SUBSURFACE CHLOROPHYLL
MAXIMUM
NORTIIEAST
PACIFIC OCEAN1
IN THE
G. C. Anderson
Department
of Oceanography,
University
of Washington,
Seattle
98105
ABSTRACT
A well-developed
subsurface chlorophyll
maximum is present during summer in oceanic
,waters off the Oregon coast. It appears to be formed at depth by a photosynthetically
active
phytoplankton
community
that is adapted to low light intensity.
It may be present and
may affect the distributions
and concentrations
of primary production,
oxygen, and nutrients over large areas of the North Pacific Ocean.
INTRODUCTION
It has been known for several years that
the maximum concentration of chlorophyll
may be found at depth between a seasonal
pycnocline and the permanent halocline
in oceanic waters off the Washington and
Oregon coasts ( Anderson 1964). Until recently, little was known of its distribution
and concentration.
This subsurface chlorophyll maximum begins beyond the continental shelf and extends seaward. It
should not be confused with a coastal
chlorophyll maximum which appears over
the continental
shelf and is associated
with upwelling.
The subsurface maximum
chlorophyll
layer has been charted in detail from two
cruises of the RV Thomas G. Thompson.
These cruises, during August 1966 and July
1968, covered large arcas off the Oregon
coast extending seaward to a distance of
more than 250 nautical milts ( 470 km). In
1966, standard hydrographic casts with 6liter plastic water samplers were made to
collect seawater samples at closely spaced
depths within the interval where the chlorophyll maximum occurs. Acetone extracts
of chlorophyll were made according to the
UNESCO procedure (UNESCO 1966). At
a very few stations, these data revealed
large concentrations of chlorophyll a (up
to 20 times greater concentration than in
surface waters) between 50 and 75 m. At
most other stations, much lower values
were recorded in this depth zone indicating that the maximum was patchy and
that it was probably confined to such a
thin layer that water samplers were missing
the peak concentration.
In 1968, the subsurface chlorophyll
maximum was again
investigated in the same area using a recently developed method for in vivo measurement of chlorophyll a by fluorescence
( Lorenzen
1966).
Vertical
profiles
of
chlorophyll concentration were made at 78
stations by pumping seawater, with submersible pump and host, to a shipboard
fluorometer
( Strickland 1968). Photosynthesis was measured in situ by 14C uptake
at one station (Steemann Nielsen 1952).
At each station measurements were made
of nitrate, phosphate, silicate, and dissolved oxygen concentrations
( Strickland
and Parsons 1965). Vertical profiles of
salinity and temperature were made with
a salinity-temperature-depth
recorder (Bissett-Berman Corp. ) , Particle counts were
made with a Coulter counter ( Model B )
( Sheldon and Parsons 1967).
RESULTS
AND
DISCUSSION
The subsurface chlorophyll maximum is
a well-developed
feature during summer
in all oceanic waters investigated off the
is
Oregon coast. Its peak concentration
confined to a relatively thin layer, typically
found between 55 and 65 m (Fig. l)depths not ordinarily sampled by routine
1 Supported
by U.S. Atomic Energy Commission Contract
AT ( 45-1) -1725 ( Rcf : RLO-1725
hydrographic casts. It is located in a layer
130). I thank Dr. C. A. Barnes and Mr. E. E.
of water bounded by two discontinuity
Collias For their helpful discussions and assistance.
layers, a seasonal halocline and pycnocline
Contribution
No. 468 from the Department
of
above and the permanent halocline and
Oceanography,
University
of Washington,
Seattle.
386
SUBSURFACE
CHLOROPHYLL
PHOTOSYNTHESIS (mg C mm3 day“)
CHLOROPHYLL (mg rnm3x IO-‘)
MAXIMUM
IN
PACIFIC
SALINITY
32.0
387
OCEAN
(%/..)
