PHYTOPLANKTON
CONCENTRATION
PRODUCTION
AND CHLOROPHYLL
IN THE BEAUFORT
CHANNEL,
NORTH CAROLINA’
Richard B. Williams and Marianne
Bureau of Commercial
Fisheries,
Radiobiological
Laboratory,
B. Murdoch
Beaufort,
North Carolina
ABSTRACT
A yearlong study was conducted on phytoplankton
at the mouth of a shallow estuary.
Photosynthesis
had a pronounced
seasonal cycle, without major irregularities,
that followed
the cycle in water temperature.
Gross photosynthesis
at the surface ranged from 0.12 to
0.72 mg C liter-l day-l. Daily photosynthesis
at 50, 25, and 10% of surface illumination
averaged 104, 85, and 58% of the surface value. Respiration ranged from 0.01 to 0.29 mg
C liter-’ day” and averaged 40% of surface gross photosynthesis.
Annual gross production
was 113 g C/m” for a 1.0 m water column (average depth of the estuary) and 225 g C/m”
for the entire euphotic zone. Annual respiration
was 0.39 g C/m”. This low production
was ascribed to shallowness and turbidity.
The phytoplankton
was chiefly nannoplankton
(centric
diatoms) and averaged 2 x 10’
cells/liter.
Nannoplankton
also contained most of the chlorophyll
a and produced most of
the respiration and photosynthesis.
Chlorophyll
a ranged from 2.0 to 9.3 @g/liter, and averaged 3.6 ,xg/litcr from June to
October (the period of high water tcmpcratures
and high production)
and 4.8 pg/liter the
remainder of the year. The ratio of photosynthesis
to chlorophyll
a (at the surface during
daylight hours) ranged from 1.9 to 19.8 mg C( mg Chl a)-l hr-” and followed
water temperature.
Values were predicted by the equation:
log [mg C (mg Chl a)-l hr-“I = 0.138 + 0.0353 x temperature
( “C) ,
yielding a Qla of 2.25 for the ratio of photosynthesis
to chlorophyll
a.
sis made in connection with classes at Duke
University Marine Laboratory (Bartclll959;
Odum and Hoskin 1958). There are no
quantitative studies of estuarine phytoplankton for areas nearer to Bcaufort than Chesapeakc Bay 300 km to the north (Hull 1963;
Pattcn, Mulford, and Warinncr 1963; Patten
et al. 1964) and the coast of Georgia 600 km
to the south (Ragotzkie 1959; Schclsko and
Odum 1961). None of these studies treat
both the standing crop of phytoplankton
and their annual rate of production.
Since chlorophyll a is essential for photosynthesis and is easily measured, numerous
attempts have been made to relate its concentration to the rate of photosynthesis of
phytoplankton
populations
(Ryther and
Yentsch 1957, 1958; Strickland 1960). Although values for gross photosynthesis at
optimal illumination tend to cluster around
4 g C ( g Chl a) -l hr-l, the range for these
values is so broad that measurements of pigment provide only a very crude estimate of
primary production. I-Iowevcr, since values
INTRODUCTION
Much of the southeastern seaboard of the
United States is shallow estuaries. This
habitat appears to be biologically productive since it supports substantial fisheries.
Thcrc are, however, few quantitative data
concerning the primary production which
ultimately sustains these fisheries. The purposes of our yearlong study in the Beaufort
Channel were to characterize approximately
an estuarine phytoplankton
population, to
measure its production, and to determine
the uscfulncss of chlorophyll a conccntration as a means to estimate productivity
of
this population.
Previous work on the phytoplankton
of
the Beaufort area was limited to taxonomic
studies of the diatom flora by I-Iustedt
( 1955)) Manly ( 1953)) and Wolfe ( 1930),
and to a few measurements of photosynthel The work reported was carried on as a part of
a cooperative project of the U.S. Fish and Wildlife
Service and the U.S. Atomic Energy Commission.
Publication
has been approved by these agencies.
73
74
RICHAnD
B. WILLIAMS
AND MARIANNE
in the literature were obtained from widely
scattered locations representing a variety of
habitats, it seemed possible that the ratio of
photosynthesis to chlorophyll
a. would be
less variable among samples from a single
location.
