rate of change in Kd(PAR) of

0.82 Kd(PAR) per day at station 4 and lower (0.32

Kd(PAR) per day) at station 2b (Fig. 6). Note that

station 1b relates to outflow from the Cai River as

described above. The 0.82 Kd(PAR) per day is the

rate of change of Kd(PAR) for a specified time and

analogous to a clearance rate of SPM, which is the

governing parameter in relation to Kd(PAR) as shown

earlier. Residence time (T) for a substance (salt or

SPM) in an estuary is calculated by T = t * (vt/vp)

(Dyer, 1997) where t is the typical tidal period

(24.5 h), vt total volume of the estuary (0.66 km3),

and vp the tidal prism (0.15 km3). The residence time

(T) here equals 107.8 h or 4.5 days, which is quite

close to the time-scale of 5–6 days estimated above,

considering the uncertainties and the simple relation

for T.

Discussion

The dataset comprised samplings in four dry and

three wet conditions between May 2006 and April

2008. Meteorological data showed that there was no

rain 7 days or more prior to the sampling in two of

the dry samplings, whereas rainfalls of 16.5 and

49.5 mm occurred 4–5 days prior to the sampling in

the last two dry samplings. Salinity was high in the

surface waters during these two dry samplings, which

complies with the residence time of 5–6 days in the

Nha Phu whereby these two dry samplings were

considered as dry conditions. There were, in com-

parison, three samplings in wet conditions where

rainfall reached a total of 76.9 mm just prior to the

sampling day in one case, and 35.6 mm prior and

during sampling in the second. The third wet

sampling followed the heavy rainfall (144 mm)

during May 10–14 2007, which amounts to 12% of

the annual rainfall for an average year. There were, in

comparison, just a few incidents where rainfall was

St. 4 Dry

St. 4 Dry

St. 4 Dry

St. 4 Dry

St. 4 Wet

St. 4 Wet

St. 4 Wet

St. 4 Wet

St. 3-2 Dry

St. 3-2 Dry

St. 3-2 Dry

St. 3-2 Dry

St. 3-2 Wet

St. 3-2 Wet

St. 3-2 Wet

St. 3-2 Wet

0102030405060708090

100

Water CDOM Chl-a SPM

Perc

ent (

%)

Fig. 4 PAR attenuation (percent) for: SPM, Chl a, and CDOM

for various for wet and dry conditions and stations. Column 1:

st. 4 dry, 2: st. 4 wet, 3: st. 3–2 dry, 4: st. 3–2 wet

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Beam att. coeff. (m-1

)

Dep

th (

m)

4

3

2b

2a

1b

Fig. 5 Beam attenuation coefficient (m-1) at stations on May

18 2007

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30

Km

Kd

(PA

R)(

m-1

)

14 May

18 May

20 May

4

3 2b 2a

1b

Fig. 6 Diffuse attenuation coefficient Kd(PAR) (m-1) at

stations on May 14, 18, and 20 following a period of heavy

rain (142 mm) and high discharge May 10–14 2007

Hydrobiologia (2010) 644:207–216 213

123

higher than 142 mm in 4 days during the wet seasons

in 2006, 2007, and 2008. This heavy rainfall in May

outside the rainy season (September–December)

shows clearly that the distinction between dry and

rainy seasons must be considered as less rigorous.

There was a good correlation between the tidal

prediction (http://www.sailwx.info/tides) for the Cam

Ranh Bay (118 530N; 1098 120E) 40 km south of Nha

Phu, and the measured tidal time-series from Nha Phu

in January 2008. Tidal prediction for the actual days

of sampling in 2006, 2007, and 2008 were accord-

ingly used to evaluate any possible effects of the tidal

cycle in relation to time of sampling, i.e., sampling at

high or low tide or sampling during rising or falling

tide. Field sampling generally commenced around 9

a.m. in the morning and was completed around 3 p.m.

in the afternoon, where the change in water level was

typically about 0.8–1.0 m during the 6-h period. All

dry and one wet condition samplings were carried out

at falling tide but there were no differences in salinity

distributions in the estuary between sampling at rising

or falling tide in dry conditions. It is thus strongly

supposed that any effects of the tide were at a mini-

mum during dry conditions. A comparison between

wet condition sampling at falling tide (May 18, 2007)

and a period of rising tide (January 20, 2008) showed

a higher Chl a concentration at station 4 (2.56 mg Chl

a m-3), as well as slightly raised CDOM absorption

coefficients at falling tide. The only source for

CDOM is the freshwater, whereby a higher CDOM at

falling tide shows that water with a lower salinity flows

outwards at falling tide. A parcel of water will move

about 5 km horizontally during a half-tidal period

(12.41 h) with a presumed current speed of 0.1 m s-1,

which shows that only station 4 will be affected as the

station closest to the freshwater source (Fig. 1). The

standard deviation of the optical properties at this sta-

tion was, on the other hand, relatively high whereby a

tide-related variation was not discernable.

