V N U Jo u rn a l of Scicncc, E arth Scicnces 28 (2012) 181-189
Rapid assessment of water eutrophic state in Tien Yen-Ha Coi Bay by estimating Chlorophyll-a concentration using
geostatistical interpolations and MODIS data
Nguyen Thi Thu Ha’’*, Pham Thi Tuyet^, Mai Trong Nhuan', Nguyen Van Vuong',
^Faculty o f Geology, VNU University o f Science, 334 Nguyen Trai, Hanoi, Vietnam
^ VNƯ Sea and Island Research Center
R eceived 25 July 2012; received in revised form 9 A ugust 2012
A bstract. E uữophication is a natural process o f enrichm ent o f water by nutrients, how ever in many case is considered as one of the major factors of water pollution affecting coastal ecosystem.
Selection and application o f suitable indicator and m ethod to assess sea euừ ophication is utm ost im portant. This study aim s at clarifying w ater euứophic state m T ien Y en - H a Coi B ay using geostatistical methods o f 40 in situ sites data and auxiliary information from MODIS/Terra image.
Resultant predicted chlorophyll-a concenừation distribution map in Tien Yen - Ha Coi Bay produced by cokriging and euừ ophic possibility m ap com puted by indicator kriging identified clearly that the b ay is classified as natural euừophic waters. B y this study, M O D IS offers the possibility to support auxiliary information for assessment euữophic state m large water areas.
Cokrigm g and indicator kriging was proved to be effective for rapid environm ental quality assessm ent and coastal w ater m anagem ent.
Keyw ords: Eutrophications, cokriging, indicator kriging, estim ation map, probability map.
1. Introduction
Eutrophication o f coastal w aters has been considered one o f the m ajor threats to the health o f m arine ecosystem s. The different processes and effects o f coastal eutrophication are well known and docum ented [1-4]. T he m ain cause o f euừophication in coastal w aters is nutrient over-enrichm ent (nitrogen, phosphorus and silica) leading to: an increase in the frequency o f harm ful algal bloom s, shellfish contam ination, anoxic and hypoxic events and
* Corresponding author. Tel: 84-1222275688.
E-mail: [email protected]
fish kills; a loss o f ecosystem integrity, aquaculture production, fish stocks and am enity value; and changes in biodiversity [5].
Therefore, identifying the m ost efficient w ay to diagnose the euữophic state o f coastal w aters is indispensable task to protect coastal environm ent and preserve ecosystem health.
H ow ever euừophication o f coastal waters has been considered for a long tim e and many related researches have been carried-out w orldw ide, one im portant question is still to be answ ered: “H ow should prim ary production o f eutrophication be m easured or estim ated?” . Such question requires thorough scientific 181
182 N .T .T . H a e t a l / V N U Journal o f Science, Earth Sciences 2 8 (2 0 1 2 ) Ĩ 8 1 - Ì 8 9
a n a ly se s as w e ll a s c o o r d in a tio n , n e w m e th o d s and approaches in assessm ent o f coastal water euữophic state need be developed and dem onstrated. C hlorophyll-a (Chl-a) concentration is an effective m easure o f the ừophic status o f sea and land w aters, because it is re la te d s ừ o n g ly to a q u a tic p h y to p la n k to n a b u n d a n c e a n d b io m a s s . E s tim a tin g C h l-a concentration is one o f the m ost traditional and significant applications o f rem ote sensing for evaluating aquatic ecosystem s and m onitoring eutrophication [6-10]. T his study develops a m ethod for rapid assessm ent o f w ater eutrophic state in Tien Yen - H a Coi B ay by using various geostatistical interpolations o f in situ Chl-a concenfrations and auxiliary inform ation from M O D IS/T e ư a im ag e. R esultant probability map o f w ater eutrophic state and potential based on Chl-a concentrations o f this study helps delim iting zones o f high and low eufrophication. Since no other data o f sim ilar nature is available at the present time, the
obtained results w ere considered as a first baseline for coastal w ater environm ental analysis and m anagem ent o f Tien Yen - H a Coi B ay on eutrophication assessm ent.
