Limnol. Oceanogr., 45(3), 2000, 591–600
q 2000, by the American Society of Limnology and Oceanography, Inc.
Nutrient and temperature control of the contribution of picoplankton to phytoplankton
biomass and production
Nona S. R. Agawin,1 Carlos M. Duarte, and Susana Agustı́
Instituto Mediterraneo de Estudios Avanzados (CSIC-UIB), Edificio Mateu Orfila, Campus Universitario UIB, Carretera de
Valldemossa km. 7.5, 07071 Palma de Mallorca, Spain
Abstract
The observation that the relative importance of picophytoplankton is greatest in warm and nutrient-poor waters
was tested here based on a comprehensive review of the data available in the literature from oceanic and coastal
estuarine areas. Results show that picophytoplankton dominate ($50%) the biomass and production in oligotrophic
(chlorophyll a [Chl a] , 0.3 mg m23), nutrient poor (NO3 1 NO2 , 1 mM), and warm (.268C) waters, but
represent ,10% of autotrophic biomass and production in rich (Chl a . 5 mg m23) and cold (,38C) waters. There
is, however, a strong covariation between temperature and nutrient concentration (r 5 20.95, P , 0.001), but the
number of observations where both temperature and nutrient concentrations are available is too small to allow
attempts to statistically separate their effects. The results of mesocosm nutrient addition experiments during summer
in the Mediterranean Sea allowed the dissociation of the effects of temperature from those of nutrients on picophytoplankton production and biomass and validated the magnitude at which picoplankton dominates ($50%)
autotrophic biomass and production obtained in the comparative analysis. The fraction contributed by picoplankton
significantly declined (r 2 5 0.76 and 0.90, respectively, P , 0.001) as total autotrophic production and biomass
increased. These results support the increasing importance of picophytoplankton in warm, oligotrophic waters. The
reduced contribution of picophytoplankton in warm productive waters is hypothesized here to be due to increased
loss rates, whereas the dominance of picophytoplankton in warm, oligotrophic waters is attributable to the differential capacity to use nutrients as a function of differences in size and capacity of intrinsic growth of picophytoplankton and larger phytoplankton cells.
The picophytoplankton, autotrophs ,2 mm in size, contribute at least 10% to total global aquatic net primary productivity (Raven 1998). The wealth of data on picoplankton
abundance and production in the sea has led to the conclusion that these organisms play a much greater role in oligotrophic waters (Stockner and Antia 1986; Raimbault et al.
1988; Peña et al. 1990; Chisholm 1992; Magazzù and Decembrini 1995; Li 1998). This dominance would be based
on their small cell size associated to small diffusion boundary layers and large surface area per unit volume (Raven
1986). This confers small phytoplankton cells an advantage
in oligotrophic waters by leading to a greater capacity to
acquire nutrients and the efficiency in their use for growth
and maintenance (Raven 1998). However, oligotrophic waters are often warm, so the perceived dominance of picophytoplankton in these waters could derive from their greater
abundance (Li 1998) and growth (Agawin et al. 1998) in
warm waters. Although there is evidence for consistent
changes in the relative contribution of picoplankton to total
autotrophic biomass (Chisholm 1992), there is also a need
to describe and test the generality of the relationship between
1
the contribution of picoplankton to total autotroph biomass
and primary production in the sea, and between nutrient
availability and temperature. Because of the strong covariation of these factors in the sea (Li 1998), the patterns identified can only be used to formulate predictions if they are
previously validated experimentally. Moreover, it has also
been argued that picophytoplankton can also be important in
nutrient-rich waters. Except for methodological problems
(i.e., filter clogging during size fractionation, Carrick and
Schelske 1997), this problem has not been adequately addressed.
Here we examine the significance of temperature and nutrient concentrations in determining the contribution of picophytoplankton to total phytoplankton biomass and production, by examining phytoplankton communities growing
under different nutrient regimes at different geographical areas. Our goal is to test the prediction that the relative importance of picophytoplankton is the highest in warm and
nutrient-poor waters. We first establish patterns in nature
based on a comprehensive review of the data available in
the literature from oceanic and coastal estuarine areas. Because of the generally negative covariation between nutrient
concentration and seawater temperature (Li 1998), these patterns cannot provide evidence of a cause and effect relationship between nutrient availability and the relative abundance
of picophytoplankton. We have, therefore, tested the predictive power of the patterns derived from our cross-system
comparison by examining the contribution of picoplankton
to autotrophic biomass and production across a range of nutrient inputs in experimental mesocosms in the NW Mediterranean Sea.
