BULLETIN OF MARINE SCIENCE. 29(3): 406-413. 1979
CORAL REEF PAPER
SMALL-SCALE SPATIAL VARIATION IN LIGHT
AVAILABLE TO CORAL REEF BENTHOS:
QUANTUM IRRADIANCE MEASUREMENTS
FROM A JAMAICAN REEF
Willem H. Brakel
ABSTRACT
Multidirectional quantum irradiance measurements performed at various localities on the
fringing reef at Discovery Bay. Jamaica, made it possible to characterize in detail the light
micro-environment of the benthos. Waters of the open fore reef and the sheltered bay had
significantly different light transmission properties, with average attenuation coefficients of
0.06 m-I and 0.] I m-', respectively, in the upper 27 m. However. the relative amount of
light incident from different directions was the same for both sites, and remained constant
in the depth range surveyed. Details of substrate type, slope, and exposure had a marked
influence on irradiance patterns at any locality, sometimes equivalent to large differences in
depth. Bottom reflectance averaged 18% for sandy substrates and 5% for surfaces of living
coral. Vertical surfaces received about 25% of the irradiance of horizontal surfaces. Canopies
of the branching coral Acropora cervicorni,\' diminished irradiance immediately below by
nearly half.
These results indicate that light distribution on coral reefs is very heterogeneous at the
microhabitat level. This spatial heterogeneity must be taken into account in studies of benthic
photosynthetic organisms.
A striking feature of the coral reef substratum is its topographic complexity.
The reef framework is built up irregularly by the action of calcifying organisms,
resulting in a complicated relief of ridges and valleys on the scale of centimeters
and decimeters. In this paper I examine the implications of small-scale substrate
heterogeneity in coral reefs for the availability of light to benthic photosynthetic
organisms by describing a set of detailed measurements of quantum irradiance
on a Jamaican reef.
Physical oceanographers have studied the subtleties of underwater light transmission with theoretical rigor and empirical precision (Duntley, 1963; Jerlov,
1966), but the tasks of translating this body of information into biologically meaningful and readily measurable terms, and the accumulation of data relevant to
bottom-dwelling plants and animals have only recently begun to receive attention.
Roos (1967) photometrically characterized the underwater radiance distribution
on a reef in Cura9ao, and related his results to changes in the growth form of the
hermatypic coral Porites astreoides. Jaubert (1971) presented a detailed mathematical treatment of light in the benthic environment, and subsequently made
continuous irradiance recordings on a Madagascar reef, demonstrating the effect
of topographic relief on the light microclimate (Jaubert and Vasseur, J 974). Weinberg (1976) recently summarized many concepts of underwater light transmission
relevant to marine ecologists and described a method for estimating the total
irradiance at any site starting from relatively few measurements taken with a
narrow-band Sensor.
Still lacking, however, is detailed information on the angular distribution of
photosynthetically-active
light available to benthic reef organisms. Such data,
which are necessary for comparisons of marine productivity and for investigations
of photosynthetic adaptations and calcification in corals and algae, are given in
406
BRAKEL:
LIGHT ON CORAL REEF
this paper. I also propose a simple, objective method for characterizing
relative scale the light microclimate at any underwater locality.
407
on a
METHODS
Investigations were conducted during July and August on the north coast of Jamaica at the Discovery Bay Marine Laboratory, as part of a broader study of coral ecology (Brakel, 1976). The
structure and zonation of north Jamaican reefs has been described by Goreau and Goreau (1973). I
measured available light over a wide range of localities on the fore reef and the sheltered bay behind
the reef crest using a Lambda Instruments LI92S underwater quantum sensor and L185 meter. The
sensor measures the quantum flux in the photosynthetically-active range of 400-700 nm, which is the
light measure preferred for biological studies (Tyler, 1973; Kirk, 1977). The sensor is cosine-corrected
to provide a measure of the irradiance in ILEinsteins m-2sec-1 passing through a plane. The meter
was mounted in an Ikelite 5910 instrument housing so that the entire apparatus could be operated
underwater by a SCUBA diver.
