Light penetration in water is affected by plankton, algae, and dissolved and suspended matter. As a consequence, the composition of backscattered light from below the air-sea interface is determined by the nature of the constituents in the water column. In contrast to the absorption spectrum of chemically pure chlorophyll in solution, algae suspensions absorb and scatter light more uniformly throughout the visible part of the electromagnetic spectrum. Because of their spectral absorption and scattering properties plankton concentration can be estimated by measuring the spectral backscattered radiance over water. Experiments using this approach were performed in upwelling regions along the northwest coast of Africa

Curran, R. J.

Szekielda, K.

English

NASA Technical Reports Server

Paper M 15 
BIOMASS IN THE UPWELLING AREAS ALONG THE NORTHWEST COAST 
OF AFRICA AS VIEWED WITH ERTS-1 
Karl-Heinz Szekielda, University of Delaware, Robert J. Curran, Goddard Spaoe 
Flight Center 
Light penetration i n  water is affected by plankton, algae,  and 
dissolved and suspended matter. As a consequence, the composition of 
backscattered l i g h t  from below the air-sea in te r face  i s  determined 
by the nature of the consti tuents i n  the water column. I n  contrast  
t o  the  absorption spectrum of chemically pure chlorophyll i n  solut ion,  
algae suspensions absorb and scatter l i g h t  more uniformly throughout 
the v i s ib l e  pa r t  of the electromagnetic spectrum. 
spec t r a l  absorption and sca t te r ing  propert ies  plankton concentration 
can be estimated by measuring the spectral ,  backscattered radiance over 
water. 
regions along the North-West Coast of Africa. 
Because of t h e i r  
Our experiments using t h i s  approach were performed i n  upwelling 
The North-West Coast of Africa was chosen f o r  several reasons: 
1. From the oceanographic point of view knowledge of the NW 
Coast of Africa is very poor. 
We are collaborating w i t h  the CINECA program which is an 
in te rna t iona l  oceanographic cooperation providing access to  
multiship cruises  f o r  ground. t ruth.  
Climatological conditions favor space observations. 
2. 
3. 
1385 
Original photography may. be ~ u t x  
EFQQS Data Center - 
l D i i t  and Dakota 
Siwx Falls, $0 
.... l W A . &  ._ 
https://ntrs.nasa.gov/search.jsp?R=19730019622 2020-03-17T08:51:15+00:00Z
4. No r i v e r  discharge appears i n  the  upwelling area thus changes 
i n  sea color are produced by plankton organisms. 
A typ ica l  analysis of wind speed and direct ion f o r  the test si te 
These data were obtained during 1967 onboard the is shown i n  Fig. 1. 
H. M. S. "Hydra". 
The Alize,  blowing i n  a northeast d i rec t ion  along the coast of 
Spanish Sahara and Mauritania is predominant the whole year. 
consequence of t h i s  wind system, the Capary Current is present during 
A s  a 
much of the year i n  a southwest direction. Wind stress and the 
geostrophic sloping of the isotherms leads t o  upwelling i n  the near 
coastal  zone especial ly  a t  Cape S i m ,  Cape Ghir, Cape Juby, Cape Blanc, 
and Cape Verde. 
increased nut r ien t  transport  from the deeper layers  causes extremely 
high concentrations of chlorophyll. 
a r id ,  v i s i b i l i t y  through the atmosphere favor observation by satellite. 
Fig. 2 shows a composite of data fo r  the re f lec ted  so la r  energy between 
0.7-1-1 pm as col lected by channel 7 of the MSS. 
obtained during ERTS-1 overpass of the NW Coast of Africa between 32"N 
and 10"N. The area covered includes Morocco, Spanish Sahara, Mauritania, 
Senegal, Gambia, Portuguese Guinea, and Guinea. 
The euthrophication of the productive layer  by 
Since the adjacent reg$ons are 
The frames were 
The following color composites of channels 4, 5 ,  and 7 give evidence 
of the types of climatic conditions encountered i n  the tiest site. 