2. M a te ria ls a n d M eth o d s The study area
Tien Yen - H a Coi Bay IS adjacent to China - V ietnam border and connected to the South China Sea by five channels (Fig. 1). The most rem arkable feature o f the bay IS its shallowness:
the sea depth generally ranges from 2 to 5 m.
A nother feature is that the bay has a strong dium al tide regim e w ith m axim um tida!
am plitude o f 5 m [11]. Therefore, aquatic ecosystem and environm ent o f the bay are dom inated m ainly by oceanographic factors such as tide, waves, and near-shore currents.
21.67'’N
0 10 20 km
1_I__ I__ I__I__ I__ I__ I__I
Figure 1. Location o f T ien Y en - H a Coi B ay and positions o f 40 sam pling points for w ater sampling.
N .T .T . Ha et a i / V N U lo urnal o f Science, Earth Sciences 28 (2 0 1 2 ) Í S Ì - Ĩ 8 9 183
A decline in the environm ental and ecological systems in Tien Yen - Ha Coi Bay has been reported, including loss o f seagrass m eadows, phytoplankton abundance and w ater contam ination [12]. One o f serious environm ental threats on the bay w ater environm ent is euữophication process: its state and cause. Therefore, assessing w ater eutrophic state in Tien Yen - Ha Coi Bay IS an indispensable task to protect the bay environm ent and preserve ecosystem health.
W aĩer sam pling and analysis
Sea w ater in Tien Yen - Ha Coi Bay was sam pled at 40 points using a speed-board on 6 July 2010 with a Global Positioning System (G PS) receiver to locate the points. These points, shown in Fig. 1, w ere selected to investigate the environm ental conditions all over the bay. The samples were taken at 50 cm w ater depth using V an D om w ater sampler, filled in 1 liter cleaned bottles at the constant tem perature of 4 "c storage and transported to a laboratory. W ater sam pling w as can ied out w ithin 10 hours before/after tim e o f image acquiring on 6 July 2011. Speed-boat w as used for fast data collection.
Concentration o f Chl-a in w ater was determ ined by standard Specfa-ophotometric m easurem ent m ethod [13]. Firstly, w ater sam ples were filtered by a pre-w ashed 47m m glass fiber filter and then exừ acted into 90%
acetone. The Chl-a and phaeophytin concentrations in the extract w ere determ ined by specữophotom eter at 750 nm and 664 nm before acidification, and 750 nm and 665 nm after acidification. D escriptive statistics o f the resultant Chl-a concenừations o f the 40 samples are sum m arized in Table 1.
M O D IS d a ta
T h e T e ư a s p a c e c r a ft p a s s e s o v e r T ie n Y e n - H a C o i B a y a t a b o u t 3:2 0 G M T (1 0 :2 0 local tim e ) e a c h d ay . T h is tim e IS s u ita b le fo r a c q u irin g s a te llite im a g e ry to c o m p a re w ith in situ w ater quality. M O D IS /T eưa level IB image data acquired on the same date as the w ater sam pling (6 July 2010), w hich were calibrated at-aperture radiances for the 36 bands a n d g e o -lo c a te d fo r W S G -8 4 N 4 8 o f th e Ư T M sy ste m , w e re u s e d to e s tim a te C h l-a c o n c e n tra tio n . A c c o rd in g to H a a n d K o ik e (2011) [14], it was identified that dark-object subừaction (DOS) by Chavez (1988) [15] was a suitable atm ospheric correction m ethod for the M ODIS data o f Tien Yen - Ha Coi Bay.
T h e re fo re , th is s tu d y u s e d o b ta in e d re fle c ta n c e after DOS as data for cross-estim ation.
