Abramova: Composition Abundance and Population Structure of Spring-Time Zooplankton
165
dominating. In the samples taken from Buorkhaya Bay in October-November this species was
not observed. In May-June 1996 copepodits of 1-3 stages of genus Pseudocalanus are
showing frequent occurrence in mesozooplankton compositions. So far we failed to make their
species identification. Abundance of these copepods in the samples was varying between 1.5
and 16% of the overall organismic abundance in the samples. Among mature individuals of
this genus females P. minutus and P. major and mature males P. minutus were recorded.
Assuming that copepodits belonged to one species, particularly P. minutus, the structure of his
population in May could be represented as following (Table 4).
Judging from the Table 4, reproduction of this species had started prior to our observations
and proceeded in May-June. McLaren et al. (1987) had described the life cycle of P. minutus
from Bedford basin in Nova Scotia, Canada, pointing to the fact that male specimens of this
species had been recorded in samples only in late winter-early summer. The authors concluded
that P. minutus is monocyclic, spending most part of the year, with the exception of the above
period, in a diapause at the 5th copepoditic stage. Our data also suggests that in the shelf of
Laptev Sea P. minutus is reproducing in late winter- early summer. However, as evidenced by
the literature, this species is reported to reproduce in August-September in New-Siberian
Islands shallows (Pavshtiks, 1990; Abramova, 1996). The above evidences allow us to suggest
the existence of two generations of P. minutus in Laptev Sea in a year. Accounting for possible
errors in identification of representatives of this genus, one can definitely assume in any case
the fact of reproduction of a certain species of Pseudoealanus genus in May-June in the shelf
zone of Laptev Sea.
Table 3: Sex-age structure of D. bungei population at Laptev Sea shelf, May 1996
Development stage
Females
Males
CS
C4
C3
C2-CI
Nauplii
% of population
18.1
6.S
S.S
0.8
0.3
1.3
67.S
Table 4: Sex-age structure of P. minutus population at Laptev Sea shelf, May 1996
Development stage
Females
Males
CS
C4
C3
C2-C1
Nauplii
% of population
7.9
I.S
2.3
0.4
1.1
68.1
18.2
Small copepod O. similis whose population consisted of copepodits of 5th-6th stages, larvae
of Polychaeta belonging to at least two species and youngsters of Sagitta may be also attributed
to widespread forms of spring-time zooplankton.
Elements of freshwater fauna, especially the Rotifera are sufficiently common for plankton in
surface water of low salinity of the shelf of Laptev Sea and are directly associated with arrival
and distribution of Lena-River water. Species diversity of spring-time zooplankton is
comparatively modest. Part of the species of estuarine-arctic complex characteristic for the shelf
zone of Laptev Sea, such as laschnovia tolli, Limnoealanus macrurus were lacking at this
season. By our observations, mature specimens 1. tolli are occurring in samples from
Buorkhaya Bay between September and January, those of L. macrurus between August and
February; occurrence of single mature specimens may be reported sometimes even in March. In
the beginning of this period, L.maerurus population is represented by nauplii and copepodits of
various stages - among which C5 is predominating in abundance. In August mature individuals
account for less than 10% of the population abundance. Gradual maturing of the remaining part
165
dominating. In the samples taken from Buorkhaya Bay in October-November this species was
not observed. In May-June 1996 copepodits of 1-3 stages of genus Pseudocalanus are
showing frequent occurrence in mesozooplankton compositions. So far we failed to make their
species identification. Abundance of these copepods in the samples was varying between 1.5
and 16% of the overall organismic abundance in the samples. Among mature individuals of
this genus females P. minutus and P. major and mature males P. minutus were recorded.
Assuming that copepodits belonged to one species, particularly P. minutus, the structure of his
population in May could be represented as following (Table 4).
Judging from the Table 4, reproduction of this species had started prior to our observations
and proceeded in May-June. McLaren et al. (1987) had described the life cycle of P. minutus
from Bedford basin in Nova Scotia, Canada, pointing to the fact that male specimens of this
species had been recorded in samples only in late winter-early summer. The authors concluded
that P. minutus is monocyclic, spending most part of the year, with the exception of the above
period, in a diapause at the 5th copepoditic stage. Our data also suggests that in the shelf of
Laptev Sea P. minutus is reproducing in late winter- early summer. However, as evidenced by
the literature, this species is reported to reproduce in August-September in New-Siberian
Islands shallows (Pavshtiks, 1990; Abramova, 1996). The above evidences allow us to suggest
the existence of two generations of P. minutus in Laptev Sea in a year. Accounting for possible
errors in identification of representatives of this genus, one can definitely assume in any case
the fact of reproduction of a certain species of Pseudoealanus genus in May-June in the shelf
zone of Laptev Sea.
Table 3: Sex-age structure of D. bungei population at Laptev Sea shelf, May 1996
Development stage
Females
Males
CS
C4
C3
C2-CI
Nauplii
% of population
18.1
6.S
S.S
0.8
0.3
1.3
67.S
Table 4: Sex-age structure of P. minutus population at Laptev Sea shelf, May 1996
Development stage
Females
Males
CS
C4
C3
C2-C1
Nauplii
% of population
7.9
I.S
2.3
0.4
1.1
68.1
18.2
Small copepod O. similis whose population consisted of copepodits of 5th-6th stages, larvae
of Polychaeta belonging to at least two species and youngsters of Sagitta may be also attributed
to widespread forms of spring-time zooplankton.
Elements of freshwater fauna, especially the Rotifera are sufficiently common for plankton in
surface water of low salinity of the shelf of Laptev Sea and are directly associated with arrival
and distribution of Lena-River water. Species diversity of spring-time zooplankton is
comparatively modest. Part of the species of estuarine-arctic complex characteristic for the shelf
zone of Laptev Sea, such as laschnovia tolli, Limnoealanus macrurus were lacking at this
season. By our observations, mature specimens 1. tolli are occurring in samples from
Buorkhaya Bay between September and January, those of L. macrurus between August and
February; occurrence of single mature specimens may be reported sometimes even in March. In
the beginning of this period, L.maerurus population is represented by nauplii and copepodits of
various stages - among which C5 is predominating in abundance. In August mature individuals
account for less than 10% of the population abundance. Gradual maturing of the remaining part