Corresponding author (ieanar@clust.uib.es).
Acknowledgements
This study was funded by the European Commission under the
ELOISE programme COMWEB (contract MAS3-CT96-0045). N.
S. R. Agawin is supported from a fellowship of the Agencia Española de Cooperación Internacional. Our sincere gratitude to A.
Lucea for the nutrient analyses, E. Benavent and C. Sánchez for
phytoplankton measurements, E. Cuñado for help with the literature
review, C. A. Juan for assistance in the operation of the mesocosms,
and X. Moran and J. Gasol for help in the 14C scintillation counting.
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Agawin et al.
Methods
We searched the published literature for size-fractionated
data on phytoplankton production and biomass as Chl a concentration. The picophytoplankton fraction threshold varied
from ,1 to ,3 mm among the various studies. Biomass and
production were recorded as mg Chl a m23 and mg C m23
d21, respectively. Data were compiled per unit volume and
production as daily rates. When data in the literature are
given per unit area (vertically integrated over the euphotic
zone), they were transformed to volumetric values taking
into account the depth of the euphotic zone reported. The
conversion of integrated abundance to volume abundance
may have possibly introduced error. For biomass data, 16%
are converted values, whereas for production data, 29% are
converted values. We tested this possibility by identifying
the data on the plots, which suggested no differential behavior compared to the remaining data. Production values given
in hourly rates were transformed to daily rates by multiplying by an average of 12 daylight hours. Inorganic nutrient
concentrations (NO3 1 NO2, NH4, PO4) and temperature data
were also recorded when provided.
The experimental test of the response of the relative contribution of picophytoplankton to total phytoplankton biomass and production to increased nutrient inputs was gathered during a mesocosm experiment in the NW
Mediterranean Sea (Blanes Bay, 41839.909N, 2848.039E).
The experiment conducted during summer (18 June–8 July
1997), consisted of seven mesocosms (14 m in height, 4.2
m 2 cross-sectional area, with an effective volume of 33 m3),
suspended from a platform moored at a site 1 km offshore
from the Bay of Blanes at a depth of 35 m. The experiment
consisted of a gradient of nutrient additions from 0 to 0.5,
1, 2, 4, 8, and 16 times the estimated mean nutrient input to
the Bay of Blanes (about 0.64 mM N d21, Duarte et al. 2000),
reaching a volumetric input of 10.1 mM N d21 at the highest
dosage. Mass balance calculations allowed the estimation of
the atmospheric input of nutrients during the experiment to
be approximately 0.005 mM N d21 (Duarte et al. 2000). Nitrogen (N), phosphorus (P), and silicon (Si) were added at a
constant ratio (20N : 7 Si : 1 P), determined empirically to be
the average nutrient ratio in the nutrient loading to Blanes
Bay (Duarte et al. unpubl. data). The nutrients were added
as ClNH4 (N), KH2PO4 (P), and Na2SiF6 (Si) every other day
during the 21 d of the experiment. Further details are provided in Duarte et al. (2000). Integrated water samples of
50 liters were collected on alternate days for biological and
chemical (dissolved nutrient concentrations) analyses.
Primary production of the ,2-mm fraction and whole seawater were determined in each mesocosm every 4 to 5 d
from the initiation of the experiment. For each nutrient level,
a subsample (500 ml) was filtered through 2-mm (pore size)
polycarbonate filters. The filtration procedure proved to be
effective in fractionating picophytoplankton (mostly Synechococcus sp.) as 93% of the population passed through the
2-mm PC filters. Three (two clear and one dark) 125-ml PC
Nalgene bottles were dispensed, each with 120 ml of the
,2-mm fraction, while another set of three bottles were filled
with whole seawater of the same mesocosm. One milliliter
of 14C solution in varying concentrations (23, 20, 15, 10, 5
mCi), depending on the increase of phytoplankton biomass
and expected carbon uptake, was added in each bottle. The
light bottles were incubated at 200 mE m22 s21 and placed
along with the dark bottles in an incubator chamber with
temperature control (adjusted to the in situ water temperature). After 3 h of incubation, samples were filtered through
0.45-mm Millipore filters, and filters were fumed over concentrated HCl to remove traces of inorganic C. Radioactivity
on the filters was measured with a liquid scintillation counter, with correction for quenching, and converted to C incorporation rates as described by Strickland and Parsons
(1972). All materials were acid-cleaned prior to use.
