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 significantly 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) and other climatological factors( Sarkar et al 2020). Seasonal fluctuation of water depth was significant different due to rainfall of the region and latitudinal connectivity of river channel. Temperature is one of the influencing factors of chemical and biological process of ecosystem (Cremona et al., 2021), fluctuations in temperature were related to regional climatic conditions and air temperature. The pH value in both the lakes were recorded towards alkaline side with limited fluctuation indicating optimum level BIS (2003) for productivity. The pH is a vital component for biochemical function of aquatic ecosystem (Jena et al., 2013). Slightly higher pH value in Akaipur lake is influenced by oscillation of metabolic activity of aquatic organism, and photosynthetic activity (Saha et al., 2021)). The coverage of macrophytes in lakes also attributed to diurnal fluctuation in pH value (Rameshkumar et al., 2019). Comparatively lower pH value in Khalsi might be due to decomposition of macrophyte which is in corollary to the study of Tang et al. (2019). Bala and Mukherjee (2010) observed similar finding in Nadia wetlands of West Bengal. Dissolved oxygen (DO) is one of the health indicators of aquatic ecosystem. DO of both lakes were well within acceptable range (above 5mg/L) for ecological wellbeing. Although DO fluctuation is varying everyday depends on temperature (Ouhmidou et al., 2015). DO concentration in both the lakes was lowest in monsoon and highest in winter season. In lower temperature solubility of oxygen in water increases and photosynthetic activity of aquatic plants is also increases (Hasler et al., 2009; Pedersen et al. 2013). Hardness and alkalinity are comparatively higher in Khalsi, considered as one of the important parameters for secondary productivity (Raju et al. 2014).
Chl a is one of the most important components to assess overall algal biomass of lakes and classifying to their trophic level (Gregor and Marˇsalek 2004). Chl a was comparatively very high in pre-monsoon (PRM) in both the lakes due to decreased water depth of lakes. Annual average Chl a was higher in 2015-16, may be indication towards the eutrophication of lakes. Enclosure culture (pen culture) practice in 2015-16 in both the lakes might have produced higher PO4−P, which can lead to increasing eutrophication, similarly observed in Baiyangdian lake by Wang et al. (2013). Although, nutrient concentration (NO3−N and PO4−P) was increased from 2014-15 to 2015-16 in both the lakes, however, a slight higher increased deviation was reported in Akaipur as compared to the Khalsi. The enhanced organic nutrients particularly, NO3−N and PO4−P due to unscientific pen culture practices in Akaipur in relation to the total water spread area of the lake as compared to the Khalsi during 2015-2016 that was coinciding the results of (Beveridge, 1984). The highest concentration of NO3−N during monsoon (June to September) in the present study may be also due to the allochthonous organic input and the decomposition of the aquatic macrophytes and jute retting (Ghosh and Biswas, 2018). Nutrient (NO3−N and PO4−P) concentration was expected reason for accelerating particularly, during second year of studies, indicating transition from high mesotrophic to slight eutrophic. N:P value have decreased significantly in 2015-16 in both the lakes might be both nutrients NO3−N and PO4−P have equally contributed towards eutrophication. Tang et al. (2019) suggested N as to be only limiting factor in Baiyangdian lake and McCarthy et al. (2007) P as limiting factor in Taihu lake, China for eutrophication. The deviation of the result in our 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 significant 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). Three common species Filinia longispina, Brachionus fulcatus, Keratella cochlearis of rotifer was recorded throughout the study period. Sharma et al. (1992) noted Filinia longispina is as eurytopic alkaline species. Two commonly occurring species Brachionus fulcatus, Keratella cochlearis are of the genus Brachionus and Keratella, considered as cosmopolitans and eurytopic species (Branco et al. 2002; Kumari et al., 2017) due its broader ecological value, thermal tolerance and geographical distribution (Bennett et al., 2019). Rotifer species Filinia longispina, Brachionus rubens, B. fulcatus, B. forficula, Keratella cochlearis, K. tropica, Bosmina longirostris and Polyarthra vulgaris were present in both the lakes with varying degree of abundance. Keratella cochlearis and K. tropica have been recorded from many freshwater bodies such as ponds to reservoir (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. 