This study measures the concentrations of phytoplankton chlorophyll and yellow substance in the coastal waters of the Gulf of Maine. Sea surface observations attempt to delineate the principal biochemical parameters responsible for sea surface color. It is shown that the reddish-brown water changed to a blue-green in the open gulf

Owen, W. P.

Yentsch, C. S.

English

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f0 IL S'
UNIVERSITY OF MASSACHUSETTS
MARINE STATION
(NASA-CR-139145) OCEAN COLOR, A THREE N75-1141
COMPONENT SYSTEM? (Massachusetts Univ.,
Gloucester.) 18 p HC $3.25 CSCL 08J
Unclas
G3/43 02581.
7
" 1863"
OCEAN COLOR -
A THREE COMPONENT SYSTEM?
Research report
N.A.S.A. grant #NAS-5-21813
Hodgkins Cove -
Box 128 - Lanesville Station
Gloucester, Massachusetts 01930
(617) 281-1930
https://ntrs.nasa.gov/search.jsp?R=19750003339 2020-03-23T03:03:11+00:00Z
OCEAN COLOR, A THREE COMPONENT SYSTEM ?
By
Charles S. Yentsch
and
William P. Owen
University of Massachusetts Marine Station
Box 128, Lanesville Station
Gloucester, Massachusetts 01930
INTRODUCTION
Most of the light energy entering the sea is absorbed by
the water or-subst-ances--in-the water, particulate and dissolved.
PhytoplAnkton are the most obvious particulate absorber while
the yellow organics (Gelbstoff) is the dominate dissolved mater-
ial. By knowing the general features of light transmission or
absorption of these substances, one can empirically model ocean
color, that is, predict color change from a change in concentration
of either absorbing substance ( Yentsch, 1960). When both sub-
stances are present in natural waters, which is frequently the
case, then the prediction of the causes of color change is
complicated because both substances absorb short wave lengths
in a similar manner.
Generally speaking, the biochemical relationships between
phytoplankton and yellow substances is poorly known and hence
the ability to predict the distribution vis-a-vis of either
substance throughout the oceans is hampered. There have been
numerous physical-chemical relationships suggested between
phytoplankton and yellow substances. But since much of the
yellow substance is associated with fresh water (river inflow)
which carries a wide variety of other compounds that can
influence phytoplankton growth, these suggestions are difficult
to test.
In this study we have measured the concentrations of
phytoplankton chlorophyll and yellow substance in the coastal
2waters of the Gulf of Maine. The questions we are trying to
resolve concern:
1) Relationships of these substances, that is, how do
concentrations of either substance vary with respect to one
another? Is there a positive, negative, or no correlation
between the two substances?
2) Is the relationship between absorbing characteristics of
particulate matter and spectra of backscattered light compatible?
That is, can one explain the spectral.characteristics of back-
scattered light by the absorbing characteristics of particulates?
Since the study represents a cooperative effort with
scientists at Goddard interested in the high altitude detec-
tion of water zolor, sea surface observations were laid on
to delineate areas of high and low phytoplankton productivity
with emphasis on the seaward extent of areas of high produc-
tivity.
SEA SURFACE OBSERVATIONS
Dr. Hovis, using a variety of remote sensing equipment,
overflew the area on July 7 and 8, 1972. We had planned
the overfly tb directly coincide with the sea-surface obser-
vations made from the research vessel R.V. Bigelow, operating
out of Cape Ann. Realistically, this was not always possible;
communication with the N.A.S.A. aircraft was sporadic and
some areas had low level cloud cover.
In this report, we attempt to delineate the principal
biochemical parameters responsible for sea surface color.
Some of these observations such as phytoplankton chlorophyll
were made continuously while underway aboard the Bigelow.
Other measurements, such as dissolved yellow substances
and the spectra of upwelled light were made at selected
stations. The choice of the location for these measure-
ments was dependent upon previous sampling by our research
group in the area.
3.
The data was obtained along three legs of stations
(Fig. 1). Leg 1 extends from the mouth of the Merrimack
River to the sea. Leg 2 extends from the seaward extent
of Leg 1 to the tip of Cape Cod. Leg 3 covers the region
between Cape Cod-Bd ad--G-Iicester.
OBSERVATIONAL TECHNIQUES
1) Phytoplankton chlorophyll was measured continuously
by pumping surface water through the Turner model 111
fluorometer. Excitation wave length was 450 nm; emission
was measured at 670 nm. Output was recorded on a 10 MV
stripchart recorder. The fluorometer was "zeroed" every
hour using distilled water, and the unit calibrated against
total chlorophyll measured by extractive technique. The fluore-
scence trace was reduced from the stripchart recorder for
presentation in this document.
2) The dissolved yellow substances in surface waters
was measured at 350 nm in a spectrophotometer using air as
a reference. The sample was prefiltered with a one micron
filter. The values used in this report are the percentage
of light absorbed over the 10 cm light path.
