Deep-Sea Research I 50 (2003) 557–571
Temporal and spatial variation in stocks of autotrophic and
heterotrophic microbes in the upper water column of the
central Arctic Ocean
Evelyn B. Sherr*, Barry F. Sherr, Patricia A. Wheeler, Karen Thompson
College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Bldg, Corvallis, OR 97330-5503, USA
Received 12 June 2002; received in revised form 12 November 2002; accepted 4 February 2003
Abstract
As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and
heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November
1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low
but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass
began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline
of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom
consisted of diatoms and phytoflagellates, mainly 2 mm-sized Micromonas sp.; the two subsequent blooms were
dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was
31711. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in
tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate
Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d1
for bacteria and of 0.11 d1 for Micromonas. Heterotrophic protists included flagellates, ciliates, and dinoflagellates,
with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 mm) and microplankton (Hmicro, 20–200 mm)
size classes.
r 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Microplankton; Phytoplankton; Bacteria; Protists; Flagellates; Biomass; Arctic Ocean; Canada Basin
1. Introduction
The 1997–1998 SHEBA/JOIS (Surface Heat
Budget of the Arctic Ocean/Joint Ocean Ice Study)
experiment (Perovich et al., 1999; Melnikov et al.,
2002; Macdonald et al., 2002) afforded an unique
*Corresponding author. Fax: +1-541-737-2064.
E-mail address: [email protected] (E.B. Sherr).
opportunity to investigate temporal and spatial
variation in abundance and biomass of pelagic
microbes in the permanently ice covered Arctic
Ocean. Information regarding the standing stocks
of microbial plankton in this major region of the
sea is relatively sparse. Past ice camp studies have
demonstrated a strong seasonality for phytoplankton, with an initial bloom in late June to early July
and one or two peaks during the growing season,
0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0967-0637(03)00031-1
558
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
which typically ended by early to mid-September
(English, 1961; Pautzke, 1979). A more detailed
analysis of autotrophic and heterotrophic microbes was carried out during the July–September
1994 Arctic Ocean Section (AOS) (Booth and
Horner, 1997; Gosselin et al., 1997; Rich et al.,
1997; Sherr et al., 1997). There is very little data on
pelagic communities in the Arctic Ocean during
winter.
The goals of the plankton biology program of
the SHEBA/JOIS expedition included determining
variation in standing stocks and activity of pelagic
microbes (bacteria, phytoplankton, and heterotrophic protists) during the dark winter months as
well as the growing season and assessing the timing
and magnitude of response of heterotrophic
microbes to the spring bloom. Here we report
changes in abundance and biomass of autotrophic
and heterotrophic microbes in the upper 120 m
of the water column during the ice camp drift.
We also determined general size ranges and
taxonomic categories of autotrophic and heterotrophic protists.
2. Materials and methods
2.1. Sample collection
Between October 19, 1997 and September 28,
1998, the SHEBA/JOIS ice camp drifted with the
permanent pack ice from its initial position in the
southern Canada Basin (75 280 N, 143 400 W) to
the Mendeleyev Basin (80 110 N, 166 050 W), traveling over the Northwind Ridge and Chukchi
Plateau from February to mid-summer, and
drifting into the Mendeleyev Basin in late summer
(Perovich et al., 1999; Macdonald et al., 2002;
McLaughlin et al., 2003; SHEBA website, http://
sheba.apl.washington.edu). Water samples for
determination of nutrient and chlorophyll concentrations, and of microbial abundance and biomass,
were collected in the upper 120 m of the water
column, with 5-l Niskin bottles deployed on a wire
through an ice-hole, on an 8 d schedule throughout
the year, with more frequent sampling in the upper
50 m for nutrient and chlorophyll a concentrations
during summer (June to September).
2.2. Measurement of inorganic nutrients and
chlorophyll a
Freshly collected seawater samples were processed for macronutrients nitrate+nitrite, silicate,
and phosphate with a Technicon Autoanalyser
(Atlas et al., 1971). Water samples for chlorophyll
a were filtered onto Whatman GF/F glass fiber
filters. Chlorophyll concentrations were determined fluorometrically, after 24-h extraction in
90% methanol at 5 C (Parsons et al., 1984).
2.3. Determination of microbial abundance and
biomass
Procedures for analysis of microbial biomass
were similar to those used during the 1994 AOS
(Sherr et al., 1997). Briefly, samples for microbial
enumeration were preserved by a three-step
procedure: 0.05% final concentration alkaline
Lugol solution, followed by the addition of 0.1%
final concentration of 3% sodium thiosulfate and
2% final concentration of borate-buffered formalin (Sherr and Sherr, 1993). Samples were allowed
to sit for at least 4 h to allow protist cells to
harden, and processed within 24 h. Aliquots of the
preserved samples were stained with DAPI
(25 mg ml1 final concentration) for 7–10 min, and
then filtered onto 25 mm diameter black-stained
membrane filters (Poretics, Inc.). For each water
sample, duplicate 50-ml subsamples were filtered
onto 0.8 mm pore size filters for enumeration of
o20 mm sized protists, and duplicate 150 ml
subsamples were filtered onto 3.0 mm pore size
filters for enumeration of >20 mm sized protists.
Two additional 10-ml aliquots of each sample were
filtered onto 0.2 mm pore size filters for bacterial
counts. Filters were removed from the filtration
towers under low vacuum, then mounted with
Resolvet low viscosity immersion oil onto glass
slides with No. 1 coverslips. Samples were stored
in slide boxes at 20 C on the ship until returned
on dry ice to a 40 C freezer in the laboratory at
Oregon State University. All slides were inspected
within 1 year of collection.
Bacteria and protists were enumerated via direct
counts with a Zeiss Universal microscope outfitted
for epifluorescence microscopy with a 100 W
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
mercury lamp and Zeiss filter set 47 77 02 (G 365
excitation filter/LP 420 barrier filter) for UV light
excitation of DAPI-stained cells, and Zeiss filter
set 47 77 09 (BP 450–490 excitation filter/LP 520
barrier filter) for blue light excitation of chlorophyll autofluorescence. Bacteria and o20 mm
sized protists were enumerated at 1000 , and
>20 mm protists at 160–400 (Sherr et al., 1993).
