Environmental Characterization of Two Ecologically Distinct Gangetic Oxbow Lakes Using Zooplankton Taxonomic Indices In Ecological Monitoring: A Comparative Approach

In recent years closed and seasonally open oxbow lakes and river oodplains are subjected to eutrophication due to disconnection from river that compromises the hydro-ecological functions. This requires systematic studies to assess the ecological health of the water bodies using different indices and suggest appropriate strategies to manage the resources. The zooplanktons are closely link with surrounding environment throughout their life cycles and change rapidly in their growth and population when changes occurs in the surrounding, hence act as potential indicator of eutrophication. The present study examined the assemblage pattern of zooplankton community and trophic state of two ecologically distinct oxbow lakes based on eco-hydrological characteristics and community structure of rotifers and planktonic crustaceans seasonally over a period of 2 years. Comprehensive trophic state index (mTSI), rotifer trophic state index (mTSI ROT ) and Crustacean based indices (TSI CR ) were used to assess the degree of eutrophication. The Kruskal-Wallis test conrmed the heterogeneity of the eco-hydrological factors between the oxbow lakes. A total of 68 zooplankton species were identied of which rotifers (69%), cladocerans (18%), copepods (3%), ostacods (3%) and protozoans (7%). Seasonal uctuation of relative abundance and frequency of species in both lakes have been well presented. Both lakes showed transition from high meso-eutrophic to moderately eutrophic state with mTSI, mTSIrot and mTSIcr values of 54.90±11.71 and 56.95±15.64, 59.55±4.54 and 60.26±4.48, 55.79±4.76 and 60.00±4.03 in Khalsi (seasonally open) and Akaipur (closed) respectively. The Canonical Correspondence analysis (CCA) revealed water quality parameters comparison NO 3- N, water temperature and pH have contributed more in enhancement of abundance of eutrophication indicator species Brachionus and Keratella. The TSIs values indicated a prompt aquaculture sheries management measures like stocking of planktivorous sh (Labeo catla and Puntius sp.) for both the lakes before they reach in plunged state at which their restoration might become a challenge. An overview of worldwide use of rotifer and crustacean based indices in assessment of TSI has also been synthesized. The use of these zooplankton indices to evaluate the trophic status of the ecologically distinct lakes is highly recommended for water quality assessment and management. Based on this study strategies could be developed to plan and manage oodplain oxbow lakes for sheries enhancement programme as well as as conservation of biodiversity. at the Khalsi lake in PM 2015-2016 and the maximum average value 8.9 was recorded in the Akaipur lake during POM 2015-2016. The results show that EC follows a signicant spatial variation (P< 0.05) between the lakes. The recorded the highest EC Value (416 µS/cm) during PRM 2015-2016 in Khalsi. The minimum average value of 117 µS/cm was recorded in WIN 2014-2015 in the Akaipur. The average levels of DO show a very marked spatial variation between the lakes (P < 0.05). The highest DO concentration was recorded in WIN season 2014-2015 in both the lakes and minimum in MON 2-15-16. The mean SD levels show a great variation (P < 0.01). The Akaipur were recorded very low levels of transparency (SD) in both the years of study during PRM compare to Khalsi in same season. The maximum level in Khalsi was recorded in PM 2014-2015 with 127 cm. our result shows that the Khalsi is more transparent than Akaipur. The variation of Alk (total alkalinity during our study is very remarkable (P <0.01). The minimum average value was recorded in the POM 2014-2015 in Akaipur and maximum PRM 2015-2016 in Khalsi. The average value of Hard (total hardness) shows that there is spatial heterogeneity between the lakes (P <0.01). The highest mean value (170 mg/L) of Hard was recorded in PRM 2015-2016 in the Khalsi and minimum (34.6 mg/L). The Khalsi recorded very high level of PO 4− P content than Akaipur during 2014-2015. The mean value of PO 4− P content signicantly varies between the lakes (P<0.05). The maximum PO 4− P content was recorded during MON 2015-2016 in Akaipur followed by POM in Khalsi for the same year. The variation mROT CR in Akaipur lake was ranged greater varies from meso-eutrophic to hyper eutrophic condition. Single peak value in mROT CR was observed during monsoon of