The KSPM(PAR) was derived by: KSPM(PAR) =

Kd(PAR) – (Kw(PAR) ? KCDOM(PAR) ? KChl a(PAR))

although with some uncertainties and especially in

environments of high scattering by the suspended

particles (Kirk, 1994). Scattering was not measured in

Nha Phu, but the obtained Kd(PAR) specific SPM

attenuation coefficient of 0.038 m2 g SPM-1 was

comparable to literature figures (0.03–0.05 m2 g

SPM-1) (Di Toro, 1978; Vant & Davies-Colley,

1986). This strongly indicates that scattering and

absorption of the SPM in Nha Phu were not very

different from that in other estuaries, which justi-

fies the partitioning method as above. Further, the Chl

a specific absorption coefficient—KChl a* = 0.029

m2 mg Chl a-1—(Lund-Hansen, 2004) applied in the

present Kd(PAR) partitioning complies with literature

results where it varies between 0.016 and

0.031 m2 mg Chl a-1 (Di Toro, 1978; Vant, 1990).

The coefficient was derived in a temperate estuarine

environment but depends foremost on phytoplankton

species and sizes (Fujiki & Taguchi, 2002). The

CDOM-specific absorption coefficient—a(PAR) =

0.221 CDOM440—derived by (Pfannkuche, 2002)

was applied in the Kd(PAR) partitioning, as CDOM is

definitely an absorber of light. The partitioning

showed that SPM governed the PAR attenuation at

station 4 both in wet (79%) and dry (88%) conditions,

but only at stations 3, 2b, and 2a in wet conditions

(63%), whereas Chl a governed at stations 3, 2b, and

2a in dry (54%). PAR attenuation by water and

CDOM were both of less significance. Optical

properties in the Nha Phu were clearly governed by

SPM and less by CDOM as compared to four Florida

tropical lagoons where PAR attenuation related to

CDOM in the water varied between 5 and 22% as

compiled by Christian & Sheng (2003). Results in the

more saline but still estuarine temperate Arhus Bay

showed that 17% of the PAR attenuation on average

was due to absorption by CDOM (0.232 m-1), 32%

due to Chl a (3.3 mg Chl a m-3), 42% to SPM

(4.5 g m-3), and 9% to water absorption (Lund-

Hansen, 2004). Numbers in brackets are average

concentrations of substances and absorption coeffi-

cients. The clear differences between Arhus Bay and

the Nha Phu reflect the fact that the Arhus Bay does

not receive freshwater directly from a catchment area

and SPM concentrations are accordingly much lower

in Arhus Bay.

The station 4 Kd(PAR) figures are comparable to

those obtained (Kd(PAR) = 3.5 m-1) at high precip-

itation and high water flow in a New Zealand estuary

(Vant, 1990), whereas stations 2b to 1a are typical

for estuarine environments such as the Kattegat

(Kd(PAR) = 0.57–0.15 m-1) (Lund-Hansen, 2004)

and other New Zealand estuaries (Kd(PAR) =

0.3–0.11 m-1) (Vant, 1990). A similar range in Kd(PAR)

was also found in the Swan River, Perth (Kostoglidis

et al., 2005). Kd(PAR) varied between 2.50 m-1 at

station 4 and 0.46 m-1 at station 2b during wet

214 Hydrobiologia (2010) 644:207–216

123

conditions in January 2008. This spatial range equals

the difference between base flow (dry) and flood

(wet) in the NZ Whitiangi River (Vant, 1990), which

emphasizes the strong spatial variation in the Nha

Phu analogous to dry and wet conditions.

The present wet and dry SPM concentrations were,

in comparison, very similar to those measured in the

Guadiana Estuary during low (2–4 g m-3) and high

flow (20–45 g m-3) periods (Cravo et al., 2006).

