Journal of Plankton Research Volume 3 Number 2 1981
Ecological investigations on the zooplankton community of
Balsfjorden, northern Norway: population dynamics of the
euphausiids Thysanoessa inermis (Kroyer), Thysanoessa raschii
(M.Sars) and Meganyctiphanes norvegica (M.Sars) in 1976 and
1977
S.Falk-Petersen & C.C.E.Hopkins
Aquatic Biology Group, Institute of Biology and Geology, University of Tromsif,
P.O. Box 790, 9001 Tromsd, Norway.
(Received December 1979; accepted September 1980)
Introduction
Fjords in general offer several advantages for conducting marine biological
research (Brattegard, 1980), and particular fjords may be especially suitable for
studies of deepwater pelagic communities (see Matthews & Sands 1973). The proximity of Balsfjorden to the University of Tromso led to the choice of this fjord
by scientists wishing to study the ecology of a semi-enclosed ecosystem. This
"Balsfjord Project" began as an interdisciplinary research programme concentrating on fish, shell-fish, hydrography and plankton (Anon 1978).
The zooplankton investigations in Balsfjord are primarily aimed at elucidating
the life-histories of the dominant zooplankters. This involves quantitative
descriptions for such fundamental features as growth, life-span, and generation
and reproductive cycles. Many of the details of the life-history of boreal/arctic
zooplankters are still inadequately known. The secondary, and long term aim of
the investigations is to calculate production and to construct energy budgets for
the chief species in Balsfjorden. Incorporated in these studies are attempts to
detect and measure the effects of physical environmental factors (such as solar
radiation and hydrography), and primary production and phytoplankton
© IRL Press Limited, 1 Falconberg Court, London W1V 5FG, U.K.
Ill
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Abstract. The population dynamics of the euphausiids Thysanoessa inermis (KrOyer), T. raschii
(M.Sars), and Meganyctiphanes norvegica (M.Sars) have been followed in Balsfjorden in 1976 and
1977. Seasonal variations in length-frequency distributions, growth in carapace length, sex-ratio, and
spermatophore production and attachment are presented and discussed in relation to changes in
hydrography and phytoplanlcton standing-crop. An annual generation of T. inermis and T. raschii
was spawned in April and May. Eggs and larvae of M. norvegica were not found in Balsfjorden, indicating that recruitment occurs from outside the fjord. T. inermis and T. raschii reached maximum
carapace lengths of 7-8 mm and 6-7 mm respectively and had life-spans of c. 2 years 3 months. M.
norvegica had a life-span of c. 2 years 6 months and reached a maximum carapace length of c. 12.5
mm. In both T. inermis and T. raschii 0-group underwent the greatest length increases from May to
October, I-group from March to August and II-group from April to June. The population structure,
growth patterns and growth periods of M. norvegica were difficult to discern. The phytoplanlcton cycle appears to be the dominant factor regulating both growth and spawning of the Thysanoessa spp in
Balsfjorden, while temperature has no obvious influence.
S.Falk-Peterwn & C.C.E.Hopldiu
Materials and Methods
Study area
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standing-stock, on the various zooplankters.
One of the major problems facing plankton in boreal/arctic regions is the large
seasonal change in incident solar radiation and the shortness of the primary production season. This has resulted in adaptations, and the development of
strategies, especially as regards growth and reproduction, to meet the problems of
overwintering (Dunbar 1968, 1970). Of particular importance in attempting to
clarify these strategies at both the individual level and at the population level is
the measurement of basic body components/parameters (eg. length, wet weight,
dry weight, carbon, nitrogen, protein, lipid, ash, caloric value etc.) and the formulation of length-weight relationships. This leads to the determination of indices of' 'condition'' and forms the basis for production and energy budget determinations.
Euphausiids have a fundamental position in marine food-chains in high
latitudes (Mauchline & Fisher 1969; Backus 1978) yet studies of their ecology, in
the sub-arctic and arctic are few. However, in the Atlantic sector, relevant studies
have been conducted in coastal areas of Norway, Scotland, and eastern Canada
(Wiborg 1968, 1971; Matthews 1973; Mauchline 1960, 1966; Berkes 1976;
Jorgensen & Matthews 1976) and in Icelandic waters and in the Barents Sea
(Einarsson 1945; Drobysheva 1957). Studies on the growth of euphausiids are
reviewed by Mauchline & Fisher (1969).