G eostatistical m ethods Co-Kriging
C o-K riging (CK ) is a form o f kriging that involves m ultiple variables. It is considered a
‘hy brid’ K riging technique (i.e. non-stationary geostatistical m ethod). In fact, the m ethod is a m ultivariate extension o f Kriging that allows inclusion o f m ore readily available and inexpensive attributes in the prediction process [16]. CK uses auxiliary variables and takes into account additional coưelated inform ation betw een variables to im prove spatial prediction, w hich requires an estim ation o f cross- variogram s. W ith CK the estim ated value at an unsam pled location is a linear w eighted sum o f all o f the variables being exam ined (i.e. two or more). For two variables, the m odels are:
z ^ { x ) = ụ ^ {x ) + s^{x ) (1) z , { x ) = ụ , { x ) + e ,{x) (2)
184 N T T . H a e t a i Ị V N U jo u r n a l o f Science, Earth Sciences 2 8 (2 0 1 2 ) 1 8 1 -189
W here ỊẦ^ and are unknow n m ean values (constants) and and £ị^ are random eư ors.
Each o f these sets o f random e ư o rs m ay exhibit autocoưelation and cross-correlation betw een the datasets, which the procedure attempts to model. C o-K riging uses this cross-coưelation or covariance to im prove the estim ation o f Z ^ { x ). The covariance is calculated according to the following formula:
c ( 5 „ s ^ ) = cov(Z(5,), Z(s^)) (3) w here cov is the covariance o f two random variables. Covariance is a scaled version o f coưelation. So, w hen tw o locations, Si and Sj, are close to each other, you expect them to be sim ilar and their covariance (a coưelation) will be large. As S i and S j get farther apart, they becom e less similar, and their covariance becom es zero.
Indicator K riging
Indicator K riging (IK) is a non-param etric geostatistical m ethod for estim ating the probability o f exceeding a specific threshold value, 2*, at a given location. In indicator Kriging, the stochastic variable, Z(u), is ừansfom ied into an indicator variable with a binary d istìb u tio n , as follows:
I { u - Z ị k ) ^ { (1, i f Z(u) < z,k, k = 1, 2,...m @ 0, otherw ise) (4)
The expected value o f I(u; Z t), conditional to n surrounding data, can be expressed as:
E[l{u-, z J ( « ) ) ] = P r a ỗ
{ Z i u ) < z , \ i n ) } = F { u - , z , \ ( n ) ) p{ u; Zi^\{n)) = \ - F { u ; I («))
(
5) (
6)
W h e re F (u ; Zk\(n)) is th e c o n d itio n a l c u m u la tiv e d is trib u tio n fu n c tio n o f Z (u ) <
Z ịk, an d P (u ; Z;t|(n)) is th e p r o b a b ility that Z (u ) > Z ị k . A t a n u n s a m p le d lo c a tio n , Uo,
e s tim a tio n m u s t u se in d ic a to r k rig in g and th e in d ic a to r e s tim a to r, / (uo; Zk), a c c o rd in g to:
=
(
7)
; = 1
w h e re I(uj, Z 0 re p re s e n ts th e v a lu e s o f th e in d ic a to r a t m e a s u re d lo c a tio n s . Up j = 1 ,2 ,..,/i, a n d X,j(Zk) is th e w e ig h tin g fa c to r o f I(uj, ZfJ in e s tim a tin g f (uo; Z 0 .
Results and Discussion
The atm ospheric co ư ectio n m ethods, DOS, were applied to the M OD IS 36 bands data to obtain surface reflectance (R ự s (Ẵ ))o ĩ the Tien Yen - Ha Coi Bay w ater surface entire. Table 1 sum m arizes descriptive statistics o f obtained Rrs(Ằ)dX tw o bands: visible green band (band 12, at 551 nm) and blue band (band 9, at 443 nm), using M O D IS /T eưa im age at 1 km pixel size w ith total num ber o f pixels is 392.
Table i. D escriptive statistics w ater sam ples datasets o f Chl-a concenừations (mg/m^) and obtained reflectance o f bands 9 and 11 M O D IS/T eưa Im age after DOS
Item Param eter
Chl-a (m g/m ’)
^ .( 4 4 3 ) (M O D IS band 9)
/?„(551) (M O D IS ban d 12)
N um ber o f data 40 392 392
A verage 12.5 0.045 0,047
M edian 12.7 0.043 0.047
M axim um 16.5 0.090 0.095
M inim um 8.1 0.025 0.014
Standard deviation 1.8 0.011 0.015
Range 8.4 0.066 0.081
N .T .T . Ha et a i / V N U Journal o f Science, Earth Sciences 2 8 (2 0 1 2 ) 1 8 1 - Ì 8 9 185
Fig. 2 show s a strong relationship between C hl-a concentrations o f w ater sam ples and ratio o f tw o reflectances at M ODIS band 12:
R„(551), versus band 9: R^,(443) (correlation coefficient r = 0.78). This relationship confirm ed a high possibility o f cokriging in prediction C hl-a concentrations from M O D IS/T erra image data (R,,(551)/R,,(443)).