A light determination at any site on the reef involved a series of measurements made with the
sensor aimed at various angles from the vertical (0°, 45°, 90°, and 180°) in each of the four cardinal
directions. In each of these orientations the sensor was held 0.5 m above the substrate. In this manner
I was able to quantify the variation in sunlight impinging on surfaces facing in any direction. These
data thus constitute approximations of the three-dimensional underwater irradiance patterns on a
coral reef. Depth, time, substrate type, and cloud cover were also recorded. Surface measurements
were taken hourly on several days for comparison with the underwater values.
RESULTS
General Trends in the Data
Over 1,200 separate light measurements were made during the research period.
A computer print-out of the data set is available from the author on request.
Because of differences in time of day and weather most of the raw data are not
directly comparable before standardization. To illustrate important trends in the
data I therefore first present radial plots of a small number of selected light
determinations that, because of identical timing and meterological conditions, can
be so compared. Measurements taken in the north-south and east-west planes
are plotted separately. These measurements were taken at angular intervals of
45°, but I have drawn smooth lines connecting all points representing different
angles at a single locality, since experiments with gradual sweeps with the sensor
through all angles indicates that the irradiance values do in fact vary continuously
with changes in angle.
Figure I shows the quantum irradiance pattern above the water surface and at
-9 and -27 m, taken on several clear, calm mornings at about 9:45 h. I will use
these and subsequent plots to illustrate the major features of underwater irradiance distribution. These trends, which will be substantiated mathematically in a
later section, are summarized below.
The irradiance pattern at any given depth is a prolate spheroid. This indicates,
as is to be expected, that surfaces facing directly upward receive considerably
more light than slanted surfaces. A horizontal surface at - 27 m under the conditions of Figure I, for example, would receive about 154/oLE m-2sec-1, whereas
a vertical surface facing north would receive only 39 /oLE m-2sec-1, or 25% of the
horizontal value.
The plots are asymmetrical. These irregularities arise for several reasons: (a)
The morning sun's position produces a characteristic bulge toward the east; (b)
The overall morphology of the fringing reef extending in an east-west direction
causes surfaces on the fore reef facing northward (out to open sea) to receive
more light than comparable surfaces facing southward (in toward the bulk of the
408
BULLETIN
OF MARINE SCIENCE.
VOL. 29. NO.3,
1979
,
1
,0
,,
I
N
5
I
I
•
N
I
I
"
5
,- ".-
I
I
0/9
I
W
E'
W
Figure I. Logarithmic polar plot of angular irradiance distribution in the north-south and east-west
planes. Measurements were taken at about 9:45 h on clear mornings at -9 and -27 m. Axes are
marked ofT in logarithmic units, i.e., a point 4 units from the origin represents an irradiance value of
e" or 55 /LE m-2sec-'.
Figure 2. Same data as in Fig. I, standardized by setting irradiance on horizontal surfaces equal to
100'%and expressing all other measurements as a fraction thereof. Axes are marked ofTlogarithmically
at the 10% and 100'%values.
reef); (c) Topographic features within a few meters of the measurement location,
such as large coral heads, cause irregularities as well.
The irradiance pattern at -27 m is much smaller than the -9 m plot. This
dramatizes the attenuation of light with depth, which is due to a combination of
absorption and scattering. In this particular example the zenith irradiance at - 27
m is 31% of that at -9 m, and 8% of that above the surface of the water.
Despite large differences in total irradiance between the -9 m and -27 m sites,
the relative irradiance pattern remains the same. This is seen more clearly in
Figure 2. The same irradiance data are presented as in Figure 1, only here all the
irradiance values at one site are expressed as a fraction of the zero slope value
at that site. All three depths show the same general pattern, allowing for differences in bottom topography. Thus in the 0 to - 27 m depth range at Discovery
Bay the relative angular irradiance distribution appears to remain constant.
Bottom characteristics strongly influence the irradiance pattern. The irradiance
values in Figure 1 were taken 0.5 m above substrates covered primarily by living
coral. These same measurements were repeated moments later above an adjacent
sand plane, but under otherwise identical conditions. The original measurements
BRAKEL: LIGHT ON CORAL REEF
N
,
,i
,
,,
s
409
N
5
E
w
,
,
-
....•...•... .•..