33.5
33.0
32.5
SIGMA - t
o-23
--
-7-’
24 ---,
--
- 25
I
-- -.+L7.--
i
i
i
-0’
I
II
DISSOLVED
OXYGEN (ml liter-‘)
se---- SIGMA -
t
FIG. 1. Vertical
distributions
of chlorophyll
a,
photosynthesis,
and dissolved oxygen at 45,” 10’ N
lat, 126” 56’ W long, 26 July 1968. The right
ordinate
represents
depths to which
specified
amounts of subsurface light penetrate,
expressed
as percentage of light at the sea surface.
usually
pycnocline
below, corresponding
to depths of about 40 m and 100 m respectively ( Budinger, Coachman, and Barnes
1964) (Fig. 2). Th cse conditions suggest
that the maximum would be subject to
little loss by mixing from above. The
maximum layer, although continuous over
the area investigated, is somewhat patchy.
Maximum chlorophyll concentrations were
generally 3 to 10 times those in surface
waters. Inspection of chlorophyll measurements made from hydrographic
casts on
summer cruises in 1964 and 1965 (Department of Oceanography 1966, 1967) cxtending out to more than 700 nautical miles
(1,300 km) off Oregon shows that the maximum is continuous seaward, although peak
concentrations
were probably missed at
most stations.
The chlorophyll
maximum observed at
this depth is most likely a result of phytoplankton growth there. Processes that cxplain chlorophyll
maxima in other areas
have involved consideration of cells sinking
from above and subsequent concentration
at depth (Riley, Stommel, and Bumpus
1949; Steele and Yentsch 1960), although
in situ formation was recently used to explain the presence of a chlorophyll maximum in the mctalimnion of a lake in Japan
( Ichimura, Nagasawa, and Tanaka 1968).
,1,TEMPERATURE
(“C)
FIG. 2.
Vertical
distribution
of temperature,
salinity, and density (sigma-t)
at 45” 10’ N lat,
126” 56’ W long, 26 July 1968.
Off the Washington and Oregon coasts,
after the formation of the seasonal pycnocline during spring, high phytoplankton
photosynthesis
in upper waters reduces
nitrate to undetectable concentrations;
as
a result, phytoplankton
concentration declines and remains low throughout summer
(Anderson 1964; Stcf,insson and Richards
1963). Beneath the upper pycnocline, nutrient concentrations increase with depth
an d remain relatively high. Light transmission, although low, is enhanced by the
paucity of phytoplankton above. Photosynthesis exhibited a peak in the maximum
layer at the same depth as that of chlorophyll and was dctcctable throughout the
388
G. C. ANDERSON
region even though light was exceedingly
low at the lower depth ( less than 0.1% of
surface light at 90 m) (see Fig. 1). The
more than threefold increase in chlorophyll
at the maximum over that found in upper
waters probably does not rcprescnt a corresponding increase in phytoplankton
volumc. Particle counts using the Coulter
counter showed little more than a twofold
increase in particle volume at the maximum. Assuming that the difference is due
to phytoplankton,
these results suggest
adaptation of the cells to low light by an
increase in their chlorophyll
content; the
increase is well within the range of values
reported from physiological studies of pigment concentration
adaptation by algae
(Steemann Nielsen and Jorgensen 1968).
Therefore, although the maximum appears
to be formed by growth of the phytoplankton within its recorded depth zone, a large
part of the chlorophyll
increase may result from adaptation to low light. Steele
(1964) suggested that midwater chlorophyll
maxima in the Gulf of Mexico may be due
to an increase of chlorophyll rather than to
an accumulation of plants due to sinking.
