J. Exp. Mar. Biol. Ecoi., 1990, Vol. 143, pp. 139-146
139
Elsevier
JEMBE 01498
Observations on double chlorophyll maxima in the vicinity
of the Fraser River plume, Strait of Georgia, British
Columbia
W. P. Cochian ‘, P. J. Harrison
‘v2, P. J. Clifford ’ and K. Yin 1
‘Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
of Oceanography, 2Department of Botany, University of British Columbia, Vancouver. B&s-h
Columbia, Canada
(Received 15 February 1990; revision received 6 July 1990; accepted 23 July 1990) zyxwvutsrqponmlkjih
Abstract: During a cruise to study nutrient and phytoplankton dynamics in the vicinity of the Fraser River
plume, vertical profiles of in vivo fluorescence, temperature, salinity, nitrate and silicate were taken. Double
chlorophyll maxima at 4-6 and 12-14 m were observed onIy during the late flood tide at two stations on
one day and at one station the fallowing day during the 1l-day cruise. Ac~omp~y~g vertical profiles of
nitrate and silicate at two stations showed concentration minima at depths corresponding approximately
to the two chlorophyll maxima. The upper chlorophyll maximum occurred at the base of the thermochne
and halocline and at the beginning of the nitracline, while the lower, chlorophyll maximum was not
associated with any features in the temperature or salinity vertical profiles, although minima in nitrate and
silicate were associated with it. The most plausible explanation for the observed double chlorophyll maxima
is that the upper chlorophyll maximum formed in plume water while the lower chlorophyll maximum
originated in waters seaward of the plume and was advected to the plume area during flood tides and
possibly entrained into the plume as the freshwater plume flowed seaward. Hence both maxima probably
formed due to nitrogen limitation of the surface waters, but at two different depths and flood tides
superimposed the two maxima on top of each other due to density differences in the two water masses. These
observations suggest that double chlorophyll maxima may only be observed in this riverine plume front
during late flood tides and only if a relatively deep (i.e., deeper than the plume maximum) chlorophyll
maximum is formed in waters beyond the plume. Other plausible mechanisms of double chlorophyll maxima
formation are discussed.
Key words: Chlorophyll maximum; Estuarine; Fluorescence; Nutricline; Plume
In vivo fluorescence is a standard technique used to estimate ph~opl~kton
Chl zyxwvutsrqp
a
concentration (Lorenzen, 1966) and it is frequently used as an index of ph~op~~kton
biomass. The advantage of the in vivo fhrorescence technique in field studies is that it
can be used to quickly obtain a vertical profile of Chl a in the euphotic zone by simply
lowering a hose through the water column and pumping the profiled water to a
shipboard fluorometer.
Correspondence address: W. P. Cochlan, Marine Biology Research Division, A-002, Scripps Institution
of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA.
0022-0981~90/$03.50 0 1990 Eisevief Science Publishers B.V. (Biomedical Division)
1411
W.P.COCHLAN
ETAL.
Subsurface fluorescence maxima are generally Chl u maxima and these subsurface
chlorophyll maxima (SCM) are often phytoplankton biomass maxima, but not usually
primary productivity maxima (Cullen & Eppley, 1981). A SCM is a common feature
of many vertical profiles of Chl a and these SCM have been classified on the basis of
their fo~ation (Cullen & Eppley, 198 1; Cullen, 1982). The SCM are observed in most
oceans during periods when the water column is stratified, and generally are found in
close proximity to the l-5 “/, surface it-radiance level (e.g., Cullen & Eppley, 198 1; Kimor
et al.. 1987) and strong vertical gradients including the nitracline, thermocline and
pycnocline (Cullen, 1982, and references therein). The often close association between
the nitracline and the SCM suggests that the growth of phytoplankton above and below
the SCM may be limited by nitrogen and light, respectively.
In this paper we report the first observations of two distinct SCM within a common
water column with accomp~ying vertical profiles of temperature and salinity, and
ambient concentrations of nitrate and silicate. These observations were made during a
study of the primary productivity (Harrison et al., submitted) and nitrogenous nutrition
(Cochlan et al., in prep.) of phytoplankton in the Strait of Georgia, British Columbia.