-- ----'
,
W
I
/
,,
--
.•.
:
,/
--,,"
Figure 3. lrradiance patterns over coral substrates (solid lines) and over sand (broken lines) at -9
and -27 m. Axes as in Fig. \.
Figure 4. Midmorning irradiance data collected on comparable days at -15 m in the bay (B) and on
the fore reef (F). Axes as in Fig. \.
taken over coral are shown as solid lines in Figure 3. Superimposed with broken
lines are the over-sand readings. It is evident that the clean white carbonate sands
typical of reef environments can greatly alter the light microclimate. In this particular example, a downward-facing surface at -9 m above coral received considerably less light than a comparable surface at -27 m above sand. Hence a
change in substrate type more than cancels out an 18 m difference in depth, when
only total quantum irradiance and not spectral composition are taken into account.
The water behind the reef crest, in Discovery Bay proper, has strikingly different light transmission properties from that of the fore reef. In Figure 4 are
given irradiance plots from measurements made on comparable days at about
10:45 h and at a depth of -15 m. In the bay (B), zenith irradiance is only 44% of
that on the fore reef (F).
Quantification of Results
Attenuation of Light with Increasing Depth.-The
trends characterized in a qualitative manner using a few examples can be described with more precision. This
was done by statistical analyses of the entire data set. Calculations of the attenuation characteristics of the water were based on standardized zenith irradiance
(downward irradiance) data. To make all observations comparable I first deleted
from the data set all light measurements made while there was significant cloud
410
BULLETIN OF MARINE SCIENCE,
VOL. 29, NO, 3, 1979
cover. To correct for the time of day I assumed that the sun is displaced 15°from
the zenith for every hour before or after local noon, While the sun is less than
40° from the zenith (8 < 40°) reflection at the air-sea interface is minimal, both
theoretically and empirically (Dietrich, 1957: 63). More than 95% of the sun's
direct rays continue down into the water column, but at a smaller angle, cp,
defined by Snell's law as:
cp = sin-l(sin
811.333).
Observations taken with the sun more than 40° from the zenith were thus deleted.
The actual pathlength, d, taken by the sun's rays to an underwater site at depth,
z, is found by simple geometry to be:
d
= z/cos 8
Finally, to compensate for diurnally varying surface light intensities, all underwater irradiance values were expressed as a percentage of the surface reading
at the time of measurement. All percent of surface irradiance data were then
plotted logarithmically against the pathlength computed from the depth and solar
elevation (Fig. 5). All points from the fore reef fell along one line of constant
attenuation, and all data from inside the bay clustered around another line. The
regression equations for these two lines are:
In I
In I
=
=
4.41 - O.lId
4.20 - 0.06d
(bay)
(fore reef).
The absolute values of the slopes, 0.11 m-l and 0.06 m-l are the attenuation
coefficients for the two water masses. Jerlov (1977) has presented data showing
that the attenuation for quanta in the 350-700 nm range varies greatly with depth
in the upper 5 m of the water column, but with increasing depth the coefficient
rapidly approaches a constant, which is the attenuation coefficient for blue light.
The small standard errors (0.186 and 0.159, respectively) about the two regression
equations for the Discovery Bay data indicate that in practice a straight line
adequately describes quantum attenuation for most biological studies.
Analysis of variance was used to test for the significance of the difference
between the two attenuation lines. This was done by computing the F ratio of
residuals from all the data pooled over the residuals when bay and fore reef points
were regressed separately. F(2, 27) was 61.7, indicating that the difference was
significant (P < 0.01).
Effect of Surface SLope.-The above equations allow us to estimate the relative
irradiance on a horizontal surface at any depth. We can go further and specify
the angular distribution as well, if we accept the assumption that the shape of the
irradiance pattern remains constant with increasing depth even while the actual
intensities change. The results shown in Figure 2 and all the other data from
Discovery Bay indicate that this assumption holds for the upper 27 m of the water
column.