The contribution of picophytoplankton biomass to total
autotrophic biomass was derived from the fraction of picophytoplankton chlorophyll biomass to total phytoplankton
chlorophyll. Phytoplankton Chl a concentration was determined using the method of Parsons et al. (1984). A variable
amount of water (50 to 500 ml depending on phytoplankton
biomass) was filtered through Whatman GF/F filters, which
later were homogenized. The pigments were then extracted
in 90% acetone for 6 h and refrigerated in the dark. Following extraction, fluorescence was measured in a Turner Designs fluorometer calibrated with pure Chl a (Sigma). Picoplankton chlorophyll biomass was determined by
converting picoplankton biovolume measurements to chlorophyll using the value 3,645 g Chl m23 cell volume (Barlow
and Alberte 1985). Picoplankton biovolume was calculated
as the product between abundance and average cell volume
based on the coccoid shape of picophytoplankton (e.g., Synechococcus sp.). Abundance and cell size (diameter) of picophytoplankton (cells ,2 mm) were determined by flow
cytometric analysis of fresh, unfractionated samples with a
FACSCALIBUR (Becton-Dickinson) flow cytometer. Cell
size was calculated from the FSC (Forward Scatter) data of
the picophytoplankton cells and calibrated using fluorescent
beads of various sizes. Analyzing algal cultures of known
sizes of Synechococcus sp. (1.3 mm) and Chlorella sp. (2.7
mm) (sizes determined from epifluorescent microscope measurements) together with latex beads of various sizes (nominal sizes of 0.5 to 4.5 mm) on the flow cytometer proved
the appropriateness of our size calibration procedure. Cells
larger than 2 mm were counted under epifluorescence and
optical inverted microscope, depending on size, and the cell
volume was calculated from the measured dimensions of the
cells (cf. Agustı́ et al. in prep.).
The relationships between the total and relative biomass
and production of picoplankton and nutrient concentrations
and temperature were described by linear regression of arithmetic or log-transformed values of the variables. The variance explained by the fitted models and the significance of
the fit were tested by analysis of variance (ANOVA, Sokal
and Rohlf 1981).
Results
The data set compiled (available at the Web site
www.aslo.org/lo/pdf/volp45/issuep3/0591a1.pdf) included 38
Picoplankton biomass and production
published reports at different areas encompassing a wide
range of nutrient conditions (from oligotrophic open oceans
to rich upwelling areas and estuaries) and representing all
major oceans and seas. The data set is comprised of samples
of different depths (0 to 200 m) from a wide variety of
locations. Total phytoplankton biomass (Chl a) compiled
from the literature ranged from 0.02 to 46.6 mg m23, although most values were ,2 mg m23. In less productive
waters, the picoplankton fraction had higher biomass than
the nano- and microphytoplankton fraction, whereas in more
productive waters, the larger phytoplankton had higher biomass than the picoplankton (Fig. 1A), as described by the
regression in Table 1. Picophytoplankton biomass (chlorophyll) also increased as total phytoplankton biomass increased but increased slower (H0: slope 5 1; t-test, P ,
0.001) than the biomass of the total phytoplankton community (Table 1). This indicated that the mean and maximum
fraction of picoplankton to total phytoplankton biomass decreased significantly (Table 1) as total phytoplankton biomass increased in more productive waters (Fig. 1B).
Total phytoplankton production ranged from 0.002 to
2078 mg C m23 d21, and the picophytoplankton fraction had
higher production in less productive waters than the nanoand microphytoplankton (Fig. 2A). In more productive waters, the larger phytoplankters have higher production than
the picoplankton (Fig. 2A). The relationship between the primary production of the picoplankton and the rest of the larger phytoplankton was described by the linear regression in
Table 1. The picophytoplankton production also increased
with increasing community production but increased slower
(H0: slope 5 1; t-test, P , 0.001) than total phytoplankton
production (Table 1). As a result, there was a tendency for
a significant (Table 1) decline of the mean and maximum
fraction of picoplankton production as total production increased (Fig. 2B). Concurrent estimates of picoplankton and
total phytoplankton production and biomass allowed the calculation of P/B or turnover rates of both picoplankton and
the nano- and microphytoplankton fraction or total phytoplankton. Turnover rates of picoplankton increased linearly
with increasing nano- and microphytoplankton or total phytoplankton turnover (Table 1), although it increased slightly
slower (H0: slope 5 1; t-test, P , 0.01) than the rest of the
larger phytoplankton (Fig. 3) or than the total phytoplankton
turnover. These relationships included the three outlier data
points at the lower left side of Fig. 3 in the calculations,
which come from data in the Arabian Sea (Jochem et al.
