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Proc Zool Soc DOI 10.1007/s12595-013-0071-6

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L K A TA

RESEARCH ARTICLE

Distribution and Abundance of Macrozoobenthic Species in Some Tropical Brackishwater Wetlands of West Bengal, India Mousumi Roy • N. C. Nandi • S. Banerjee D. Majumder

•

Received: 30 October 2012 / Revised: 29 April 2013 / Accepted: 6 May 2013 Ó Zoological Society, Kolkata, India 2013

Abstract Benthos inhabiting brackishwater ecosystems is subjected to transitional environment of freshwater and saltwater conditions. In the present paper the effects of environmental variables were studied along with anthropogenic activities, selecting two man-made fishery systems (bheri) and one natural estuarine system at Canning town, West Bengal, India. 11 water parameters and five sediment parameters were studied. Qualitative study of macrozoobenthos indicates that natural estuarine ecosystem harbours 57 species of nine groups while brackishwater impoundments are inhabited by 17–20 species only. The population density of the commonly occurring macrobenthic species reveals variation with respect to season and sites. Pearson’s correlation coefficient analysis and canonical correspondence analysis results suggest that more than 15 macrozoobenthic species bear significant correlation with one or more water and sediment parameters, within which 12 species showed significant correlation in estuarine

M. Roy (&)  N. C. Nandi Ecology Division, Zoological Survey of India, ‘M’ Block, New Alipore, Kolkata 700 053, India e-mail: [email protected]; [email protected] S. Banerjee Aquaculture Research Unit, Department of Zoology, Ballygunge Science College, University of Calcutta, Kolkata 700 019, India D. Majumder Department of Statistics, Bidhanchandra Krishi Vidyalaya, Mohanpur, Nadia, WB, India Present Address: M. Roy Department of Zoology, Kishore Bharati Bhagini Nivedita College, 148, Ramkrishna Sarani, Vivekananda Pally, Behala, Kolkata 700 060, WB, India

ecosystem indicating more environmental stress in impoundments than estuary. Keywords Estuarine ecosystem  Man-made impoundments  Macrozoobenthos  Water  Sediment

Introduction Brackishwater or estuarine ecosystems are transitional environment between saltwater and freshwater systems. The mixing of the saline and freshwater in this ecosystem plays important role in the distribution and abundance of fauna. Factors influencing the distribution, community structure and abundance of macrozoobenthos have widely been investigated by several investigators abroad (Reice 1980; Cowell and Vodopich 1981; Ravera 2001; Argu¨elles et al. 2002; Smith et al. 2003; Denisenko et al. 2003; Kaller and Hartman 2004; Peeters et al. 2004; Viðinskiene¨ 2005; Ogunwenmo et al. 2005; Magni et al. 2008). In India, Stoliczka (1869) made the pioneering study of brackishwater animals of Gangetic delta. Estuarine macroinvertebrates of Indian coast have been studied by several scientists like Kurian (1972), Harkantra et al. (1980), Harkantra and Parulekar (1991), Subha Rao et al. (1995) and Nandi and Das (2003). In West Bengal, Bhunia (1979), Nandi (1983), Subba Rao et al. (1987), Patra et al. (1990), Chaudhuri et al. (1994) and Mukherjee et al. (2007) have made valuable studies on different ecological aspects of mudflat, brackishwater and estuarine systems. Brackishwater man-made impoundments used for prawn and fish culture, locally called bheries in South 24 Parganas district, West Bengal, was investigated by Datta and Sarangi (1980). Despite this general ecological interest, few published data (Roy and Nandi 2012) are available in tropical

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countries to date to assess the impact of different environmental factors on the distribution pattern and abundance of macrozoobenthos in different brackishwater wetlands. Anthropogenic activities like fisheries operation by destructing mangrove vegetation, waste discharge to wetlands and above all chemical pollution affecting wetland biodiversity by destruction and alteration of natural habitats of a number of micro and macroinvertebrates. This investigation was initiated in three different brackishwater wetlands over a period of 2 years with the aim of finding whether the water and sediment parameters along with differential anthropogenic activities in two man made wetlands and in one natural habitat had any influence on the distribution pattern of macrozoobenthic species in tropical brackishwater ecosystem.