In Balsfjorden the species Thysanoessa inermis (KrOyer), T. raschii (M.Sars),
and Meganyctiphanes norvegica (M.Sars) are often found in large quantities in
zooplankton sound scattering layers (Hopkins et al. 1978; Hopkins & Evans
1979) and are amongst the dominant vertical migrators (Hopkins & Gulliksen
1978). Euphausiids are also dominant items in the diets of pelagic fish in
Balsfjord (Pearcy et al. 1979). A study was therefore conducted to elucidate the
population dynamics, growth, and breeding strategies of the eupausiids in
Balsfjorden in relation to primary production and hydrography. This paper
describes the population dynamics of the euphausiids T. inermis, T. raschii, and
M. norvegica from Balsfjorden, northern Norway in 1976 and 1977.
Balsfjorden is c. 40 km long and lies south east from TromsO. Shallow sills (10-30
m) in the north and west (Tromsosund, Sandnessund, and Rystraumen) limit
water exchange. The sampling station was located in the deepest part of the fjord
near Svartnes (69°21'N 19°06'E) where the depth is 180-190 m (Fig. 1). The bottom at the sampling station is flat and consists of fine mud.
Environmental parameters
Incident and subsurface irradiance, nutrient analyses of the sea water, primary
production, chlorophyll a, and phytoplankton cell counts at the sampling station
during the study period are given by Eilertsen (1979). Temperature and salinity
depth (TSD) profiles were taken in 1976 with Nansen reversing water bottles and
in 1977 with a Neil-Brown Instruments Mark III CTD sonde.
178
Population dynamics of enpbaosilds
Location of Balsfjorden, northern Norway.
Zooplankton sampling
The present investigation is based upon quantitative plankton samples from 63
oblique net hauls collected on 30 daytime cruises from May 1976 to July 1977.
The water column was sampled with a Beyer's Low Speed Midwater Trawl
(BLSMT) (Designed by F. Beyer, Institute of Marine Biology, University of Oslo)
(mouth 0.7 m2, mesh 1 mm) and two types of Bongo nets (mouth 0.2 m2, mesh
200 pin; mouth 0.05 m2, mesh 500 /tm). Flow was measured with either a HydroBios flowmeter or a TSK (Tsurumi-Seiki-Kosakusho Co., Ltd.) flowmeter. Depth
was determined using either a time-depth recorder or an acoustic Furuno Trawl
Sonde.
The samples collected with the BLSMT were frozen in liquid nitrogen (- 196°C)
whilst those from the Bongo-nets were preserved in 4% formaldehyde with hexamine buffer. If necessary, the material was sub-sampled either with a LeaWiborg splitter (Wiborg 1951), or with a whirling flask and Stempel pipette.
Measurements of carapace length (CL) were taken as shown by Matthews (1973).
Year classes
An individual is defined as belonging to O-Group from hatching to 31 March the
following year, and year-groups change after this date each year (see Tesch 1971).
T. inermis and T. raschii were caught in large numbers with the Bongo-nets, but it
was necessary to use the BLSMT to sample larger numbers of M. norvegica.
The various generations could often be separated on the basis of length179
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Fig. 1.
>.Q
f
n
6.0,
>
*
7.0/
A 1
1977
33.75 3 3 .- 5
33.0
Fig. 3. Salinity isopleths from April 1976 to October 1977. Svartnes, Balsfjorden. •-sampling dates, • sampling depths with Nansen reversing bottles. In
1977 continuous profiles were taken with a CTD-sonde
1976
33.75
Fig. 2. Temperature isopleths from April 1976 to October 1977. Svartnes, Balsfjorden. A-sampling dates, • sampling depths with Nansen reversing bottles.
In 1977 continuous profiles were taken with a CTD-sonde.
20H
0-1
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p
p
a?
i
Population dynamics of euphausilds
frequency distributions. The different generations follow each other so that
Generation I (GI) is the oldest and G IV the youngest. When necessary the data
were further analysed with the help of probability paper (Harding 1949) and
tested for goodness of fit with a Kolmogorow-Smirnow test (see Sokal & Rohlf
1969).
Results
Environmental parameters
Dinoflagellatei
Diatoas
Photocystis
Total c t l l
cr
cr
UJ
a.
to
UJ
o
10*-
o
10J-
10'
M
I
Fig. 4. Seasonal variations in the phytoplankton standing stock February to May 1977. Svartnes,
Balsfjorden. After Eilertsen, 1979.