The Nha Phu dry condition concentrations were

comparable to SPM concentrations in the estuarine

Arhus Bay (Lund-Hansen, 2004). SPM concentra-

tions reached up to 190.2 g m-3 at the Gio River

(Fig. 1) station during wet conditions on January 24,

2008 (Table 1). The 190.2-g m-3 is similar to

average SPM concentrations in the Mekong River

(*250 g m-3) (Wolanski et al., 1996) and low flow

concentrations in the Red River (200 g m-3) (Van

Maren & Hoekstra, 2004). Chl a concentrations were

on average very low for both wet (0.35 mg Chl a

m-3) and dry (0.48 mg Chl a m-3) conditions, as

compared to temperate estuaries as Arhus Bay with

an annual range between 9.8 and 1.3 mg Chl a m-3

(Lund-Hansen, 2004). The Irish Shannon estuary

showed a similar high (2.0–18.0 mg Chl a m-3) Chl

a concentration range (McMahon et al., 1992).

The present station 4 CDOM absorption coeffi-

cients were similar to those obtained in the estuarine

Arhus Bay (*0.23 m-1) (Lund-Hansen, 2004), in

New Zealand estuaries (*0.1–0.6 m-1) (Vant, 1990),

and in the freshwater-influenced Clyde Sea (Bowers

et al., 2000). Absorption coefficients (0.02–0.08 m-1)

at outer stations (2b–2a) were comparable to those

from more open coastal waters such as the New

Zealand shelf (Pfannkuche, 2002) and northern North

Sea (Højerslev et al., 1996). The high (1.96 m-1) Gio

River wet conditions absorption coefficients were, on

the other hand, typical of lakes and inland dams (Kirk,

1994). The Nha Phu beam attenuation coefficients are

comparable to those of the New Zealand estuaries at

base flow and dry conditions (Vant, 1990).

Conclusions

The diffuse attenuation coefficient Kd(PAR) varied

strongly between wet and dry conditions as well as

spatially between the head and mouth of the estuary.

Kd(PAR) was generally governed by variations in

SPM concentrations during both wet and dry condi-

tions. Chl a concentrations were quite low and

showed only a little variation between wet and dry

conditions. CDOM absorption coefficients were

higher at the estuary head in wet conditions. The

residence time of 4.5 days as derived from changes

in Kd(PAR) corroborated a calculated value of

5–6 days. The main differences, as compared to

temperate estuaries, were the high significance of

SPM in relation to Kd(PAR) as compared to Chl a.

The second was the far more episodic character of the

wet conditions with high rainfall and freshwater flow

into the estuary in 4–5 days followed by longer

periods of dry conditions.

Acknowledgments Director Bui Hong Long and staff at the

Institute of Oceanography, Nha Trang, Vietnam, are thanked

for their good cooperation and fine hospitality during the

project ‘‘Hydrodynamics of Wet and Dry Estuaries’’ financially

supported by Danish Research Council contract. Nr. 272-06-

0582. Mr. Thrien is thanked for help during the field work.

References

Barthel, K., R. Rosland & N. C. Thai, 2009. Modelling the

circulation on the continental shelf of the province Khanh

Hoa in Vietnam. Journal of Marine Systems 77: 89–113.

Bowers, D. G. & E. G. Mitchelson-Jacobs, 1996. Inherent

optical properties of the Irish Sea determined from

underwater irradiance measurements. Estuarine, Coastal

and Shelf Science 43: 433–447.

Bowers, D. G., G. E. L. Harker, P. S. D. Smith & P. Tett, 2000.

Optical properties of a region of freshwater influence—

The Clyde Sea. Estuarine, Coastal and Shelf Science 50:

717–726.

Christian, D. & Y. P. Sheng, 2003. Relative influence of

various water quality parameters on light attenuation

in Indian River Lagoon. Estuarine, Coastal and Shelf

Science 57: 961–971.

Cloern, J. E., 1987. Turbidity as a control on phytoplankton

biomass and productivity. Continental Shelf Research 7:

1367–1381.

Cloern, J. E., 1991. Annual variations in river flow and primary

production in the South san Francisco Bay estuary (USA).

In Elliot, M. & Ducrotoy (eds), Estuaries and Coasts:

Spatial and Temporal Intercomparisons. Olsen and Olsen,

Fredensborg: 91–96.

Cravo, A., M. Madureira, H. Felicia & M. J. Bebianno, 2006.

Impact of outflow from the Guadiana River on the dis-

tribution of suspended particulate matter and nutrients in

the adjacent coastal zone. Estuarine, Coastal, and Shelf

Science 70: 63–75.

Di Toro, D. M., 1978. Optics of turbid estuarine waters:

approximations and applications. Water Research 12:

1059–1068.

Hydrobiologia (2010) 644:207–216 215

123

Dyer, K. R., 1997. Estuaries—A Physical Introduction. Wiley,

Chichester: 195 pp.

Falkowski, P.G. & J.A. Raven, 2007. Aquatic Photosynthesis,

2nd ed. Princeton University Press, Princeton: 351 pp.