Materials and Methods Study Area In the present investigation, three selected brackishwater sites, viz. Matla Estuary (ME, natural estuarine mudflat), Viswabank Bheri (VB, man-made semi derelict fishery system of 22 ha, exhibiting moderate human interference) and Nikarighata Bheri (NB, man-made semi-intensive shrimp culture impoundment of 45 ha, suffering with high human activities) situated in between the latitude 22°17.6910 –22°18.0490 N and longitude 088°40.5170 – 088°40.7000 E were selected from Canning, South 24 Parganas district, West Bengal, India and studied with respect to water and sediment characteristics as well as macrozoobenthos. Fortnightly surveys were carried out during 2004–2006 from these three brackishwater sites in coastal West Bengal (Fig. 1). Collection, Preservation and Identification of Samples Samples of water and sediment were collected from three selected sites in triplicate form using standard methods (Piper 1966; Jackson 1973; Trivedy and Goel 1984; APHA 1998). 11 physico-chemical parameters of water, viz., temperature, pH, conductivity, transparency, total alkalinity, hardness, dissolved oxygen, salinity, phosphate, nitrite and nitrate and five physico-chemical parameters viz. temperature, pH, salinity, percentage of organic carbon and organic matter were determined fortnightly for each site for a period of 2 years (APHA 1998; Jackson 1973). Nine samples, three from each of the three selected stations of each site were collected each time using a box type sampler (15 cm 9 15 cm 9 10 cm) and were washed thoroughly in a standard sieve of 0.5 mm mesh size (Jonasson 1955; Havgaard 1973). All the organisms were sorted and

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Fig. 1 Google images (Geo Eye 2008) showing the sampling sites at Canning, West Bengal. [1 Matla Estuary (ME, 22°17.8380 N, 088°40.7000 E); 2 Viswabank Bheri (VB, 22°17.8430 N, 088°40.6270 E); 3 Nikarighata Bheri (NB, 22°17.6910 N, 088°40.5170 E)]

preserved in 4 % formalin or 70 % alcohol. Identifications of benthic organisms were made by consulting taxonomic references (Fauvel 1953; Naidu 1965; Alcock 1968; Subba Rao 1993; Kurian and Sebastian 1993) and expert taxonomists of different groups. Identified species were kept as reference collection for future study and in case of confusion, if any, respective specialists were consulted. In this study the benthic fauna obtained in the sampler was expressed as number of organisms per meter square using the following formula as outlined by Welch (1948). n ¼ O=a  s  10; 000 where, n is the number of organisms/m, O is the number of organisms counted, a is the area of the sampler and s is the number of replicates taken. Data Analysis The whole statistical analysis was carried out with the help of a relevant software programme under SPSS version 10.0. Pearson’s correlation coefficients were calculated to evaluate the parametric relationships between the abiotic and biotic factors supposedly in interaction. The two tailed

Proc Zool Soc

of water temperature, pH, DO, alkalinity, transparency and phosphate content were observed in Matla estuary. BOD (2.54 mg/l) and nitrite were recorded lowest in Nikarighata Bheri. In Viswabank Bheri lowest value of salinity, conductivity and hardness were observed.

‘t’ test was used to test the correlation coefficients at 5 and 1 % level of significance. Such analysis was repeated for each site and for each species separately. Canonical correspondence analysis (CCA) was performed to assess the distinct relationship between the distribution of macrozoobenthic species and environmental variables using CANOCO 4.5 (Ter Braak and Smilauer 1998). The unimodal forward selection of environmental variables with the Monte Carlo permutation test (P B 0.05, 499 permutations) was used in order to select variables significantly affecting the species distribution. The variable with the highest and significant contribution was included in the present analysis through repeated procedure. The total variation present in the species data is called total inertia and is given by the sum of all constrained eigenvalues.

Sediment The nature of bottom sediment has a selective influence upon the quality of benthic fauna for which five sediment parameters were analysed. The average variations in the sediment characteristics of three studied brackishwater wetlands are presented in Table 2. Sediment temperature in the fortnightly studied brackishwater wetlands varied from 30.83 °C in Matla Estuary to 32.22 °C in Nikarighata Bheri. The peak in temperature varied from site to site. In most of the sampling sites the high range in temperature was observed in pre-monsoon to the onset of monsoon. The pH varied from 7.78 in Nikarighata Bheri to 7.95 in Viswabank Bheri. The total salinity of sediments ranged from 0.94 to 1.23 % in these wetlands. The highest amount of salinity was observed in Nikarighata Bheri due to low level of water. Mean organic carbon (%) varied from 0.41 % in VB to 0.57 % in ME. The highest percentage of organic carbon in the mudflat was observed at the end of premonsoon season (June–July). The organic matter content varied from 0.71 to 0.98 % in these three sampling sites.

Results Water The mean values of eleven physico-chemical parameters of water estimated over a period of 2 years are summarized in Table 1. Maximum mean water temperature (32.07 °C), salinity (14.86 %), conductivity (29.48 mS/cm) and transparency (48.28 cm) values were recorded in Nikarighata Bheri whereas highest amount of pH (8.8), DO (8.3 mg/l), BOD (2.99 mg/l), total alkalinity (424.22 mg/l) and phosphate content (0.27 mg/l) were found in Viswabank Bheri. Hardness of water fluctuated from 2,136.14 mg/l in Viswabank Bheri to 2,720.5 mg/l in Matla estuary. Average nitrite content (0.03–0.10 mg/l) was higher in estuary than the man-made impoundments. In contrast, lowest amount