3) The amount of visible light attenuated by particu-
late matter in the surface water was measured using the
method of Yentsch (1962). One liter of water was filtered,
and the filtrate was scanned spectrophotometrically in the
region of 350-750 nm, using a plain filter as reference.
4) The spectra of upwelled light was scanned between
350-750 nm using a "downlooking" monochromator/photodetector
combination which could be submerged just under the sea
surface. Spectra resolution of this unit is about 2 nm
over the entire region, while photometric accuracy (repeti-
tive scans) is about 5%.
5) Water temperature was recorded continuously, and
samples were taken for salinity approximately every 30
minutes during steaming.
4.
SPATIAL DISTRIBUTION OF PHYTOPLANKTON CHLOROPHYLL
JULY 8 - 9 1972
Along this coast, large growths (blooms) of phytoplankton
occur from nutrient enrichment either by upwelling of deep
waters or by pollution. The distribution of phytoplankton
chlorophyll along Leg 1, Merrimack River to sea (Fig. 2) is
an example of coastal enrichment by river inflow. In this
case, nutrients are supplied by the polluted river water.
As the fresh water mixes with coastal in the tidal estuary,
there is a net transport of this mixture seaward. The timing
is such that maximum growth probably occurs somewhere near
the mouth of the estuary and/or near the coast. In these
regions, chlorophyll concentrations near the coast are as
high as one ever observes in this area. A more representa-
tive level (normal) of chlorophyll concentration for this
season occurs at the seaward stations. Water samples revealed
that the phytoplankton bloom consisted of the common diatom
species Skeletonema costatum. The width of the bloom (50%
of maximum) was about seven nautical miles. The decrease
of chlorophyll seaward was exponential, the rate of change
leaving the maximum, going seaward, gradually decreased.
This pattern of change is associated with increasing salinity
in this section, which suggests that population size is the
result of a gradual dilution of enriched river water moving
seaward. At stations two and three, copepods were extremely
abundant, suggesting that grazing pressure was also important.
Chlorophyll concentrations somewhat less than 0.2 g/l
were recorded from the first 25 miles along Leg 2 (Fig. 3),
which roughly paralleled the coast. However, off Cape Cod
in the region of Stellwagen Bank, another phytoplankton
bloom was encountered. This bloom was composed of an equal
mixture of the diatoms Skeletonema costatum and Thalassiosira.sp.
The shape of the change in concentration with distance was
somewhat more symmetrical than the coastal bloom; however,
the width of the bloom (50% of maximum) was about the same.
5.
The cause of this bloom must be some vertical mixing process
associated with the hydrography of the area. Physical evi-
dence is the lower water temperatures associated with the
bloom. Most of Massachusetts Bay is thermally stratified
at this time of year. Lower surface temperatures can only
result from upward mixing of deeper waters.
Because of low level cloud cover, Leg 3 (Cape Cod to
Wilkinson Basin) was abandoned. Leg 4 crossed Massachusetts
Bay on a northeast course, ending in Gloucester Harbor. On
this leg, a small increase in phytoplankton chlorophyll was
encountered in the central region of the Bay (Fig'. 4). This
could have been a portion of the bloom observed on Leg 2.
In the coastal waters off Cape Ann, another massive bloom was
encountered. From past experience, we recognize this bloom
to be a rather persistent feature at this time of year. It
is largely the result of the wind system which generates
coastal upwelling.
RELATIONSHIP OF YELLOW SUBSTANCE TO PHYTOPLANKTON
CHLOROPHYLL ALONG LEG 1
First, and probably the most important point is that
the amount of yellow substance (Fig. 2) is inversely related
to salinity. Thus, the pattern of distribution must closely
represent the mixing ratio between river and coastal ocean
water. The second point is that Merrimack waters carry large
amounts of nitrogen and phosphorus. Nitrogen is in limiting
quantities in the coastal waters at this time. Therefore,
the presence of fresh water means enrichment, and since
fresh water and yellow substance are correlated, then the
presence of yellow substance also means enrichment. The net
result is that there is a positive correlation between yellow
substance and phytoplankton chlorophyll.
6.
CHARACTERISTICS OF VISIBLE LIGHT ATTENUATION BY PARTICULATE
MATTER ON LEG 1
The red absorption band of chlorophyll a (672 nm) is
apparent in the spectra of particulate matter in the section.(Fig. 5)
Also, the influence of the chlorophyll Soret band and caro-
tenoids can be seen on the background of high short wave
length attenuation. The high attenuation of short wave
length has been seen in other samples of ocean particulate
matter (see Yentsch, 1962). The difference between these
spectra and pure phytoplankton (Fig. 6) suggest that the
additional particulate absorber is very similar optically
to dissolved yellow substances. Or to turn the statement
around, not all of the yellow substances are dissolved,
there are "particulate yellows".
The interrelationship between the absorbers can be
summarized as follows:
