1. Hydrobiologia 210: 225-232, 1991.
0 1991 Kluwer Academic Publishers. Printed in Belgium. 225
Phytoplankton and zooplankton of the Westport River Estuary,
Massachusetts (USA)
Walter J. Conley ’ & Jefferson T. Turner*
Biology Department, Southeastern Massachusetts University, North Dartmouth, MA 02747, USA (*author
for correspondence): ‘present address: Department of Marine Science, University of South Florida, 140
Seventh Av. S., St. Petersburg, FL 33701-5095, USA
Received 27 September 1989; in revised form 17 May 1990; accepted 26 June 1990
Abstract
Zooplankton and phytoplankton samples were simultaneously collected at approximately biweekly
intervals over most of an annual cycle in the Westport River Estuary, Massachusetts. Phytoplankton
numbers were overwhelmingly dominated throughout the study by athecate nanoplankton c 5 pm in
diameter. The zooplankton was primarily composed of copepod nauplii. Periods of occurrence of other
zooplankters such as adult copepods, marine cladocerans, meroplankters and ctenophores were similar
to those recorded for adjacent estuaries. Our results emphasize the abundance of smaller plankters that
have been historically undersampled.
Introduction
There have been numerous field investigations of
plankton seasonality and community structure in
estuarine waters of the northeastern United
States. These include studies of phytoplankton
(Smayda, 1973; 1980; 1983; Karentz & Smayda,
1984; and references therein), microzooplankton
(Sanders, 1987; Verity, 1987, and references
therein), and net zoopla$ton (Jeffries & Johnson,
1973; Turner, 1982, and references therein).
There have been fewer studies in which plankters
from multiple trophic levels were synoptically
sampled and identified in order to infer inter-
relationships with biotic and abiotic factors
(Deason & Smayda, 1982; Durbin & Durbin,
1981; Peterson, 1986; Turner et al., 1983; and
references therein).
During investigations of the relative importance
of herbivorous and carnivorous feeding in two
species of omnivorous estuarine copepods,
Conley & Turner (1985) combined laboratory
studies of feeding with field sampling of phyto-
plankton and zooplankton populations in the
Westport River Estuary, Massachusetts. Total
amounts of phytoplankton and copepod nauplii
were presented in terms of carbon, but taxonomic
data on fluctuations of major components of the
plankton community were not. Accordingly, we
here present patterns of abundance and com-
munity structure of phytoplankton and zoo-
plankton over most of an annual cycle. These are
the first such data for the Westport River Estuary,
and aside from an inventory of finfish, shellfish,
and marine angiosperm resources (Fiske et al.,
1968), the only published biological data for this
system.
Methods
Collections were made l-5 times per month from
11 April 1980 until 15 November 1980. Ice and/or

2. 226
Fig. 1. The Westport River estuary. All samples were col-
lected at Station A.
gale-force winds precluded further sampling from
an open skiff. All collections were from a single
station (Station A, Fig. l), at the surface (water
depth less than 4 m). Surface water was collected
with a bucket and 400 ml samples for phytoplank-
ton analyses were preserved with Lugol’s solution.
Salinity was measured with a refractometer, and
temperature was recorded from each bucket
sample. Zooplankton was collected in horizontal
surface tows by simultaneously-towed 73 pm-
mesh and 363 pm-mesh nets. All collections were
in daylight. To prevent clogging, the 73 pm-mesh
net was towed for 30 sec. In order to collect
sufficient numbers of larger copepods for feeding
studies (Conley & Turner, 1985), the 363 pm-
mesh net was towed for 3-5 min. Prior to pre-
servation, ctenophores and medusae were se-
parated by screening, and ctenophores were
measured for volume displacement. After identi-
fication, ctenophores and medusae were dis-
carded overboard, and the remainder of each
zooplankton sample was preserved in 5% for-
malin : seawater solution.
Zooplankton samples were reduced to aliquots
of 500-1000 animals with a Folsom plankton
splitter. Phytoplankton samples were con-
centrated by a factor of ten by sedimentation, and
aliquots were enumerated microscopically in a
Fig. 2. Surface temperature and salinity.
Sedgwick-Rafter cell. Aliquots of at least 500 cells
were counted to obtain k 10% error (Guillard,
1973). Linear dimensions of phytoplankton cells
were measured with an ocular micrometer, cell
volumes were estimated using geometric formulae,
and phytoplankton carbon was determined using
the volume :carbon conversions of Mullin et al.
(1966).
Results
Salinity, temperature and phytoplankton
Surface temperature ranged from 8 “C on 11
April to 25 “C on 4 August 1980 (Fig. 2). Salinity
varied little (30.0-32.5%,), with minimum values
after ice melt in early spring (Fig. 2).
