2.1 Study area
Two Oxbow lakes Khalsi and Akaipur located 25059’54.02’’N 88038’27’’E and 23005’14.14’’N 88042’56.22’’E, West Bengal, India, respectively (Fig. 1) were selected for study during March 2014- February 2016, Whereas the period were divided as pre-monsoon (March to May), monsoon (June to August), post-monsoon (September to November) seasons and Winter (December- February) respectively. Major activities in the catchments of both lakes include; intensive agriculture, habitat for birds, cattle, orchards and human habitation. These lakes are subjected to culture based fisheries management practices involving stocking of both indigenous and exotic carp seed to provide livelihood support to the 500 and 300 fishers families, respectively (Meena et al., 2019). Both the lakes are located in the lower Gangetic deltaic regions that subjected to the influence of runoff from agricultural land and domestic wastes, silt materials from the catchment due to jute retting, bathing, washing etc. Both the selected lakes are ecologically distinct. Akaipur oxbow lake has lost its perennial connectivity with the flushing channel, while Khalsi oxbow lake still maintains a feeble connectivity with the adjacent rivulet facilitating annual flushing. Dominant submerged macrophytes (Hydrilla verticillata, Ceratophyllum demersum, Vallisneria sp. and floating aquatic vegetations (Eichhornia crassipes, Pistia stratiotes, Nelumbo nucifera, Salvinia molesta, Lemna etc.) have occupied a greater part of Khalsi wetland (40-45%) distinguishing it from Akaipur, where only sporadic occurrence of aquatic plants was noticed (10-15%) during the study period. Floating and submerged macrophyte is providing setleter and breeding habitat for many small indigenous fish species in lakes. Water Framework Directive (WFD) has provided guidelines for standard sampling frequency to effectively monitor biotic and abiotic variables of a lake (EC, 2000). The sampling frequency to address the seasonal variability and inter-annual changes are followed as per guidelines of WFD.
2.3 Plankton data collection and analysis
The sampling protocol for zooplankton was followed as prescribed by Sharma and Sharma (2012) respectively. Taxonomic identification to the lowest possible taxon using standard keys was carried out following standard literature Koste (1978), Battish (1992), Segers (1995) and Nogrady & Segers (2002) for rotifers Edmondson (1959), Victor and Fernando (1979), Sehgal (1983), Benzie (2005) and Dussart Defaye (2001) for cladoceans and copepods. Verification and confirmation of taxonomic nomenclature was follows for rotifers International Commission on Zoological Nomenclature (http://iczn.org/lan/rotifer), the Cladocerans (Kotov et al. 2013) and Copepods (Walter and Boxshall 2018). Abundance of zooplankton was expressed as ind/l. Aliquot of 5ml sample out of 50ml sample (50 ml sample concentrated from 5litres of lake water) were taken randomly after mixing, used for counting in a Sedgewick-Rafter chamber under an inverted microscope (Zeiss-Winkel) .
Biovolumes of zooplankton were assessed following equations based on geometrical formulae best fitted for each body shape (Ruttner-Kolisko 1977). Thirty organisms of each selected species were selected randomly of same taxon, length, width and height were measured and best fitted geometrical shape formulae were applied. Biovolume of each taxon was converted to fresh weight assuming a specific density of (1*10−6). The fresh weight (FW) was later converted to dry weight (DW) (Schindler and Noven 1971; Bottrell et al. 1976). Biomass of ostracods was calculated following Lehette and Hernández-León (2009). Biovolume of Copepod nauplii were assumed 0.400 µg as suggested by Hawkins and Evans (1979). Zooplankton biomass calculated by multiplying wet weight of individual zooplankton with abundance. Phytoplankton biomass was assessed based on chlorophyll a estimation following Vörös and Padisák (1991).
To determine the ecological indices, number of zooplankton taxa present in each season was included in calculation excluding copepod nauplii and unidentified zooplankton. The commonly used index for biological system was Shannon-Wiener diversity index (H'), computed to characterize species diversity of zooplankton community. Shannon-Wiener diversity index was used both for comparing two distinct water bodies on temporal and spatial scale and to evaluate the health of water bodies. Index value greater than 3 indicates clean water while values in the range of 1–3 are indicates moderate pollution (Mason 1996).