During inspection of both bacterial and nanoplankton samples, it was noted that coccoid
cyanobacteria were virtually absent from all
samples. Each protist cell counted was sized for
biovolume calculations with a calibrated ocular
micrometer and grouped into general taxonomic
categories: diatoms, mixed species nanoflagellates,
choanoflagellates, dinoflagellates, and ciliates.
Two taxa of phytoflagellates, Micromonas sp.
and Phaeocystis sp. (probably Phaeocystis poucheii), were identified to genus level based on past
experience with phytoplankton in this region of
the Arctic Ocean (Booth and Horner, 1997; Sherr
et al., 1997).
Bacterial biomass was estimated from a value of
20 fg C cell1, from the prior results of Sherr et al.
(1997). Protistan carbon biomass was estimated
from empirically determined biovolumes using
equations suggested by Menden-Deuer and Lessard (2000). Based on a comprehensive review of
the literature, Menden-Deuer and Lessard derived
C: volume relationships for marine protists, one
for protists excluding diatoms (pg C cell1=
0.216 volume in cubic microns raised to the
power 0.939 [C ¼ 0:216 V 0:939 ]) and one for
diatoms (pg C cell1=0.288 volume in cubic
microns raised to the power 0.811 [C ¼
0:288 V 0:811 ]). These equations result in C:
volume factors of 0.20 pg C mm3 for 2 mm
diameter protists, 0.15 pg C mm3 for 10
mm diameter protists, 0.13 pg C mm3 for 20 mm
diameter protists, and 0.060 pg C mm3 for 20 mm
diameter diatoms.
Stocks of nutrients and of chlorophyll a were
integrated for the upper 50 m of the water column,
since the more frequent summer profiles only
included depths to 50 m. In contrast, microbial
biomass data were based on the routine, 8-d
interval profiles that sampled depths to 120 m,
including depths of 40 and 60 m. In these profiles,
559
higher summer microbial biomass extended to
60 m depth; thus we integrated biomass over the
upper 60 m. Chlorophyll a concentrations at
depths >50 m were o0.2 mg l1, so there would
be little difference in phytoplankton biomass for a
0–50 m versus a 0–60 m integration depth. We also
calculated winter and summer average cell abundances of the various microbial stocks for two
depth intervals: 0–40 m (summer euphotic zone),
and 80–120 m (depths beneath the euphotic zone).
3. Results
3.1. Variation in nutrient and chlorophyll a
concentrations
During the SHEBA/JOIS experiment, the icestation passed from the deep, oligotrophic Canada
Basin to relatively shallower regions of the Northwind Ridge and Chukchi Plateau, and then finally
over the deep Mendeleyev Basin (Fig. 1C). This
path crossed a hydrographically dynamic part of
the Arctic Ocean, with three major identified
regimes: (1) the south-central Canada Basin from
Fall, 1997 to early winter, 1998; (2) the Northwind
Ridge and Chukchi Plateau from February to midsummer; (3) the Chukchi Plateau and Mendeleyev
Basin from June to September (Macdonald et al.,
2002; McLaughlin et al., submitted) (Fig. 1C).
Nutrient distributions in the upper 120 m encountered during the ice camp drift were influenced by
change in water mass characteristics, especially by
variation in relative contribution of Pacific and
Atlantic-origin water masses (McLaughlin et al.,
submitted). In the fall and early winter, nitrate+nitrite stocks were o0.5 mM in the upper 50 m, but
increased to 2–7 mM after the ice camp traversed
the Northwind Ridge (Fig. 1A). Silicate stocks
increased similarly. Higher nitrate and silicate
content of water over the Northwind Ridge and
Chukchi Plateau was likely due to water of Pacific
origin (McLaughlin et al., submitted). Starting in
mid-June, nutrient stocks began to decrease in the
upper 50 m, coincident with an increase in
chlorophyll a stocks as the spring bloom began
(Fig. 1A and B). Stocks of both nitrate+nitrite
and of silicate integrated over 0–50 m showed a
560
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Fig. 1. Variation in concentrations of (A) nitrate+nitrate (mM) and (B) chlorophyll a (mg liter1) in the upper 120 m of the water
column, and in (C) bathymetry during the SHEBA/JOIS experiment. Dots indicate profile sampling depths. The approximate
demarcations of the three hydrographic regimes encountered during the ice camp drift are indicated. Regime 1—south-central Canada
Basin from fall, 1997 to early winter, 1998; Regime 2—Northwind Ridge and Chukchi Plateau from February to June; Regime 3—
Chukchi Plateau and Mendeleyev Basin from June to September.
progressive decline from May 29 to September 25,
while stocks of phosphate showed little seasonality
(Fig. 2). In the upper 20 m, nitrate+nitrite decreased from an average of 2.8 mM to 0.01 mM, and
silicate from an average of 8.7 mM to 1.7 mM, from
June 15 to August 30.
3.2. Variation in microbial stocks
All groups of pelagic microbes appeared to
exhibit a strong response to the large annual
amplitude in solar insolation, with low biomass in
spring and higher biomass during the short
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
autotrophs varied dramatically in summer because
of blooms. Heterotrophic protists made up, on
average, 86% of total protistan biomass in winter,
and 44% in summer.
silicate
0-50 m nutrient stock, mM
561
600
400
3.3. Distribution of protist biomass
nitrate + nitrite
200
phosphate
0
347
32
82
133
182
232
Day of year
Fig. 2. Change in integrated stocks (mM) of nitrate+nitrite,
silicate, and phosphate in the upper 50 m of the water column
during the SHEBA/JOIS experiment.
summer growing season (June–September)
(Fig. 3). Microbial biomass was concentrated in
the upper 40–60 m of the water column (Figs. 1B
and 3). There was a rapid increase in phytoplankton stocks in June (beginning about day 140), after
snow cover melted from the ice surface (Fig. 4).