181
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The seasonal changes in temperature, and salinity in Balsfjorden are shown in
Figs. 2 & 3 respectively. Low winter temperatures, storage of freshwater as snow
and ice during winter, and the subsequent melting in spring and summer are some
of the most important factors determining the hydrography of Balsfjorden. The
winter situation is characterised by cold (c. 2°Q, moderately saline (33-34 °/oo)
and homogeneous water masses. In summer river discharge and heating leads to
stratification and a homogeneous surface layer with a salinity of < 33.00 °/oo
and temperatures rising from c. 3° up to 12°C from May to September. During
summer and autumn there is an increase in temperature, and decreases in salinity
and density in the deeper water masses. Fresh water run-off and the decreasing
density in the fjord-basin leads to an inflow of warmer, more saline, high density
S.Falk-Petersen & C.C.E.Hopklns
coastal water over the sill. These water masses can be discerned in the deeper part
of the fjord-basin from April to August.
The spring phytoplankton bloom in Balsfjorden starts in late March and
reaches a peak in late April (Fig. 4). There is a summer minimum in primary production which lasts from May until the middle of August. A new autumn production peak occurs in the beginning of September. The annual primary production
in 1977 was 110 gm C m ~ 2 year (Eilertsen, 1979). Primary production starts with
a mass increase of Phaeocystis pouchetii. This alga is dominant throughout the
primary production period except for a short time at the height of the spring
bloom when larger diatoms dominate (Eilersten 1979).
Euphausiid life cycles
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The breeding season of euphausiids can be determined by the presence or absence
of spermatophores, changes in the sex ratio, the appearance of eggs and larvae in
the plankton, and the size of the larvae. The eggs are fertilised by spermatophores
transferred from males to females. Drobysheva (1957) and Mauchline (1960) consider the transferrence of spermatophores to occur just before the spawning
period for T. inermis and T. raschii, whilst there can be an appreciable delay between spermatophore transferrence and spawning for M. norvegica.
The percentage of adult euphausiids carrying spermatophores in Balsfjorden
are shown in Fig. 5. T. inermis and T. raschii appear to be spring breeders and
the pattern of spermatophore attachment suggests a peak spawning period in the
beginning of April, at the start of the spring phytoplankton bloom. There was no
spermatophore attachment in M. norvegica females between early July 1976 and
January 1977, while males had spennatophores continuously from August 1976
to July 1977.
One year-old T. inermis and T. raschii developed their secondary sexual
characters during the period of rapid growth in May 1977. The importance of
these one year-old T. inermis and T. raschii in terms of recruitment to the
breeding population was investigated. In order to do this data for spermatophore
occurrence for May were analysed separately for one year-old and two year-old
euphausiids (Table I). Spermatophore occurrence suggested that very few females
of T. inermis and T. raschii took part in breeding as one year olds. Relatively few
one year-old T. raschii males developed spermatophores whilst most T. inermis
did. The results for one year-old male T. inermis are unfortunately based upon
few specimens. The sex ratio varies around 50% throughout the year.
The growth patterns of the different generations of euphausids can be determined from their length-frequency distributions. Larvae of T. inermis and T.
raschii were not identified to species and the data have been "pooled" in Fig. 6 &
7. T. inermis and T. raschii show a clear bimodal size-frequency distribution
throughout the study period (Fig. 6 & 7), whilst this is only occasionally apparent
for M. norvegica. (Fig. 8). The data indicate that T. inermis and T. raschii reached an age of 2 years 3 months whilst M. norvegica may possibly reach an age of
more than 2 years 6 months. Four generations could be identified for each of the
three species studied.
182
Population dynamics of enphansiids
ThyianotAia invmu,
80-
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1977
Fig. 5. Thysanoessa inermis, T. raschii and Meganyctiphanes norvegica. Seasonal variations in the
percentages of adult males and females carrying spennatophores. Svartnes, Balsfjorden.
The first generation (G I) of T. inermis and T. raschii was already dying out
when sampling started and no individuals were detected after July 1976. The second generation (G II) could be followed through the whole sampling period and
maximum numbers were encountered from December to March. Individuals of
the third generation (G III) were first recorded on 21 May 1976 and were
numerically dominant until April 1977. The fourth generation (GIV) recruited to
the sampled population between 16 and 25 May 1977 and were the numerically
dominant generation after June 1977.
183
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80-
S.Falk-Pctenen * C.C.E.Hopkina
Table I. Thysanoessa inermis and T. raschii. Percentage of individuals with spermatophores in
Generations II (2 year old) and III (1 year old) during May 1977. n = number of individuals
examined, °7o « percentage of individuals with spermatophores.