o -s
Figure 2. R elationship betw een C hl-a concentration o f w ater sam ple and ratio o f two reflectances at M ODIS band 12 (551 nm) versus band 9 (443 nm).
Experim ental covariance produced from th e s e tw o d a ta s e ts w a s a p p ro x im a te d b est by the exponential model with a nugget effect o f 0 .1 2 m g / m \ a sill o f 16.6 m g / m \ a n d a ra n g e o f 17 km (Fig. 3A). To check the spatial estim ation accuracy o f CK with a cross- validation, a scattergram that represents the re la tio n s h ip b e tw e e n th e in situ ^ C h ỉa - v alu e a n d th e p re d ic te d C K v a lu e u sin g th e Cchia data w as produced (Fig. 3B). The re s u lta n t m e a n e ư o r o f th e p re d ic tio n is c lo se to 0 (0.005 mg/m^) and the RM SE is relatively low (0 .9 9 m g /n v '), w h ic h c o n firm s th e high c a p a b ility o f C K . T h e re s u lta n t C K d istrib u tio n o f Chl-a concentration and the kriging variance map are depicted in Fig. 4 and 5. The kriging variances are negligible over the entire bay.
Only a sm all zone outside the bay, where no in situ data w as collected, has a relatively high variance, but its value is under 10% o f the estim ated (^Chia. Such variance trend also supports the correctness o f the CK estimation.
c 10 Predicted 10-
•• Model + Averfl8«d 0istar>ce(0^r«e). h 10 Measured 10'
Figure 3. (A ) C ovariance and exponential m odel (curve) o f the in 5/7uChl-a concentrations andR „(551)/R „(443).
( B ) Scattergram for cross-validation o f cokriging prediction. The 45-degree line is superim posed.
186 N .T .T . H a et al. / V N U Journal o f Science, Earth Sciences 2 8 (2 0 1 2 ) Ĩ 8 Ĩ - 1 8 9
Prediction Chl-adừtribution map produced by CK The spatial distribution o f Chl-a concenứations w ithin Tien Y en - H a Coi Bay was predicted by C K interpolation (Fig. 4).
Basing on the map, Chl-a concentrations on 6 July 2010 ranged from 8.1 to 1 6.5 m g/m ^ w hich are 4 to 7.5 tim es o f the eutrophic level, 2.21 mg/m^of Sim boura et al. [17]. Generally, concentrations o f Chl-a over 14.3 mg/m^ occur
in local estuaries at the m ouths o f the Ha Coi and D am Ha Rivers and along the coast from M ong Cai to H ai Ha. The m iddle bay also contains high C hl-a from 13.8 to 14.3 mg/m^
from Cai Chien to Hai Ha districts. On the confrary, the concentrations becom e low tow ard the w est near Cai B au Island w ith a m inim um o f 8.1mg/m^ in the channels connected w ith the outer sea.
21.55»N
107.45‘F.
ÍOS.DO-E
C h l-a (n ig /m ^
■ i 15.15- 16.50 14.32- 15.15
H i
H i ) 3 .4 9 - 13.81 13.29- 13.49 12.97- 13,29 i S 12.45- 12.97 11.62- 12.45 1 0 .2 7 -1 1 .6 2 8 . 1 0 - 10.27
21.15-’N
Figure 4. Spatial distribution of Chl-a concenứations produced by cokriging.