Seasonal increases in stocks of bacteria and of
heterotrophic protists (Fig. 3A and C) were nearly
simultaneous with increase in autotrophic protists
(phytoplankton) (Figs. 1B and 3B). Stocks of
bacteria and of heterotrophic protists remained
relatively high in the upper 60 m throughout the
summer, even after phytoplankton biomass had
declined (Fig. 3A and C).
Table 1 presents average winter (November–
May) and summer (June–September) values of
abundance (cells ml1) and volumetric biomass
(mg C m3) for bacteria and for autotrophic and
heterotrophic protists in nanoplankton and microplankton size categories. To clearly distinguish
seasonal effects, we averaged values for two depth
zones: the summer euphotic zone (upper 40 m),
and subeuphotic depths (80–120 m). Stocks of
bacteria and of heterotrophic protists approximately doubled from winter to summer in the
euphotic zone. Bacterial stocks showed no increase
at the lower depth interval. Autotrophic protists
exhibited an approximately 20-fold increase in
numbers and biomass during summer compared to
winter stocks. Cell numbers and biomasses of
Variations in depth-integrated (0–60 m) protistan biomass from January to September 1998 are
shown in Fig. 5. Three discrete peaks in biomass of
autotrophic protists were observed, the first from
the end of June to the first week of July, and the
other two during the last two weeks of July
(Fig. 5A and B). These peaks mirrored the three
peaks in chlorophyll concentration found during
summer (Fig. 1B and 4). Autotrophic protist
biomass was much higher than biomass of heterotrophic protists during these blooms (Fig. 5A).
Since microplanktonic diatoms have a lower C:
volume ratio compared to nanoplanktonic cells
(Menden-Deuer and Lessard, 2000), in terms of
carbon biomass all three blooms were dominated
by o20 mm sized cells (Fig. 5B). Over the entire
growing season, diatoms averaged 24711% of the
total biomass of autotrophic cells. Heterotrophic
protist biomass was fairly evenly divided between
nanoplankton and micro-plankton size classes
during the winter, but during the summer nanoplanktonic heterotrophic protists (Hnano) showed
a sharp peak in biomass coincident with the initial
spring bloom (Fig. 5C).
A striking result was the small size of most
protistan cells, both heterotrophic and autotrophic. From 72% to 95% (average 87%) of
heterotrophic flagellates were o5 mm in size.
Picoflagellates (1.5–2 mm preserved size) dominated the Hnano assemblage throughout the year,
ranging in abundance from 100 to 1600 cells ml1,
and averaging 52% of total Hnano numbers.
Autotrophic protists were also numerically dominated by o5 mm sized cells, which made up 44–
99% (average 95%) of cells in the phytoplankton
assemblage during the growing season. A large
proportion of small phytoplankton cells were 2 mm
sized Micromonas sp., which had a maximum
abundance of 28,000 cells ml1 during the initial
spring bloom, and were present at abundances of
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Day of Year
A. Bacteria
30
80
130
180
230
20
Depth, m
40
4
11
4
6
345
6
562
4
4
4
8
4
60
4
5
4
80
2
100
2
120
B. Autotrophic protists
345
30
80
130
180
230
80
10
10
20
30 20
Depth, m
40
60
60
40
80
20
100
0
120
C. Heterotrophic protists
345
30
80
130
180
230
20
Depth, m
10
20
40
15
60
10
80
5
100
120
Dec 11
Jan 30
Mar 21
May 10
Jun 30
Aug 20
3
Fig. 3. Variation in carbon biomass (mg C m ) of planktonic microbes in the upper 120 m of the water column during the SHEBA/
JOIS experiment: (A) Heterotrophic bacteria, (B) Autotrophic protists (phytoplankton), and (C) Heterotrophic protists. Dots indicate
profile sampling depths.
1000–10,000 cells ml1 in the upper 40 m during
the rest of the growing season.
3.4. Carbon:chlorophyll ratio
Analysis of the biomass of autotrophic protists
in water samples for which chlorophyll a concen-
tration was also determined during the SHEBA/
JOIS experiment allowed calculation of empirical
carbon:chlorophyll (C:Chl) ratios for phytoplankton in the central Arctic Ocean. C:Chl ratios were
calculated for 36 discrete water samples collected
in the upper 60 m from 19 May to 22 September
(Table 2). Phytoplankton biomass varied from 1.3
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
snow cover melt
[------]
1.5
300
1.0
200
0.5
100
0
Phytoplankton biomass, gC m-2
Short wave radiation, Watts m-2
400
0.0
296
346
31
81
131
181
233
281
Day of Year
Fig. 4. Variation in short wave radiation (dotted line, W m2,
measured with an Eppley Precision Spectral Pyranometer,
range of 295–2500 nm), and in 0–50 m integrated phytoplankton biomass estimated from chlorophyll a concentrations and
an average C:chl a ratio of 31 (solid line), during the SHEBA/
JOIS experiment. The period during which winter snow cover
melted from the ice surface is indicated. Data on radiation and
ice melt was provided by the SHEBA Project Office, Applied
Physics Laboratory, University of Washington.
to 112 mg C m3, and chlorophyll a concentration
from 0.07 to 4.3 mg m3, in these samples. The
average C:Chl ratio determined from this sample
set was 31711. The C:Chl ratio was biased
upward by values greater than 50 calculated for
July 7 and July 31 data. Without these high values,
the average C:Chl ratio was 2878. The lowest
ratio was 13, found for phytoplankton at 20 m on
May 31 and at 40 m on July 23 (Table 2).
3.5. Taxonomic composition of protists
Autotrophic protists o20 mm in size were
mainly phytoflagellates, including Micromonas
sp, the flagellated form of Phaeocystis pouchetii,
other haptophytes, and occasional cryptomonads
and small autotrophic dinoflagellates. Diatoms
occurred, but were not abundant, in the nanoplankton fraction. Microplanktonic autotrophs
were mostly diverse species of diatoms, including
centric and pennate species previously identified by
Booth and Horner (1997) in this region. Abundance of autotrophic dinoflagellates was typically
less than 1 cell ml1, but did show a peak of
563
25 cells ml1 at a depth of 20 m during the initial
spring bloom. Autotrophic ciliates, Mesodinium sp.,
were rare, 0–3 cells ml1. Phagotrophic ciliates with
sequestered chloroplasts were common in summer,
but were included with the heterotrophic protists.
