International Journal of Marine Science 2016, Vol.6, No.28, 1-9
4
The minimum oxygen concentration was noticed at station 2 with maximum at station 9 (Table 1). The inorganic
nitrate concentration was noticed in the range between 3.5 and 19µmol/l
-1
with an average of 6.9 ±4.74 µmol/l
-1
with the lowest nitrate content procured at station 6 and the highest value observed at station 9 (Table 1). The
nitrite content was found from 1.58 to 7.5µmol/L
-1
with an average of 3.24 ±1.71µmol/l
-1
. The lowest and highest
nitrite was noticed at stations 6 and 9, respectively (Table 1). The phosphate concentration was found to vary
between 6.95 and 29.5µmol/l
-1
with an average of 10.75 ± 7.09µmol/l
-1
with the minimum value at station 4,
whereas the maximum value at station 2 (Table 1). The reactive silicate concentration varied from 7.9 to
32.0µmol/l
-1
with an average of 14.96±7.98µmol/l
-1
. Minimum silicate was recorded at station 2, whereas the
maximum was obtained at station 5 (Table 1).
Gulf of Mannar region comprises 114 species of zooplankton belonging to diverse group’s
viz
., 53 species of
calanoid copepod, 17 species of cyclopoid copepod, 16 species of ciliata, 8 species of crustacean larval forms, 7
species of harpacticoid copepod, 2 species of chaetognatha, 2 species of mollusca, 2 species of echinodermates, 2
species of foraminifera, 2 species of chordates, 1 species of decapoda, 1 species of appendiculariae, 1 species of
amphipods, 1 species of polychaete larvae, 1 species of ctenophores and 1 species of hydromedusae (Table 2).
The maximum (83) species and minimum (34) species were observed from the stations 4 and 5 & 6 respectively.
Among 114 species were found, the order copepoda was the most abundant taxon of which the calanoids
Acartia
spinicauda, Acartia danae, Pseudodiaptomus aurivilli, Eucalanus elongatus, Labidocera acuta, Nannocalanus
minor
and
Paracalanus parvus
the other species dominated were the cyclopoid
Corycaeus speciosus, Dioithona
rigida, Oithona similis, Metis jousseaumei
along with
Favella brevis, Tintinnopsis directa
, barnacle nauplii,
bivalves larvae and gastropods larvae (Table 2).
It is inferred from the present study that, zooplankton calanoid copepods has highest percentage composition
68.1 % at station 5 whereas cyclopoid copepod has 39.5% at station 7 (Fig. 2). Similar observation was made
earlier in other regions such as Lakshadweep Island (Achuthankutty
et al
., 1989), Straits of Malacca (Rezai
et al
.,
2004), Palk Strait (Jayasiri, 2007), Gulf of Thailand (Maiphae and Sa-ardrit, 2011), Parangipettai coast
(Santhanam
et al
., 2013) and Southwest coast (Jeyaraj
et al
., 2015). Calanoid copepods were the most dominant
group in all stations except station 3, both in terms of diversity and density during the investigation period. This
might be due to calanoid copepods could able to adapt to the prevailing environmental situations besides
continuous breeding habit (Kowenberg, 1993 and Chandramohan,
et al
., 1999).
The zooplankton taxa observed in various study sites ranged from 34 to 83 numbers with an average of 48.77 ±
15.51 numbers. The minimum taxa was observed in station 5 and the maximum was noticed in station 4 (Table 1).
The zooplankton population density observed in various study sites. It was noticed that, population density ranged
from 1913 to 11733 Nos./m
3
with an average of 6008 ±2986 Nos./m
3
. The maximum zooplankton density could
be attributed to mainly by the nutrient enrichment and it might be due to high density of phytoplankton. But
minimum might be due to oceanic currents and low primary productivity in the waters (Goswami, 1982 and
Bhattathiri and Devassi, 1981). The variation of zooplankton density might be due to the Dial Vertical Migration
of the species (DVM) and influence of oceanic currents (Pai
et al
., 2010). Higher population density with more
number of copepods species were earlier observed by Maruthanayagam and Subramanian (1999) in Gulf of
Mannar region.
The diversity and evenness of zooplankton are studied around the study sites. The diversity was ranged between
2.51 and 3.97 bits /individual with an average of 3.21 ±0.43 (Table 1). The minimum diversity was observed from
station 5 and the maximum was noticed from station 4. The high species diversity values at station 4 may be due
to high copepod density which indicates the stable hydrological conditions and phytoplankton density (Goswami
and Rao, 1981). Low species diversity at station 5 might be due to single species dominance. Evenness ranged
between 0.83 and 0.96 with an average of 0.92 ±0.04 an unequal distribution of the zooplankton species were
observed at station 5 which causes the lowest evenness at this place. However, highest evenness values were
obtained at station 4 which indicated that the species were equally distributed (Table 1)