1993). These data are valid, and the slopes of the regression
between picoplankton and the larger phytoplankton turnover
rates were statistically not significantly different with and
without the three data points (P . 0.05, T9 method in unplanned comparison among regression coefficients, Sokal
and Rohlf 1981). Picoplankton turnover rates were correlated
both with the larger phytoplankton and total phytoplankton
production and increased linearly with increasing nano- and
microphytoplankton production and total phytoplankton production (Table 1). The increase however, was also slower
(H0: slope 5 1; t-test, P , 0.01) than the larger phytoplankton fraction or than the total phytoplankton production did.
Picophytoplankton biomass tended to increase at its maximum in very low nutrient concentrations in the water (NO3
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Fig. 1. (A) The relationship between picophytoplankton biomass and the nano- and microphytoplankton biomass (mg Chl m23)
in different waters. Solid and empty symbols represent data points
recalculated from integrated biomass (16% of total data points) and
volumetric values, respectively. The dashed line indicates the 1 : 1
correspondence ratio, and the solid line indicates the fitted linear
regression line as described in the text and in Table 1. (B) The
relationship between the percent contribution of picophytoplankton
fraction to total phytoplankton biomass (mg Chl m23). Solid symbols represent the mean percentage within increasing bins and their
standard error, and the solid line indicates the envelope determined
by fitting a curve to the maxima of the bin intervals.
1 NO2 , 1 mM) and tended to reach a plateau or even
declined at higher nutrient concentrations (Fig. 4A). Consequently, the fraction of picoplankton biomass to total phytoplankton biomass reached its maximum in very low nutrient concentrations in the water (NO3 1 NO2 , 1 mM, Fig.
4B) but generally declined at higher nutrient concentrations
(Table 1). The pattern of picoplankton primary production
Nutrients (NO3 1
NO2)
Temperature
Nano- and microphytoplankton turnover
Nano- and microphytoplankton primary
production
Total phytoplankton
turnover
Total phytoplankton
primary production
y 5 0.56(60.24) · x 1
14.55(64.45), r 2 5 0.04,
P , 0.05
log(y) 5 20.19(60 05) ·
log(x) 1 1.05(60.07), r 2
5 0.16, P , 0.05
log(y) 5 0.66(60.03) ·
log(x) 2 0.66(60.02), r 2
5 0.50, P , 0.05
log(y) 5 0.41(60.03) ·
log(x) 2 0.62(60.02), r 2
5 0.30, P , 0.05
Total phytoplankton
biomass
P . 0.05
ln(y) 5 20.34(60.03) ·
ln(x) 1 3.07(60.04), r 2 5
0.20, P , 0.05
y 5 223.59(61.04) · log(x)
1 25.80(60.89), r 2 5
0.49, P , 0.05
Picoplankton biomass
Independent variable
Nano- and microphytoplankton biomass
% contribution of
picoplankton to
total biomass
% contribution of
picoplankton to
total primary
production
Picoplankton
turnover
y 5 24.96(60.64) · ln(x) 1 y 5 0.23(60.10) · x 1
46.93(61.97), r 2 5 0.13,
30.48(64.46), r 2 5 0.02,
P , 0.05
P , 0.05
y 5 215.28(61.15) · log(x) log(y) 5 0.49(60.05) ·
log(x) 1 0.84(60.05), r 2
1 47.26(61.42), r 2 5
0.32, P , 0.05
5 0.34, P , 0.05
log(y) 5 0.89(60.04) ·
log(x) 1 0.03(60.06), r 2
5 0.71, P , 0.05
log(y) 5 0.65(60.05) ·
log(x) 1 0.29(60.08), r 2
5 0.47, P , 0.05
y 5 0.66(60.18) · x 1
y 5 1.36(60.23) · x 1
y 5 1.82(60.06) · x 1
1.13(62.83), r 2 5 0.11, P
6.32(63.70), r 2 5 0.24, P
7.87(69.34), r 2 5 0.18, P
, 0.05
, 0.05
, 0.05
y 5 24.53(11.29) · log(x) +
18.13(60.90), r 2 5 0.25,
P , 0.05
log(y) 5 0.84(60.02) ·
log(x) 2 0.44(60.03), r 2
5 0.74, P , 0.05
log(y) 5 0.63(60.03) ·
log(x) 2 0.07(60.04), r 2
5 0.55, P , 0.05
Picoplankton
primary
production
Table 1. Summary of regression coefficients and their significance as tested by ANOVA between data compiled from the literature. Column heads are dependent variables.