First, all angular measurements at any site were expressed as a percentage of
the irradiance on a horizontal surface at that site. The four observations (north,
east, south, and west) for each zenith angle were then averaged and plotted
against slope in Figure 6. Separate symbols were used for measurements over
coral and over sand because it was already known (Fig. 3) that substrate type
strongly affects the shape of the irradiance pattern. The relationship between
irradiance and substrate slope is not easily described by a simple regression
equation, but by drawing a smooth line between the points one can interpolate
l!ranhicallv for all slones.
BRAKEL:
411
LIGHT ON CORAL REEF
w
u
z
0
«
~
0::
0::
10
I
~
w
-l
I
100
CJJ,~
50
%~
:r:
I20
Z
20
W
N
~
a.:
~t~
10
LL
30
"BAY
0
"FORE-REEF
IZ
U
CD
10
20
50
100
PERCENT OF SURFACE IRRADIANCE
0::
w
a..
CD
:.&'-.0
o
0
0
°SAND
oCORAL
5
w
5
0
0
45
em
90
135
180
SLOPE OF SUBSTRATE
Figure 5. Downward irradiance (logarithmic scale) versus path length (distance, in meters, sun's
direct rays had to travel underwater). By plotting data this way, measurements taken at different
times on different days can be combined to characterize the optical properties of the water column.
Note nonlinear attenuation at depths less than 5 m.
Figure 6. Relation between the slope of a substrate and the quantum irradiance it receives. Irradiance
data were standardized by expressing them as percentages of the downward irradiance at the time
and place the observation was made. Each point is the mean of 4 measurements in the 4 cardinal
directions.
Effect of Substrate Type.-The effect of substrate type on the angular irradiance
distribution was demonstrated qualitatively in Figure 3. When all available data
were averaged, the mean bottom reflectance (ratio of incident to reflected irradiance) was found to be ]8.2% (SO = 7.9, N = 21) for sandy substrates and 5.0%
(SO = 1.3, N = 10) for living coral.
Effect of Exposure.-Another
source of variation in the availability of light at the
microhabitat level is the exposure of a site, i.e., whether it is covered or shaded
by overhanging coral, sponges, algae, or other sessile macrobenthos. A feature
of many Caribbean coral reefs are vast thickets of the slender branching staghorn
coral, Acropora cervicornis, which form a canopy under which many other organisms thrive. To see how this open branching coral growth affects the light
regime beneath it I measured downward irradiance just above such a thicket and
on the substrate immediately below it. One typical result indicated that, at -]5
m, Acropora reduced the light incident on a horizontal surface from 392 to 210
JLE m-2sec-1, or by nearly half.
Applications of the Light Model
The relationship between irradiance and the variables depth, water clarity,
bottom slope, substrate type, and exposure, which are summarized above, constitute a model for the availability of light to coral reef benthos. The model can
be used to predict the light micro-environment anywhere on a reef. The attenuation coefficients given above are only valid for Discovery Bay at the season
that this research was done; for other localities the attenuation properties of
the water column must first be determined. Attenuation for quanta typically varies
from 0.04 m-I to 0.16 m-I in the -5 to -30 m depth range for waters of optical
types I to III (Jerlov, 1977).
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BULLETIN
OF MARINE SCIENCE. VOL. 29. NO.3.
1979
To examine the importance of microhabitat setting to available light more
closely, it is instructive to turn the model around and ask: what changes in microhabitat are required to bring about, say, a doubling of irradiance? This was
done by trial-and-error insertion of hypothetical microhabitat parameters for substrate type, slope, and exposure into the model. The following microhabitat
changes would result in a doubling of incident light on an underwater surface:
(1) changing the slope from 60 to 0 (horizontal); (2) changing the surrounding
substrate from coral to sand (applies only to a surface oriented 115 from the
vertical and thus facing downward); and (3) changing the exposure of a site by
moving it out from under a canopy of staghorn coral.
The same doubling of light input could be realized by holding the microhabitat
constant, but by changing macroenvironmental factors: (1) moving a site at -18
m in the bay to an equivalent depth on the fore reef; (2) decreasing the depth
from -18 to -12 m (applies only in the bay); and (3) decreasing the depth from
-18 to -6 m (applies only on the fore reet).
This simple exercise demonstrates that subtle shifts in microhabitat may, in
0
0
0
their consequences for total quantum irradiance, be as important to reef benthos
as a move from one reef zone to another or as a depth change of many meters.
