Download LIGHT FIELD FLUCTUATIONS IN THE PHOTIC ZONE

NOTES AND COMMENT
tique
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Pelagos, 2: l-32.
1967. Research on phytoplankton
and
pelagic protozoa in the Mediterranean
Sea
from 1953 to 1966. Oceanog. hlarine Biol.
Ann. Rev., 5: 205-229.
AND J. LECAL.
1960. Plancton unicelluiaire
recolte dans l’ocean Indien
par le
Charcot
( 1950)
et le Norsel
( 1955-56).
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EPPLEY, R. W., R. W. HOLSIES, ASD J. D. H.
STRICKLAND. 1967. Sinking rates of marine
phytoplankton
measured with a fluorometer.
J. Exptl. Marine Biol. Ecol., 1: 191-208.
FOURNIER, R. 0.
1966. North Atlantic
deepsea fertility.
Science, 153 : 1250-1252.
KRUMBEIN, W. C., AND F. J. PETTIJOHN.
1938.
Manual of sedimentary
petrography.
Appleton-century-crofts,
New York. 549 p.
MCNOWN, J. S., AND J. MALAIKA.
1950. Effects of particle shape on settling velocity at
low Reynolds
numbers.
Trans. Am. Geophys. Union, 31: 74-82.
MENZEL, D. W.
1967. Particulate
organic carbon in the deep sea. Deep-Sea Res., 14:
229-238.
-,
AND R. F. VACCARO. 1964. The mea-.
697
surement of dissolved
organic and particulate carbon in seawater.
Limnol. Oceanog.,
9 : 138-142.
MULLIN, M. M., P. R. SLOAS, AND R. W. EPPLEY.
1966.
Relationship
between
carbon
content, cell volume, and area in phytoplankton.
Limnol. Oceanog., 11: 307-311.
PARKE, hI. 1961. Some remarks concerning the
Brit. Phycol. Bull., 2:
class Chrysophyceae.
47-55.
PARSONS, T. R. 1963. Suspended organic matter in sea water, p. 203-239.
In hi. Sears
[ed.], Progress in oceanography,
v. 1. Pergamon, New York.
REDFIELD, A. C., B. H. KETCHU~I, AND F. A.
RICHARDS.
1963.
The influence
of organisms on the composition of sea water, p, 27In hl. N. Hill [ed.], The sea, v. 2.
77.
Interscience,
New York.
RILEY, G. A., D. I?AN H~ERT,
AND P. J.
WANGERSKY. 1965. Organic
aggregates
in
surface and deep waters of the Sargasso Sea.
Limnol. Oceanog., 10: 354-363.
S~ERDRUP, H. U., hl. W. JOHNSOS, ASD R. H.
FLEI~ISG.
1942.
The oceans.
PrenticeHall, Englewood
Cliffs, N. J. 1087 p.
LIGHT FIELD FLUCTUATIONS IN THE PHOTIC ZONES
It is known that the underwater light field
upper layers of the photic zone. Here we
surrounding marine organisms strongly in- wish to discuss these latter fluctuations
fluences their growth rate and other as- under clear sky conditions and to suggest
pects of their life (Yentsch 1963, 1965).
their possible physical and biological sigThis has led to many studies of the undernificance.
water light field
(Preisendorfer
We wish to thank Prof. A. Ivanoff for
1961;
Ivanoff, Jerlov, and Waterman 1961; Jerlov
offering several valuable suggestions,
1964; Tyler and Smith 1967). One propEXPERIMENTAL
MEASUREMENTS
erty of the light field that is seldom
AND RESULTS
analyzed is the large, rapid, temporal
Measurements of the light field fluctufluctuations in underwater irradiance due
ations were carried out using an underto atmospheric and sea surface effects
(Schenck 1957; Dera and Olszewski 1967). water irradiance meter with a selenium
Atmospheric effects, as when clouds ob- photocell, a 525 rnp filter having a 50 rnp
struct the direct rays of the sun, can cause passband, and a fast recorder. The meter
was positioned from a fixed post, and
a decrease in the light intensity incident
the instantaneous signal was recorded for
on the sea surface by as much as a factor
about 10 min. The depth of the meter was
of 10. These atmospherically
induced
changes are of low frequencies and are then changed and the readings repeated.
transmitted to great depths in the sea. From these records the magnitude of the
AE (2)
about the
Fluctuations
caused by the refraction of irradiance fluctuations
light from the complex wave structure of mean value of irradiance E (2) at a depth
the surface are characterized by higher
2 was determined.
The measurements
frequencies and are confined mostly to the were carried out in the coastal waters of
South Florida and on the shallow Bahama
l Contribution
No. 981 from the Institute
of
Banks where fixed posts were available.
Marine
Sciences,
University
of Miami.
This
Thus, the results are for relatively turbid
work was supported
by the National
Science
water and small waves. The results of
Foundation.
698
SOTES
FRACTIONAL
10
1LII
20
AE(Z,/s(Z,
FLUCTUATION
30
40
50
60
i
ASD
CO?\fhfEXT
(%)
70
6
FRACTIONAL
60
1
FLUCTUATION
AS(Z)/i%Z)
(%)
90
I
4
1
FIG.