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Agawin et al.
Picoplankton biomass and production
595
Fig. 3. The relationship between the turnover rates of the picoplankton fraction and that of the nano- and microphytoplankton
community. The dashed line indicates the 1 : 1 correspondence ratio,
and the solid line indicates the fitted linear regression line as described in the text and in Table 1.
Fig. 2. (A) The relationship between picophytoplankton production and the nano- and microphytoplankton production (mg C
m23 d21) in different waters. Solid symbols and empty symbols
represent data points recalculated from integrated production (29%
of total data points) and volumetric values, respectively. The dashed
line indicates the 1 : 1 correspondence ratio and the solid line indicates the fitted linear regression line as described in the text and in
Table 1. (B) The relationship between the percent contribution of
picophytoplankton fraction to total phytoplankton production (mg
C m23 d21). Solid symbols represent the mean percentage within
increasing bins and their standard error, and the solid line indicates
the envelope determined by fitting a curve to the maxima of the
bin intervals.
with increasing nutrient concentrations was not as clear as
that of its biomass due to the smaller number of observations
(n 5 39, Fig. 4C), but generally, picoplankton primary production was relatively high at lower nutrient concentrations
(NO3 1 NO2 , 1 mM), although the smoothed data set also
showed high picoplankton production at intermediate nutri-
ent levels (NO3 1 NO2 ; 16 mM). However, the fraction of
picoplankton production to total phytoplankton biomass increased at its maximum in very low concentrations in the
water (NO3 1 NO2 , 1 mM, Fig. 4D), but generally declined
at higher nutrient concentrations (Table 1). Picophytoplankton primary production, but not biomass, was significantly
linearly correlated with temperature (Table 1, Fig. 5A, 5C).
The fitted regression between picophytoplankton primary
production and temperature resulted in a Q10 of 1.85, meaning that picoplankton primary production almost doubled
with a 108C rise in temperature. Consequently, the fraction
of picophytoplankton production increased with increasing
temperature (Table 1, Fig. 5D). However, temperature was
only weakly correlated with the fraction of picophytoplankton to total phytoplankton biomass (Table 1, Fig. 5B), although picoplankton turnover rates were significantly linearly correlated with temperature (Table 1). Temperature may
be as important as nutrient concentrations in explaining the
variability in the primary production of picophytoplankton
and its contribution to the total phytoplankton production.
Water temperature and nutrient concentrations were negatively correlated (r 5 20.95, P , 0.001, n 5 40), but the
number of observations where both temperature and nutrient
concentrations were available is too small (only 10% of the
data set) to even allow attempts to statistically (path analysis) separate their effects. Hence, we investigated the role of
nutrients in a mesocosm experiment in Blanes Bay, NW
Mediterranean, where the effect of a gradient of increasing
nutrient inputs on picophytoplankton production and biomass and their contribution to total phytoplankton biomass
and production was tested in summer.
Water temperature during the mesocosm experiments was
relatively constant (;21 6 18C), because the bags were suspended in situ. Autotrophic biomass ranged from 0.09 to
40.2 mg Chl m23, and primary production ranged from 11
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Agawin et al.
Fig. 4. The relationship between the (A) picoplankton biomass (mg Chl m23), (B) percent contribution of picophytoplankton fraction
to total phytoplankton biomass, (C) picoplankton primary production (mg C m23 d21), (D) percent contribution of picophytoplankton fraction
to total phytoplankton production, and NO3 1 NO2 concentrations (mM) in different waters. Solid lines represent the underlying trend
described by locally weighted scatterplot smoothing (Cleveland 1979).
and reached to 4,367 mg C m23 d21 at high nutrient loading.