Introduction
The oxbow lakes are characterized by shallow depth, high sediment nutrient, high productivity, well mixing of nutrient (Janseen et al. 2014;Tang et al. 2018Tang et al. , 2019. These lakes offer a plethora of ecosystem services for livelihood and nutritional security to the riparian communities ; Das et al., 2021). These provide safety from natural calamities, serve as habitat for larval rearing, nutrient recycling, water for domestic and irrigation purpose, shing activities and also considered as "Kidney of ecosystem" due to its e cient sinking capacity to inhale major agricultural runoff ( This is promising tool in management of lake, understanding about the trophic status and features of lake can been ascertain.
Rotifer and crustaceans based indices for evaluating the degree of eutrophication and relationship of index with environmental factors are promising for shallow lakes. In the view of above, the present investigation was an attempt to determine spatio-temporal patterns and trends of physico-chemical factors by perusal of zooplankton structured taxonomic indices for revealing spatiotemporal dynamics mediated intensity of eutrophication in two lower Gangetic Oxbow lakes.
Thus, lake managers and policy makers to develop guidelines for ecosystem based sheries management approach for oxbow lakes could utilize outcomes of the study.

Study area
Two Oxbow lakes Khalsi and Akaipur located 25 0 (10-15 %) during the study period. Floating and submerged macrophyte is providing setleter and breeding habitat for many small indigenous sh 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.
Biovolumes of zooplankton were assessed following equations based on geometrical formulae best tted 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 tted geometrical shape formulae were applied. Biovolume of each taxon was converted to fresh weight assuming a speci c 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).

Environmental factors
For each site three environmental parameters such as Water temperature, pH, conductivity was measured using a multiparameter pcstestr 35 (Eutech) in situ:, water transparency was measured by using Secchi-disc. Dissolved oxygen were measured following modi ed Winkler's method (Strickland and Parsons 1972 To determine the ecological indices, number of zooplankton taxa present in each season was included in calculation excluding copepod nauplii and unidenti ed 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: Where, Pi is the probability of the i th species The Margalef's richness index (Margalef, 1958) indicates species richness computed by considering number of species.
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 Where, H'= 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 andAdamovich et al. 2016). The wetland trophic status classi ed on a numerical scale between 0-100 is given in Table 1. The individual trophic state index (TSI) of Chlorophyll a (Chl-a), total phosphorus (TP) and Secchi disk transparency (SD) were calculated as follows.

Variations among environmental factors
During the study period, the physical and chemical water quality parameters were obtained for two years, the stations of

Diversity indices
The  (Fig. 4a and b) (Fig. 4a&b). The variation mROT CR in Akaipur lake was ranged greater varies from meso-eutrophic to hyper eutrophic condition. Single peak value in mROT CR was observed during monsoon of 2014-2015.
Analyses of Pearson's correlations among taxonomic diversity indices D, J, H`, and trophic status indices mTSI ROT and mTSI CR with environmental parameters were given in the Table 4. The mTSI ROT of both lakes were found to exhibit a signi cantly positively correlation with PO 4 − P and Chl a and signi cantly negatively correlated with N:P and DO concentration. The mTSI CR were also signi cantly negatively with N:P and positively with PO 4 − P and Chl a correlated in both lakes. This indices were also negatively signi cantly correlated with Temp and DO in Khalsi lake and insigni cant in Akaipur lake. The Shannon Wiener (H`) is signi cantly positively correlated with DO concentration in both lakes but negatively with PO 4 − P and Chl a concentration. Pielou's eveness (J) of both the lakes were highly signi cantly negatively correlated with water depth and positively with EC. The signi cance of Margalef index (D) of both lakes were highly variable.  CCA was carried out to identify the important environmental factors in uencing the zooplankton abundance in Khalsi and Akhaipur lake. The rst axis of the CCA (CCA1) explained 23% of the total variation of the model and 15% of the variability was explained by the second component. The rst two component CCA indicated that the environmental parameters including phosphate, Secchi disk depth (transparency), alkalinity, hardness, electrical conductivity, chlorophyll a and water depth are the important parameters in uencing the zooplankton abundance in Khalsi lake. On the other hand nitrate, temperature, pH, N:P and DO are the important parameters in uencing the zooplankton abundance in Akhaipur lake. CCA also indicated that the assemblage pattern of zooplankton in Akhaipur lake was distinct from Khalsi lake (Fig. 5).