MATERIALS
AND
METHODS
The cruise was conducted from 20 to 3 1 July 1987 in the Strait of Georgia, British
Columbia, in the vicinity of the Fraser River plume during a spring-neap tidal cycle
(Fig. 1). The hydrography/circulation (LeBlond, 1983) and biology (Harrison et al.,
1983) of the study area have been described previously and a detailed account of our
sampling methods and analytical techniques is given in Clifford et al. (1989).
Underway measurements of temperature and salinity, determined with an InterOcean
5 14A CSTD equipped with model 5 13D probe, were used to define the riverine plume
boundaries and select three main productivity stations: the inner plume (Stn. 3) the
mid-plume (Stn. 2) and the outer plume (Stn. 1) (Fig. 1). At these stations continuous
vertical profiles (0 to w 20 m) of temperature, salinity, in vivo fluorescence, nitrate plus
nitrite and silicate were obtained. A darkened hose (1.5 cm ID) was fixed to the CSTD
probe, and as the probe was lowered (z 1 m - min -- ’ ). water was pumped to the ship’s
deck with a mRoy FR162-144 diaphragm pump ( a 1 1. min - ‘). Water from the vertical
profiles was pumped through a Turner mode1 ill ~uorometer (equipped with a flowthrough cell) to measure in vivo fluorescence, and a Technicon autoanalyser II to
measure ambient concentrations of nitrate plus nitrite (Wood et al., 1967) and silicate
(Armstrong et al., 1967). These instruments were connected to an analog to digital
converter, and the data stored and plotted using an IBM personal computer to give
real-time plots of vertical profiles of the parameters (Jones et al., submitted). Pumping
time lags and machine analysis delays were measured in order to coordinate sample
values from the vertical profnes with the actua1 position in the water column as
determined by bottle casts; the depths were generally within + 0.5 m of each other.
141
DOUBLE CHLOROPHYLL MAXIMA. STRAIT OF GEORGIA
5
n
I
-_
STRAIT
\
1
II
-_,
I
15 zyxwvutsrqponmlkjihgfedcbaZYXWVU
20 km
IO
.._
:.
:
i;
OF GEORGIA
48;45
123”40
W
t23*
20’ w
Fig. 1, Map of study area showing Fraser River plume in southern portion of Strait of Georgia, and three
main stations (Stn. 1, outer plume; Stn. 2, mid-plume; Stn. 3, inner plume). Inset shows study area in
relation to southern coastline of British Columbia.
A sample for phytoplankton species composition was collected at 12 m at Stn. WP6
from the outlet attached to the pumping system. The sample was preserved in acid
Lugol’s solution (Parsons et al., 1984) and stored in the dark. A lo-ml subsample was
set&d for 24 h and counted using an inverted microscope at 200 and 500 x magnification (UtermUhl, 1958).
142
W.P.COCHLAN E'I‘AL
Subsurface light measurements were determined with a Lambda Instruments
LI-185B light meter equipped with a LI-192s underwater quantum sensor (2 rc).
RESULTS AND DISCUSSION
Water circulation in the Strait of Georgia is strongly affected by daily tides (LeBlond,
1983). During a flood tide, water from the Pacific Ocean enters the Strait through the
southern entrance and moves up the east side due to the Coriolis effect. On an ebb tide,
water moves down the west side of the Strait and out to the ocean. Therefore, the Fraser
River plume moves out into the Strait and northwards during a flood tide, and out and
southwards during an ebb tide. The size and position of the plume is also affected by
spring and neap tides on a fortn~ghtly cycle. The plume is largest during spring tides and
may easily reach Stn. 1, depending on the volume of the river discharge (maximum in
June) and wind velocity. During the neap tide the plume is much smaller (Z 50?,
reduction) and frequently reaches only to Stn. 2 during late July. The zone between the
spring and neap tide extent of the plume is referred to as the estuarine plume, and this
zone has the highest productivity due to nutrient input from the river and nutrient
entr~nment (Harrison et al., submitted). The zone from the river to the boundary
formed by the plume on a neap tide is lower in primary pr~u~tjvity, since it is always
covered by the turbid, riverine plume.