DISCUSSION
The quantum irradiance patterns shown in Figures 1-3 and the relationships
derived from the standarized data show that aspects of the microhabitat-e.g.,
slope, substrate type, exposure-greatly
influence the amount of light available
to benthic organisms. Moreover, the effects of microhabitat setting may easily
be large enough to overshadow irradiance differences due to differences in depth
or differences in the optical properties of the water column. These results imply
that a knowledge of the fine details of a photosynthetic organism's micro-environment are crucial to an understanding of its biology and ecology.
In this paper I have only begun to describe the complexities of light distribution
on reefs. No mention has been made, for instance, of changes in spectral composition, the possible heterogeneity of the water column, or seasonal changes at
the study area. Yet even the simple model of light availability I have proposed
may help to explain some features of benthic ecology. I have found, for instance,
that the seemingly haphazard distribution of colony shape types of the coral
Porites at Discovery Bay begins to fall into a light-related pattern once the microhabitat is taken into account (Brakel, 1976).
Just as light distribution is strongly influenced by microtopography, it is likely
that other physical factors of the reef environment-wave
energy, currents, sedimentation-also
vary at the micro-environmental level. The benthic reef environment should thus be viewed as a mosaic of microhabitats. Furthermore, these
microhabitats may be ephemeral. If small-scale factors such as slope, exposure,
and substrate type determine the nature of a microhabitat patch, then small-scale
disturbances may alter the habitat suddenly and drastically. These results add to
the growing evidence (Connell, 1978) that, to sessile organisms, a coral reef is
spatially and temporally a heterogeneous environment.
ACKNOWLEDGMENTS
I thank D. Wethey for technical advice, and J. Ramus, G. Rosenberg, and V. TunniclitTe for
comments on the manuscript. Financial support and equipment were provided by the Biology Department, Yale University, and the Discovery Bay Marine Laboratory. This research is part of a
dissertation submitted to the Graduate School of Yale University in candidacy for the degree of
Doctor of Philosophy, and is contribution no. 174 from the Discovery Bay Marine Laboratory.
BRAKEL:LIGHTONCORALREEF
LITERATURE
413
CITED
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corallite structure of Porites on a Jamaican reef. Ph.D. diss., Yale Univ. 256 pp.
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199: 1302-1310.
Dietrich, G. 1957. Allgemeine Meereskunde: eine Einfiihrung in die Ozeanographie. Gebriider Borntraeger, Berlin. 492 pp.
Duntley, S. Q. 1963. Light in the sea. J. Opt. Soc. Amer. 53: 214-233.
Goreau, T. F., and N. I. Goreau. 1973. The ecology of Jamaican coral reefs. II. Geomorphology,
zonation, and sedimentary phases. Bull. Mar. Sci. 23: 399-464.
Jaubert, J. 1971. Etude et mesure d'un facteur ecologique: I'eclairement; Realisation d'un appareil
enregistreur. Tethys 3: 205-246.
---,
and P. Vasseur. 1974. Light measurements: duration aspect and the distribution of benthic
organisms in an Indian Ocean coral reef (Tulear, Madagascar). Proc. Sec. Int. Coral Reef Symp.
2: 127-142.
Jerlov, N. G. 1966. Optical oceanography. Elsevier, Amsterdam. 194 pp.
---.
1977. Classification of sea water in terms of quanta irradiance. J. Cons. Int. Explor. Mer 37:
281-287.
Kirk, J. T. O. 1977. Use of a quanta meter to measure attenuation and underwater reflectance of
photosynthetically active radiation in some inland and coastal south-eastern Australian waters.
Aust. J. Mar. Freshw. Res. 28: 9-21.
Roos, P. J. 1967. Growth and occurrence of the reef coral Porites astreoides Lamarck in relation
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72 pp.
Tyler, J. E. 1973. Lux vs. quanta. Limnol. Oceanogr. 18: 810.
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DATE ACCEPTED: September 13, 1978.
ADDRESS: Department of Zoology, University of Nairobi, P. O. Box 30197, Nairobi, Kenya.