During winter, phytoplankton were present as
nanoflagellates, mainly Micromonas sp. and unidentified haptophytes, at abundances of hundreds of
cells ml1, plus >20 mm diatoms and pigmented
dinoflagellates at abundances of about 1 cell ml1.
Phytoplankton composition changed dramatically during the growing season (Fig. 6). Phytoplankton biovolume during the initial bloom was
composed about equally of >20 mm diatoms and
o20 mm phytoflagellates, primarily 2 mm Micromonas. The two later blooms were dominated by
flagellated Phaeocystis, 4–6 mm in size, at abundances of 5000–18,000 cells ml1. Microplanktonsized flagellates, mostly autotrophic dinoflagellates,
were not an important component of the phytoplankton assemblage during the blooms (Fig. 6).
Nanoplankton-sized heterotrophic protists were
a mixed species assemblage of non-pigmented
flagellates. Most of these flagellates, especially the
very small, 2 mm sized species could not be
identified. Choanoflagellates were present, but
generally rare, with abundances ranging from 0
to 150 cells ml1 (average 13 cells ml1). Non-armored heterotrophic dinoflagellates 10–20 mm in
size, including spherical gymnodinoid forms and
spindle-shaped Katodinium-like species, were found
at an average abundance of 17 cells ml1 (range 1–
60 cells ml1). Non-pigmented cryptomonad-like
flagellates similar to the species Leucocryptos
marina (Vors, 1992) were occasionally observed,
and o20 mm ciliates occurred in the nanoplankton
fraction at an average abundance of 1 cell ml1.
Heterotrophic protists in the microplankton size
class were dominated by non-pigmented dinoflagellates, which made up, on average, 83% of the
numerical abundance and 63% of the biomass of
microzooplanktonic protists. Heterotrophic dinoflagellates >20 mm in size were present throughout
the year at abundances of 1–12 cells ml1, and
included both armored Protoperidium-like species
as well as spindle-shaped non-armored forms.
Ciliates occurred at abundances of 0.1–2 cells
ml1, and were mostly spherical or elongate
564
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Table 1
Comparison of winter (Nov.–May) and summer (June–Sept.) cell abundances (cells ml1) and biomasses (mg C m3) in 0–40 m and 80–
120 m depth zones, of bacteria, autotrophic protists, and heterotrophic protists (ranges of values in parentheses)
Abundance (cells ml1)
Bacteria
0–40 m
80–120 m
Autotrophic protists
0–40 m
2–20 mm
20–200 mm
80–120 m
2–20 mm
20–200 mm
Heterotrophic protists
0–40 m
2–20 mm
20–200 mm
80–120 m
2–20 mm
20–200 mm
Biomass (mg C m3)
Winter
Summer
Winter
Summer
1.7670.28 105
(1.32–2.86)
1.2770.26 105
(0.72–2.07)
3.2370.70 105
(1.9–6.7)
1.1170.19 105
(0.72–1.78)
3.570.6
(2.6–5.9)
2.670.5
(1.4–4.1)
6.471.4
(3.8–13.3)
2.270.3
(1.4–3.6)
2307230
(3–1500)
0.970.4
(0.05–2.6)
560075100
(100–28,000)
23716
(0.7–85)
0.4670.36
(0.02–2.1)
0.3470.34
(0.0–3.4)
10.9711
(1.0–112)
2.172.1
(0.4–15)
ND
1778
(3–42)
2.371.5
(0.8–5.6)
ND
0.1170.07
(0.01–0.21)
0.1870.18
(0.2–0.48)
ND
ND
260780
(90–490)
6.1–1.3
(3.0–9.7)
5907310
(210–2300)
8.072.3
(2.5–18)
1.970.6
(0.7–4.6)
1.871.0
(0.6–9.2)
4.572.7
(1.0–18)
3.371.6
(10–11)
ND
150727
(110–200)
3.471.5
(1.4–6.0)
ND
1.370.4
(0.9–2.2)
0.8370.3
(0.4–1.1)
ND
ND
ND=no data.
spirotrichs. During the summer, mixotrophic
ciliates with sequestered chloroplasts were frequently observed. Tintinnids were rare. Other
microplanktonic protists included unidentified
heterotrophic flagellates at an average abundance
of 0.2 cells ml1.
3.6. Contents of protistan food vacuoles
During summer, ingested phytoplankton cells
could be identified as chlorophyll autofluorescence
in the food vacuoles of phagotrophic protists.
During the initial spring bloom heterotrophic
nanoflagellates, including abundant 2 mm sized
cells and choanoflagellates, commonly ingested
Micromonas. In the later Phaeocystis-dominated
blooms, heterotrophic nanoflagellates were also
observed with ingested phytoplankton prey. Ciliates and non-armored dinoflagellates were frequently observed with ingested phytoflagellates
and smaller-sized diatoms. Thecate dinoflagellates
were rarely found with recognizable food vacuole
contents.
3.7. Microbial growth rates
We were able to calculate microbial growth
rates from the abundance data in two cases: for
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
5
autotrophic
gC m
-2
4
3
2
heterotrophic
1
565
These estimates should be considered realized
growth rates (r) for the prevailing in situ conditions of environmental parameters and mortality
processes; intrinsic growth rates (m) of bacteria and
Micromonas at these temperatures, substrate concentrations, and light conditions would be expected to be higher.
