Water Quality Study of the Baltic Sea by Optical Remote Sensing Methods

523

WATER QUALITY STUDY OF THE BALTIC SEA BY OPTICAL REMOTE SENSING METHODS

J. Lokk, A. Purga I n s t i t u t e o f Thermophysics and E l e c t r o p h y s i c s Academy o f Sciences o f t h e Estonian S.S.R. INTRODUCTION O p t i c a l remote sensing methods enable us t o save t i m e i n studying t h e spatial

and temporal

v a r i a b i l i t y o f oceanic p r o p e r t i e s ,

and t o c o l l e c t

simultaneous data f o r l a r g e area. I n t h i s paper, we discuss t h e p o s s i b i l i t y o f u s i n g measurements o f t h e upward s p e c t r a l radiance t o study the d i s t r i b u t i o n o f suspended and d i s solved m a t t e r i n t h e sea. t h e research vessel R/V

The experiments were performed d u r i n g c r u i s e s o f "Ayu-Dag"

i n t h e B a l t i c Proper, and from aboard a

h e l i c o p t e r i n c o a s t a l areas. DISCUSSION

The study o f t h e s p a t i a l d i s t r i b u t i o n s o f v a r i o u s c o n s t i t u e n t s i n t h e sea by remote sensing methods i s somewhat l i m i t e d by t h e f a c t t h a t we can o n l y measure d i r e c t l y t h e o p t i c a l l y a c t i v e m a t t e r i n t h e water, phytoplankton pigments,

suspended matter, and y e l l o w substance.

such as The quan-

t i t y o f o t h e r substances can o n l y be estimated i n an i n d i r e c t way, u s i n g

known r e l a t i o n s h i p s between these substances and t h e o p t i c a l l y a c t i v e matter.

Many authors simply use t h e c o r r e l a t i o n s between t h e c o n c e n t r a t i o n o f

a given substance and sea b r i g h t n e s s i n one o r two s p e c t r a l bands.

This

method can o n l y be used under c e r t a i n c o n d i t i o n s , f o r we know t h a t b r i g h t ness i s n o t o n l y a f u n c t i o n o f water q u a l i t y b u t i t i s a l s o s t r o n g l y corr e l a t e d w i t h t h e downward s p e c t r a l i r r a d i a n c e . radiance v a r i e s g r e a t l y i n t h e B a l t i c area.

The downward s p e c t r a l i r For a d i r e c t study o f t h e

c h a r a c t e r i s t i c s o f water masses, we use t h e s p e c t r a l radiance index, p ( A ) , d e f i n e d by t h e expression:

Br(A)

= Bo(A) where B r ( A )

i s t h e sea radiance toward t h e n a d i r p o i n t and BO(A)

i s the

d i f f u s e radiance. When measurements a r e made t o study suspended and d i s s o l v e d matter i n t h e sea, r e f l e c t i o n s from t h e sea surface contaminate t h e data.

However, i n

s t u d i e s o f t h e c o n d i t i o n s o f t h e sea surface ( o i l s l i c k s , waves, e t c . ) ,

the

524

sun g l i t t e r s c o n s t i t u t e t h e main s i g n a l .

Experiments show t h a t , on a cloud-

l e s s day and w i t h a h i g h sun, about 40% o f t h e sea s u r f a c e i s covered w i t h sun g l i t t e r s i n t h e B a l t i c Sea.

I n order t o o b t a i n r e l i a b l e i n f o r m a t i o n on

subsurface l a y e r s i n t h e presence o f sun g l i t t e r s , we have t o make measurements when t h e h e i g h t o f t h e sun i s l e s s t h a n 50" o r t o i n c l i n e t h e r a d i o meter a t some angle from t h e n a d i r p o i n t i n t h e d i r e c t i o n f a c i n g t h e sun. Assuming t h a t a l l r a d i a t i o n r e g i s t e r e d i n near i n f r a r e d i s r e f l e c t e d o n l y from t h e water surface, we can then estimate c o r r e c t i o n s f o r r e f l e c t e d l i g h t i n t h e o t h e r s p e c t r a l bands. An a l t e r n a t i v e method i s t o use, lowing

expression

(Lokk

and

as a f i r s t approximation, t h e f o l -

Pelevin,

1978;

Pelevin,

Pelevina:'

and

Kel b a l ikhanov, 1979):

where R(A)

denotes t h e d i f f u s e s p e c t r a l radiance index, B&(A)

the zenith

p o i n t s p e c t r a l radiance and 0.02 i s t h e value o f t h e Fresnel c o e f f i c i e n t f o r o r t h o g o n a l l y f a l l i n g l i g h t beam radiance.

The u n d e r l y i n g assumption i s t h a t

t h e z e n i t h p o i n t and a 20° area around i t have equal b r i g h t n e s s .

