Indian Journal of Marine Sciences Vol. 28, September 1999, pp. 297-301

Zooplankton ecology of the Bahuda estuary (Orissa), east coast of India

Sujata Mishra & R C Panigrahy

Department of Marine Sciences, Berhampur University, Berhampur-760 007, Orissa, India

Received 24 February 1998, revised 20 April 1999

Ecology of zooplankton in Bahuda estuary. a shallow tidal estuary situated along the south Orissa coast, was investigated during December 1990-November 1992. Zooplankton volume, density, speFies composition and the relative abundance of major constituents showed well marked seasonal variations. The annual cycle showed bimodal oscillations with a major peak during May/June and secondary peak in November. The distribution of zooplankton was signiticantly intluenced by variations in salinity. A total of 100 species of holoplankton and 7 meroplankton were encountered. Copepods' constituted the most dominant group. They were represented by 58 species belonging to 32 genera of 25 families. Tintinnids formed the second richest group comprising 23 species. The other major holoplankters were ostracods. lucifers, amphipods. decapods, c1adocerans. siphonophores. chaetognaths, dolioloids and salps. The monsoon season erop was represented by a mixture of some oligo-stenohaline, limnetic and euryhaline forms .

Zooplankton fonns the vital links in the pelagic food chain. Many micro zooplankters also constitute the major food item of the larvae of crustaceans, molluscs and fishes. Thus, abundance of zooplankton practically acts as an ideal index to assess the fertility of a water mass. Furthennore, many zooplankton species are used as the water quality indicators including pollution. In recognition of their multiple utility emphasis has been given to aquire more and more knowledge on the ecology of the marine and estuarine zooplankton. The latest reviews by Millerl and Madhupratap2 have considerably updated our knowledge on the ecology of estuarine zooplankton. The zooplankton studies in Indian waters are mainly reported from the Cochin backwater3

.4, Mandovi-Zuari estuarine complex5,6,

Vellar estuary7,8, Hooghly estuary9,10 and Goutami­Godavari estuary 1 1 • But studies relating to the zooplankton ecology from Orissa coast are meagre and mostly limited to the Chilka lake l2

,13 and Rushikulya estuary 14. The present paper deals with species composItion, seasonal distribution and relative abundance of major groups of zooplankton in the Bahuda estuary, where such studies were not made earlier. The results of this study highlights the seasonal changes in zooplankton community structure in a shallow tidal estuary.

Materials and Methods Bahuda estuary (Iat. 19°IO'N and long. 84°20'E) is

a shallow (max. depth 4.5 m) tidal estuary opening into the Bay of Bengal at Patisonapur (Ganjam,

Orissa)(Fig.I). The general morphology and hydrological features of this environment have been reported earlierl5 .

Surface zooplankton samples were collected at monthly interval from three stations (Fig. I) during December 1990- November 1992 in the forenoon hours (08-11 AM), . by filtering 1000 litre of water using plankton net of mesh size 0.076 mm. The plankton samples were preserved in 4% buffered formaldehyde in seawater immediately after their collection. The water column depth, transparancy and temperature were measured at each station simultaneously. Salinity estimation of surface water was made adopting the Knudsen'sl6 titrimetric method, and for the estimation of dissolved oxygen (DO), modified Winkler'sl7 method was employed. The zooplankton volume was determined by

Fig. I-Map oflhe Babuda estuary showing sampling stations.

298 INDIAN 1. MAR SCI., VOL. 28, SEPTE~BER 1999

displacement method. The residual plankton on the filter were resuspended in 4% formaldehyde solution and was used for qualitative analysis.

Larger organisms such as copepods of families Pontallidae, Eucalanidae and Euchaetidae, chaetog­naths, dolioloids and salps were sorted out, identified and counted. The remaining plankton mixture was made to exactly 100 mI, and from this aliquot, 1 mI was transferred to a Sedgwick-Rafter cell. Species wise counting was made scanning the Rafter cell. The process of counting is repeated three times for each sample. The sum of the total micro zooplankton density (mean of three counts) and macrozooplankton density is taken as the bulk density (no m·3) of zooplankton in a given sample. From the bulk density and species wise density, the relative abundance (%) of major taxa was determined.