Macrozoobenthos A total of 66 macrozoobenthic species including six unidentified species belonging to four phyla comprising nine groups viz., Nemartina, Cnidaria, Polychaeta, Oligochaeta, Crustacea, Insecta, Gastropoda, Bivalvia and Pisces

Table 1 Physico-chemical parameters of water in different sites of Matla estuary Parameter

Brackishwater wetland sites VB

Temperature (°C)

NB

31.56 ± 3.23 (24.75–37.5)

ME

32.07 ± 3.48 (24.0–39.5)

30.37 ± 2.54 (23.95–35.55)

pH

8.80 ± 0.39 (8.3–9.9)

8.71 ± 0.50 (8.0–10.2)

8.32 ± 0.49 (7.50–9.5)

Dissolved oxygen (mg/l)

8.30 ± 1.52 (5.95–11.90)

6.61 ± 1.35 (3.55–9.00)

6.15 ± 1.45 (3.60–10.00)

BOD (mg/l)

2.99 ± 1.60 (0.90–7.50)

2.54 ± 1.12 (0.30–5.40)

2.78 ± 1.15 (0.55–4.70)

Salinity (%)

13.53 ± 7.64 (4.50–28.00)

14.86 ± 8.22 (5.00–32.95)

14.21 ± 7.87 (3.50–27.70)

Conductivity (mS/cm) Alkalinity (mg/l) Hardness (mg/l) Transparency (cm)

24.15 ± 14.25 (7.30–53.30) 424.22 ± 113.33 (240.24–600.60)

29.48 ± 12.50 (14.73–61.60) 406.49 ± 95.73 (240.24–610.61)

27.32 ± 11.95 (7.90–45.50) 299.05 ± 99.43 (170.18–550.55)

2,136.14 ± 1,013.43 (792.54–4,105.5) 2,649.82 ± 1,374.48 (714.0–4,641.0) 2,720.52 ± 1,602.10 (214.2–5,033.7) 44.50 ± 24.46 (10.9–89.5)

48.28 ± 15.11 (23.6–76.2)

Phosphate (mg/l)

0.27 ± 0.02 (0.25–0.30)

0.26 ± 0.02 (0.25–0.30)

15.21 ± 5.12 (7.8–29.9) 0.25 ± 0.06 (0.00–0.30)

Nitrite (mg/l)

0.04 ± 0.08 (0.00–0.25)

0.03 ± 0.04 (0.00–0.15)

0.10 ± 0.07 (0.00–0.23)

Nitrate below detection level: data given in parentheses indicate range VB Viswabank Bheri; NB Nikarighata Bheri; ME Matla Estuary

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Proc Zool Soc Table 2 Data [mean ± SD (range)] on physico-chemical parameters of sediment in different study sites at Canning Parameter

VB

NB

ME

Temperature (°C)

30.83 ± 3.03 (22.50–34.50)

31.12 ± 3.33 (24.25–35.25)

32.22 ± 2.75 (25.50–38.00)

pH

7.95 ± 0.32 (7.40–8.70)

7.78 ± 0.28 (7.30–8.20)

7.83 ± 0.22 (7.45–8.20)

Salinity(%)

1.01 ± 0.56 (0.00–2.00)

1.23 ± 0.51 (0.05–2.00)

0.94 ± 0.63 (0.00–2.50)

Organic carbon (%)

0.41 ± 0.17 (0.20–0.90)

0.43 ± 0.22 (0.09–1.19)

0.57 ± 0.21 (0.31–1.07)

Organic matter (%)

0.71 ± 0.28 (0.34–1.55)

0.75 ± 0.37 (0.15–2.05)

0.98 ± 0.36 (0.53–1.84)

(Table 3) were collected during 2004–2006 from different brackishwater habitats surveyed from Matla estuary of coastal West Bengal. Among the nine major groups, gastropods represented highest diversity of 28 species. Mudflats of Matla river at Canning, was inhabited by highest diversity of 57 species. Viswabank Bheri and Nikarighata Bheri harbored low diversity, representing 20 and 17 species respectively. This may be due to anthropogenic activities and also due to impact of prawn culture practices. Damselfly larvae, dragonfly larvae and Pythia plicata were only found in Viswabank Bheri whereas dipteran larvae, Theodaxus oualaniensis and Auricula sp. occurred only in Nikarighata Bheri. Out of a total of 66 macrozoobenthic species under nine major groups collected from the studied brackishwater wetlands, ten species (Cerithidea cingulata, Telescopium telescopium, Stenothyra deltae, Stenothyra blanfordiana, Assiminea brevicula, Assiminea beddomeana, Tornatina estriata, Gangetia miliacea, Neritina violacea, Thiara paludomoidea, Ampelisca sp.) were of common occurrence in these three habitats. The fortnightly collected population density of some commonly occurring species are pooled and presented season wise in Table 4. Population densities of these macrozoobenthic species differed from species to species and also from site to site. Cerithidea cingulata showed higher densities in the manmade impoundments (bheries) than in the mud flat of Matla river. Population density of this species was recorded highest (831 no./m2) in Viswabank Bheri during monsoon season. On the contrary, Cerithidea alata of the same gastropod family was abundant only in the mudflat with its mean population density of 339–1,124 no./m2. Telescopium telescopium was also abundant in the mudflat in premonsoon to monsoon season than the man-made impoundments. Population density of this common species was recorded maximum 667 no./m2 in the estuary in premonsoon season. On the other hand, population density of Stenothyra deltae was higher in the bheries than the mudflat. In Viswabank Bheri, Stenothyra deltae showed highest population (2,575 no./m2) with a mean of 112–1,307 no./m2. Stenothyra blanfordiana of the same family also showed the same trend and was common only