Species diversity and homogeneity were calculated using the Shannon-Wiener diversity index H' (Shannon and Wiener, 1949) using the following equations:
$${\text{H}}^{{\prime }}=-{?}_{\text{i}}\text{P}\text{i} (\text{l}\text{n}\text{P}\text{i})$$
,
Where, Pi is the probability of the ith species
The Margalef's richness index (Margalef, 1958) indicates species richness computed by considering number of species.
\(\text{D}=\left(\text{S}-1\right)/\text{l}\text{n}\) N,
Where, D= Margalef's richness index, S= number of species and N= Total number of individuals
Evenness index J' (Pielou, 1966), which indicates the degree of evenness of distribution of species in a sample, was determined by following formula
$$\text{J}{\prime }=\text{H}{\prime }/\text{l}\text{n} \left(\text{S}\right)$$
,
Where, \(H\text{'}\)= Shannon diversity index and S = total number of species in a sample, across all samples in database.
The numeric trophic state index (TSI) was used for qualitative assessment of trophic states for the lakes (Carlson 1977 and Adamovich et al. 2016). The wetland trophic status classified on a numerical scale between 0-100 is given in Table 1.
Table 1
The topographic details and land use pattern of Oxbow lakes
Information | Khalsi | Akaipur |
Area of lakes (Ha) | 65 | 28 |
Coordinate | 25059’54.02’’N 88038’27’’E | 23005’14.14’’N 88042’56.22’’E |
Bioclimate | Semi-Aride | Semi-Aride |
Pre-monsoon season | March-May | March-May |
Monsoon season | June-August | June-August |
Post-monsoon | September-November | September -November |
Winter | December-February | December-February |
Mean depth (cm) | 177 | 160 |
Managed by society | KhalsiUdvastuMatsyjiviSamanvayaSamittee Ltd | AkaipurDwarvasini Fishermen Cooperative Society Ltd |
River connectivity | Icchamati river | Ganga river |
Water salinity | Freshwater | Freshwater |
Connectivity with river channel | Seasonally open | Closed |
Macro-vegetation | Floating, submersed and marginal | Floating, submersed and marginal |
Water use | Irrigation, Fisheries, Domestic, Cattle bathing, washing and Jute rating | Fisheries, Cattle bathing, washing and Jute rating |
Fisheries management | Culture based fisheries, enclosure culture | Culture based fisheries, enclosure culture |
Dominant species | Small Indigenous fish Pethia sp. Puntius sp. Chanda sp. | Small Indigenous fish Pethia sp. Puntius sp. Chanda sp. |
mTSI= (TSI Chl−a + TSI TP + TSISD)/3
The individual trophic state index (TSI) of Chlorophyll a (Chl-a), total phosphorus (TP) and Secchi disk transparency (SD) were calculated as follows.
(1) TSI Chl−a = 9.76ln (Chl-a) + 30.91
(2) TSI TP = 14.43ln (TP) + 4.15
(3) TSISD = -14.39ln (SD) + 59.91
Rotifer is one of the biotic components, which is used as an indicator of aquatic ecological health (Ejmont-Karabin, 2012). Rotifer community structure have been used for rotifer trophic status index (TSIROT) by following equations given by (Ejmont-Karabin, 2012)
(1) TSIROT1 = 5.38ln (Nr) + 19.28; where Nr number of rotifer (ind/l)
(2) TSIROT2 = 5.38ln (B) + 64.47; where B biomass of rotifer (mg ww /l)
(3) TSIROT3=3.85(B/ Nr)−0.318
(4) TSIROT4=0.144 TECTA + 54.8; percentage of spineless form (tecta) in abundance of Keratella cochlearis
(5) TSIROT5=0.203IHT+ 40.0; number of species contributed to the high trophic status indicator group
The mean value of TSI (mTSIROT) obtained from of above five equation is used as an indicator of Ecological status of lakes.
Crustacean based indices (TSICR) was developed by Ejsmont-Karabin & Karabin (2013) for estimation of Crustacean based trophic status indices has followed for Trophic index of lakes is given below;
(6) TSICR1 = 25.5 Nr0.142 ; where Nr number of crustacean abundance (ind/l)
(7) TSICR2 = 57.6B0.081; where B total wet cyclopoid biomass (mg/l)
(8) TSICR3=40.9CB0.097; percentage of cyclopoid biomass in the total crustacean biomass
(9) TSICR4=58.3(CY/CL)0.071; CY/CL: ratio of cyclopoids (CY) to cladoceran biomass (CL)
(10) TSI CR5=5.08Ln (CY/CA) + 46.6; ratio of cyclopoids (CY) to calanoid biomass (CA)