Description
of area
The Beaufort Channel is the smaller of
two passageways connecting the broad, shallow estuary of the Newport River with the
Beaufort Inlet and thus with the ocean.
The channel is 90 to 400 m wide, 3 km long,
and 4 to 10 m deep, The tide, with a mean
amplitude of 0.8 m, generates turbulent currents in the channel swift enough to carry
sand grains to the surface fro,m depths of
scvcral meters. The estuary of the Newport
River is over 6 m deep in a few channels,
but averages only 0.8 m at low tide and 1.3
m at high tide. Its surface area at low tide
is 3.6 km3 and at high tide 5.0 km2. Its volume at low tide is approximately half that
at high tide. The ocean near the Bcaufort
Inlet is also relatively shallow; the 20-m isobath lies 15 km offshore. Bottom sediments
in both the Bcaufo,rt Channel and the estuary range from mud to sand. Much of the
intertidal area is salt marsh; there is little devclopment of submerged macroscopic plants.
Except after heavy rains, freshwater inflow
in the area is minor in comparison with the
tidal exchange. The Newport River drains
a mixture of forest and agricultural lands;
the estuary and the Beaufort Channel receive wastes from fish processing plants and
raw sewage from communities totaling several thousand people.
MATERIALS
AND METHODS
Our study consisted of 26 measurements
made from December 1962 to December
1963. Water samples were dipped from the
surface of the Beaufort Channel at Pivers
Island at approximately the same time of
day (0960) at one- and three-week intervals;
thus spaced, successive measurements were
at approximately opposite stages of the tide.
A surface sample was representative of the
entire water column because turbulence
Produced bv tidal currents largely precluded
B. MURDOCII
stratification.
The samples were strained
through No. 10 netting to remove zooplankton and part of the water was refiltered
through No. 25 netting to remove the net
plankton. The No. 25 netting had openings
ca. 60 SL.square that retained the largest
cells and chains of small cells. Photosynthesis, respiration, and the standing crop of
plankton were estimated separately for the
coarsely filtered and the finely filtered water
to determine the relative importance of the
net and nannoplankton.
When the samples
were taken for productivity
measurements,
water temperature was measured to the
nearest degree with a dial thermometer and
salinity to the nearest part per thousand
Secchi disc readings
with a hydrometer.
were made daily.
Photosynthesis and respiration were measured by the light- and dark-bottle technique of Gaarder and Gran ( 1927). Dissolved oxygen was measured by Winkler
titration, following generally the procedures
of Strickland and Parsons ( 1960). Changes
in dissolved oxygen were converted
to
changes in organic carbon using the relationship formulated by Ryther ( 1956) : 1.0
mg oxygen is equivalent to 0.30 mg carbon.
Seawater was siphoned into 125-ml glassstoppered bottles. Three dark bottles were
placed in running seawater for 24 hr, and
two light bottles were suspended at each of
four depths in the Beaufort Channel for the
same period. This period of incubation produced, in general, changes in the conccntration of dissolved oxygen large enough to be
readily measurable, but not large enough to
alter markedly the environment in the bottics. Preliminary experiments indicated that
the rate of respiration in the bottles remained constant for at least 24 hr, and use
of an entire day averaged any differences
in the rate of photosynthesis arising from
diurnal periodicities in the physiology of the
plankton, The pairs of light bottles were
located at depths where they received approximately 100, 50, 25, and 10% of the surface illumination.
The depths were detcrmined from the extinction coefficient of the
water which was estimated from a Secchi
disc measurement by use of the formula of
PHYTOPLANKTON
PRODUCTION
AND
CI~LOROI’HYLL
IN
BEAUFORT
CHANNEL
75
I
JANw
JUL
SEP
FIG. 1, Secchi disc reading, salinity, and water
temperature.
FIG. 2. Gross photosynthesis and respiration at
Iour light levels.
Poole and Atkins ( 1929) : extinction coefficient = 1,7/Secchi disc measurement in m.