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in the bheries with highest mean population density of 306 no./m2 in the undisturbed Viswabank Bheri. Gangetia miliacea exhibited its higher abundance (104–402 no./m2) in the undisturbed man-made impoundment (Viswabank Bheri). In the impoundments, Assiminea brevicula was present in pre-monsoon to monsoon season whereas it was absent in the estuary during these two seasons and exhibited highest density (244 no./m2) in the mudflat in postmonsoon season which might be related with their migratory habit to choose suitable habitat like mudflat for reproduction. Another small gastropod species Tornatina estriata was common only in the undisturbed Viswabank Bheri in pre-monsoon and post-monsoon season (3–33 no./ m2). Among the crustaceans Ampelisca sp. was common mostly in the bheries with maximum density of 326 no./m2 in Viswabank Bheri. Population densities of these commonly occurring macrozoobenthic species indicate that these species are sensitive to cultural practices or anthropogenic activities and are susceptible to natural conditions. Likewise some polychaete (Dendronereis arborifera and Talehsepia annandalei), freshwater oligochaete (Limnodrilus hoffmeisteri and Bothrioneurum iris), crustaceans (Varuna litterata, Metapograpsus latifrons, Scylla serrata and Palaemon styliferus), gastropods (Neritina violacea, Assiminea beddomeana and Thiara paludomoidea) and bivalves (Donax incarnatus, Theora opalina and Pelecyora trigona) were seasonally or rarely present in either one of the three studied brackishwater sites which might be related to their habitat preference. Relationship between macrozoobenthos and environmental variables Pearson’s correlation coefficients were calculated to determine a relationship between every abiotic (physicochemical parameters of water and sediment) and biotic variables i.e. density of fairly abundant species. Only significant correlations of Pearson’s correlation coefficient of abiotic factors and biotic components of fortnightly surveyed brackishwater wetlands are analysed for three sampling sites which revealed the effect of individual parameter on the benthic fauna at different levels of probability.

Proc Zool Soc Table 3 Occurrence (?) of macrozoobenthic species from sampling sites of coastal West Bengal Groups/species

Table 3 continued Groups/species

VB NB ME

VB NB ME IV. Mollusca

I. Nemartina Unnamed species

Gastropoda -

-

?

-

-

?

II. Cnidaria Unnamed species III. Annelida Polychaeta

Neritina (Dostia) violacea (Gmelin)

?

?

?

Theodaxus (Clithon) oualaniensis Lesson

-

?

-

Littoraria (Littorinopsis) scabra scabra (Linnaeus) -

-

?

Littoraria (Littoraria) undulata Gray

-

-

?

Stenothyra deltae (Benson)

?

?

?

Dendronereis arborifera Pefers Dendronereis estuarina Southern

-

-

? ?

Stenothyra blanfordiana Nevill

?

-

?

Gangetia miliacea (Nevill)

?

?

?

Namalycastis fauveli Rao

-

-

?

Assiminea beddomeana Nevill

?

?

?

Talehsapia annandalei Fauvel

-

-

?

Assiminea brevicula (Pfeiffer)

?

?

?

Lumbrineris sp.

-

-

?

Thiara (Mainwaringia) paludomoidea Nevill

-

-

?

Glycera sp.

-

-

?

Cerithidea (Cerithidea) obtusa (Lamarck) Cerithidea (Cerithideopsilla) cingulata (Gmelin)

?

?

? ?

Bothrioneurum iris Beddard

-

?

?

Cerithidea (Cerithidea) alata (Philippi)

-

-

?

Limnodrilus hoffmeisteri Claparede

?

-

?

Telescopium (T.) telescopium (Linnaeus)

?

?

?

Natica tigrina (Roeding)

-

-

?

Thais blanfordi (Nevill)

-

-

?

Oligochaeta

IV. Arthropoda Crustacea Balanus amphitrite Darwin

-

-

?

Nassarius stolatus (Gmelin)

-

-

?

Uca rosea (Tweedie)

-

-

?

Tornatina estriata (Preston)

?

?

-

Scylla serrata Forskal

-

-

?

Auricula sp.

-

?

-

Scylla tranquibarica (Fabricius)

-

-

?

Pythia plicata (Ferussac)

?