The phytoplankton assemblage was dominated
(usually > 95 %) in terms of number and carbon
Fig. 3. Phytoplankton biomass (expressed as carbon).

3. 227
content by small (< 5 pm diameter) athecate In addition to the nanoplankton, other phyto-
nanoplankton. For the most part, it was impos- plankters were sporadically abundant. The dino-
sible to identify these cells to genus and species. flagellate Peridinium trochoideum ( = Scrippsiella
Total phytoplankton abundance (as carbon) fluc- trochoidea) reached levels of 68 cells ml - l in
tuated between 53 and 207 PgC liter - ’ (Fig. 3). September, and equalled or exceeded the carbon
Table 1. Phytoplankton taxa.
Taxa 4/l 1 4125 5116 6106 6/l 1 6124 l/O9 l/29 8/04 S/20 8127 9/13 9130
X X
X
X
X X
X X
X X X
X
X X
Silicoflagellates
Distephanus speculum X X
Dictyocha fibuia X
Diatoms
Achnanthes longtpes X X
Asterionella glacialis X
Amphiprora sp.
Bacillaria paradoxa
Bacteriastrum delicatum
Bidduiphia aurita X
Chaetoceros spp.
Corethron hystrix
Coscinodiscus spp. X X X
Diploneis smithii
Ditylum brightwellii X
Fragilaria sp. X
Grammatophora marina X X
Guinardia jlacida X
Gyrosigma spp. X X
Leptocylindrus danicus
Licomorphora spp. X X X
Melosira sulcata X X
Navicula spp. X X X
Nitzschia closten’um
N. longissima X X X
N. reversa X
N. seriata
Rhabdonema adriaticum
Rhizosolenia setigera
Skeletonema costatum X
Striateha unipunctata
Thalassionema nitzschoides
Thalassiothrix frauenfeldii
X X X X
X X X
X X
X X X X X
X X X X X X X
X X X X X
X X X
X
X
X
X X X
X X X X X X
X
X
X
Dinoflagellates
Ceratium minutum
C. tripos
Dinophysis acuminata
Gonyaulax sp.
Gymnodinium nelsoni
Peridinium depressum
P. trochoideum
Prorocentrum micans
P. minutum
X
X
X
X
X X X
X
X X X
X X X
X X
X X
X
X X X X
X X X X
X X X X X X X
X X X

4. 228
contribution of the nanoplankton. In August the
diatom Skeletonema costatum (maximum abund-
ance of 1603 cells ml- ‘) was in the same order
of magnitude, but never exceeded the carbon
contribution of the nanoplankton. Also in late
August, the diatom Leptocylindrus danicus (maxi-
mum abundance of 54 cells ml- ‘) contributed
19.7% of the carbon content. All other phyto-
plankton species were present in amounts so low
(0.1-10.0 cells ml- ‘) that quantitative data for
each species would have unacceptable error. In
some cases, presence of these taxa was based
upon observation of only a single cell. Therefore,
abundance data for these taxa are not presented,
but rather a list with dates of occurrence
(Table 1).
Zooplankton
The zooplankton collected in the 73 pm-mesh
nets was numerically dominated by copepod
nauplii. They comprised 24-98% of the animals
collected (Fig. 4), and reached maximum numbers
of 80 304 m- 3 on 29 May. Throughout the study,
nauplii were primarily those of the genus Acartia,
and species composition of nauplii generally re-
flected that of copepod adults. Fluctuations of all
other organisms, most of which were copepodites,
generally mirrored abundance of nauplii (Fig. 4).
The maximum concentration of zooplankters
occurred on 29 May, with 165 361 animals m- 3.
Fig. 4. Total zooplankton collected by the 73 pm-mesh
net.
The larger zooplankton collected in the 363-
,um-mesh net were generally dominated by adult
copepods, although various other animals were
intermittently abundant (Table 2). Throughout
most of the spring and early summer the copepods
Acartia hudsonica and Pseudocalanus sp. were
usually dominant (Fig. 5; Table 2).
Due to taxonomic problems within the genus
Pseudocalanus (Corkett & McLaren, 1978), no
attempt was made at the time of analysis to assign
specimens of this genus to species. However, a
recent taxonomic reanalysis of the genus Pseudo-
calanus by Frost (1989) reveals that either
P. moultoni or P. newmani could be present in the
Westport River estuary. The Pseudocalanus speci-
mens from the present study (collected in 1980)
are not longer available for reexamination. None-
theless, in recent collections (1987-1990) from
adjacent waters of Buzzards Bay, Pseudocalanus
specimens examined thus far were all P. newmani.
This distinction was based upon absence of
mediodorsal urosomal sensilla on adult females
(see Frost, 1989, p. 541 and p. 543).