The presence of this well-developed
maximum over an extensive region of the
North Pacific Ocean can be expected to
have a considerable
effect on related
oceanographic features and processes; for
example, it should be significant to studies
of primary production in general and to the
distributions of oxygen and nutrients in the
North Pacific Ocean.
when primary
production
Ordinarily,
measurements are made on oceanographic
cruises, the selection of sampling depths is
based on a rather arbitrary definition of a
euphotic zone; that is, the region extending
from the surface to a depth where 1%
surface light occurs, the latter depth commonly thought of as being near to compensation depth. Because net photosynthesis
by definition is zero at compensation depth,
measurements are usually made only at
depths shallower than the 1% level. Consequently, a large part of the primary production may not be measured in regions
where phytoplankton communities adapted
to low light exist at considerable depth. In
this instance, compensation depth, assuming 14C uptake measures close to net photosynthesis ( Strickland 1960)) extends even
below the 0.1% light level and 15% of the
total primary production lies below the 1%
depth. Based on a value of 0.3 cal cm-2
min-l ( 24-hr mean) for incident radiation
measured during 26 July 1968 and assuming that less than half would penetrate the
surface of the ocean, light energy at most
would be 1.50 X 1O-3 cal cmd2 min-l at the
1% light level. This compares favorably
with a commonly used value of 1.45 x 10 3
cal cm-2 min-l for compensation intensity
for Coscinodiscus (Jcnkin 1937)) but both
values are too high by an order of magnitude for the present cast.
Over large areas of the North Pacific
Ocean during summer, a subsurface oxygen
maximum is found beneath the seasonal
pycnocline at a depth corresponding to the
lower part of the winter mixed layer, similar to the depth of peak concentrations of
chlorophyll in the area of this investigation.
It is best developed as a broad band (35”
to 45” N lat) extending across the entire
North Pacific Ocean (Kitamura 1958; Reid
1962). Its occurrence has been explained
by summer loss of oxygen above the maximum layer owing to warming
of the
upper waters (Reid 1962; Pytkowicz 1964);
it has also been suggested that offshore
movement
and subsequent
sinking
of
coastal upwelled water enriched by oxygen
from photosynthesis at the surface contributes significantly to the observed maximum
off the Washington
and Oregon coasts
( Stefansson and Richards 1964).
From these results, it is suggested that
the oxygen maximum is formed largely by
photosynthesis, at least in areas where the
chlorophyll
maximum is present.
That
summer loss of oxygen in upper waters
dots occur is not disputed, but it appears
invalid to attribute the total oxygen incrcment to this process, for the following
reasons. Surface dissolved oxygen for the
area reaches a maximum value of about
6.8 ml liter-1 during early spring (Pytkowicz 1964). In summer 1968, the oxygen
SUBSURFACE
CHLOROPHYLL
increment was 1.6 ml liter-l (surface 6.0,
maximum 7.6) but only half of this amount
represents a loss and is confined to waters
above the seasonal pycnocline ( Fig. 1). An
equal amount of oxygen is gained by the
maximum layer and can be explained by
photosynthesis. From the data presented in
Fig, 1, the contribution of oxygen by photosynthcsis at the chlorophyll maximum is
approximately
0.2 ml liter-l month-l.
If
the chlorophyll maximum begins to form
shortly after the start of thermal stratification in April, the total amount of oxygen
contributed by photosynthesis by the end
of July would be 0.6 ml liter-l. This value
is in reasonable agreement with the observed gain of 0.8 ml liter-l, especially
when one considers that only a single series
of in situ photosynthesis measurements is
available, that photosynthesis was not measurcd at precisely the depth of maximum
chlorophyll (62 m), and that dissolved oxygcn was not measured during spring 1968.2
It is conceivable that the oxygen increment
between 35” and 45” N lat is greater than
the generally reported values of l-2 ml
liter-l since routine hydrographic sampling
would likely miss peak concentrations in
the relatively thin oxygen maximum layer.
Also, it is known from this study that the
chlorophyll concentration may be at least
three times greater than the value used in
the above computation.
Thus, if one assumcs a corresponding increase in photosynthesis, the oxygen increment could be
much larger.
Other features of the chlorophyll maximum can be clucidatcd if it is reasonable
to assume that the distribution
and concentration of the oxygen maximum reflects
the distribution of chlorophyll.