-

-

Pseudosesarma edwardsi (deMan) Varuna litterata (Fabricius)

-

-

? ?

Onchidium tenerum (Stoliczka)

-

-

?

Onchidium tigrina Stoliczka

-

-

?

Metapograpsus latifrons White

?

?

?

Metapograpsus maculatus H. M. Edwards

-

-

?

Anadara granosa (Linnaeus)

-

-

?

Metaplax dentipes (Heller)

-

-

?

Modiolus striatulus (Hanley)

-

-

?

Metaplax intermedia deMan

-

-

-

Metaplax distincta H. M. Edwards

-

-

?

Macoma birmanica (Philippi) Theora opalina (Hinds)

-

-

? ?

Myomenippe hardwickii (Gray)

-

-

?

Donax incarnatus Gmelin

?

-

?

Macrobrachium rosenbergii (deMan)

?

?

?

Polymesoda (Geloina) bengalensis (Lamarck)

-

-

?

Palaemon (Exopalaemon) styliferus H. M. Edwards

?

?

?

Meretrix meretrix (Linnaeus)

?

-

?

Metapenaeus monocerus (Fabricius)

-

?

-

Pelecyora trigona Reeve

-

-

?

Ampelisca sp.

?

?

?

-

-

?

-

-

?

20

17

57

-

-

?

Unidentified species Insecta

Bivalvia

V. Pisces Boleophthalmus boddarti (Pallas) Total (66 species)

Coleoptera Canthydrus morsbachi (Wehncke) Guignotus flammulatus Sharp

-

?

?

Guignotus inconstans Regimbart

-

-

?

Paracymus evanescens Sharp

-

-

?

Heterocerus sp.

-

-

?

Damselfly larvae

?

-

-

Dragonfly larvae

?

-

-

-

?

-

Odonata

Diptera Dipteran larvae

Polychaete species (Namalycastis fauveli, Talehsapia annandalei and Dendronereis arborifera) which were abundant only in the mudflat exhibited significant correlation with one of the five water parameters (temperature, transparency, DO, BOD and alkalinity). The freshwater oligochaete species Limnodrilus hoffmeisteri interestingly exhibited significant positive correlation (r = 0.32, p \ 0.05) with salinity in Viswabank Bheri, whereas another oligochaete species Bothrioneurum iris showed correlation with temperature and alkalinity in Nikarighata Bheri and Matla Estuary. Crustacean species population

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Proc Zool Soc Table 4 Season-wise mean population density (nos/m2) of dominant macrobenthic species in three selected brackishwater sites

Species/site

Premonsoon

Monsoon Mean

Postmonsoon

Mean

SD

SD

Mean

SD

Cc

556

±148.22

831

±91.10

Tte

39

±46.16

18

±6.18

Sd

682

±434.73

112

±164.26

Sb

306

±364.82

14

±27.00

36

±32.04

Abr

216

±347.67

2

±3.50

0

±0.00

Gm

402

±463.71

104

Te

33

±52.86

0

±0.00

3

±5.50

Nv

7

±7.68

13

±15.36

9

±2.63

Tp

6

±7.14

8

±6.40

0

±0.00

11 5

±13.09 ±3.50

Viswabank Bheri

Abe A sp.

2 96

±3.50 ±155.92

±96.47

486 6 1,307

251

2 164

±141.73 ±8.04 ±1,036.62

±213.79

±3.50 ±118.72

Ta

2

±3.50

0

±0.00

2

±3.50

Lh

15

±29.50

0

±0.00

0

±0.00

Cc

595

±126.36

690

±92.64

560

±99.58

Tte

31

±23.06

15

±25.05

11

±9.56

Sd

231

±174.47

127

±135.88

479

Sb

38

±29.85

4

±7.50

0

±0.00

Abr

34

±63.92

2

±3.50

0

±0.00

Gm

147

±131.24

50

±36.99

15

±29.50

Nv

5

±3.50

2

±3.50

2

±3.50

Abe

7

±10.37

0

±0.00

2

±3.50

A sp.

54

±59.37

2

±3.50

67

±120.50

Cc Tte

0 178

±0.00 ±326.83

145 11

±38.68 ±22.00

11 0

±22.00 ±0.00

Sd

0

±0.00

11

±22.00

67

±77.17

Abr

0

±0.00

11

±22.00

244

±459.63

Gm

139

±249.03

22

±44.50

0

Ca

339

±262.03

1,124

Di

134

±170.42

0

±0.00

0

To

0

±0.00

0

±0.00

622

Pt

33

±66.50

0

±0.00

0

±0.00

Vl

44

±62.70

0

±0.00

33

±66.50

Ml

22

±25.40

0

±0.00

0

±0.00

Nikarighata Bheri

No decimal figure was used in the table as the specimen was not present in the wetland in fraction Abe, Assiminea beddomeana; Abr, Assiminea brevicula; A sp., Ampelisca sp.; Bi, Bothrioneurum iris; Ca, Cerithidea alata; Cc, Cerithidea cingulata; Da, Dendronereis arborifera; Di, Donax incarnatus; Gm, Gangetia miliacea; Lh, Limnodrilus hoffmeisteri; Ml, Metapograpsus latifrons; Nv, Neritina violacea; Ps, Palaemon styliferus; Pt, Pelecyora trigona; Sb, Stenothyra blanfordiana; Sd, Stenothyra deltae; Ss, Scylla serrata; Ta, Talehsepia annandalei; Te, Tornatina estriata; To, Theora opalina; Tp, Thiara paludomoidea; Tte, Telescopium telescopium; Vl, Varuna litterata]

