International Journal of Marine Science 2016, Vol.6, No.30, 1-8
3
The mesozooplankton samples were collected using bongo net (mesh size 200 μm) with a mouth area of 0.28 m
2
.
A calibrated flow meter (General Oceanics model number-2030 R, 2012) was attached to the net and was towed
horizontally just below the surface at a fixed speed of approximately 1 knot for 10-15 minutes. The samples were
fixed in 4 % buffered formalin and 95% ethyl alcohol. Zooplankton biomass was estimated by displacement
volume method and expressed as ml.m
-3
(Harris
et al.,
2000, Johnson and Allen 2005). Dissolved oxygen
concentration was estimated by modified Winkler’s method and Mohr-Knudson method for measuring salinity
(Strickland and Parsons 1972, Grasshoff
et al.,
1999). Free carbon dioxide was estimated by APHA (2005). The
water samples for estimation of the inorganic nutrients, nitrate-nitrogen, nitrite-nitrogen, ammonia-nitrogen,
phosphate-phosphorus and silicate-silicon were acidified with conc.HNO
3
and analyzed based on standard
methods (Grasshoff
et al.,
1999). Samples were sorted at group level for major zooplankton taxa (Omori and
Ikeda 1984, Tait 1981, Todd and Laverack 1991) enumerated and density was expressed in ind.m
-3
. Each copepod
was identified to species level using standard keys (Sewell and Seymour 1947, Kasturirangan 1963, Wellershaus
1974). For determination of sex ratio, 150 individuals were removed from each other and sexed. The sex ratio
[number of males (M) to number of females (F)] was tested by the Chi- square analysis (X
2
) using the software,
Statistical Package for the Social Sciences
(
SPSS) version 20.
3 Results
The average mesozooplankton density recorded in Ashtamudi estuary was 150533 ind.m
-3
. Highest density of
mesozooplankton was recorded in Ashramam (251733 ind.m
-3
), followed by Ashtamudi (115933 ind.m
-3
) and
lowest was in Kavanadu (83933 ind.m
-3
). In Ashramam, swarm was composed of very small numbers of
organisms other than the members of swarming species of calanoid copepods (97%). Fish eggs contributed 2.4%
of the total planktonic population while other organisms such as cyclopoid copepods (0.2%), crustacean nauplii
(0.3%), molluscan larvae (0.1%) and cumaceans were scarce in the sample. In Kavanadu, calanoid copepods
contributed 85.3% followed by crustacean nauplii (9.6%) and cyclopoid copepods (2.6%), whereas in Ashtamudi,
calanoid copepods contributed 69.3% of the total zooplankton population followed by crustacean nauplii (9.4%),
cladocera (6.6%), cyclopoids (3.6%), copepodids (3.6%) and fish eggs (2.2%).
The calanoid copepod density was 244200 ind.m
-3
in Ashramam; represented by Family Acartiidae (98%) and
Paracalanidae (2%) (Figure 2). Swarming of
Acartia southwelli
(34%) was observed along with
Acartia plumosa
(16%)
, Acartia centrura
(16%),
Acartia tropica
(16%),
Acartia bilobata
(9%) and
by
Bestiolina similis
(9%) in
Ashramam (Figure 3). Adult copepods generally constituted the major part of the swarm; especially,
A. southwelli,
A. plumosa,
and
A. centrura
that consisted almost entirely of adults. Adults constituted more than 85% of the
copepods in the swarms, reported in Ashramam. Among adults, females generally outnumbered males in the
swarms and
Acartia
species, representing mostly adults showed extreme values of adult sex ratios.
Sex ratio of
the swarm was 1:3 in the Ashtamudi estuary during the study period. Chi square test (X
2
) indicates that the sex
ratio significantly differed from the expected sex-ratio of 1:1. Therefore, the overall sex –ratio of calanoid
copepod was significantly in favor of females (X
2
=9.8; p<0.01). The sex ratio in adult calanoid copepod
populations was typically biased with the dominance of females.
Figure 2 Variation in mean density (ind.m
-3
) of
Acartia
species in Ashtamudi estuary during the study period
34%
16%
9%
16%
9%
16%
Acartia southwelli
Acartia centrura
Acartia bilobata
Acartia plumosa
Bestiolina similis
Acartia tropica