Results and Discussion Hydrography-The annual cycle of surface water

temperature exhibited a bimodal pattern of distribution (Fig. 2). The recorded minimum and maximum temperature values are 24°C and 32°C in 1991 and 25°C and 32SC in 1992. Higher temperature values were noticed during premonsoon season. Salinity showed wide range of fluctuations (2.7 and 33.9xlO,3 during 1991 and 2.7 and 36.3xlO'3 during 1992), but exhibited unimodal annual cycle (Fig. 2). High salinity observed during permonsoon season was reduced to 2.3x 10,3 in the monsoon period. Increase in salinity was noticed from October till the maximal value was attained in May. Dissolved oxygen contents ranged from 3.51-5.62 mrl during 1991 and 1.22-5.74 mrl during 1992. No definite pattern of annual cycle is discernible with respect to dissolved oxygen (Fig. 2).

ZoopLankton biomass-The plankton volume . ranged from 1.2 to 6.0 ml m,3 during 1991 and from 1.2 to 7.0 mI m,3 during 1992 (Fig. 3). Significant positive correlation (p<O.OO I) was noticed between zooplankton volume and density. High plankton standing stock was encountered during premonsoon season than the other times of the year. The average plankton volume as well as density during the study period were 3.5 ml m,3 and 4526] org m,3 respectively. The zooplankton counts ranged from 6920-88610 org m'3 during 1991 as against 4725-88750 org m,3 during ] 992. The pattern of distribution of zooplankton indicated significant differences between stations. But, the annual cycle at all the stations showed bimodal oscillations with

"'-E z w ~ x 0 0 lJJ > . ...J 0 III III B

~s;2

x ·

>-I-

Z -' « III

w a:: ::J I­« a:: w CL ~ W

.1-

Sin 1 <>- Sin 2 +- Sin 3 _ 7

10

22~~~~~~~~~~~~~~~ OJ FMAMJJASONOJ FMAMJJASON

1991 MONTH 1992

Fig. 2-Monthly variations of salinity. temperature and dissol ved oxygen in Bahuda estuary.

peaks once during May/June and again in December/January. Minimum density was noticed in August/September. Bimodal pattern.of annual cycle with low production during rainy seasons is · a common feature of estuarine, lagoonal and nearshore environments along Indian coasts.

ZoopLankton composition-A total of 100 species of holoplankton and 7 species of meroplankton were identified.

Protozoa-24 species of protozoans were identified from the collections, of which 23 species belong to Tintinnida (Tintinopsis tubulosa. T. cylindrica, T. nordqvisti, T. sacculus. T. mortensenii. T. beroidea, T directa, T. butschLii. T. bermudensis. T. tocantinensis, T. Lohmanni. T. uruguayensis, Dictyocysta seshoiyai, Eutintinnus tenuis. MetacyLis jorgensenii, Fa vella philippinensis. F. brevis. CoxLiella ampLa, Helicostomella longa. Tintinnidium incertum. Rhabdonella sp., Codonellopsis cstenfeldii. Amphorellopsis acuta, Leptotintinnum SimpLex and I species of Dinofiagellata (NoctiLuca scintillans). Tintinopsis sp. and Favella sp. constituted the major protozoan components almost round the year. Species

MISHRA & PANIGRAHY: ZOOPLANKTON ECOLOGY 299

'(Jr------------------,Io Density f3

B Volume

6

M- 2 'E

>­.... ~ I. w o

~ 2 .... ~

II=U:H~ 1=1 ~

51)

8

I.

2

0 SI2

2

0 511

~O~~~~~~~~~~

8 N 8 8

6 6

I.

2 2

o . .., .. __ _ . 0 OJ Ft.:ItiMJJ ASON.DJ FMAMJ JASON

1991 MONTH 1992

~-:-E

E

z 0

~ Z <t: ....I a.. 8 N

Fig. 3-Monthly variations of zooplankton volume and density in Bahuda estuary.

diversity (H') index of tintinnids varied from 0.80 to 2.36, the minimal and maximal values being reported in the monsoon and premonsoon seasons respectively. The species composition and mode of abundance of tintinnids is comparable to those of the Vellar estuarl.

Coelenterata-The coelenterates were represented by hydromedusae, scyphomedusae and siphonophores viz., Obelia sp., Porpita porpita. Aurelia aurita, Diphyes dispar and Lensia sp. Siphonophores were found only during December-June, while the medusoid forms occurred during September­November. They were absent -in the monsoon season, when the estuary was flooded with freshwater.