0
(a)
2.5
Anano
< 20 µm
gC m
-2
2.0
1.5
1.0
Amicro
> 20 µm
0.5
0.0
(b)
1.0
Hnano
< 20 µm
gC m
-2
0.8
0.6
Hmicro
> 20 µm
0.4
0.2
0.0
0
(c)
50
100
150
Day of year
200
250
Fig. 5. Variation in integrated carbon biomass (g C m2) of
protists in the upper 60 m of the water column from January to
September, 1998: (A) Comparison of biomass of heterotrophic
(dashed line) and autotrophic (solid line) protists, (B) Comparison of biomass of autotrophic protists in the nanoplanktonic
(Anano, 2–20 mm, solid line) and microplanktonic (Amicro, 20–
200 mm, dashed line) size classes, (C) Comparison of biomass of
heterotrophic protists in the nanoplanktonic (Hnano, solid line)
and microplanktonic (Hmicro, dashed line) size classes.
bacterioplankton and for Micromonas sp. in the
upper 20 m of the water column over periods
spanning the initial spring bloom. The abundance
of bacterioplankton increased exponentially from
May 7 to June 24, as did Micromonas cell
abundance from May 22 to June 23. Regressions
of natural logarithm of cell abundance with time
during these periods (Fig. 7) yielded an estimate
for bacterial growth rate of 0.025 d1 (doubling
time of 28 d) and an estimate for Micromonas
growth rate of 0.11 d1 (doubling time of 6.1 d).
4. Discussion
4.1. Variation in abundance and biomass of
microbial plankton
Stocks of all components of the microplankton
were concentrated in the upper 40–60 m of the
water column, and showed a strong seasonal
response to the short growing season in the high
Arctic (Figs. 1B, 3 and 4). Autotrophic protists
(phytoplankton) had the largest amplitude in
seasonal abundance and biomass. The phytoplankton included diatoms and Phaeocystis, which
constitute a large fraction of planktonic autotrophs in both northern and southern polar
systems (El-Sayed and Fryxell, 1993; Smith,
1994; Bigidare et al., 1996; Dennett et al., 2001).
We also found that 2 mm phytoflagellates, identified as the prasinophyte alga Micromonas sp., were
a large fraction of the initial spring bloom.
Micromonas-like prasinophytes have been previously found to be abundant in the Northeast
Water polynya off Greenland (Booth and Smith,
1977) and in the central Arctic Ocean (Booth
and Horner, 1997). Phytoplankton in the picoplankton fraction, typically prasinophytes and
prymnesiophytes, can also be important in the
Southern Ocean (El-Sayed and Fryxell, 1993;
Agawin et al., 2002).
During summer blooms, peak abundances of
Micromonas were 28,000 cells ml1, and of Phaeocystis were 18,000 cells ml1. These are high values
for eukaryotic phytoplankton in oligotrophic
ocean environments. For example, in the NE
Pacific, abundances of autotrophic nanoflagellates
were on the order of 2000 cells ml1 year-round
(Boyd et al., 1995). During the 1994 AOS
expedition across the polar cap, Booth and Horner
(1997) reported maximum abundance values of
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
566
Table 2
Calculated carbon:chlorophyll a ratios for phytoplankton during the 1998 growing season in the central Arctic Ocean
Date
Day of year
Sample depth (m)
Chl a (mg m3)
Biomass (mg m3)
C:Chl a
May 19
139
May 22
142
May 31
June 8
June 17
151
159
168
June 23
July 7
174
188
July 15
July 23
196
204
July 31
212
Aug. 14
226
Aug. 20
232
Aug. 28
240
Sept. 7
250
Sept. 14
257
Sept. 22
265
10
20
4
10
20
20
10
10
20
10
10
20
40
4
10
20
40
10
40
10
20
40
10
40
10
20
40
60
10
40
10
40
4
10
30
50
60
0.23
0.21
0.18
0.18
0.16
0.21
0.33
0.59
0.44
1.41
0.51
0.64
0.12
0.35
0.44
4.3
0.16
0.23
0.21
0.11
1.6
0.21
0.09
0.38
0.11
0.12
0.35
0.07
0.09
0.20
0.07
0.15
0.12
0.13
0.08
0.11
0.08
3.6
3.8
4.8
4.7
3.6
2.6
11
25
17
48
36
34
4.3
7.9
12
112
2.0
14
11
3.3
58
4.1
3.4
6.4
2.4
4.1
12
1.3
3.8
4.1
2.9
3.3
2.2
3.2
3.1
4.6
1.7
15
18
26
26
23
13
33
42
38
34
70
53
36
23
27
26
13
62
52
31
36
19
36
17
21
33
33
19
40
21
40
23
18
25
40
41
21
10,000 cells ml1 for 2 mm phytoflagellates (tentatively identified as Micromonas pusilla) and of
470 cells ml1 for flagellated Phaeocystis pouchetii.
Agawin et al. (2002) reported pico-phytoflagellate
abundances from o1000 to B10,000 cells ml1,
with abundances of >20,000 cells ml1 found at
one station, in the Bransfield Strait, Antarctia. Our
intensive sampling program through the growing
season allowed us to observe bloom peaks that
might have been missed by previous shorter-term
sampling efforts.
Our empirically calculated carbon:chlorophyll
ratios, which ranged from 13 to 70 and averaged
31711, were similar to the C:Chl ratios, range of
13 to 59, average of 28710 (excluding one very
low value of 4), estimated by Booth and Horner
(1997) for central Arctic phytoplankton in summer. These relatively low C:Chl ratios are typical
for phytoplankton adapted to low irradiances
(Smith and Sakshaug, 1990).
Heterotrophic bacteria approximately doubled
in abundance in the euphotic zone from a winter
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Fig. 6. Proportional taxonomic composition of phytoplankton
during the three major bloom periods of the growing season, in
terms of biovolume: D=diatom biovolume (clear bars);
NF=nanoflagellate (cells o20 mm) biovolume (striped bars);
MF=microflagellate (cells >20 mm) biovolume (solid bars).
Ln bacterial cells ml -1
13.5
13.0
r = 0.025 d -1
Td = 28 d
12.5
12.0
11.5
125
135
145
155
165
175
Ln Micromonas cells ml -1
(a)
(b)
11
10
9
r = 0.11 d -1
Td = 6.1 d
8
low of 1.8 105 cells ml1 to a summer high of
3.2 105 cells ml1, with a maximum abundance
of 6.7 105 cells ml1 (Table 1). Bacterioplankton
abundances during the summer of 1998 were lower
than the bacterial abundances (3.8–11.7 105 cells
ml1, average 4.871.3 105 cells ml1) we had
previously reported for the upper 50 m of the
water column in the central Arctic Ocean during the growing season (Sherr et al., 1997).