Fujiki, T. & S. Taguchi, 2002. Variability in chlorophyll a

specific absorption coefficient in marine phytoplankton as

a function of cell size and irradiance. Journal of Plankton

Research 24: 859–874.

Gallegos, C. L., D. L. Corell & J. W. Pierce, 1990. Modelling

spectral diffuse attenuation, absorption, and scattering

coefficients in a turbid estuary. Limnology and Ocean-

ography 35: 1486–1502.

Højerslev, N. K., N. Holt & T. Aarup, 1996. Optical mea-

surements in the North Sea Baltic Sea transition zone. I.

On the origin of the deep water in the Kattegat. Conti-

nental Shelf Research 16: 1329–1342.

Jerlov, N. G. 1976. Marine Optics, 2nd ed. Elsevier Ocean-

ography Series, 14. Elsevier, Amsterdam: 371 pp.

Jørgensen, B. B., 1996. Case study—Aarhus Bay. In Jørgensen, B.

B., K. Richardson (eds) Coastal and Estuarine Studies, Vol.

52. American Geophysical Union, Washington: 241 pp.

Kirk, J. T. O., 1994. Light and Photosynthesis in Aquatic Eco-

systems. Cambridge University Press, Cambridge: 401 pp.

Kostoglidis, A., C. B. Pattiaratchi & D. P. Hamilton, 2005.

CDOM and its contribution to the underwater light climate

of a shallow, microtidal estuary in South-western Australia.

Estuarine, Coastal and Shelf Science 63: 469–477.

Lund-Hansen, L. C., 2004. Diffuse attenuation coefficients

Kd(PAR) at the estuarine North Sea Baltic Sea transition:

time-series, partitioning, absorption, and scattering.

Estuarine, Coastal and Shelf Science 61: 251–259.

Lund-Hansen, L. C. & H. M. Sørensen, 2009. Parameterization

of surface irradiance and primary production in Arhus Bay,

SW Kattegat, Baltic Sea. Hydrobiologia 620: 173–179.

Lund-Hansen, L.C., T.J. Andersen, M.H. Nielsen & M. Pejrup,

2010. Suspended matter,Chl-a, CDOM, grain-sizes, and

optical properties in the arctic fjord-type estuary,

Kangerlussuaq, West Greenland during summer. Estuaries

and Coasts (Accepted)

McMahon, T. G., R. C. T. Raine, T. Fast, L. Kies & J. W.

Patching, 1992. Phytoplankton biomass, light attenuation

and mixing in the Shannon estuary, Ireland. Journal of the

Marine Biological sAssociation U.K. 72: 709–720.

Pfannkuche, J., 2002. Optical properties of Otago shelf waters:

south island New Zealand. Estuarine, Coastal and Shelf

Science 55: 613–627.

Philips, E. J., T. C. Lynch & S. Badylak, 1995. Chlorophyll a,

tripton, color, and light availability in a shallow tropical

inner-shelf lagoon, Florida Bay, FL. Marine Ecology

Progress Series 127: 223–234.

Smith, R. C. & K. S. Baker, 1981. Optical properties of the

clearest natural waters (200–800 nm). Applied Optics 20:

177–184.

Strickland, J. D. H. & T. R. Parson, 1972. A practical Hand-

book of Seawater Analyses. Ottawa, Canada: 235 pp.

Van Maren, D. S. & P. Hoekstra, 2004. Seasonal variation of

hydrodynamics and sediment dynamics in a shallow

subtropical estuary: the Ba Lat River, Vietnam. Estuarine,

Coastal and Shelf Science 60: 529–540.

Vant, W. N., 1990. Causes of light attenuation in nine New

Zealand estuaries. Estuarine, Coastal and Shelf Science

31: 125–137.

Vant, W. N. & R. J. Davies-Colley, 1986. Relative importance

of clarity determinants in Lakes Okaro and Rotorua. New

Zealand Journal of Marine Freshwater Research 20: 355–

363.

Wolanski, E., 2007. Estuarine Ecohydrology. Elsevier,

Amsterdam: 157 pp.

Wolanski, E., N. N. Huan, L. T. Daos, H. N. Nhan & N. N.

Thuy, 1996. Fine-sediment dynamics in the Mekong River

Estuary, Vietnam. Estuarine, Costal and Shelf Science 43:

565–582.

Zhang, S. P. & B. Wang, 2008. Global summer monsoon rainy

seasons. International Journal of Climatology 28: 1563–1578.

216 Hydrobiologia (2010) 644:207–216

123