Arthropoda-Arthopods formed the most dominant group of zooplankton and were represented by 64 species. Copepoda alone comprised of 58

species, while others such as amphipoda, mysidaceae, decapoda, euphasiida, c1adocera and ostracoda were represented by only one species each (Leueothoe spiniearpa, Mesodopsis orientalis. Lucifer hasseni. Euphusia sp., Evadne tergestina, Philomedes sp.) . Calanoid copepods were represented by 33 species (Nannocalanus minor, Eucalanus elongatus. E. crassus, E. subcrassus. Paracalanus pravus, P. aeuleatus, Acroealanus longieornis. A monachus, A. gracilis, Scolecithrix danae, Centropages darsi­pinatus, C. Jurcatus, C. trispinosus. Pseudodiaptomus aurivilli, P. annandalei, P. binghami. Temora turbinata, T. discaudata, T. longieornis. Metaealanus aurivilli, Candacia armata, C. bradyi, Labidocera acuta, L. minuta, Pontella spinipes, P. andersoni, Pontellopsis hardmani, Acartia spinieauda, A. ttrythraea, A. clausi, Lucicutia Jlavicornis. Tortanus gracilis, T. Joreipatus) while 15 species (Oithona rigida, O. simplex, O. similis. O. brevi­corn is, O. linearis, Oncaea venusta. O. eonfiera, Coryeaeus danae, C. calus, Copi/ia mirabilis. Sapphirina ovatolanceolata, S. nigromaculata. Halicylops tenuispina, Mesocyclps sp., Cyclops sp.) were from Cyclopoida and 10 species (Longipedia weberi, Mierosetella norveglca, M. rosea, Maerosetella oeulata, M. gracilis. Clytemnestra seutellata, Euterpina aeutifrons, Tigriopus sp., Metis jousseaumei, Laophonte sp.) from Harpacticoida. Species composItIOn of copepods showed conspicuous seasonal variations. The premonsoon_ crop remained rich with neretic forms together with the typical euryhaline and brackishwater components. But during monsoon season, only a few oligostenohaline and limnetic species like Cyclops, Mesoeyclops and Helicyclops were seen abundantly . Conspicuous variation of copepod diversity was observed due to the salinity changes. However, 26 species have been reported as resident species because of their frequent occurrence almost throughout the yearl 8. The species diversity (H') values of copepods varied from 1.29 to 3.50 with higher values during premonsoon season . The other crustaceans were mostly limited to premonsoon and postmonsoon seasons, and were totally absent during the monsoon period.

Chaetognatha-Chaetognaths were represented by two species, namely Sagitta bedoti and S. enflata. While, S. bedoti occurred throughout the year, except when the estuary was flooded with fresh water, occurrence of S. enflata was limited to December­May.

300 INDIAN 1. MAR SCI., VOL. 28, SEPTEMBER 1999

Urochordata-Doliolum, Oikopleura dioica, Salpa fusiformis and Fritillaria borealis were the main representatives of urochordates found in p'lankton catches. Their occurrence was also limited to the postmonsoon and premonsoon seasons.

.during November-December and May-June.. This could be ascribed to the recruitment of meroplankters from the neighbouring coastal wa.ter by tidal migfation. Similar instances of tidal migration of fish

. eggs and fish larve were reported earlier from the , Rushikulya estuari4. Meroplankton-The meroplankton fauna mainly

comprised of crustacean larvae i.e. Nauplii, Zoea, Mysis and Megalopa; larvae of polychaetes, fish eggs and fish larvae. Also, a good number of molluscan veiigers were found in plankton catches. The abundance of larval plankton exhibited marked seasonal variations. Larvae of crustaceans and polychactes were found almost throughout the year except when the estuary was flooded with silt borne freshwater. This type of larval distribution was also reported from HoohulylO and Rushikulya l4 estuaries. The fish eggs and fish larvae were more abundant

Relative abundanc,e-The population density of zooplankton showed considerable spatial and temporal variations depending upon the previling environmental conditions. The relative abundance of major groups showed conspicuous monthly variations (Table I). Copepods, however, had emerged as the most dominant group throughout the study period. They shared about 62.66-89.78% of the total zooplankton counts. The second , In order of abundance, are the tintinids, which contributed between 4.78 and 20.49% of the total zooplankton

Month

December

January

February

March

April

May

June

July

August

Sep.tember

. October ' .