Our summer 1998 bacterial abundance values
are similar to the range of abundances of
2–5 105 cells ml1 found by Cota et al. (1996) in
water samples taken over the continental shelf and
slope of the Chukchi Sea and Canada Basin in
August 1993.
Heterotrophic protists showed an early spring
increase in biomass, before the initial spring
bloom, perhaps due to differences in water mass
history during the SHEBA/JOIS drift (Fig. 3). The
summer stocks of heterotrophic protists (Hnan,
210–2300 cells ml1; Hmicro, 2.5–18 cells ml1)
were similar to protistan abundances found during
the summer 1994 AOS expedition (Hnan 160–
1900 cells ml1, Hmicro 2–15 cells ml1) (Sherr
et al., 1997). The abundances of protists in this
region are comparable to those reported for other
oligotrophic regions of the world ocean, for
example the Sargasso Sea (Caron et al., 1995)
and the Ross Sea, Antarctica (Dennett et al.,
2001). The general taxonomic composition of
heterotrophic protists was similar to that found
in the central Arctic Ocean during the summer of
1994, and to the protistan community described
for Antarctic waters (Garrison and Gowing, 1993;
Dennett et al., 2001). These results confirm the
importance of 2 mm-sized heterotrophic flagellates
in the Hnano, and heterotrophic dinoflagellates in
the Hmicro, size categories.
4.2. Survival of planktonic microbes during the
Arctic winter
7
6
135
567
145
155
165
175
Day of Year
Fig. 7. Regressions of natural logarithm of cell abundance
versus time in the upper 20 m during the initial spring bloom
period, from which growth rates were determined for (A)
bacteria and (B) Micromonas.
Virtually all identifiable groups of microbial
cells in the plankton were present in the upper
120 m during winter sampling. Bacterioplankton
maintained relatively high winter cell abundance
(Table 1), although the cell-specific activity
assessed via rate of uptake of tritiated leucine
568
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
was much lower than in summer (Sherr and Sherr,
2003). Heterotrophic bacteria are able to enter a
state of low metabolism (starvation survival) when
substrates are scarce (Morita, 1997). In contrast,
phytoplankton stocks were low during the winter,
with average cell abundances of only several
hundred per ml, about an order of magnitude
lower than winter cell abundances of phytoflagellates in the NE subarctic Pacific (Boyd et al.,
1995). All components of the phytoplankton
assemblage were present during winter, although
identifiable cells of Phaeocystis, which was an
important component of summer blooms, were
rare. The biomass stocks of heterotrophic protists
were higher than those of either bacteria or
autotrophic protists during winter (Table 1), and
included all categories of identified cells. Phagotrophic protists should be able to continue feeding
during the winter. Some phytoflagellates, such as
the prasinophyte Micromonas sp., which was
abundant during summer blooms, may also be
capable of phagotrophy (Gonzalez et al., 1993;
Caron, 2000) and thus have a survival advantage
over non-phagotrophic phytoplankton. These results are likely generally applicable to ice-covered
Arctic systems during winter, although the fact
that the SHEBA camp was in a different hydrographic regime during summer compared to winter
is a complication for seasonal interpretation of our
data set.
4.3. Response of planktonic microbes during the
spring bloom
Despite the cold water temperatures (1.0 C to
1.8 C) that prevailed year-round in the central
Arctic, both bacteria and heterotrophic protists
increased in biomass over the same time period as
did the phytoplankton during the spring bloom,
without any significant lag (Fig. 3). Bacterial cellspecific activity also increased rapidly during the
spring bloom (Sherr and Sherr, 2003). During the
initial bloom period, the autotrophic picoflagellate
Micromonas sp. grew at a rate of 0.11 d1,
equivalent to a doubling time of about 6 d. This
rate is at the low end of the growth rates that
Smith (1994) estimated for Phaeocystis pouchetii
blooms in the Greenland Sea (0.2570.20 d1
(range of 0.13–1.05 d1, doubling times of 5.3–
0.7 d).
Bacterioplankton abundance increased at a
slower rate, 0.025 d1, equivalent to a doubling
time of about 28 d. Bacterial growth rates during
June, independently estimated from rates of
incorporation of tritiated leucine, were as high as
0.02–0.04 d1 for specific depths in the euphotic
zone (Sherr and Sherr, 2003). Rich et al. (1997)
previously estimated bacterial growth rates ranging from 0.05 to 0.5 d1 (doubling times of 14–
1.4 d) in the central Arctic Ocean during summer,
also based on rates of leucine incorporation.
However, the rates of leucine incorporation by
planktonic bacteria during the 1994 AOS expedition (Rich et al., 1997) were about an order of
magnitude higher compared to rates measured
during summer of the SHEBA year (Sherr and
Sherr, 2003). A comparison of data collected in
1994 (Rich et al., 1997) and in 1998 (this paper,
and Sherr and Sherr, 2003) suggests there may be
significant inter-annual variability both in stocks
and in growth rates of bacterioplankton in the
central Arctic Ocean.
Our calculated in situ growth rates based on
observed increases in cell number over time should
be less than the intrinsic rate of increase at in
situ conditions, as heterotrophic nanoflagellates
were abundant during the spring bloom and
were capable of ingesting both bacteria and
Micromonas (Sherr et al., 1997). Viral lysis could
also have represented a source of mortality
for bacterioplankton and for phytoplankton
(Fuhrman and Suttle, 1993; Suttle, 1994; Steward
et al., 1996).
4.4. Inferred food web relationships
Inspection of food vacuole contents during the
growing season showed that all size classes of
heterotrophic protists, from the smallest nanoflagellates to the largest ciliates and dinoflagellates,
consumed phytoplankton. In particular, the abundant 2 mm-sized heterotrophic flagellates, as well as
choanoflagellates, were routinely observed with
ingested Micromonas. These groups of flagellates
have been thought to prey primarily on heterotrophic or autotrophic bacteria rather than on
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
eukaryotic phytoplankton (Caron, 2000). We had
previously noted phytoplankton ingestion by
o5 mm heterotrophic flagellates as well as by
dinoflagellates and ciliates in the central Arctic
Ocean during the summer of 1994 (Sherr et al.,
1997). It is clear that the entire spectrum of
heterotrophic protists can be significant consumers
of phytoplankton in the Arctic Ocean; this trophic
pathway needs to be quantified.