Other copepod species varied in abundance
with season. In spring and early summer these
included Centropages hamatus, Temora longicornis,
Eurytemora herdmani, Tortanus discaudatus, Cen-
tropages typicus and Oithona colcarva (Figs. 5 and
6). As numbers of A. hudsonica declined in late
spring and early summer, its congener A. tonsa
increased in abundance to become the dominant
copepod throughout most of the late summer and
Fig. 5. Abundant holoplankton collected by the 363 pm-
mesh net.

5. 229
Table 2. Dominant taxa collected by the 363 pm mesh net.
Date Copepods Percent of total Dominant Taxa
Meroplankton Cladocerans
4/11 93.9 03.3 00.0 Acartia hudsonica 74.2
4125 99.1 00.0 00.0 Pseudocalanus spp: 57.9
5116 98.2 0.16 00.0 Acartia hudsonica 52.4
5129 96.9 00.2 01.8 Pseudocalanus spp. 42.8
6106 70.3 29.2 00.3 Acartia hudsonica 60.5
6/11 62.2 37.4 00.3 Acartia hudsonica 35.4
6124 81.8 11.6 00.0 Acartia hudsonica 63.8
7109 28.8 62.0 02.6 Decapod larvae 57.8
7129 16.6 81.9 04.2 Gastropod veligers 46.9
8104 05.7 54.3 00.0 Decapod larvae 48.6
8/12 18.3 12.2 67.9 Penilia avirosmk 67.5
8120 05.1 79.6 13.6 Decapod larvae 74.5
8122 21.6 41.0 36.9 Penilia avirostris 36.9
8127 07.2 06.5 87.7 Penilia avirostti 87.7
9103 29.8 41.1 27.2 Decapod larvae 34.5
9113 30.2 05.4 62.5 Penilia avirostris 62.5
9122 81.3 02.2 16.3 Acartia tonsa 78.9
9130 86.6 03.6 09.9 Acartia tonsa 84.6
10/07 36.8 29.6 05.6 Acartia tonsa 24.0
ll/ 5 96.2 00.0 00.0 Acartia tonsa 25.0
11/15 96.3 02.5 00.0 Acartia tonsa 16.0
fall (Table 2; Fig. 5). Other moderately-abundant cladoceran Penilia avirostris. Meroplankters com-
summer/fall copepods included Labidocera prised as much as 82% of total animals, and
aestiva, Centropages typicus, Eurytemora herdmani P. avirostris peaked at 700 m - 3 (87 % of total) on
and Oithona spp. (Fig. 6). 27 August.
During mid-summer organisms other than
copepods frequently were the most abundant
zooplankters in the 363 pm-mesh samples
(Table 2; Fig. 7). These included various mero-
plankters such as gastropod veligers, decapod
larvae (principally brachyuran crab zoea), and the
Other taxa were present at various times of the
year but never abundant. These included
(Table 3) the copepods Calanus Jinmarchicus,
Diosaccus tenuicornis, Longepedia coronata and
several other unidentified harpacticoids; clado-
cerans of the genera Evadne and Podon; ostracods
Table 3. Occurrence of less-abundant organisms collected by the 363 pm mesh net (numbers m- ‘).
4/ll 4/25 5/16 5/29 6/06 6/12 6/24 7/09 7/29 8/04 8/12 g/20 8/22 g/27 9/03 9/13 9122 S/30 IO/O7 II/OS II/IS
Evadne spp.
Podon spp.
Euconchoecia spp.
Calanus
tinmarchicus
Paracalanus
crassirostris
Longepedia
coronata
Diosacchus
tenuicornis
Mysid shrimps
Idotea baltica
ldotea phosphora
Sagitta elegans
29.7 0.8 0.9 4.4 0.9 1.0 2.4 6.0 0.4
0.9 0.7
1.9 3.4
3.7 1.9
5.5 0.7 0.5 0.3
6.4 0.6 I.1
I.9 0.7
192.3 10.4 0.4 1.6 4.2 7.1 6.3 10.6 3.6 5.5 0.7 0.2 0.3
3.6 3.3 0.7
0.3
I.9 0.7

6. 230
Date
Fig. 6. Less-abundant copepods collected by the 363 pm-
mesh net.
of the genus Euconchoecia; the isopods Idotea
baltica and I. phosphorea; the chaetognath Sagitta
elegans; unidentified mysids; and various mero-
plankters (Fig. 7) such as barnacle nauplii and
cyprids, polychaete and gastropod trochophores,
gastropod veligers, echinoderm plutei, bryozoan
larvae, and fish eggs and larvae.
Ctenophores, mostly Mnemiopsis leidyi but also
Pleurobrachia pileus, were abundant from mid-
summer through early fall (Fig. 8). They did not
occur in net tows until late July, but were observed
as early as 24 June. Ctenophores peaked on 25
August at 55 ml me3 (Fig. 8), and were so dense
that all other zooplankton sampling had to be
suspended due to net clogging. Cyanea capillata
and various other jellyfish were also frequently
observed in summer. However, they were infre-
quently collected, and in low numbers, thus they
were not enumerated.