northern boundary of the test si te includes the region of the Anti A t l a s  
mountains i n  Morocco. 
The 
Figure 3 shows the coast  of Morocco south of the 
1386 
upwelling areas of Cape Sim and. Cape Ghir. 
gathered from the satell i tes Nimbus 2, 3, and 4 and our recent analysis  
of NOAA 2 data, t h i s  s l i d e  shows the northern l i m i t  of the cloud f r e e  
region of the test site. Par t  of the test si te includes a r i d  regions 
o r  desert .  Figure 4 shows p a r t  of Senegal south of Cape Blanc with 
the Island Tidra included. The area shown i n  t h i s  f igure i s  also pa r t  
of an ongoing program t o  study the chemical impact of eol ian dust from 
the Sahara on the nut r ien t  budget of the ocean. 
w a s  used t o  determine the geographic path of the transported sand. 
r e su l t s  of t h i s  pa r t  of the study w i l l  be discussed i n  
From s tudies  made using data 
The ERTS-1 imagery 
The 
thes i s  form. 
The only important r i v e r  discharge i n  the test s i te  is from the 
Gambia River which is shown i n  Figure 5. 
south i n  agreement with the movement of the Canary Current. 
ing of the r i v e r  mouth from north t o  south, in evidence by closer  analysis  
of the figure,  i s  probably a long term evdlutianary process reaul t ing 
from the Canary Current. 
The river discharge is  t o  the 
The s h i f t -  
The chlorophyll d i s t r ibu t ion  as obtained from di rec t  measurements 
aboard research vessels during August 1972 i s  shown i n  Fig. 6. 
w a s  invest igated by the Spanish R. V. "Cornide de Saveedra" i n  the North 
and the  Br i t i sh  R. V. "Discovery" i n  the South. 
chlorophyll i n  the outer  sec t ion  near the Canary Islands i s  representative 
of the open ocean with values around 0.5 pg-1 
concentrations were found i n  the upwelling area near Cape Juby with 
values above 8 pg.1 . Thus, the chlorophyll concentration f o r  t h i s  
This area 
The concentration of 
-1 . High chlorophyll 
-1 ' 
1387 
test site ranges over two orders of magnitude. 
maximum concentrations were found up t o  16 pg.1 
I n  plankton blooms 
-1 . Generally speaking, 
the  whole coastal  area shows eutrophication with chlorophyll concen- 
t r a t ions  above 0.5 pg.1-l which is s t i l l  one order of magnitude higher 
than i n  the open ocean. 
Maximum gradients of chlorophyll concentration were always found 
t o  be close t o  the  coast  with highest  concentration near Cape Juby. 
Fig. 7 shows the display of channel 4 from the MSS during the operation 
of "Cornide de Saveedra" i n  t h i s  area. 
increased plankton concentration as evidence 'by backscatterad l i g h t  i n  
The gray scale indicates  
the 0.5-0.6 pm region of the MSS. Channel 5, 6, and 7 showed ne i ther  
cloud contamination nor sun g l in t .  For more de t a i l s ,  p a r t  of the 
encircled area was investigated with gr id  p r i n t  maps from the d i g i t a l  
values found on computer compatible tapes. 
To derive the posi t ion of the coast l ine with the radiance measure- 
ments, we established the frequency histogram of a l l  data. 
p ic ture  consisted of 585 lines with 810 samples per  l ine with 47,380 
points. An average gray level w a s  1.14 m i l l i w a t t s / c m  /steradian. The 
frequency histogram f o r  channel 4 is shown i n  Fig. 8, where a l l  data 
were normalized t o  the maximum of 100. 
The t o t a l  
2 
The two maxima i n  the frequency d is t r ibu t ion  indicate  the d i f fe ren t  
gray levels f o r  water and the adjacent continent. 
1.17 t o  about 1.56 mw/cm2/ster. 
The high radiance of 
-1 (i.e., 60 t o  80 d i g i t a l  uni ts)  
1388 
corresponds t o  an albedo of from 0.254 t o  0.339 and shows the re f lec t ive  
properties of the  deser t  region. Gray leve ls  between 0.51 and 0.98 
mw/cm-2/ster.-1 (i.e., 26 t o  50 d i g i t a l  un i t s )  correspond t o  albedo 
levels of 0.110 t o  0.213 and can be assigned t o  the radiance of the 