Matla Estuary

±738.48

406

±0.00 ±181.76 ±0.00 ±1,244.50

Ss

0

±0.00

0

±0.00

44

±62.70

Ps

11

±22.00

11

±22.00

0

±0.00

A sp.

11

±22.00

0

0

0

±0.00

Da

22

±44.50

0

±0.00

33

±42.56

Ta

56

±66.56

0

±0.00

33

±66.50

Bi

0

±0.00

0

±0.00

22

±25.40

(Varuna litterata, Palaemon styliferus, Metapograpsus latifrons and Metapenaeus monocerus) was influenced at least by one of the three parameters viz., BOD, NO2 and salinity. Among 28 macrozoobenthic gastropod species collected from three sites of Canning region 12 species

123

±680.51

were significantly correlated with at least one of the nine water parameters (salinity, conductivity, BOD, alkalinity, temperature, PO4, DO, hardness and transparency). Salinity mainly exerted influence on gastropod species population. The bivalve species Meretrix meretrix was significantly

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negatively correlated with water temperature and alkalinity. Sediment played a vital role in regulating biotic components of brackishwater wetlands. In the mudfalts of Matla Estuary, polychaetes (Talehsapia annandalei and Namalycastis fauveli) were mostly correlated with either sediment temperature or organic enrichment of the soil. The oligochaete species (Bothrioneurum iris) in Nikarighata Bheri only exhibited correlation with soil temperature (r = -0.49, p \ 0.01). Three crustacean species (Metapograpsus latifrons, Palaemon styliferus, Ampelisca sp. and an unidentified isopod species) and one insect species (Guignotus flammulatus) population were influenced mostly by organic enrichment of the soil. Soil salinity, soil temperature and mainly the organic enrichment of the sediment exerted influence on the population density of 9 gastropod species in the natural and man-made brackishwater wetlands studied from Canning. Neritina violacea was correlated only with soil pH in Viswabank. In general, 15–16 macrozoobenthic species in manmade impoundments showed significant correlation with one or more water and sediment parameters. In Nikarighata Bheri, BOD of water exerted significant correlation with eight macrobenthic species whereas soil salinity showed significantly negative correlation with five macrobenthic species in Viswabank Bheri. Stenothyra blanfordiana in both the brackishwater bheries revealed significant correlation with maximum six abiotic factors. In the mudflat of Matla Estuary, 12 macrobenthic species were significantly correlated with at least one of the eight water parameters and also with at least one of the five sediment parameters. The CCA with forward selection of environmental variables suggests that macrozoobenthic species like Metapograpsus latifrons, Assiminea brevicula, Donax incarnatus, Tornatina estriata, Stenothyra blanfordiana, Gangetia miliacea, Talehsapia annandalei and Ampelisca sp. were associated with higher salinity range of water. Soil temperature had a higher score on the second axis than soil salinity indicating greater importance of temperature. Macrozoobenthic species like Cerithidea cingulata, Telescopium telescopium, Assiminea beddomeana, Thiara paludomoidea, Stenothyra deltae, Neritina violacea and Metapenaeus monocerus were mostly effected by these factors. In case of Nikarighata Bheri, the first two canonical axes account for 28.1 % of the variance of the species data and 62.6 % of the species-environment relationships. The eigen-values of the first two canonical axes are k1 = 0.162 and k2 = 0.101. Nine abiotic factors (water salinity, conductivity, transparency, alkalinity, phosphate, soil temperature, soil salinity, organic carbon and organic matter content of sediment) exhibited direct significant relationship on macrozoobenthic community. Figures 2 and 3 show that the species like Stenothyra blanfordiana,

Gangetia miliacea, Tornatina estriata, Assiminea brevicula and Telescopium telescopium were positively related with the environmental variables like water salinity, conductivity, alkalinity, transparency, phosphate and soil temperature. Stenothyra blanfordiana was more affected with Phosphate content, Tornatina estriata was highly related with water salinity whereas Neritina violacea was mainly

1.0

Paleomon styliferus Meretrix meretrix

Talehsapia annandalei Ampelisca sp

Metapenaeus monocerus Neritina violacea

Soil salinity Cerithidea cingulata

Stenothyra deltae Gangetia miliacea

Telescopium telescopium

Tornatina estriata Stenothyra blanfordiana Donax incarnatus

Assiminea beddomeana Thiara paludomoidea.