The titration values for oath group of
duplicate or triplicate bottles were averaged
and the group means were used to calculate
changes in carbon. Replicates rarely diffcred by more than 0.13 mg O&iter (equivalent to 0.4 mg C/liter).
This level of precision was similar to that reported by Patten
et al. (1964) for the light- and dark-bottle
method.
The abundance of algal cells and the
concentration of chlorophyll a in material
retained by HA Millipore82
filters were
used to estimate standing crop of phytoplankton. Ccl1 counts were made on freshly
collected, unpreserved samples by examination of the filter after clearing with immersion oil. Chlorophyll
a was extracted
with 90% acetone and estimated with a
Beckman model DU spectrophotometer following the method of Strickland and Parsons (1960). The optical densities obtained
with the spectrophotometer were converted
to pigment concentrations
by means of
nomograms (Duxbury and Yentsch 1956)
representing the equations of Richards with
Thompson (1952).
Measurements of insolation were obtained
from two, sources. A 50-junction Epplcy
pyrhcliometer
connected to a Varian G-IO
recorder was in operation for 18 of the measurements. For the remaining tight mcasurements, radiation values were taken from
data oE the U.S. Wcathcr Bureau (Climatological Date-National
Summary) for Cape
IIattcras-the
station nearest Beaufort.
2 Registered trademark, Milliporc Filter Corporati on, Bedford, Massachusetts.
RESULTS
Physical
data
Conditions in the Beaufort Channel are
summarized in Fig. 1. Water temperature
followed a seasonal cycle with a midwinter
low of 4C and midsummer high of 28C.
Salinity ranged from 24 to 36%0,with lowest
values in the winter. The general pattern of
alternately higher and lower salinities reflected the state of the tide when the sample was collected-higher
salinities at high
tide and lower salinities at low tide. Secchi
disc readings ranged from 0.5 to 2.5 m and
averaged 1.4 m. Extinction coefficients obtained from these ranged from 3.4 to 0.68
and averaged 1.2. The depth for the bottom
of the euphotic zone (the level of 1% of surface illumination)
obtained from this average coefficient was 4.6 m, indicating that
the entire water collumn lay within the cuphotic zone over most of the estuary most
of the time. There were generally greater
transparencies during summer and fall,
Photosynthesis (Fig. 2) has a pronounced
seasonal cycle with high values during the
summer and early fall and low values
throughout the remainder of the year. There
was no suggestion of a spring bloom or
other brief pulse of high production.
The
range for gross photosynthesis at the sur-
76
RICHARD B. WILLIAMS
AND MARIANNE
face, 0.12 to 0.72 mg C liter-l day-l, was
within that previously recorded for fertile
inshore areas (Patten 1961; Riley 1941;
Strickland 1960). The pattern of alternately
higher and lower values for successive measurements was, like salinity, associated with
the stage of the tide when the water was
collected-high
values with low-tide samples and low values with high-tide samples.
This pattern indicated that the estuarine
water was more productive than the ocean
water. Differences in productivity between
the estuarine and oceanic water were, as in
the High Venice Lagoon (Vatova 1961))
smaller in winter than in summer,
Throughout the year, maximum photosynthesis for the 24-hr day was obtained at
either 100 or 50% of surface illumination;
25 and 10% illumination usually yielded successively lesser amounts (Fig, 2). These results agreed with a previous observation
that a phytoplankton population exposed to
changing illumination
because of turbulent
mixing, adapted to the higher levels of illumination ( Ryther and Menzel 1959).
In each experiment, measurements of photosynthesis at 50, 25, and 10% of surface illumination were expressed as percentages
of surface photosynthesis and these were
pooled into a single curve ( Fig. 3). Photosynthesis at these lower light intensities
relative to photosynthesis at surface illumination varied widely between successive
measurements and had no seasonal cycle.
Average values ranged from 104% at 50% of
surface illumination
to 58% at 10% of surClear days produced
f ace illumination.
relatively greater subsurf ace photosynthesis
than cloudy days, but the differences were
small, and except for the largest (that at
25% illumination)
not significant (Fig. 3).