Microplanktonic protists are known to ingest
heterotrophic as well as autotrophic nanoflagellates (Verity, 1991; Solic and Krstulovic, 1994;
Sherr and Sherr, 2000), and grazing by microzooplankton may have been at least partly
responsible for the cycling of heterotrophic nanoflagellate biomass during summer (Fig. 3C). Because of their larger cell sizes, microplankton
protists are, in turn, more likely to be subject to
grazing by mesozooplankton than are heterotrophic nanoflagellates (Stoecker and Capuzzo,
1990). We speculate, as for the 1994 AOS results
(Sherr et al., 1997; Thibault et al., 1999), that
copepod grazing could have affected summer
biomass fluctuations observed for the microzooplankton.
4.5. Major conclusions
(1) Microbial abundance and biomass in the
central Arctic Ocean showed a strong response to the large annual amplitude in solar
radiation. Phytoflagellates were the most
important component of phytoplankton
blooms, especially after the initial spring
bloom in which diatoms were also abundant
in terms of biovolume. Availability both of
light (spring) and of macronutrients (late
summer) appeared to be limiting to phytoplankton growth in the ice-covered central
Arctic Ocean.
(2) Heterotrophic microbes: bacteria and protists,
were present in the upper water column
throughout the winter, had B2-fold higher
biomass during the summer, and increased in
concert with phytoplankton during the initial
spring bloom. Stocks of both bacteria and
heterotrophic protists remained high throughout summer and into early fall.
569
(3) Heterotrophic protists appeared to be feeding
primarily on phytoplankton of all size categories, but were likely also significant grazers
of bacteria, and probably represented an
important food resource for zooplankton.
Acknowledgements
We are indebted to the captains and crews of the
Canadian Coast Guard research vessel Des Grosielliers, the SHEBA support staff of the Applied
Physics Laboratory, University of Washington,
and our Canadian colleague Dr. Harold (Buster)
Welch for their invaluable logistics support of the
ocean biology program during the SHEBA year.
We thank Julie Arrington and Andy Ross for their
dedicated technical assistance at the SHEBA drift
camp, and Julie Arrington for assistance with
microscopic analysis and data work-up of plankton samples. Funding was provided by NSF Grant
OCE 9708088 to P. Wheeler, B. Sherr, and E.
Sherr.
References
Agawin, N.S.R., Agusti, S., Duarte, C.M., 2002. Abundance of
Antarctic picophytoplankton and their response to light
and nutrient manipulation. Aquatic Microbial Ecology 29,
161–172.
Atlas, E.L., Hager, S.W., Gordon, L.I., Park, P.K., 1971. A
practical manual for use of the Technicon Autoanalyser in
seawater nutrient analyses. Department of Oceanography,
Oregon State University, Corvallis, OR. Revised OSU
Technical Report 215, Ref. No. 71–22, 48pp.
Bigidare, R.R., Iriate, J.L., Kang, S.-H., Karentz, D.,
Ondrusek, M.E., Fryxell, G.A., 1996. Phytoplankton:
quantitative and qualitative assessments. In: Ross, R.M.,
Hofmass, E.E., Quentin, L.B. (Eds.), Foundations for
Ecological Research West of the Antarctic Peninsula.
Antarctic Research Series, Vol. 70. American Geophysical
Union, Washington, DC, pp. 173–198.
Booth, B.C., Horner, R.A., 1997. Microalgae on the Arctic
Ocean Section, 1994: species abundance and biomass. DeepSea Research II 44, 1607–1622.
Booth, B.C., Smith, W.O., 1977. Autotrophic flagellates and
diatoms in the Northeast Water Polynya, Greenland:
summer 1993. Journal of Marine Systems 10, 241–261.
Boyd, P.W., Strom, S., Whitney, F.A., Doherty, S., Wen, M.E.,
Harrison, P.J., Wong, C.S., Varela, D.E., 1995. The NE
570
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Pacific in winter: I. Biological standing stocks. Marine
Ecology Progress Series 128, 11–24.
Caron, D.A., 2000. Symbiosis and mixotrophy among
pelagic organisms. In: Kirchman, D.L. (Ed.), Microbial Ecology of the Oceans. Wiley–Liss, New York,
pp. 495–523.
Caron, D.A., Dam, H.G., Kremer, P., Lessard, E.J., Madin,
L.P., Malone, T.C., Napp, J.M., Peele, E.R., Roman, M.R.,
Youngbluth, M.J., 1995. The contribution of microorganisms to particulate carbon and nitrogen in surface waters of
the Sargasso Sea near Bermuda. Deep-Sea Research I 42,
943–972.
Cota, G.F., Pomeroy, L.R., Harrison, W.G, Jones, E.P., Peters,
F., Sheldon Jr., W.M., Weingartner, T.R., 1996. Nutrients,
primary production and microbial heterotrophs in the
southeastern Chukchi Sea: Arctic summer nutrient depletion and heterotrophy. Marine Ecology Progress Series 135,
247–258.
Dennett, M.R., Malthot, S., Caron, D.A., Smith Jr., W.O.,
Lonsdale, D., 2001. Abundance and distribution of phototrophic and heterotrophic nano- and microplankton in the
southern Ross Sea. Deep-Sea Research II 48, 4019–4037.
El-Sayed, S.Z., Fryxell, G.A., 1993. Phytoplankton.
In: Friedmann, E.M. (Ed.), Antarctic Microbiology.
Wiley–Liss, New York, pp. 65–122.
English, T.S., 1961. Some biological oceanographic observations in the central North Polar Sea, Drift Station Alpha
1957–1958. Arctic Institute of North America, Research
Paper 13.
Fuhrman, J.A., Suttle, C.A., 1993. Viruses in marine planktonic
systems. Oceanography 6, 51–63.
Garrison, D.L., Gowing, M.M., 1993. Protozooplankton. In:
Friedmann, E.M. (Ed.), Antarctic Microbiology. WileyLiss, New York, pp. 123–165.