Discussion
The ecological state of water quality is highly variable in shallow lakes which are disconnected from river, exclusively depend on physico-water quality parameters. The studied both oxbow lakes were signi cantly different based on electrical conductivity, phosphorus concentration, DO, pH, alkalinity and hardness. Differences in hydrological parameters may be attributed to riverine connectivity of lakes (Amoros and Bornette, 2002)  study in contrast to this study might be due to the fact that supply of both nitrogen and phosphorus from atmosphere deposition, non-point source as agriculture runoff is potentially a signi cant source of increasing nutrients loads in lakes.
The pattern of zooplankton community structure and abundance are very important for the maintenance of the ecological health of the aquatic ecosystem. Zooplanktons are considered to be the ecological indicators of aquatic environment (Neto et al. 2014). Rotifers respond quickly to aquatic environmental changes due to their short life cycle and are therefore used as indicators of overall health or condition (Carriack and Schelskek, 1977 (Doukhandji and Arab, 2017). The seven species of rotifers of both lakes have wide range of pollution tolerance (Sládeček 1983;Sharma et al. 1992, Branco et al. 2002Kumari et al., 2017). Genus Keratella and Brachionus is considered as indication of eutrophic state of the lake due to their close association with nutrient rich surrounding environment, have been con rmed by many authors (Branco et  Measuring zooplankton Species diversity is one of the most important characteristics of aquatic ecosystem to maintain stability as means of coping with any environmental changes. We have compared the value of various indices between two lakes. We have found that lowest diversity indices H′ and D from Akaipur during winter. Diversity have been decreased due to unscienti c enclosure culture practice and lack of connectivity with main river channnel that might have corroborated factors such abiotic factor, natural predation by copepod and cladoceran, competition for food and increasing The mTSI ROT and mTSI CR values were determined annually for both the lakes but spatial as well as temporal difference was insigni cant. Rotifer community among the zooplankton shows quick response to the environmental changes, is considered as highly suitable foe assessing degree of eutrophication, have been con rmed by many authors

Conclusion
Based on the scienti c evidences, rotifers proved their strong sensitivity of nutrient load in freshwater ecosystem. The present investigation advocate the suitability of rotifers based TSIs which is rst of its kind, integrating with physicochemical mTSIs to assume and validate biological indices to suggest an indicator species for determination of water quality and ecosystem health of the lakes. The mTSI CR and TSI based combination of environmental factors have revalidated our nding to assess degree of eutrophication of oxbow lakes. The study highlights the importance of understanding riverine connectivity of lakes and short term impact of enclosure culture on degree of eutrophication of lakes. The present investigation could further help us to understand the phenological mechanisms of water quality and rotifer abundance for scienti c management. Nutrient concentration PO 4 − P, temperature, Chl a, EC, Hard, Alk and SD was found to be most signi cant in uencing factor in both lakes. Based on the nding following points can be concluded.
Khalsi, a partially open wetland has more sinking capacity as compared to Akaipur (closed wetland) that that left a marginal scope for continue enclosure culture practices on long term basis and suggested a holistic investigation on impact assessment.
Khalsi has more sh production potential and essential water quality parameters for open water sheries in optimal range emphasizing upon the scienti c management and ecosystem based approach for sustainable sh production to the riparian communities.
Akaipur is required immediate intervention in terms of restricted irrational culture practices and to cutoff point source of organic load as per the sinking capacity of the wetland.
Akaipur might be succeeding toward super eutrophication if same practices continue for long term without considering scienti c management.
Present study is rst of its kind to validate the physico-chemical and biotic factors with rotifer and crustacean based indices thus advocating that rotifers can be considered as excellent indicator organism for ecological assessment, water quality monitoring and assessment of degree of eutrophication for better policy decisions.
Thus, addressing the issues related to the eco-hydrological alteration taking place in the closed and semi-closed lakes for sheries enhancement and carry forward ecosystem services in sustainable manner for wellbeing of the riparian community.
Declarations Acknowledgment This work was nancially supported by, ICAR-CIFRI Under Institute project. The Author is grateful towards the Director ICAR-CIFRI for providing necessary facility. The authors also thank Bandana Das Ghosh for their help with map drawing. We would like to thank Mr. Y. Ali and Mr. Bablu Naskar for their assistance in sampling. The authors are also acknowl-edge constructive suggestions of anonymous reviewers and constructive comments which helped us in strengthening the manuscript.
Authors Contributions:

Figure 1
The Khalsi and Akaipur oxbow lakes and their sampling locations