Assiminea brevicula

Limnodrilus hoffmeisteri

Water Salinity Soil temperature

-1.0

Metapograpsus latifrons

-1.0

1.0

Fig. 2 Ordination diagram obtained from the canonical correspondence analysis (CCA) showing the position of macrozoobenthic species and environmental variables performed on data set of VB. The length of the arrow is a measure of the importance of the variable and the arrowhead points at the direction of increasing influence

0.6 Neritina violacea

Transparency Soil salinity Assiminea beddomeana, Conductivity Cerithidea cingulata Bothrioneurum iris Alkalinity Stenothyra deltae Metapenaeus monocerus Soil temperature Stenothyra blanfordiana, PO4 Gangetia miliacea

Water Salinity Telescopium telescopium Tornatina estriata Assiminea brevicula Ampelisca sp.

Organic Carbon

Organic Matter

Thiara paludomoidea

-1.0

Palaemon styliferus -1.0

0.4

Fig. 3 Ordination diagram obtained from the canonical correspondence analysis (CCA) showing the position of macrozoobenthic species and environmental variables performed on data set of Nikarighata Bheri. The length of the arrow is a measure of the importance of the variable and the arrowhead points at the direction of increasing influence

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affected by water transparency. Ordination diagram (Fig. 3) also revealed that the population density of Cerithidea cingulata, Stenothyra deltae, Metapenaeus monocerus, Bothrioneurum iris and Assiminea beddomeana was increased by the rise of soil salinity values whereas the population of Ampelisca sp., Thiara paludomoidea and Palaemon styliferus flourished with the increase of the organic carbon and organic matter content of sediment. In river bed (Matla Estuary), the first two canonical axes (Fig. 4) account for 13.2 % of the variance of species data and 100 % of the species-environment relationships. The Eigen values of the two first canonical axes are k1 = 0.326 and k2 = 0.276. Two environmental variables viz. total alkalinity and hardness of water exerted significant effect on the macrozoobenthic invertebrates. The ordination diagram (Fig. 4) confirmed the effect of hardness mainly on the Telescopium telescopium, Varuna litterata, Gangetia miliacea, Pelecyora trigona, Donax incarnatus and Metapograpsus latifrons whereas alkalinity exerted positive effect on Talehsapia annandalei, Namalycastis fauveli, Dendronereis arborifera, Amphipod and Bothrioneurum iris population. On the other hand, population density of Stenothyra deltae, Assiminea brevicula, Cerithidea cingulata, Scylla serrata, Cerithidea alata and Myomenippe hardwickii decreased with the increase of these environmental parameters.

Bothrioneurum iris

1.0

Dendronereis arborifera

Namalycastis fauveli

Stenothyra deltae

Assiminea brevicula Myomenippe hardwickii

Alkalinity

Cerithidea cingulata Cerithidea alata

Talehsapia annandalei

Scylla serrata

Ampelisca sp. Telescopium telescopium Palaemon styliferus

Metapograpsus latifrons Donax incarnatus

Varuna litterata Gangetia miliacea Pelecyora trigona

-1.0

Hardness -0.6

1.2

Fig. 4 Ordination diagram obtained from the canonical correspondence analysis (CCA) showing the position of macrozoobenthic species and environmental variables performed on data set of Matla Estuary. The length of the arrow is a measure of the importance of the variable and the arrowhead points at the direction of increasing influence

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Discussion Among the 66 macrozoobenthic species, 40 species showed their occurrence only in mudflat which might be related with their preference of river bed as habitat and water current due to strong tidal flow. It seems three species viz. Limnodrilus hoffmeisteri, Donax incarnatus and Meretrix meretrix appeared in Viswabank Bheri due to uncultivated condition of this impoundment for last 3 years. Abundance of most predominant gastropod mollusks (Telescopium telescopium, Stenothyra blanfordiana and Neritina violacea) in manmade brackishwater impoundments was positively effected by the water temperature. But in the riverine ecosystem, the water temperature and sediment temperature negatively influenced the growth of several macrobenthic species (Namalycastis fauveli, Sesarma tetragonum, Gangetia miliacea, Stenothyra deltae and Assiminea brevicula) which supports the findings of several workers (Logan and Maurer 1975; Paul and Nandi 2003). Water and soil pH exerted negative influence on the pollution sensitive crustacean population in man-made brackishwater impoundment of the present study, while in the mudflat region, soil pH showed positive influence on some crustacean species (Metapograpsus latifrons and Sesarma tetragonum). Adholia et al. (1990) found negative influence of sediment pH on gastropods and oligochaetes. Berner (1976) opined that sediment pH control the benthic community. Dissolved oxygen (DO) showed positive correlation with the abundance of polychaetes and crustaceans in the mudflats of Matla river. Paul and Nandi (2003) reported the same relation between DO and Polychaeta in Hugli river of India. Polychaeta, Crustacea and Gastropoda population decreased with the increase of BOD i.e. from the increase of pollution with anthropogenic activities. Ampelisca sp. (Crustacea) and a gastropod (Cerithidea alata) were abundant in the polluted site of manmade system and estuary respectively which suggests that these species are pollution tolerant. The variations in the occurrence of bivalve species in the estuary might be due to their variation in breeding season, salinity tolerance, and ecotoxicology (Bhattacharya et al. 2003; Sarkar et al. 2007). The finding of positive relationships between water salinity and gastropods, crustaceans and biomass corroborated with the view of Josefson and Hansen (2004). Soil salinity negatively affected the population of gastropods, crustaceans and total benthos in the present investigation. Higher soil salinity reduces the decomposition rate (Bandopadhyay and Burman 2006) of organic matter which may affect the feeding habit of benthic organisms. Pearson’s correlation coefficient result of the present investigation showed that some macrobenthic gastropods (Stenothyra blanfordiana, Gangetia miliacea and Neritina violacea) in bheri systems were positively correlated with the conductivity of water as they were also positively related with the water salinity. The abundance of gastropods