The fraction contributed by picoplankton to total primary
production and to total phytoplankton biomass declined significantly (r 5 20.87 and 20.95, respectively, P , 0.001)
as total phytoplankton production and biomass increased in
the mesocosm experiments in Blanes Bay (Fig. 6). The ambient nutrient concentrations inside the mesocosm bags
ranged from 0 to 5 mM NO3 1 NO2 (Fig. 7). The fraction
of picoplankton to total phytoplankton primary production
and total phytoplankton biomass during the mesocosm experiment initially increased until nutrient concentrations
reach to almost 1 mM NO3 1 NO2 and declined exponentially (P , 0.001) at concentrations .1 mM NO3 1 NO2
(Fig. 7).
Discussion
The results show a general exponential decline in the percent biomass and primary production of picophytoplankton
in productive waters, both when represented by total community biomass and production and by nutrient concentrations .1 mM NO3 1 NO2. The values obtained allowed a
rough calculation of the contribution of picophytoplankton
to global marine primary production. The predicted percent
contribution of picophytoplankton at the average primary
production in the ocean of about 360 mg C m22 d21 (Longhurst et al. 1995) is 39% of the daily primary production of
the community, assuming most of this production to be confined to the upper 75 m of the ocean (i.e., an average of
about 5 mg C m23 d21). Similar calculations indicate that the
contribution of picophytoplankton biomass to global phytoplankton biomass (estimated at 1 pg C [Falkowski et al.
1998] or about 37 mg C m23) is predicted to be 8%, assuming a carbon to chlorophyll ratio of 50 (Harris 1986). Hence,
our results indicate, when scaled up, that picophytoplankton
contribute 39% of the planktonic primary production of the
global ocean, representing 19 pg C yr21, with only 8% of
the photosynthetic biomass there, or about 0.08 pg C. They
Picoplankton biomass and production
597
Fig. 5. The relationships between the (A) picoplankton biomass (mg Chl m23), (B) percent contribution of picophytoplankton fraction
to total phytoplankton biomass, (C) picoplankton primary production (mg C m23 d21), (D) percent contribution of picophytoplankton fraction
to total phytoplankton production, and temperature. Solid symbols represent the mean percentage within 38C bins and their standard error.
Solid lines indicate the fitted regression lines of the raw data points.
achieve this high contribution to the global primary production with that comparatively small biomass by virtue of their
rapid turnover, calculated from the numbers above to be 0.64
d21, which is five times faster than the turnover of total phytoplankton biomass in the oceans (once per week, Falkowski
et al. 1998). In the mesocosm experiment conducted, the
turnover of picophytoplankton also exceeded that of total
phytoplankton by an average of fivefold, except at the highest nutrient loading.
Whereas picophytoplankton dominates ($50%) the biomass and production of sparse (Chl a , 0.3 mg m23), nutrient-poor (NO3 1 NO2 , 1 mM), and warm (.268C) waters, they represent ,10% of autotrophic biomass and
production in rich (Chl a . 5 mg m23), highly productive
(.200 mg C m23 d21), and cold (,38C) waters. Strong covariation between nutrient concentrations and seawater temperature precluded, however, any inferences on the causal
mechanisms underlying these patterns, as observed in the
past (Li 1998). Both temperature and nutrients are statistically significantly related to the contribution of picoplankton
to the total phytoplankton in the compiled data set, but their
effects cannot be statistically separated. However, there is
evidence that the robust patterns observed in nature are derived from the combined effect of these two factors. However, we chose to address the role of nutrients experimentally, because the role of temperature is already supported
by robust evidence, such as the observations that picophytoplankton are scarce in polar waters (Li 1998) and that high
temperatures are required for maximum growth in culture
(Moore et al. 1995) and in the field (Agawin et al. 1998, Li
1998), even in areas receiving significant nutrient inputs in
summer (Agawin et al. 1998). Besides, the effect of temperature is not amenable to relevant experimental test. Had
we been able to alter temperature in the mesocosm between
5 and 308C, we would have no doubt induced considerable
mortality of the summer assemblage present, therefore inducing confounding effects that would have rendered the
experiment inconclusive. The mesocosm experiment conducted, supported the existence of a direct effect of increasing nutrient inputs on a reduction of the relative contribution
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Agawin et al.