2002; Kumari 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 confirmed by many authors (Branco et al., 2002; Kumari et al., 2017; Doukhandji and Arab, 2017; Smaoune et al., 2020). Genus Polyarthra was less abundant in both the lakes due to high nutrient availability, less tolerance and other environmental factors. García-Chicote et al. (2018) was reported as indicator of mesotrophic state, while other available reports say the occurrence of Polyarthra in both oligitrophic and eutrophic lakes ( Obertegger et al., 2008; Obertegger et al., 2014; Karpowicz et al., 2020). In the present study, in winter (higher PO4−P), Polyarthra were absent, therefore this genus is indicator of oligrophic to mesotrophic state. Genus Trichocerca was recorded only in post monsoon and winter, when nutrient concentration was higher. Castro et al. (2005) was also reported the genus from eutrophic environment and agreed with our finding. Genus Bosmina was relatively abundant in throughout the seasons except in monsoon. In monsoon season, generally flow of water is higher compare to other seasons and Cladocerans usually prefer lentic water (littoral zone with macrophytes) than lotic water (Maia-Barbosa et al. 2008; Jeong et al (2015), therefore the genera was absent in both lakes in monsoon. Only cladocerans Ceriodaphnia cornuta, Moina branchiate and M. micrura were recorded during monsoon seasons in Khalsi because of their preference towards clear water and macrophyte dominant reasons. Mergeay et al (2006) have also found similar result and confirm their association in macrophyte dominated reasons in shallow Kenyan lakes lakes of southern Kenya. More abundance of copepods in monsoon and post-mansoon indicated well mixing of nutrients and favorable temperature. Yin et al. (2018) also reported higher abundance of copepod in mesotrophic lake. Copepod adult have greater preference towards higher nutrient rich environment than naupli. Mathews et al. (2018) have reported that favorable temperature range 25-280C for copepod growth and reproduction copepod. In this context monsoon and post-mosoon season is most favorable season for higher abundance of copepods. Ostracod communities have close association with aquatic macrophytes (Matsuda et al., 2015) and higher ecological tolerance (Kiss, 2007). Centropyxis, Diffulgia and Arcella were dominant in Khalsi but Centropyxis in Akaipur, throughout the study period, all these protozoans are having wide range of tolerance of nutrients, found in oligotrophic to eutrophic conditions (Madoni 2011).
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 unscientific 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 eutrophication (Marcus, 2004; Perbiche-Neves et al., 2016; Arcifa et al., 2020).
The mTSIROT and mTSICR values were determined annually for both the lakes but spatial as well as temporal difference was insignificant. Rotifer community among the zooplankton shows quick response to the environmental changes, is considered as highly suitable foe assessing degree of eutrophication, have been confirmed by many authors (Jekatierynczuk-Rudczyk et al., 2014; Dembowska et al., 2015; Ejsmont-Karabin et al., 2016; Wen et al., 2017; Smaoune et al., 2020). In this study trophic state index was assessed based on rotifer and crustacean groups. The mTSIROT and mTSICR found positively significantly relationship with phosphorus concentration (PO4−P) in both the lakes. Similar finding was reported by Xiong et al. (2016) from two subtropical lakes. Although, there are cumulative effect of many abiotic factors in changing the trophic dynamics of ecosystem PO4−P and Chl a showed significantly positive relationship towards eutrophication.
The composition and diversity of zooplankton species with higher trophic state have well explained by combination of environmental parameters with Pearson’s correlations and CCA analysis. García-Chicote et al. (2018) explained the dominance of some of the species in ecological stressful environment. Our result also shows dominance of genus Brachionus influenced by NO3−N rich environment also supported by favourable temp and pH. According to Branco et al. 2002; Kumari et al., 2017 dominance of genus Brachionus is a considerably good indicator of eutrophication of the lakes. Present study also reveals the fluctuation in species composition with high level of eutrophication and agreed by Obertegger and Manca (2011). The density of rotifers much more prevalent in both the lakes