Gonzalez, J.M., Sherr, B.F., Sherr, E.B., 1993. Digestive
enzyme activity as a quantitative measure of protistan
grazing: The acid lysozyme assay for bacterivory. Marine
Ecology-Progress Series 100, 197–206.
Gosselin, M., Levasseur, M., Wheeler, P.A., Horner, R., Booth,
B.C., 1997. New measurements of phytoplankton and ice
algal production in the Arctic Ocean. Deep-Sea Research II
44, 1623–1644.
Macdonald, R.W., McLaughlin, F.A., Carmack, E.C., 2002.
Freshwater and its sources during the SHEBA drift in the
Canada Basin of the Arctic Ocean. Deep-Sea Research I 49,
1769–1785.
McLaughlin, F., Carmack, E., Macdonald, R., Melling, H.,
Swift, J., Sherr, B., Sherr. E., 2003. On the juxtaposition of
Atlantic and Pacific-origin waters in the Canada Basin,
1997–1998. Deep-Sea Research I, submitted for publication.
Melnikov, I.A., Kolosova, E.G., Welch, H.A., Zhitina, L.,
2002. Sea ice biological communities and nutrient dynamics
in the Canada Basin of the Arctic Ocean. Deep-Sea
Research I 49, 1623–1649.
Menden-Deuer, S., Lessard, .E.J., 2000. Carbon to volume
relationships for dinoflagellates, diatoms, and other protist
plankton. Limnology and Oceanography 45, 569–579.
Morita, R.Y., 1997. Bacteria in Oligotrophic Environments.
Chapman & Hall, New York, 529pp.
Pautzke, C.G., 1979. Phytoplankton primary production below
the Arctic ice pack: an ecosystems analysis. Ph.D. Thesis,
University of Washington, 180pp.
Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A manual of
chemical and biological methods for seawater analysis.
Pergamon Press, Toronto.
Perovich, D.K., Andreas, E.L., Curry, J.A., Eiken, H., Fairrall,
C.W., Grenfell, T.C., Guest, P.S., Intrieri, J., Kadko, D.,
Lindsay, R.W., McPhee, M.G., Morrison, J., Moritz, R.E.,
Paulson, C.A., Pegau, W.S., Persson, P.O.G., Pinkel, R.,
Richter-Menge, J.A., Stanton, T., Stern, H.M., Tucker III,
W.B., Uttal, T., 1999. Year on ice gives climate insights.
Eos, Transactions, American Geophysical Union 80 (481),
485–486.
Rich, J., Kirchman, D.L., Gosselin, M., Sherr, E., Sherr, B.,
1997. High bacterial production, uptake and concentration
of dissolved organic matter in the central Arctic Ocean.
Deep-Sea Research II 44, 1645–1663.
Sherr, E.B., Sherr, B.F., 1993. Preservation and storage of
samples for enumeration of heterotrophic protists. In:
Kemp, P., Sherr, B., Sherr, E., Cole, J. (Eds.), Current
Methods in Aquatic Microbial Ecology. Lewis Publ., NY,
pp. 207–212.
Sherr, E.B., Sherr, B.F., 2000. Phagotrophy in aquatic
microbial food webs. In: Newell, S. (Ed.), Manual of
Environmental Microbiology II. ASM Press, Washington,
DC, pp. 409–418.
Sherr, E.B., Sherr, B.F., 2003. Community respiration/production and bacterial activity in the upper water column of the
central Arctic Ocean. Deep-Sea Research I, submitted for
publication.
Sherr, E.B., Caron, D.A., Sherr, B.F., 1993. Staining of
heterotrophic protists for visualization via epifluorescence
microscopy. In: Kemp, P., Sherr, B., Sherr, E., Cole, J.
(Eds.), Current Methods in Aquatic Microbial Ecology.
Lewis Publishers, Boca Raton, FL, pp. 213–228.
Sherr, E.B., Sherr, B.F., Fessenden, L., 1997. Heterotrophic
protists in the central Arctic Ocean. Deep-Sea Research II
44, 1665–1682.
Smith, W.O., 1994. Primary productivity of a Phaeocystis
bloom in the Greenland Sea during spring, 1989. In:
Johannessen, O.M., Munch, R.D., Overland, J.E. (Eds.),
The Polar Oceans and their Role in Shaping the Global
Environment. American Geophysical Union, Washington,
DC, pp. 263–272.
Smith, W.O., Sakshaug, E., 1990. Polar phytoplankton. In:
Smith, W.O. (Ed.), Polar Oceanography, Part B. Academic,
San Diego, CA, pp. 477–525.
Solic, M., Krstulovic, N., 1994. Role of predation in controlling
bacterial and heterotrophic nanoflagellate standing stocks in
the coastal Adriatic Sea: seasonal patterns. Marine Ecology
Progress Series 114, 219–235.
Steward, G.F., Smith, D.C., Azam, F., 1996. Abundance and
production of bacteria and viruses in the Bering and
Chukchi Seas. Marine Ecology Progress Series 131, 287–300.
E.B. Sherr et al. / Deep-Sea Research I 50 (2003) 557–571
Stoecker, D.K., Capuzzo, J.McD., 1990. Predation on protozoa: its importance to zooplankton. Journal of Plankton
Research 12, 891–908.
Suttle, C.A., 1994. The significance of viruses to mortality
in aquatic microbial communities. Microbial Ecology 28,
237–244.
Thibault, D., Head, E.J.H., Wheeler, P.A., 1999. Mesozooplankton in the Arctic Ocean in summer. Deep-Sea
Research I 46, 1391–1415.
571
Verity, P.G., 1991. Measurement and simulation of prey uptake
by marine planktonic ciliates fed plastidic and aplastidic
nanoplankton. Limnology and Oceanography 36, 729–750.
Vors, N., 1992. Ultrastructure and autecology of the marine,
heterotrophic flagellate Leucocryptos marina (Braarud)
Butcher 1967 (Katablepharidaceae/Kathabelpharidae) with
a discussion of the genera Leucocryptos and Katablepharis/
Kathablepharis. European Journal of Protistology 28,
369–389.