Fig. 6. The relationships between the percent contribution of
the picophytoplankton fraction to total phytoplankton (A) primary
production (mg C m23 d21) and (B) biomass (mg Chl m23) during
the mesocosm experiment in Blanes Bay. Different symbols indicate
data from the mesocosms of different nutrient loading rates, as indicated in the figure. Solid lines indicate the fitted linear regression
line described as (A) ln(y) 5 6.13(60.40) 2 0.81(60.08)·ln(x), r 2
5 0.76, P , 0.05, and (B) ln(y) 5 2.63(60.06) 2
0.96(60.04)·ln(x), r 2 5 0.90, P , 0.05.
of picophytoplankton to the autotrophic planktonic community. Specifically, the experiment confirmed the dominance ($50% of autotrophic biomass and production) of picophytoplankton in sparse (Chl a , 0.3 mg m23), and
nutrient-poor (NO3 1 NO2 , 1 mM) waters, whereas they
represent ,10% of autotrophic biomass and production in
rich (Chl a . 2 mg m23), highly productive (. 120 mg C
m23 d21) environments. Therefore, these results support previous suggestions that the importance of picophytoplankton
is greatest in warm, oligotrophic waters. They also support
the suggestion that in oligotrophic areas, the flow of C and
cycling of elements in the planktonic food web is dominated
by picophytoplankton (Fenchel 1987; Legendre and Rassoulzadegan 1995). However, the strong covariation between
temperature and nutrient concentration and the increasing
importance of picophytoplankton with increasing temperature (e.g., Li 1998), suggest that the food chain of temperate
waters must shift seasonally from a dominance of larger phytoplankton cells in winter to a dominance of picophytoplank-
Fig. 7. The relationship between the percent contribution of the
picophytoplankton fraction to (A) total phytoplankton primary production, and (B) total phytoplankton biomass and ambient nutrient
concentrations during the mesocosm experiment in Blanes Bay. Solid lines represent the underlying trend described by locally weighted
scatterplot smoothing (Cleveland 1979).
ton in summer, consistent with existing reports for the NW
Mediterranean (Mura et al. 1996; Agawin et al. 1998).
The relative contribution of picophytoplankton to total
phytoplankton production or biomass declines in high-nutrient or productive waters following the much greater increases of production and biomass of the larger phytoplankton
cells. These results, derived from the compiled literature
data, were validated using the mesocosm experiment in NW
Mediterranean waters, which expanded the range of primary
production to almost 4,500 mg C m23 d21. The much greater
increases of production of larger phytoplankton cells may be
due to their higher intrinsic capacity for growth (mmax of
diatoms can reach an average of 2.2 d21, Banse 1982) than
do picoplankton; thus, when released from the constraints of
nutrient diffusion limitation in high nutrient environments,
they could outcompete the picoplankton. The reduced rela-
Picoplankton biomass and production
tive contribution of picophytoplankton in productive waters
may not be wholly ascribed to the seemingly inhibitory effects of high nutrient concentration to picophytoplankton
production because their production increases together with
that of the whole autotrophic community, although they do
so more slowly. This indicates that the reduced contribution
of picophytoplankton in warm productive waters must be
explained through increased loss rates (e.g., strong grazing
pressure) there. In contrast, the dominance of picophytoplankton in warm, oligotrophic waters is attributable to differential capacity to acquire nutrients.
Growth of phytoplankton cells can be limited by the rate
at which molecular diffusion can supply nutrients to the cell
surface (Munk and Riley 1952; Pasciak and Gavis 1974).
The minimum nutrient concentration to sustain phytoplankton populations has been proposed to increase as the fourth
power of cell radius when phytoplankton sinking loss is balanced by the diffusion-limited growth (Thingstad 1998), implying that this minimum nutrient concentration would be
about 10,000-fold higher for phytoplankton 5 mm in radius
compared to picoplankton 0.5 mm in radius. The minimum
nutrient concentrations to sustain populations that can be calculated following the formulation of Thingstad (1998) are,
however, well below the detection limits of available methods, so that pico- and microphytoplankton populations
should be able to coexist even in the most oligotrophic natural waters. However, the growth rates these groups can sustain under oligotrophic conditions will differ greatly. The
nutrient concentrations required to sustain the fastest growth
of picophytoplankton in nature (about 2 d21, Agawin et al.