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and polychaetes increased with high total alkalinity content of water in the studied shell-fish culture farms and in the Estuary confirms the reports of several workers (Dutta 1978; Paul and Nandi 2003). Okland (1990) found hardness of water to explain most of the differences in gastropod population. Hardness acted as a negative factor on the abundance of Crustacea and biomass in studied sites. This parameter has been reported by Paul and Nandi (2003) to affect the distribution of Polychaeta and Gastropoda. The relationship between water transparency and gastropod species (Stenothyra blanfordiana) in the shrimp culture pond was positive. But Polycheata and Crustacea group prefer low transparency or much turbid water in both the bheries and mudflats being burrowers. Increase in the phosphate content of water was responsible for the decrease in Crustacea population. In contrast, phosphate content exerted positive correlation on some gastropod molluscs (Assiminea beddomeana and Telescopium telescopium) in the bheries. Shetty et al. (1988) found enrichment of bottom fauna due to nitrite of water which confirms present investigation. The organic carbon and organic matter contents exerted positive influence on all groups of benthic abundance mostly in the bheries. In the riverine ecosystem the crustaceans were negatively influenced by organic carbon. According to Paul and Nandi (2003), this type of negative relationship on Crustacea group may be due to bacterial decomposition and decline in available oxygen. Kumar (2002) reported that the accumulation of organic matter in mangrove sediments depends largely on the sediment particle size. Differences in the interrelationship of crustacean with sediment organic matter in the bheries and mudflats might be due to textural differences. Among the 11 significant water parameters, BOD, salinity and alkalinity played most significant role on the population density of predominant macrobenthic species in the brackishwater wetlands. Soil salinity, organic carbon and organic matter in sediment were highly significant factors for population fluctuation of burrowing macrobenthic species (Donax incarnatus, Varuna litterata, Metapograpsus latifrons, amphipod, Talehsapia annandalei) in the estuary. Pearson’s correlation coefficient suggests that polychaetes, crustaceans and bivalves of brackishwater wetlands are highly related with sediment characters, either positively or negatively, whereas gastropods are related mostly with water parameters. These impacts are mainly due to infaunal and epifaunal nature of the benthos and their preference for specific habitat or substratum. This result finds support from the investigation of several workers (Logan and Maurer 1975; Berner 1976; Dutta 1978; Adholia et al. 1990; Paul and Nandi 2003; Josefson and Hansen 2004; Roy and Nandi 2012). The findings on the interrelationship of predominant benthic groups with environmental variables indicate that gastropods have flourished with the higher salinity

concentration of water and soil except in those sites where Cerithidea (Cerithideopsilla) cingulata was dominant as it was negatively related with salinity. It was also observed that most of the crustaceans were abundant in high organic matter concentrated sediment and Polychaeta were abundant in highly oxygenated water and in higher alkaline zone. Desai and Krishnankutty (1967) found that the salinity and substratum quality played dominant roles in quantitative distribution of benthos in backwaters as observed in this study. Qualitative study revealed those benthoses are more diverse in natural wetlands (estuary) than in man-made bheri system. Gastropoda was the dominant group in man-made brackishwater bheries, while Crustacea and Gastropoda were predominant in the estuary. The most dominant benthic species recorded in the tropical Brackishwater wetlands studied in this investigation were Cerithidea cingulata and Stenothyra deltae in the bheries and Cerithidea alata in the estuarine system. Quantitative study showed that the unmanaged man-made wetland system had higher benthos density over well managed bheri system due to management practices and anthropogenic activities. Illegal slum dwellers around the mud flats of Matla river are using the bank of the man-made impoundments as domestic sewage disposal site which lead to the eutrophication of the wetlands and damaging the natural habitats for many invertebrates. Destruction and alteration of natural habitats have led to the elimination of many invertebrates mainly the crustaceans from that place.

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