7.
8FIG. 1. Depth variation
tuations
in irradiance
for
water clarity.
2.
clownwelling
0.59 111-l.
of the fractional
flucseveral couditions
of
such measurements for se\-era1 values of
water clarity as reflected by the attenuation coefficient of downwelling
irradiance
& ( Preiscndorfer 1961) are shown in Fig.
1, where the relative maximum amplitude
of the irradiance fluctuations S(Z)
E(Z)
are plotted as a function of depth. The
most outstanding features of these curves
are the large maximum
occurring
at
depths between the surface and about 2
m and the decay with depth below this
maximum. The decay is nearly esponential
in many cases. The position and magnitude of the maximum are strongly dependent on the clarity of the water, and from
the general structure of the cur\‘es we
would expect that in clear ocean water the
maximum would be broader and occur at
a greater depth. It must be noted here
that, because of their comples structure,
the actual magnitudes of the obscr\red
fluctuations \vill be dependent on the detector. This is understandable,
since the
diffuse screen of the irradiance meter is
cssentiall>- a spatial integrator; that is. if
one cli\-ides the detector area into say 10
smaller areas and increases the intensit!
of light incident on one of these areas b>
a factor of 10, the detector OLlti>Llt
w-ill
only increase by a factor of about t\f’o.
masking the actual intensit!,
increase.
Thus, the actual \~alues of AE (2) E( 2 )
Comparison
irradiance
between
upwelling
and
fluctuations
for Ka =
seen by microscopic marine organisms will
be much larger than that measured by
our receiver which has a surface area of
12 cmz.
The downwelling
and upwelling fluctuations measured at the same time are
compared in Fig. 2. The absence of the
pronounced maximum in the upwelling
case and the equality of the relative fluctuations in the two cases after several
optical depths are notejvorthy.
The difference between the upwelling
and downnrelling curlyes may be an indication of
the semidiffuse nature of the light field,
and the equality of the results after se\-era1 optical depths suggests that the onset
of the asymptotic light field (Preisendorfer
1961) may take place in this region. We
\Terified that the light field was in fact
nearly asymptotic at this depth by obser\Ting that little change occurred in the output of the irradiance meter when held in
a \rertical positioir and rotated through an
azimuth angle of 360”, indicating the radiance distribution was symmetric about the
\-ertical ( Lcnoble 1961) .
INFLUESCE
LIGHT
OF
FII:LD
WAVES
OS
THE
FLUCTUATIOSS
hlcasurements
of the waste structure
\vere not carried out during the present
experiments, so onlv a general discussion
of their influence can be given. \F7e will
consider a simple one-dimensional
\va\-e
surface with a \va\-e form gil-cn by
SOTES
2GT
!J = 210cos 7 x,
ASD
(1)
where L is the length of the wave and v(,
the amplitude. Now the parallel light rays
from the sun incident on this wave surface
will be refracted, and the crest of the
wave will act like a conv7erging lens and
focus the incident light somewhere belovsr
the surface. For this simple wave, one
would then at a point near the focal plane
of the “lens” observe bright flashes of light
with a frequency V = w/L, vvhere ti is the
velocity with which the wave moves across
the surface. In the first approximation, we
can calculate the focal length of such a
lens by considering the curv7ature of the
crest of the wave and using geometrical
optics. The result, assuming the refractive
index of the water to be 1.33, is
f = L”/y,n-‘.
Since the area near the crest of the wav’e
is mainly responsible for the actual focusing, one would expect this equation to be
reasonably accurate for a simple wav’e.
For the experiments
conducted,
the
waves did not have the simple structure
of equation (1); however, they were simple
enough to define an average wave length
of about 1 m and an average amplitude
of about 5 cm. Using these parameters
with equation (2), one calculates a focal
length of 2 m. It is gratifying to note that
the experimental
maximum
of relative
fluctuation for these waves is always above
this focus, as it must be due to the attenuation (in this case of beam transmittance)
of the light as it passes through the water.
DISCUSSIOS
Generally the light field fluctuations
and particularly these short period fluctuations in the upper layers of the photic
zone should have some effect on marine
organisms, especially when one considers
the large magnitude of the instantaneous
radiant energy possible for a short time at
a given point. This may influence the rate
of primary production, or it may cause a
migration toward or away from regions of
large relative fluctuations or regions of a
specific temporal spectrum of fluctuations.
699
COMMEST
These fluctuations
should be taken into
consideration in studies of primary7 production in the sea, since in the open ocean
they would penetrate more deeply into the
also may
photic zone. Such fluctuations
be useful in determining wav.e spectra for
v.ery small waves.
JERZY
HOWARD
Institute
of Marine
DERA?
R.
GORDOS
Sciences
fllld
Optical P72ysics Lahoratoq,
C7nizjersity of Mianbi,
.lliami, Florida
33149.
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On the
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beam transmittance
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N. G.
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PREI~ESDORFER,
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SCHESCK,
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