Discussion
Plankton patterns in the Westport River Estuary
were similar to those recorded for other temperate
estuaries. The phytoplankton was numerically
dominated by athecate nanoplankton (primarily
microflagellates), asin virtually every other similar
study employing microscopic examination and
proper preservation (not formalin) of phyto-
plankton samples (see Bruno et al., 1983; Durbin
et al., 1983; Turner et al., 1983). The nano-
plankton exhibited abundance pulses of approxi-
mately 2-3 weeks duration, a pattern similar to
that found by Smayda (1957) for nearby
Narragansett Bay.
‘Detrital’ particles were often abundant in
phytoplankton samples. These particles were
undoubtedly a combination of organic detritus
and resuspended inorganic bottom sediments. No
attempt was made to quantify these particles,
although they can comprise as much as 66-78x
of total particle volume in estuarine waters (Van
Valkenburg et al., 1978). Detrital particles were
sufficiently abundant in phytoplankton samples
on two dates (11 April and 6 June) to prevent
accurate counting. Therefore, phytoplankton data
from these two dates are not presented.
Direct comparisons of results in zooplankton
studies are difficult because collection methods
strongly influence results. Number and taxonomic
composition of organisms in samples is over-
whelmingly dependent upon mesh of sampling

7. 231
nets (see Turner, 1982; Turner & Dagg, 1983).
Microzooplankton, particularly copepod nauplii
and protozooplankton, are undersampled by
meshes larger than approximately 100 pm.
Copepod nauplii were the most abundant
zooplankters recorded in our samples. Our range
of naupliar abundance (1370-70 300 m- ‘) is
within the range 41-255224 m- 3 collected by
Turner (1982) in Peconic Bay, New York, using
the same mesh (73 pm). Our range is also similar
to that of 550-82 100 me3 recorded by Faber
(1966) in nearby Narragansett Bay, using a slightly
larger mesh (116 pm). Most other zooplankton
studies in adjacent waters have used coarser
meshes of nets, and these have reported sub-
stantially lower numbers (see Table 2 of Turner,
1982 and Table 4 in the errata attachment to
Turner & Dagg, 1983 - last page of Volume 3,
Number 2 of Biol. Oceanogr., 1984).
Underestimation of the numbers of copepod
nauplii and other small zooplankters has probably
distorted the view of some zooplankton as-
semblages. For instance, Hulsizer (1976) cap-
tured relatively few nauplii with 153 pm-mesh
nets, and she suggested that copepod reproduc-
tion in Narragansett Bay was limited. Also,
Hulsizer’s total zooplankton numbers (which
likely underestimate Acartiu spp. subadults) have
been used by Hitchcock & Smayda (1977) and
Deason (1980) to question earlier views (Pratt,
1965; Martin, 1970) on the importance of cope-
pod grazing pressure release for initiation of
winter-spring diatom blooms in Narragansett
Bay. However, subsequent work by Deason &
Smayda (1982) points to the importance of
grazing pressure in controlling blooms in Narra-
gansett Bay, at least during the warmer season.
Other aspects of zooplankton patterns in the
Westport River Estuary are similar to those re-
corded for other temperate estuaries. These in-
clude the seasonal alternation of Acartia species
as dominant copepods, with A. tonsa and A. hud-
sonica dominant during warm and cold seasons,
respectively, (Jeffries, 1967; Turner, 1982; Sul-
livan & McManus, 1986; and references therein);
the precipitous declines in numbers of copepods
coincident with increase in ctenophores (Deason
& Smayda, 1982; Turner et al., 1983; and refer-
ences therein); and pulses of abundance of mero-
plankton (see Turner, 1982) and the cladoceran
Penilia avirostris (Turner et al., 1988, and refer-
ences therein) during the summer and early fall.
Our results highlight the abundance of small
plankters that have been historically under-
sampled. These include athecate nanoplankton
and copepod nauplii. Athecate microflagellates,
not diatoms, were the most abundant phyto-
plankton, and copepod nauplii, not adults, were
the most abundant metazoans. Much marine
ecological theory assumes that nanoplankters are
too small to be efficiently grazed by ‘zooplankton’.
Although this may well be true for the adult
copepods that are the subjects of most zoo-
plankton feeding studies, it likely is not true for
nauplii. Unfortunately, there is a paucity of in-
formation on copepod nauplii feeding ecology
(Turner, 1984). Since nauplii are frequent prey of
ichthyoplankton and other planktonic consumers
(Turner, 1984) they may be an important conduit
through which nanoplankton primary production
is transferred to higher trophic levels.
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