We w i l l

have t h e b e s t r e s u l t s i f t h e s u r f a c e i s smooth. The two beam approximation discussed by Morel and' P r i e u r (1977) desc r i b e s t h e d i f f u s e s p e c t r a l radiance index by t h e expression:

where k i s a nondimensional c o e f f i c i e n t , p(A) t h e backward s c a t t e r i n g coe f f i c i e n t and K(A) t h e l i g h t a b s o r p t i o n c o e f f i c i e n t (absorbance). The absorbance can be c a l c u l a t e d i n t h e f o l l o w i n g way

where K ~ ( A ) denotes t h e absorbance by pure water,

a),

K

P

(A) t h e absorbance caused

(A) t h e absorbance caused Y by d i s s o l v e d o r g a n i c m a t t e r ( y e l l o w substance) and K~ t h e absorbance caused by phytoplankton pigments (mainly c h l o r o p h y l l

K

by n o n s e l e c t i v e p a r t i c l e s o r "grey" suspended m a t t e r .

Using ( 4 ) ,

equation

( 3 ) can be w r i t t e n as R(A) = k

p(A)

p(A) + KW(A)

+ K

P

(A) + K (A) + Y

KM

'

(5)

525 For water, t h e f u n c t i o n p ( A ) v a r i e s slowly.

Q u a l i t a t i v e spectral distribu-

t i o n curves o f o t h e r q u a n t i t i e s are known.

The i n f l u e n c e o f t h e d i f f e r e n t

components on t h e s p e c t r a l curve R(A)

v a r i e s w i t h t h e wavelength.

The

dominant f a c t o r s i n v a r i o u s s p e c t r a l bands can be categorized as f o l l o w s :

550-600 nm

KM' M'

500-550 and 600-680 nm 400-500 nm 350-400 nm

K,, KM,

KM,

p,, B,, B,,

(p,

denotes

the

backward

scattering

from suspended m a t t e r ) K~(A)

K~(A), K~(A)

~ ~ 0 ~1 ~, 0 B(A) 1 , .

I n t h e case o f c l e a r oceanic water, d e r e t h e i n f l u e n c e o f some components i s small, results.

t h e system o f equations based on (5) gives s a t i s f a c t o r y

However, when t h e water c o n s i s t s o f a complicated m i x t u r e o f

o p t i c a l l y a c t i v e m a t t e r ( l i k e t h e B a l t i c Sea and t h e e s t u a r i e s o f l a r g e rivers),

t h e r e s u l t s are found wanting.

Accuracy i n such c o n d i t i o n s i s

determined by t h e s i m p l i f i c a t i o n s t h a t have been made on a case by case basis.

Choosing these s i m p l i f i c a t i o n s g i v e s us t h e p o s s i b i l i t y o f f i n d i n g

more s e n s i t i v e s p e c t r a l ranges and r e l a t i o n s f o r c a l c u l a t i n g t h e q u a n t i t a t i v e d i s t r i b u t i o n o f some substances.

As an example, F i g u r e 1 shows a map

based on measurements f r o m t h e h e l i c o p t e r a f t e r a s t r o n g storm i n t h e G u l f o f Riga (Pelevin, Gruzevich and Lokk, 1980).

The map shows t h e d i s t r i b u t i o n

( i n r e l a t i v e u n i t s ) o f y e l l o w substance i n t h e sea.

The r a t i o p369/p560

used t o d e s c r i b e t h e y e l l o w substance c o n t e n t o f t h e water.

is

The measure-

ments were confirmed by a n a l y z i n g water samples c o l l e c t e d a t various s i t e s from t h e h e l i c o p t e r f o r c a l i b r a t i o n and,determination o f t h e o p t i c a l charact e r i s t i c s o f t h e main o p t i c a l l y a c t i v e substances i n l a b o r a t o r y . A more p r e c i s e method i s one which u t i l i z e s t h e whole spectrum o f l i g h t backscattered from t h e sea.

Knowing t h e o p t i c a l c h a r a c t e r i s t i c s o f t h e most

i m p o r t a n t substances o b t a i n e d i n l a b o r a t o r y experiments f o r the study area, we can e s t i m a t e t h e u n i v e r s a l s p e c t r a l curves f o r d i f f e r e n t concentrations, g i v e n by

where

PA) = p'(A)

+ K

and where we denote by

P'

526

r

Fig. 1.

O i s t r i b u t i o n o f t h e r a t i o p369/p560 f o r t h e Riga G u l f area on t h e

b a s i s o f t h e d a t a o f 18 and 19 September 1977. d e f i n e d as follows

The numerical s c a l e i s

521 K

t h e n o n s e l e c t i v e absorbance by p a r t i c l e s ,

P

p'(A)

t h e backward s c a t t e r i n g c o e f f i c i e n t f o r w a t e r w i t h suspended matter,

K

t h e p u r e w a t e r absorbance,

W

t h e r e l a t i v e absorbances b y c h l o r o p h y l l and y e l l o w substance,

KP' KY c,

r e s p e c t i v e l y , and by

s

the

concentrations

of

chlorophyll

and

yellow

substance.