During the post-spring portion of a spring-neap tidal cycle (29 July 1987) two distinct
SCM were observed at 4-6 and 12-14 m (Fig. 2). These maxima were first observed
during a series of five vertical profiles conducted over a 5-h period during a transect from
the inner to the outer plume. They were most pronounced in the outer plume (WP6 and
Stn. I), moderately evident within the plume (Stn. 3) and not evident at mid-plume
(Stn. 2 and WP7). Accompanying vertical profties of nitrate and silicate at two stations
(WP6 and Stn. 3) showed minima in their concentrations at depths corresponding to
the two SCM (Fig. 2). The upper SCM occurred at the base of the thermocline and
halocline (density is primarily salinity-driven and sigma t profiles follow salinity very
closely) and at the beginning of the nitracline. The lower SCM was not associated with
any features in the temperature and salinity vertical profiles, but coincided with minima
in nitrate and silicate concentrations. At Stn. 3 the lower SCM was mainly below the zyxwvutsrqp
1 “/, light depth, while at WP6, both the shallow and deep SCM were largely above the
lo:, surface irradiance depth. The fluorescence peaks represent Chl a concentrations of
e 1 (Fig. 2A) to 2 mg * m 3 (Fig. 2B).
The phytoplankton species composition at WP6 at 12 m was numerically dominated
by Mcromonaspusilla (Butcher) Manton et Parke, cryptomonads, Skeletonema costatum
(Greville) Cleve, Chrysochromulina spp. and Chaetoceros spp. (in order of decreasing
abundance). The total number of cells was 1.7 x lo4 cells -ml- ’ and M. pusilla composed 44% ofthe total number of cells at this depth, while S. Costa&m, Ch~sochromulina
spp. and cryptomonads each composed z 13P’,of the cell numbers. However, in terms
DOUBLE
A
CHLOROPHYLL
SALINITY
00
5
(%a)
IO
TEMP
15
20
MAXIMA,
STRAIT
(“C)
25
NITRATE
30
0
5
143
OF GEORGIA
IO
(PM 1
15
20
25
5
z
k
E
- IO
F
kl
n
15
20
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
80
1.0 0
20
40
60 zyxwvutsrqponmlkjihg
.2
.4
.6
.8
0
RELATIVE
SALINITY
8
oOI
5
20 1
0
FLUORESCENCE
(%a)
IO
’
.2
RELATIVE
TEMP
15
20
l
’
I
.4
.6
SILICATE
(“C)
25
.8
FLUORESCENCE
NITRATE
30
1.0
0.
5
0
IO
(PM)
(JJM)
15
20
20
SILICATE
25
40
(JIM)
Fig. 2. Vertical profiles of temperature
(T), salinity (S), in vivo fluorescence (F), nitrate (N) and silicate (Si)
at A: Stn. 3 (1900) and B: WP6 (1745) on 29-30 July 1987, respectively, during a late flood tide ofpost-spring
tidal cycle.
134
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
W. P. COCHLAN ET AL.
of cell biovolume or biomass, diatoms dominated (e.g., S. costutum, Chaetoceros spp.
and Thaiassiosira spp.). This species composition
is very similar to the composition
reported at the other stations (l-3) over a range of depths (O-10 m; Clifford et al., 1989;
Harrison et al., submitted). Because phytoplankton
community structure was similar
throughout the whole plume area it is likely that the species composition in the upper
SCM and lower SCM were similar, although no samples of the upper SCM were taken.
The mode(s) of formation of these double SCM are not apparent, however. there are
several possibilities based on biological and/or physical mechanisms.
A single SCM
frequently forms in the plume area, typically at a depth of 4-6 m a few days after
nitrogen is exhausted from the surface water (Clifford et al., 1989; Harrison et al.,
submitted). Therefore it is the lower SCM at 12-14 m that requires explanation. The
most plausible explanation is that the deeper SCM represents the SCM from an area
beyond the plume which has been advected into the vicinity of the plume. Waters beyond
the plume to the south, with its deeper SCM, may be transported into the plume area
during a flood tide by advection and perhaps into the surface plume by entrainment
as
the freshwater plume flows seaward. We observed that during the post-spring portion
of the spring-neap tidal cycle, two double SCM occurred at the inner and outer part
of the plume but not at the mid-plume stations (WP7 and Stn. 2) near the end of the
flood tide (an increase in tidal height of 3.2 m) on the evening of 29 July. There was no
apparent reason for the absence of the SCM at these two latter stations. Vertical mixing
could not be inferred from the salinity profiles. However, the double SCM were not
observed at Stn. 1 during the ebb tide (mornings of 29-30 July), but they were observed
again in the evening on 30 July during the latter portion of the flood tide.