The relationships between insolation, water temperature, and surface gross photosynthesis are shown in Fig. 4. The similarity of the seasonal cycles in photosynthesis
and temperature produced the obvious correlation between these variables. Measurements of photosynthesis formed two groups
-high
values obtained at 20C and above,
and lower values obtained below 20C. The
significant positive correlation between in-
B. MURDOCH
100
1
I
CLOUDY
-
I
1
AVERAGE
60
2
5
5
g
60
40
3
-I
20
0
20
40
DAILY
60
GROSS
60
100
PHOTOSYNTHESIS
120
140
(%)
3. Relative rates of photosynthesis
at perJAG.
centages of surface illumination.
The lines indicate separate averages for clear and cloudy days,
and an overall average.
The horizontal
bars indicate 95% confidence limits for the overall average.
solation and photosynthesis was, although
not obviously, probably a reflection of the
correlation between insolation and water
temperature. Both measurements of photosynthesis at temperatures below 20C and
measurements at 20C and above were made
under a wide variety of light conditions
(Fig. 4). When these groups of measurcments were analyzed separately, the correlations between photosynthesis and insolation were no longer significant.
FIG. 4. Three-dimensional
bar graph depicting
daily rates of photosynthesis
at surface illumination
in relation to water temperature
and to insolation.
Each bar represents one measurement.
The length
of the bar indicates the photosynthesis
obtained,
and the location of the bar in the horizontal plane,
the temperature
and insolation during the cxperiment.
PHYTOPLANKTON
TABLE 1.
PRODUCTION
Summary
AND
of phytoplankton
Chlorophyll n
(/-a/
liter)
LOW production
period
Dee-May
and 14 Ott-Dee
High
production
June-7 Ott
CHLOROPIIYLL
4.8
BEAUFORT
data-auerage
SF;$i
77
CHANNEL
values
Gross Pho-
Respiration
188/m3
182,/m’
318/m2
79/m”
79/m’
316/m2
547/n?
553/m’
l,196/m2
162/m’
162/m2
648/m2
readmg
I.*
(m)
1.14
At surface
For a 1.0-m water
For a 4.0-m water
column
column
period
3.6
Estimated
annual
1.46
like
At surface
Por a 1.0-m water
For a 4.0-m water
values for 125 high production
Gross photosynthesis
(g C/n-P)
Respiration
(g C/m’)
Respiration,
production
IN
photosynthesis,
z~vcra@XI
higher in warm weather,
but unlike photosynthesis, fluctuated irregularly from measurement to measurement (Fig. 2). Values
ranged from 0.01 mg C literlday-l
in midwinter to 0.29 mg C liter-lday-l
in early
October when the Beaufort Channel was
c,onspicuously polluted with wastes from
fish processing plants. Although respiration
had a highly significant colrrelation with surface gross photosynthesis, the ratio of respiration to photosynthesis ranged from 5 to
913%. Under both warm and cool conditions
(above and below 2OC) respiration averaged
approximately 40% of surface gross photosynthesis.
To estimate the rate of production in the
estuarine system at Beaufort, gross photosynthesis and respiration were calculated
for a water column 1.0 m deep (average
depth of the Newport River estuary) and
for a water column 4.0 m deep which included the entire euphotic zone (Table 1).
Rates of subsurface photosynthesis were obtained by multiplying
surface gross photosynthesis by the relative rates (Fig. 3) at 50,
25, and 10% of surface illumination,
Below
lo%, photosynthesis was assumed to be proportional to illumination.
The light depths
were estimated from the average Secchi
colmnn
column
and 240 low production
days
1.0 m
water
column
4.0 ni
water
column
113
39
225
157
disc reading. Integration
of the rates of
photosynthesis down to 1.0 or 4.0 m yielded
photosynthesis/m”.
Data from the Bcaufort
Channel were divided between a warm,
more productive period (June through 7
October) and the cool, less productive remainder of the year. These periods approximately corresponded to water temperatures
above and below 20C.
The daily rates of gross photosynthesis/m2
(Table 1) were similar to the range of values
reported from coastal waters (Ryther 1963;
Strickland 1960). The results suggest that
photosynthesis was insufficient to support
the respiration only in the deepest channels;
clsewherc there was net photosynthesis.