ocean. 
0.98 mw/cm-2/ster.-1 (albedo equals 0.213) are caused by the coastal  
features  or  the continent. 
f o r  the posi t ion of the waterline to  be indicated by radiances less 
The frequency d is t r ibu t ion  shows tha t  gray levels greater  than 
Consequently we developed a threshold level 
than 0.98 m/cmm2/ster. -1 . 
Fig. 9 shows a computer pr intout  where characters were assigned 
t o  the radiance, 
of 1 2 ,  
positon of the coastl ine.  
caused by plankton. 
Current can be seen by the symbols 0 and 8. 
mw/cm-2/ster.'1 had no assigned symbol. 
northwest corner of the display indicated low radiance and consequently 
low concentration of plankton organisms. 
Only every 12th l ine was used with a sample spacing 
The gradient v i s i b l e  by the  assigned character 0 shows the 
%e region near Cape Juby shows high radiance 
The transportation of plankton within the Canary 
Radiance less than 0.61 
The offshore area i n  the 
Fig. 10 gives the analysis of the t o t a l  values as obtained from 
The i so l ines  of 
The posit ion 
the generated g r id  p r i n t  map over an area of 30 km. 
radiances were smoothed over about e ight  gr id  points. 
of the coast l ine was established by the maximum gradient a t  the in te r face  
between continent and ocean. 
three gr id  p r in t  points from the coast  which corresponds t o  a distance 
The nearest i so l ine  t o  the coast w a s  
1389 
of about 300 materr. 
of the bathymetry near the coastl ine.  
This should avoid the most important influence 
Oceanic regions with high productivity have a th in  eutrophic layer  
and consequently the photo-penetration depth is very shallow. 
with the Secchi d i sc  i n  the upwelling area showed only five meters. 
This can be explained by the l i g h t  attenuation of plants ,  
radiance can be found i n  the mar coastal  waters where upwelling i s  
expected t o  be s t rongest l  
plankton bloom are not pers is tant .  
location. 
Readings 
High 
Repeated coverage with ERTS showed t h a t  the 
They change t h e i r  size and t h e i r  
Continuous recordings of chlorophyll concentrations during the 
ground t ru th  program i n  the upwelling area along tha NW Coast of Africa 
showed similar pat terns  as derived with ERTS-1. 
The recordings p a r a l l e l  t o  the coast showed periodic var ia t ions 
i n  the chlorophyll concentration which showed the same spacing as the 
per iodici ty  observed with ERTS-1 data. In  other words, we detected 
the real d is t r ibu t ion  s t ruc ture  of plankton with ERTS-1 as i t  could 
never be investigated with the  conventional method o r  e 
cruises.  
We were asked t o  stress the economical point of view i n  our 
invhastigations. A very simple calculat ion will demonstrate the advan- 
tage of satellite information f o r  the research i n  upwelling regions. 
D r .  W. Fisher from the United S ta tes  Geological Survay estimated the 
1390 
cost  per km2 i n  the image t o  be less than 3 cents. 
would cost  about $324. 
t o t a l  amount f o r  the satel l i te  coverage t o  be $1,000. The information 
obtained with the sh ip  was  i n  the order of $30,000 o r  roughly 30 t i m e s  
more expensive. 
data  show the high poten t ia l  and the inexpensive acquis i t ion of inform- 
a t ion  obtained with the satellite i n  our s tudies .  It is desirable  
t h a t  NASA focus more on the ac t iv i ty  i n  spacecraft  oceanography. 
The analyzed frame 
Including computer t i m e  w e  can estimate the 
I think t h a t  t h i s  estimate and the presentation of 
1391 
1 
i 139' 
I I I I I I 1 
kMS Hydra 30 Oct-12 Nov 1967 
4 // 
++y R LOU 5 
/ 
ovckcholt U) 
Figure 1 
1392 
1393 
Figure 3 
1394 
1395 
1396 
- 27' 
- 26. 
- 25. 
1397 

.... 
F i w e  8 
1399 




Biomass in the upwelling areas along the northwest coast of Africa as viewed with ERTS-1

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server