The wcakening of the oxygen maximum south of
2 Computation of oxygen production based on
phosphate utilization
lends further support to the
argument that photosynthesis
accounts for the subsurface dissolved oxygen gain during summer. The
difference
between
phosphate
concentration
occurring in the surface mixed layer before spring
stratification
and the concentration
observed at
the maximum in July is approximately
0.3 pg-at./
liter. This represents a gain of 0.9 ml liter-l dissolved oxygen, assuming a AO/AP ratio of 276: 1
by atoms (Redficld, Ketchum, and Richards 1963).
MAXIMUM
IN PACIFIC
NO;
389
OCEAN
, SiOi-
(,bg -at.
liter-‘)
--.
PO:-
(pg-at.
-7 ---Fp-’
IlterC’)
FIG. 3. Vertical
distribution
of phosphate, nitrate, and silicate at 45” 10’ N lat, 126” 56’ W
long, 26 July 1968.
35” N lat coincident with the disappearance of the permanent halocline suggests
the importance of stability to formation of
the chlorophyll
maximum.
However,
a
similar weakening of the oxygen maximum
occurs north of 45” N lat where the pcrmancnt halocline is well developed. This may
be a result of other conditions in northern
waters such as diminution in incident radiation, increased turbidity of surface waters,
and greater turbulence-none
of which favors phytoplankton
development at depth.
The persistent nutrient
deficiency
in
summer, especially nitrate depletion, in
these surface waters has been mentioned.
At the same time, below the seasonal pycnoclinc, nitrate is detectable only at depths
beneath the level of the chlorophyll maximum (Fig. 3). Hence, waters that are
mixed into the surface layer as the pycnocline deepens during summer are nitrate
deficient.
As the mixed layer depth increases, the chlorophyll
maximum may
gradually deepen leaving a zone of nitrate
deficient water between it and the upper
pycnocline. Therefore, nutrients that might
normally be supplied to surface waters by
diffusion
and mixing from below are
mostly used by the photosynthetically
ac-
390
G. C. ANDERSON
tive phytoplankton
community making up
the maximum zone. Intensive studies for
many years by Canadian oceanographers
at Weather Station “P” in the Gulf of
Alaska ( 50” N lat, 145” W long) show a
high concentration of nitrate in upper waters during summer but no marked chlorophyll concentration at depth ( McAllister,
Parsons, and Strickland 1960). Recently, it
has been shown that nitrate is present at
the surface throughout the year over much
of the subarctic northeast Pacific Ocean
but that it becomes depleted during spring
and summer south of about 45” N lat and
toward the coast (Anderson, Parsons, and
Stephens, in press). The maintenance of
high nitrate concentrations in waters north
of 45” N lat was attributed to relatively
intensive entrainment of deep water into
the upper zone coupled with a slow rate
of removal of nitrate by the primary producers. These observations agree with the
proposed relationship
of the chlorophyll
maximum to the distribution
of nutrients.
Thus, the phytoplankton maximum may act
as an efficient subsurface nutrient trap
over a large oceanic area, The result is
markedly low productivity
of surface waters during summer.
NOTE ADDED IN PROOF
Dr. P. H. Wiebe recently called to my
attention the chlorophyll
data collected
during the Ursa Major Expedition
by
Scripps Institution
of Oceanography
in
summer 1964 (Scripps Institution of Oceanography 1967 ) , Chlorophyll measurements
were made at standard dcp,ths (0, 5, 10,
15, 20, 35, 50, 75, 100, 150, 300 m ) on a
section at 155”OO’W long from Kodiak,
Alaska, to’ near 26”OO’N lat. Although peak
concentrations of chlorophyll
wcrc probably missed, the general distribution of the
subsurface maximum can be followed. The
layer was well developed from about 48”
N lat to 32” N lat with a gradual deepening from less than 50 m in the northern
tip to about 100 m in the southern section.
These observations agree well with the
boundaries suggested in this paper for the
north-south
extension of the chlorophyll
maximum and lend further support to the
suggestion that
transpacific.
its distribution
may be
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