1) There appears to be a positive correlation between
phytoplankton chlorophyll (band intensity at 670 nm) and
the "particulate yellows".
2) There is a positive correlation between the
"particulate yellows" and dissolved yellows.
3) The source of the particulate yellows is not known.
They could arise from the metabolic activities of the phyto-
plankton, or they may be a particulate form introduced with
river water.
CHARACTERISTICS OF BACKSCATTERED LIGHT ALONG LEG 1
A change in water color was visible from the deck of
the research vessel; off the Merrimack, waters were reddish
brown, whereas the open Gulf was blue-green. Spectral (Fig. 7)
characteristics of the upwelling light at Station I show a
massive reduction of short wave length of light which agrees
with the high attenuation by particulate matter at that
station. Maximum backscattered light occurred at wave
7.
lengths 600 nm or longer. As phytoplankton and yellow
substances decrease moving seaward, the maximum backscattered
light shifts from longer to shorter wave length, which is
what would be predicted, knowing the attenuation character-
istics of the particulates. The surprising thing is the
large amount of backscattered light (Station 1) at wave
lengths of 600 nm or longer. This means that in waters con-
taining large amounts of phytoplankton and yellow substances
the backscattered light spectra is categorized not only by
a loss of short wave lengths, but long wave lengths are
augmented due to scattering.
ABSORPTION VS. SCATTERING WITH REFERENCE TO IMAGERY AT
ALTITUDE
To model ocean color based on two absorbers (phyto-
plankton pigments and yellow substance) plus water is an
oversimplification. Absorbing materials other than the
photosynthetic pigments are present in large quantities
in the particulate matter. We label these as particulate
yellows. Although their absorption spectra is not known,
there is ample evidence that the spectra is not too different
from dissolved yellow substances. However, it is doubtful
that any simple scheme can be devised to predict the amounts
of particulate yellows present. The other oversimplification
concerns light scattering. It has been the practice of this
laboratory as well as others, to consider ocean color the
result of a constant level of backscattered light, where
wave lengths are selectively removed by absorption. Our
measurements show that water color also depends to a
great extent upon the depth to which light penetrates in
the water column. In areas of low transparency (e.g. Station
1), more light of lower wave lengths is backscattered than
is the case for water of high transparency (e.g. Station 4).
Since long wave lengths are strongly absorbed by water, this
means that the scatterers are very near the surface.
8.
The spectra of backscattered light is not known for all
oceanic conditions; however, we submit that a rather general
scheme to distinguish phytoplankton from yellow substances
can be worked out using broad band sensors similar to those
used by ERTS-A satellites. Our observations show that phyto-
plankton, in addition to absorbing blue wave lengths, strongly
backscatter red wave lengths. Dissolved yellow substances
alone will attenuate short wave lengths without augmentation
of long wave lengths.
The usefulness of such a predictive system is worth
documenting. We are already on record advocating the use
of chlorophyll measurement in oceanic fisheries. We feel
strongly that satellite sensing will be a useful tool for
coastal waters. Some examples are mapping the red tide,
and general studies of coastal eutrophication. In this
report, we have demonstrated how measurements of dissolved
yellow substances are used to interpret the causative factors
for phytoplankton growth (see page 5 on yellow substance dis-
tribution). The key in this exercise is the interrelationship
between the dissolved yellow substances and fresh water.
Therefore, the yellow substance offers the opportunity to
perform a flushing experiment using yellow substance as a
natural dye indicator. This oepns the possibility of making
large aerial estimates of the influence of fresh water dis-
charge in coastal areas a problem heretofore unattainable.
LITERATURE CITED
Yentsch, C.S., "The Influence of Phytoplankton Pigments on
the Color of Sea Water", Deep-Sea Research, 7 1-9 (1960)
Yentsch, C.S., "Measurement of Visible Light Absorption by
Particulate Matter in the Ocean", Limnol. Oceanog.
7: 207-217, 1962
FIGURES
Figure 1. Area of study, July 8-9
Figure 2. Leg #1, Merrimack to sea
A. temperature and salinity
B. continuous chlorophyll fluorescence
(solid line). Black dots, extracted
chlorophyll; dotted line, dissolved
yellow substance
Figure 3. Leg #2, Station 4 to Cape Cod
A. temperature and salinity
B. continuous chlorophyll fluorescence
Figure 4. Leg #4, Massachusetts Bay to Gloucester
A. temperature and salinity
B. continuous chlorophyll fluorescence
Figure 5. Attenuation of light by particulate
matter
Figure 6. Attenuation of light by four species
of cultured phytoplankton (Yentsch, 1960).
Figure 7. Spectra of backscattered light,
stations 1-4
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Ocean color, a three component system?

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