In agreement with investigations
clsewhere (Yentsch and Ryther 1959), phytoplankton production throughout the year in
the Bcaufort Channel was derived prccmincntly from the nannoplankton
( Fig, 5).
Removal of net plankton decreased surface
gross photosynthesis an average of 27% between June and 7 October, and 9% the remaindcr of the year. Results were similar at
the three lower levels of illumination.
Rcspiration, however, was increased an average
of 17% by removal of net plankton. In many
cases the increase seemed too large to have
resulted from errors in technique. The in-
RICHARD
B. WILLIAMS
AND
FIG. 5. a. Cross photosynthesis
at surface illumination and respiration
of total plankton and of
nannoplankton
alone. b. Chlorophyll
a concentration and the ratio of the average rate of gross
photosynthesis
during the dayIight hours at surface
illumination
to chlorophyll
n concentration.
crease in the filtered water might have resulted from increased bacterial activity following the removal of algae or other material releasing antibiotics.
Standing crop
There was no seasonal change in cell numbers corresponding to the cycle in productivity. Cell counts varied irregularly from
0.13 to 5.4 X lo6 cells/liter, but averaged
2 X IO6 in both warm and cool weather, Although flagellates often were present, small
centric diatoms, such as Skeletonema costatum, were the most abundant plankton
throughout the year. Filtration of the water
through No. 25 netting reduced both cell
numbers and chlorophyll a concentration an
average of 21%.
The seasonal cycle of chlorophyll a concentration suggested an inverse relationship
between chlorophyll a and gross photosynthesis ( Fig. 5)) because the average concentration was 4.8 pg,Iliter during the cool, less
productive period and 3.6 pg/liter during the
warm, more productive period ( Table 1).
The range in chlorophyll a concentration in
the Beaufort Channel, 2.0 to 9.3 pg/liter
(Fig. 5), was similar to that observed in
Chesapeake Bay (Patten et al. 1963) and
Long Island Sound (Conover 1956) except
during phytoplankton
blooms in those localities, However, chlorophyll a estimates
may at times have included significant
amounts of detrital pigment and thus may
MARIANNE
B. MURDOCH
not precisely delineate the seasonal cycle in
phytoplankton
pigment. A highly significant positive correlation between chlorophyll u concentrations and Secchi disc readings suggested that part of the pigment in
the higher concentrations was derived from
material temporarily suspended at times of
reduced transparency, and a lack of significant correlation between Secchi disc readings and surface gross photosynthesis suggested that this material was detrital.
Relationship between chlorophyll
and photosynthesis
The seasonal variation in surface gross
photosynthesis and its negative correlation
with chlorophyll a were reflected in the pronounced seasonal cycle in the rate of photosynthesis per unit of chlorophyll a (Fig. 5).
The values were obtained by dividing gross
photosynthesis over the 24-hr day by the
concentration
of chlorophyll
and by the
period of daylight (9.8 to 14.4 hr). Values
ranged from 1.9 mg C (mg Chl a)-i hr-l in
early winter to 19.8 in midsummer.
The
average for the more productive period was
12.1 mg C (mg Chl a)-l hr-l, and for the less
productive period, 4.2. The ratios of photosynthesis to chlorophyll,
although higher
than many of those previously reported,
were minima1 estimates of the ratio potentially obtainable at optimum illumination,
because insolation approximated the optimum level only briefly during the cxperiments.
There was no significant correlation between average intensity of insolation and the
ratio of photosynthesis to chlorophyll, and
the position of the seasonal cycle in this
ratio (Fig. 5) suggested that its value was a
function of water temperature rather than
of insolation. The relationship between the
logarithm of the ratio and temperature (Fig.
6) was fitted to a least squares regression:
log [mg C (mg Chl a)-l hr-l]
= 0.138 $0.0353 x temperature
( “C ) .