. November

. ' Dec'ember .' ' .. ~ january

February

March

April

May

June

July

August

September

October

November

Gr I

4.78

13.49

16.40

20.48

14.16

11.73

8.35

9.87

6.05

8.64

8.69

8.00

9.07

14.39

17.77

14.35

17.33

12.52

6.26

12.47

1l.l8

10.53

8.06

6.39

Gr 1- Tintinnida

Gr 2- Copepoda

Gr 3- Cladocera

Gr 4- Mysidacea

Table I-Relative abundance of zooplankton in Bahuda estuary (mean of three stations)

Relative abundance (%) of major zooplankters

Gr2

80.70

80.17

80.06

64.38

62 ,66

82.81

85.51

82.33

84.53

85.74

83.85

85.81

82.99

78.15

78.28

69.38

67.56

79.66

85.64

78.69

80.47

80.22

87.52

89.78

Gr3

0.02

0.01

0.09

0.10

0.02

0.006

0.02

0.03

0.03

0.17

0.Q7

Gr4 Gr5 Gr6 Gr7 Gr8 Gr9 Gr 10

0.78

0.43

0.26

0.51

1.43

0.41

0.27

0.59

0.30

0.48

0.77

0.79

0.42

1.46

1.74

0.74

0.68

1.01

0.34

0.43

0.003

0.01

0.04

0.01

0.006

0.01

0.03

0.006

0.03

0.09

0.16

0.06

0,01

0.01

Gr 5- Ostracoda

0.003

0.01

0.03

0.16

0.02

0.006

0.04

0.02

0.02

0.04

0.22

0.16

0.10

0.006

0.03

0.02

Gr 6- Euphausiacae & Decapods

Gr 7- Amphipoda

Gr 8 Chaetognatha

0.05

0.07

0.03

0.14

0.17

0.03

0.02

0.05

0.01

0.01

0.02

0.17

0.13

0.06

0.03

0.02

0.05

0.57

0.54

0.28

1.73

5,59

1.19

0.73

0.81

0.43

0.79

0.71

0.83

0.46

2.95

2.81

1.43

1.17

0.50

0.3 1

0.35

0.22

0. 18

0.24

0,39

1.04

0.16

0.13

0.06

0.15

0. 13

0. 14

0. 18

0.13

0.74

0.74

0.24

0,17

0.09

0 .02

0.08

5,09

2.88

1.57

6.39

4,47

1.66

2.56

1.02

0,9 I

2.35

1.89

2.52

2.45

1.97

4.45

1.42

2. 11

2.40

0.70

1.57

1.40

0.94

Gr 9- Doliolida & Salpida

Gr 10- Appendicularia

Gr 11- Fish eggs & larvae

Gr 12- Other meror ~ankton

Gr II Gr 12

(l.48

0,24

0. 19

0.62

1.87

0.38

0.44

1.08

1.31

0.89

0,57

0,74

0.45

0, 32

0 ,33

1.14

1.04

0.54

0.46

0.80

1.06

1,09

0.33

0,32

7,33

1. 89

0.99

4.64

7,02

1.56

1.08

4 ,13

7,19

4,72

3.77

2. 14

3,36

2.80

2.51

4.86

4.82

2.51

3. 13

5.76

7.28

6.57

1.95

1.60

, .~

.J

MISHRA & PANIGRAHY: ZOOPLANKTON.ECOLOGY 301

densi.ty. The 'quantitative abundance of all other groups showed marked variations and ·some of them were ' found absent during the monsoon period. Copepod predominance is a common event ill' zooplankton population of Indian estuaries5

-10

•

Occurrence of tintinnid swarms, however, can be considered as an interesting feature for the Bahuda estuary as similar situation has not been reported from any other estuary of the Indian coasts except the Vellar estuary8.

Zooplakton in relation to hydrography-The zooplankton distribution in estuaries is generally influenced by some physico-chemical and biological properties of the ambient water. Sarkar et al. 10 opined that in tropical estuaries, the role of temperature fluctuations become insignificant to inhibit the recruitment of zooplankton. The role of DO and pH also become insignificant except in polluted estuaries . Phytoplakton standing stock also seldom act as the limiting factor governing the abundance and distribution of zooplankton in tropical estuaries l9

.

Therefore, salinity, because of its wide range of fluctuations, can be considered as the most important factor governing the recruitment and distribution of zooplanktons in tropical estuaries. In order to assess the influence of various hydrographic features and phytoplankton standing stock on zooplankton production in Bahuda estuary, simple correlation analyses were made between the zooplankton density with ambient water temperature, salinity, DO, transparency and chlorophyll-a contents. Results of this analyses showed that there exists significant positive correlation (P <0.001 %) only between the zooplankton density and salinity. Again a record fall in population density was noticed during monsoon season, owing to the massive ingress of freshwater into the estuary. Thus, salinity only can be considered

as the key factor governing the abundance and distribution of zooplankton in this estuarine milieu .

Acknowledgement .The financial support provided to one of us (SM)

by CSIR, New Delhi is gratefully acknowledged .

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