1998) can be calculated, following the formulation in Chisholm (1992), to be 0.0004 mM P and 0.007 mM N, whereas
the fastest growth rates of microphytoplankton (average of
2.2 d21, Banse 1982) can be calculated to be about 0.04 mM
P and 0.72 mM N. Even at 10% of the maximum growth
rate, a 10-mm diameter cell would require 0.004 mM P and
0.072 mM N, 10-fold higher than what would be required to
sustain maximum growth rates of picophytoplankton. Hence
picophytoplankton should be able to maintain high growth
rates in oligotrophic waters where microphytoplankton
growth rates can be expected to be nutrient limited (Chisholm 1992). As a result, increasing nutrient availability
should stimulate microphytoplankton populations to a greater extent than picophytoplankton. This prediction from basic
principles is consistent with the results derived from the
comparative analysis presented, as well as the decline in relative importance of picophytoplankton with increasing nutrient loading determined experimentally. Specifically, data
from both the literature compilation and mesocosm experiments reveal a significant decline of the fraction of picophytoplankton to autotrophic biomass and production when the
waters reach nutrient concentrations of almost 1 mM NO3 1
NO2.
From growth kinetic studies, growth of larger phytoplankton (e.g., diatoms) is saturated at 0.9 mM N (Cañellas 1997),
which is a magnitude similar to the approximate nutrient
concentration at which larger phytoplankton is free of constraints of diffusion limitations. At these concentrations (0.7
to almost 1 mM N), growth of larger phytoplankton is expected to be saturated, and they should outcompete the pi-
599
cophytoplankton because of their higher intrinsic capacity
for growth, resulting in the decline of the fraction of picophytoplankton when reaching this range of nutrient concentrations. At nutrient concentrations ,1 mM NO3 1 NO2,
results from both the literature and mesocosm experiments
indicate an increase in the picophytoplankton fraction with
increasing nutrients. Although there is a paucity of literature
on growth kinetics of picophytoplankton, available information suggest growth of picophytoplankton to be saturated
at ambient concentrations of .0.2 mM N (Cochlan and Harrison 1991). It is therefore plausible that at very low nutrient
concentrations, the contribution of picophytoplankton increases until ambient nutrient concentrations reach the concentration when their growth is saturated and below which
the growth of larger phytoplankton is constrained by nutrient
diffusion limitation.
The general patterns described in this paper should apply
to the whole picophytoplankton fraction (Synechococcus 1
Prochlorococcus) principally because the data examined primarily consisted of size fractionated samples and thus should
contain both picophytoplankton groups. Important differences can exist among the different taxa in the picophytoplankton fraction. That the patterns apply to Synechococcus alone
is supported (for abundance patterns) from the recent papers
by Agawin et al. (1998) and Li (1998) and from the results
of the mesocosm experiment described here. Data on Prochlorococcus abundance alone, to test for the coherence of
patterns for this taxon, are still few. The differences in optimum growth temperature (288C for Synechococcus vis à
vis 248C for Prochlorococcus, Moore et al. 1995) will not
influence the general trend shown much since it has been
suggested that differences in seasonal and latitudinal distributions of Synechococcus and Prochlorococcus cannot be
explained by the temperature-dependent growth responses
observed (Moore et al. 1995). Possible differences in nutrient requirements of both groups need to be investigated because there is a paucity of knowledge on these, although it
has been suggested that Prochlorococcus may be less efficient at using low nutrient concentrations, despite its smaller
size (Chisholm 1992).
The capacity of picophytoplankton to acquire and use nutrients in resource-limited areas more efficiently than larger
cells is believed to reflect the competitive advantages associated with small size (Fogg 1986, see essay review of Raven 1998). As a function of their size, picophytoplankton
have much higher nutrient affinity, allowing them to maintain high uptake rates at low nutrient concentrations, which
confers a greater competitive advantage over larger phytoplankton in oligotrophic areas (Donald et al. 1997). In addition, the small size of picophytoplankton also confers a
greater efficiency to absorb and use the incident light compared to larger autotrophs (Agustı́ et al. 1994).
In summary, we have provided evidence, based on patterns observed in nature, that picophytoplankton’s relative
importance is greatest in warm and nutrient-poor waters. Evidence suggest that these patterns observed in nature are derived from the combined effects of both temperature and
nutrient concentrations. The results of the mesocosm experiments conducted during summer in the Mediterranean allowed the dissociation of the effects of temperature from
600
Agawin et al.
those of nutrients on picophytoplankton production and biomass and showed percent contribution of picophytoplankton
to exponentially decline when reaching almost 1 mM NO3
1 NO2. The reduced contribution of picophytoplankton in
warm, nutrient-rich waters can be explained by the differential capacity to use nutrients and the intrinsic capacity for
growth between picophytoplankton and larger phytoplankton
cells.
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Received: 16 December 1998
Accepted: 5 October 1999
Amended: 17 December 1999