The c o n c e n t r a t i o n s o f o p t i c a l l y a c t i v e substances can t h e n be o b t a i n e d by comparing t h e measured s p e c t r a l d i s t r i b u t i o n curves w i t h t h e c a l c u l a t e d curves.

The "measured"

concentrations a r e , t h e s e t o f values f o r which t h e

c a l c u l a t e d spectrum i s most s i m i l a r t o the' observed s p e c t r a l c u r v e .

Figure

2 shows a c t u a l measured s p e c t r a l c u r v e s and F i g u r e s 3, 4 and 5 show v a r i o u s e s t i m a t e d model curves. Finally, the

i t may be t h a t measurements o f c h l o r o p h y l l c o n c e n t r a t i o n by

UNESCO method and b y t h e remote method a r e i n h e r e n t l y d i f f e r e n t .

I n one

case t h e a n a l y s i s a p p l i e s t o samples from d i s c r e t e depths which may a l l be outside

t h e maximum c o n c e n t r a t i o n

layers,

whereas i n t h e o t h e r case t h e

v a r i a b l e we measure accounts f o r a l l t h e c h l o r o p h y l l i n t h e a c t i v e l a y e r (really

up t o Secchy d i s c

v i s i b i l i t y depth) w i t h d i f f e r e n t i n f l u e n c e a t

d i f f e r e n t depths. CONCLUSIONS

We have shown t h a t i n f o r m a t i o n a b o u t t h e f u l l

spectrum o f u p w e l l i n g

l i g h t i s needed f o r w a t e r q u a l i t y s t u d i e s b y o p t i c a l remote s e n s i n g methods.

In w a t e r s w i t h a c o m p l i c a t e d c o m p o s i t i o n of o p t i c a l l y a c t i v e m a t t e r (e.g. estuaries,

c l o s e d seas) i n f o r m a t i o n i s needed a b o u t t h e o p t i c a l p r o p e r t i e s

o f t h e main components p r e s e n t i n t h e a r e a ( c h l o r o p h y l l , y e l l o w substance, etc.).

528

15-

10-

05 -

I

F i g . 2.

I

400

500

I

600

nm

Measured spectral radiance.

I

1.0

I L 400

Orno

I.

500

600

nm

3. Computed spectral radiance w i t h a ) c=O.1, y=O.O, 8=0.0020; b) c=O.Ol, ~ 1 . 0 , B=O.O018; C ) ~ ~ 2 . 0~ , 0 . 0 , B=O.O020; d) ~ ~ 2 . 0yzl.0, , B=O. 0020. Fig.

529

I

c= 20 p=(01+ 1.0 1

1.0- 10

- a

051

'0.1 500

400 Fig. 4.

600

700 nm

Dependence o f t h e s p e c t r a l radiance, p(A),

f i x e d c h l o r o p h y l l c o n c e n t r a t i o n (c=Zmg.m

-3

on t h e parameter p f o r

).

-

p = 0.1 m- 1

C =(Q5+ 5.0)

.

0.5

1.0 .

0.5 .

5.0

400 Fig.

5.

500

600

Dependence o f t h e s p e c t r a l

concentration f o r

p =

0 . 1 rn

-1.

700 nm

radiance,

p(A),

on t h e c h l o r o p h y l l

530 REFERENCES Eerme, K. and J . Lokk, 1980. On t h e B a l t i c Sea water b r i g h t n e s s and c o l o u r measurements by t h e research vessel "Ayu-Dag" i n August 1977. Proc. o f t h e 1 1 t h Conf. o f B a l t i c Oceanographers, V o l . 2, Rostock. Lokk, J. and V. P e l e v i n , 1977. The i n t e r p r e t a t i o n o f t h e spectrum o f t h e u p w e l l i n g r a d i a t i o n based on the B a l t i c Sea. Proc. o f t h e 1 1 t h Conf. o f B a l t i c Oceanographers, Vol. 2, Rostock. Morel, A. and P r i e u r , L., 1977. Analysis o f v a r i a t i o n s i n ocean c o l o r . Limnol. Oceanogr. , 22: 709-722. Pelevin, V.N., M.A. Pelevina and B.F. Kelbalikhanov, 1979. Upwelling spectrum s t u d i e s from aboard a h e l i c o p t e r ( i n Russian) Opticheskie metody i z u c h e n i j a okeanov i v n u t r e n n i h vodojemov, Novosibirsk. Pelevin, V . , A. Gruzevich and J. Lokk, 1980. On t h e p o s s i b i l i t y o f e v a l u a t i n g t h e d i s t r i b u t i o n o f y e l l o w substance i n t h e sea water by t h e outcoming r a d i a t i o n spectra ( i n Russian), Svetovye p o l j a v okeane., Mos kva. Schmidt, D. and K.A. U l b r i c h t , 1978. Mass occurrence o f blue-green algae i n t h e Western B a l t i c e v a l u a t i o n o f s a t e l l i t e imagery and i m p l i c a t i o n s on marine chemistry and p o l l u t i o n . Proc. o f t h e 1 1 t h Conf. o f B a l t i c Oceanographers, Vol. 1, Rostock.