Further evidence for the interweaving of water masses hypothesis comes from the
double minima in nitrate and silicate concentrations,
corresponding
to the two SCM
(Fig. 2). Therefore the two SCM likely represent interweaving water masses with the
upper layer indicative of plume water (increased temperature and decreased salinity in
the upper 3-4 m) and the lower SCM representative
of water beyond the plume. This
interleaving of water masses has also been observed in Saanich Inlet which is near our
study site (Parslow, unpubl. data). Both maxima probably form at different depths due
to nitrogen limitation in the surface waters (Harrison et al., submitted; Cochlan et al..
in prep.), and the flood tides superimpose the SCM on top of each other due to density
differences in the two water masses. Previous studies in other parts of the Strait and
in nearby Saanich Inlet indicate that SCM usually form at 5-12 m (Harrison et al.,
1983; Parsons et al., 1983; Haigh & Taylor, in press).
A second possibility is that the double SCM are the result of successive phytoplankton blooms in the same water mass; the deepest SCM represents an older bloom
and the shallower SCM represents a more recent bloom. However, this is an unlikely
mode of formation in an area such as the plume where vertical mixing and advection
are dynamic processes.
A third possibility is that the double SCM were originally
result of selective zooplankton
grazing;
single maxima, and are the
a layer of zooplankton
at 6-10 m may have
DOUBLE CHLOROPHYLL MAXIMA, STRAIT OF GEORGIA
145
consumed a portion of the chlorophyll rn~~~
thereby producing a ph~opl~kton
minima in this zone. Zoopl~kton have been shown to aggregate ho~zont~y, with
maximum abundance at the boundary of the plume (Parsons et al., 1969; Mackas &
Louttit, 1988; Harrison et al., submitted), but it is not known whether they aggregate
vertically in such discrete layers.
A fourth possibility is that the observed double SCM are a sampling artifact, caused
by the movement of an internal wave through the water column while we were lowering
our sampling pump. If an internal wave had vertically displaced the water column, it
is conceivable that we sampled the same water mass twice at different depths (Holligan,
1978), resulting in apparent double fluorescence maxima (Mackas & Owen, 1982).
However, in addition to displacing the fluorescence signal, we would also expect the
salinity and temperature signals to be similarly displaced. Plots of sigma t vs. depth (not
shown) do not display any vertical displacement of the density structure, and we
conclude that the double SCM observed are not due to sampling artifacts.
The last plausible mech~ism of fo~ation of double SCM is by verticahy boating
dinofla~ellates or flagellates. In this case one might expect to find mainly diatoms (or
any non-migrating ph~opl~kter) in one SCM layer and migrating phytoplankton in the
other SCM. Our species composition data suggests that the double SCM did not form
by such a biological mechanism.
We took over 40 vertical profiles during July 1987 in the vicinity of the Fraser River
plume and only observed the double SCM on 2 days at three stations (Clifford et al.,
1989). However, almost all of these profiles were taken at times other than during the
late flood tide. During a cruise in this region during June 1989, we again observed double
SCM on three days at two stations; the double SCM at Stn. 3 (inner plume) occurred
at the end of the flood tide. These observations suggest that the double SCM may only
be found during the late flood portion of post-spring or neap tides, and support our first
theory of double SCM formation in which a deeper SCM, from outside of the rive&e
plume area, is tidally advected into the plume area and beneath a shallower plume SCM.
ACKNOWLEDGEMENTS
This research was funded by the Natural Sciences and Engineering Research Council
of Canada (NSERC) and a University of British Columbia Graduate Fellowship to
W. P. Cochlan. We acknowledge the technical assistance of D. Jones (computer programming and data logging), R. Haigh (phytoplankton counts), and the captain and
crew of the C. S. S. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Vector.
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136
W. P. COCHLAN ET AL
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