The slope of the regression yielded a QUI
value of 2.25 for the ratio; 95% confidence
limits were 1.86 and 2.73. Values predicted
by, the regression for individual
measure-
PHYTOPLANKTON
PRODUCTION
AND
CHLOROPHYLL
IN
ments should, in 95% of the cases, lie between one-half and twice the true value
( Fig. 6). Secchi disc readings for the observations above the regression line averaged 1.4 m and for those below the line 1.0
m, suggesting that some of the scatter arose
from variations in the ratio of phytoplankton pigment to detrital pigment.
--_-_
BEAUFORT
95% CCWWDENCE
REGRESSION
L,NE
79
CHANNEL
LIMITS
FOR
THE
95% CONFIDENCE
LlMlTS
FOP
THE
PREDICTICIN
OF PHOTOSYN
/ CHLCJ
FROM TEMPERATURE
/’
,’
/’
DISCUSSION
Although temperature, salinity, and transparency in the Beaufort Channel appear to
be suitable for the growth of phytoplankton
throughout the year, there is a pronounced
seasonal cycle in phytoplankton production
that tends to follow the water temperature
cycle. Annual cycles in the production of
marine phytoplankton,
with higher values
in summer, are characteristic of shallow temperate embayments. Such cycles have been
observed in widely separated studies-Long
Island Sound and bays along the coasts of
Massachusetts, Italy, and Denmark. They
have been ascribed to a more rapid regeneration of nutrients in warm weather following increased bacterial metabolism at elevated temperatures (Grgntved 1960; Riley
1941, 1956; Ryther 1963; Steemann Nielsen
1958; Vatova 1961) . The mechanism of this
temperature
regulation
in the Beaufort
Channel is unknown, but it may be related
to the rate of nutrient regeneration.
The seasonal and short term variations in
the phytoplankton production in the Beaufort Channel are intermediate in magnitude
among those reported in previous inshore
studies. Seasonal variation was small in
Long Island Sound ( Riley 1941)) threefold
in the Beaufort Channel and lo- to 20-fold
in the Danish Belt (Steemann Nielsen 1958)
and in the High Venice Lagoon (Vatova
1961)) although the seasonal variation in
water temperature was greater in the Beaufort Channel and Long Island Sound than
in the Venice Lagoon and the Danish Belt.
Short term variations were negligible in the
High Venice Lagoon (where each estimate
of daily production is an average of a high
and a low tide sample), two- to fourfold in
the Beaufort Channel and the Danish bays,
and up to ninefold in Long Island Sound.
I'
( ,I'
0
I'
I
IO
TEMPEP;’
I
20
1
30
IVE (Or j
6. The effect of water temperature on the
ratio of the rate of gross photosynthesis
to chlorophyll a concentration.
FIG.
Had the estimate for the High Venice Lagoon been based on alternate high tide and
low tide samples, short term variations there
would have been twofold and thus similar
to those at Beaufort. These four studies suggest that the amplitude of the seasonal cycle
in phytoplankton production is not closely
related to1 seasonal variation in water temperature and that short term variations in
phytoplankton production are least in areas
like the High Venice Lagoon and the Beaufort Channel where tidal exchanges are
large. The pattern of respiration in the
Beaufort Channel is similar to that in Long
Island Sound (Riley 1941) with an average
value close to half the gross production,
much irregular variation, and higher rates
in warm weather.
Although the rate of gross photosynthesis
per unit volume is moderately high near the
surface in the Beaufort Channel (that is,
three times that of Long Island Sound) this
80
RICHARD
B. WILLIAMS
AND
rate is insufficient
to compensate for the
shallowness and turbidity of the water, so
photosynthesis per unit area is no greater
than that in many regions of the open sea
and far below that of fertile benthic cornmunities of marsh grass or macroscopic algae
(Odum 1961; Riley 1941; Ryther 1963). Although reduction in turbidity would mark:
edly increase photosynthesis per unit area
in the deeper channels, such channels comprise only a small fraction of the Newport
River estuary. Much of the estuary is so
shallow that light adequate for rapid photosynthesis already penetrates to the bottom.