21.55-'N
I08.00“E
V ariance o f C h l-a (mg/in')^
WÊÊ
■ 1 l.2 t
H I 1.00
I Ẽ M Ổ.83 0 . 7 0 -O.s:^
0 . 5 8 - 0 . 7 0 0 .4 9 - 0 . 5 8 0.38 - 0.49
1.77 1.46 1.21 1.00
107.45«E I_________________ £ ____________________________________________________________
21.15‘N
Figure 5. Kriging variance for representing uncertainty o f estimation o f Chl-a concenữations by cokriging.
N .T .T . H a et al. / V N U jo u r n al o f Science, Earth Sciences 2 8 ( 2 0 Ì 2 ) 1 8 Ĩ - Ĩ 8 9 187
Those spatial characteristics o f Chl-a concentrations conform ed to the hydrodynam ic system in the bay that is generated by the interaction o f regional surface cuư ents, tides and waves. The local river estuaries and central bay are shielded by the islands (V an N uoc, Cai Chien, and V inh Thuc) and the tortuous coastline, and therefore the hydro-energy related to currents and w aves is w eak there.
U nder such conditions, phytoplankton, the main source o f high Chl-a concentrations, accum ulate and grow. In contrast, in the connection channels and the largely m ovable w aters affected directly by the outer sea, the hydro
energy is strong. These w aters contain abundant
am ounts o f re-suspended material, which prevent the accum ulation and growth o f phytoplankton. Thus, Chl-a concentrations becom e low in such high-energy zones.
Water eutrophic state probability map
CSTT[18] defined 10 mg/m^ o f Chl-a as the E nvironm ental Q uality Standard (EQS) for coastal waters. I f the C hl-a concenừation o f a w ater body frequently exceeds this criterion in summer, it is regarded as having a eutrophic condition. B ased on this criterion, the waters o f Tien Y en - H a Coi Bay w ere considered to be euưophic.
21.55"N
107.45-E
Cai Bau
108,00*E
P{u;\ồ)
■ ■ 0.97 - 1.00
■ ■ 0.94 - 0.97 m 0 .8 9 -0 .9 4 B | 0 .8 4 -0 .8 9 s 0.68-0.84
ggsi 0 . 56- 0.68
0.42-0.56 0.23 - 0.42 0.00-0.23
Figure 6. P robability map o f w ater euừophic state and potential in T ien Y en - Ha Coi Bay by IK.
Figure 6 shows probability o f w ater euừophic state and potential in Tien Y en - Ha Coi Bay by using IK and threshold o f 10 m g/m ^ A high probability o f euừophication (P(u;10) ranges from 0.97 to 1) w as determ ined in the bay w ater entire except for the zones in front o f Tien Yen Estuary or channel connected
the bay w ith outer sea w hich located between V an N uoc and Cai Chien islands. However, undesirable eutrophication disturbances, such as algae bloom s, generated in the w ater with concentration o f C hi-a over lOOmg/m^ [19]
have not been observed in Tien Yen - Ha Coi Bay. This bay m ay be eutrophic by natural
188 N .T .T . H a et al. / V N U jo u r n a l o f Science, Earth Sciences 2 8 (2 0 1 2 ) 181-189
processes rather than by anthropogenic causes such as waste water, thereby m aintaining a high ecological quality sim ilar to other coastal areas [20].
Conclusions
This study dem onsfrates usefulness o f two geostatistical m ethods, CK and IK, and auxiliary inform ation for coastal w ater euừophication assessm ent by considering the relationship betw een in situ data and M OD IS image reflectance, analyzing the excess o f Chl- a concentration against the environm ental standard. A case study for Tien Y en - Ha Coi Bay clarified that Chl-a concentrations in seawater exceeded high eufrophic level [17] and the bay can be m arked as natural eufrophic waters.
A nother superiority o f the com bination betw een geostaitistical m ethods and rem ote sensing data was to provide rapidly database for good understanding o f the spatial variation and distribution o f Chl-a concenừation.
Consequently, that is easy to delineate high and low risk areas o f eufrophic state and potential for regional w ater m anagem ent.
Acknowledgement
W e are grateful to Projects coded Q GTD 10.31 TRIG -A and 105.09.82.09 N A FO STED by the V ietnam ese N ational Scientific G rant for their support o f the fieldw ork and sam ple analysis. Thanks also to N A SA for providing the M ODIS data.
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