T. inermis
T. raschii
cr
9
n
«
n
G II
23
43.5
18
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26
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9
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36
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56
26.8
Growth in length
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No larvae of M. norvegica were caught in Balsfjorden and individuals of < 6
mm CL were rare. The smallest animals were caught in October 1976 (5 mm CL)
and in May 1977 (3.7 mm CL). The first generation (G I) of M. norvegica could
be followed distinctly until July 1976 and probably disappeared in the autumn.
The second generation (G II) could be followed throughout the sampling period
and was the dominant generation in 1977. It was not possible to separate G II
from G I in August, September, and October 1976. The third generation (G III)
was first detected in October 1976 and could be followed to the end of the sampling period. A single individual, caught in May 1977, was classified as belonging to
a fourth generation (G IV). M. norvegica was caught in relatively small numbers
throughout the sampling period.
Growth in euphausiids in terms of length is often based on the progression of
size-frequency histograms and expressed as time-related change in mean carapace
length. Accurate estimates using this method depend on the assumption that the
individuals comprising each generation, or cohort, recruit to the sampled population more or less simultaneously, and that the samples are taken from the same
population. The data for Balsfjorden indicate that these assumptions are valid
for T. inermis and T. raschii but recruitment and population structure for M.
norvegica are less certain.
Growth patterns for T. inermis and T. raschii are very similar in terms of variations in mean carapace length (Fig. 9). The mean carapace length for GI and GII
T. raschii seems to be c. 1 mm shorter than that for GI and GUT. inermis. Maximum carapace length for G I and GUT. inermis (7-8 mm) and T. raschii (6-7
mm) occurs in the autumn in Balsfjorden. G II T. inermis and T. raschii had a
growth period from June to September 1976 reaching carapace lengths of 6 and 7
mm respectively. Mean carapace length of G II decreased until January 1977,
before a new increase took place from April to June 1977. "Pooled" results are
presented for G III T. inermis and T. raschii until May 1977, and indicate a
growth period from May to October 1976, with a possible "stagnation" period in
mid-summer. Little growth occurred during the long winter period from October
to April 1977, but in the short time from April to July 1977 a doubling of
184
Population dynamics of enphandids
D Tkyianotiia
MAY
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Fig. 6.Thysanotssa larvae and Thysanoessa inermis. Length-frequency distributions, May 1976 to
July 1977. All data are standardized to 1000 m3. Svartnes, Balsfjorden.
185
S.Falk-Pttersai & C.C.E.Hopldju
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Fig. 7. Thysanoessa larvae and Thysanoessa raschii. Length-frequency distributions, May 1976 to
July 1977. All data are standardized to 1000 m3. Svartnes, Balsfjorden.
186
Popnlation dynamics of eupbaudldj
2MAY
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Fig. 8. Meganyctiphanes norvegica. Length-frequency distributions. May 1976 to July 1977. All
data are standardized to 1000 m3. Svartnes, Bakfjorden.
187
S.Falk-Petenefl 4 C.C.E.Hopkins
Thy&anousa i.
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2 -
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Fig. 9. Thysanoessa inermis and Thysanoessa raschii. Seasonal variations in monthly mean carapace
lengths and 95% confidence limits for generations I-IV
G I : A bongo, • BLSMT. G II : D bongo, • BLSMT. G III : O bongo, • BLSMT. • T. inermis
and T. raschii larvae caught with bongo nets. G IV: + T. inermis and T. raschii larvae caught with
bongo net. Svartnes, Balsfjorden.
188
Population dynamics of eophaosUds
carapace length occurred both for T. inermis (c. 3.3 - 6.5 mm) and for T. raschii
(c. 2.8 - 5.8 mm). G IV T. inermis and T. raschii increased their carapace length
from 0.5 - 1.5 mm between May and July 1977 (pooled data).
Seasonal variations in carapace length for M. norvegica are shown in Fig. 10.
The carapace length of G I remained constant at c. 12.5 mm. G II had a mean
carapace length of c. 7 mm during the summer and a mean carapace length of c.
12 mm during the winter. G III underwent a slight increase in carapace length
from c. 6 to 8 mm between November 1976 and July 1977.
Discussion
H-
MI-{II
E
i i
10-
Hi'
J
J
A ' S V N ' O
1976-
J
F ' M
1
1
A ' M ' J
1
J '
1977
Fig. 10. Meganyctiphanes norvegica. Seasonal variations in monthly mean carapace lengths and
95% confidence limits for generations I-IV. A = O I , D = O II, Q = G I + G II, O •= O III, + »
G IV. Svartnes, Balsfjorden.