In these areas, production per unit area
could be markedly clevatcd only by increasing the rate of production per unit volume,
The daily rate of surface photosynthesis
in the Beaufort Channel can be predicted
from chlorophyll a concentration, day length,
and water temperature by a three-step procedurc: 1) Obtain the ratio of photosynthesis to chlorophyll a from water temperature
(Fig. 6). 2) Obt ain the mean hourly rate
of photosynthesis by multiplying
this ratio
by the concentration of chlorophyll
a. 3)
Multiply the mean hourly rate by the hours
of daylight. The procedure differs from that
of Ryther and Yentsch ( 1957) principally in
that a variable ratio of photosynthesis to
chlorophyll
is substituted for their single
fixed value, and in that the rate of photosynthesis is expressed as the mean rate at
the surface during daylight hours rather
than the rate occurring at o,ptimal illumination. Adjusting the ratio of photosynthesis
to chlorophyll a for changes in temperature
increases the accuracy of prediction. Daily
rates calculated by this three-step procedure
differed from the measured rates (Fig. 2) by
an average of 26%, whereas daily rates calculated from a constant ratio of photosynthesis to chlorophyll a [the mean for the 26
samples: 6.9 mg C (mg Chl a)-l hr-l] differed
from the measured rates by an average of
58%. Use o,f the mean rate for photosynthesis during daylight hours avoids the need for
first measuring the rate at various light intensities and subsequently calculating rates
at different times of day from these mcasurements.
MARIANNE
B. iMURDOCH
An increase in the ratio of photosynthesis
to chlorophyll with increasing temperature
is present in the data of Barlow, Lorenzen,
and Myren (1963), Hepher (1962), Ichimura
(1960), Steemann Nielsen and Hansen
(1961), and Ryther and Yentsch (1957), suggesting that it is a widespread phenomenon,
at least in locations where photosynthesis is
not limited by a shortage of nutrients. Observations on the temperature dependence
of the rate of light-saturated photosynthesis
(Ichimura 1960; Talling 1957; Wassink et al.
1938) provide a possible explanation for this
increase in assimilation numbers with increasing temperature.
These studies on
planktonic algae indicated that once a certain light intensity is reached, the speed of
the dark reactions limits the rate of photosynthesis. With constant temperature, furthcr increases in illumination
above this
point of light saturation do not increase
photosynthesis. However, the dark reactions
arc speeded by increased temperature, elcvating the light intensity at which saturation
occurs, and permitting more complete utilization oE the light energy. The average Qlo
value for light-saturated
photosynthesis in
these three studies lay between 2.3 and 2.5,
values close to the Qlo of the ratio of photosynthesis to chlorophyll CIfor phytoplankton
at the surface of the Beaufort Channel.
Since it is likely that the photosynthesis of
phytoplankton exposed to surface illumination is not light limited throughout much of
the day, it is reasonable to find the rate of
photosynthesis modified by temperature.
Values far in excess of 3.7 mg C( mg Chl
a)-1 hr-l for photosynthesis per unit of chlorophyll obtained in warm weather may not
be artifacts arising from incomplete extraction of pigment, as suggested by Barlow ct
al. (1963) and by Ryther and Yentsch (1957).
Iu our study, underestimation
of this ratio
through overestimation
of phytoplankton
chlorophyll stems more probable because of
contamination by detrital pigments. Diatoms, the dominant algal group observed in
the cell counts, are completely extracted by
99% acetone (Antia et al. 1963). Although
an abundance of small plankters, inconspicuous on a cleared Millipore filter and in-
PHYTOPLANKTON
PRODUCTION
AND
completely extracted by 90% acetone, is not
impossible, our data contain no evidence of
such organisms. An alternative explanation
for the high values observed in warm
weather in the surface layers of stratified
waters (Ichimura 1960) as well as in shallow, fertile embayments is that they truly
represent the physiological condition of the
phytoplankton.
REFERENCES
ANTIA, N. J., C. D. MCALLISTER, T. R. PARSONS,
K. STEPHENS, AND J. D. H. STRICKLAND.
1963. Further measurements of primary production using a large-volume
plastic sphere.
Limnol. Oceanog., 8: 166-183.
BARLOW, J, P., C. J. LORENZEN, AND R. T. MYREN.
1963. Eutrophication
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