189
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Both T. inermis and T. raschii appeared to be maintained as relatively stable
populations in Balsfjorden and the population size of the two species were
generally similar. The population density for adult Thysanoessa was low during
summer and a consistent decrease could be seen for G II in the summer of 1977.
There was also a slight decrease in the mean carapace length in G II T. inermis
and T. raschii in the autumn of 1976. These two phenomena may be due to a
combination of factors, such as difficulty in separating G I and G II, selective
mortality or emigration, but is more likely to be related to the pattern of vertical
migration of the well defined zooplankton sound scattering layer (SSL) (Hopkins
et al. 1978). The migration of SSL's is often closely related to variations in incident light intensity (see Clarke 1971; Kampa 1971) and high light intensities in the
midnight sun period may tend to drive the adult euphausiids in Balsfjorden
against the fjord bottom, resulting in poor sampling of the adult population.
S.Falk-Petenen & C.C.E.Hopkins
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Spawning of T. inermis and T. raschii takes place in spring at water
temperatures of 0-7°C (Einarsson 1945; Drobysheva 1957; Mauchline 1966;
Berkes 1976). The data from Balsfjorden indicate that there was a consistent 2-3
week spawning period for both Thysanoessa species in 1977, peaking at the beginning of April when temperatures were 1-2°C throughout the water column.
However, in 1976 T. inermis again probably spawned in April whilst T. raschii
had a peak spawning period in the middle of May. Berkes (1976) also found indications that T. raschii spawned later than T. inermis in the Gulf of St.
Lawrence. Spawning in Balsfjorden coincided with the spring phytoplankton
bloom whilst the fjord was still in a typical winter-situation with cold, well mixed
water masses. Einarsson (1945) found a close relationship between the spawning
of northern euphausiids and the spring phytoplankton bloom, whilst no relationship could be seen between spawning and water temperature.
Mauchline & Fisher (1969) state that M. norvegica generally has a longer and
less defined breeding season than T. inermis and T. raschii and that spawning occurs during the spring as well as summer. Both males and females of M.
norvegica in Balsfjorden had spermatophores, yet in spite of this no eggs or larvae could be seen in the plankton. This agrees well with the findings of Einarsson
(1945) who considered that the northern spawning limit of M. norvegica coincides
with the 5°C isotherm at 100 m.
Drobysheva (1957) also found spermatophores on both sexes in the Barents
Sea, but larvae were registered only in the western part in the warm summer of
1954; she therefore assumed that M. norvegica does not reproduce in the Barents
Sea and that recruitment occurs partly from the warmer Norwegian Atlantic
Current.
M. norvegica, being found only as adults in Balsfjorden, is likely to depend entirely on recruitment from external coastal waters, possibly associated with the
northwards flow of the Norwegian Coastal Current.
In Balsfjorden only a small percentage of T. inermis females become sexually
mature when they are one year old, whilst all males and females of both T.
inermis and T. raschii have reached sexual maturity as two year olds. The declines
in the percentages of males in the adult populations of T. inermis between April
and May and T. raschii and M. norvegica between May and July in Balsfjorden
may be associated with breeding mortality. Sexually mature T. inermis and T.
raschii when two years old in Balsfjorden had a carapace length of 6-7 mm and
5-6 mm respectively, and this is similar to the size of first time spawners (two year
olds) in the sub-arctic region as classified by Einarsson (1945).
Growth of the Thysanoessa species in Balsfjorden, in terms of changes in
carapace length, is closely related to the phytoplankton cycle. 0- and I-group T.
inermis and T. raschii had rapid growth periods in both the spring and autumn,
with a possible less intense growth period during the primary production
minimum in the summer. I-group Thysanoessa started their length increase in
April whilst II-group started theirs in May. This difference may be associated
with the phytoplankton succession; the small one-celled alga Phaeocystis
pouchetii underwent a large increase in March/April whilst the larger diatoms
were dominant in late April.
190
Population dynamics of euphausiids
Balsfjorden is a cold-water fjord (Saelen 1950), and summer heating does not
affect the middle and deep water masses before July. This warming of the main
water masses occurs after the onset of the breeding season and main growth
period of the euphausiids. The phytoplankton cycle appears to be the dominant
factor regulating both growth and spawning of the Thysanoessa species in
Balsfjorden while temperature has no obvious influence.
Acknowledgements
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