WATER QUALITY CHARACTERISTICS
In the present study, the two selected beels were found to have low transparency during the summer period (particularly May- June), which is attributed to the wind action and phytoplankton bloom. Various workers have also reported similar seasonal fluctuations in lake water transparency (Michael, 1969; Kumar, 1985).
Kumar (1985) calculated stratified temperature in a beels eco-system in West Bengal's Nadia district. The water temperature closely matched the temperature of the atmosphere in the beel habitats studied (Tables − 1). Variation in water temperature was quite distinct during different seasons throughout the study. According to Bhowmik (1988), maximum and minimum temperatures in West Bengal beels and baors ranged from 17.5 to 32.0 0C, which is consistent with the current research. Rai and Dutta Munshi (1989) have also reported that the presence of macrophytes profoundly influences water temperature. The present study also confirms the same. According to Banerjea (1967); Jhingran (1989) in water bodies with high organic contents in bottom mud, large-scale mortality takes place in summer months especially after a shower or cold wind.
The prime life-bearing gas in aquatic media was within the moderate range of 6.4 to 10.8 ppm (Table- 1). The reason for the maximum stratification of oxygen in monsoon may be attributed to the high rate of surface mixing of atmospheric oxygen due to the showering of a raindrop (Banerjea, 1967). It is interesting to mention that the higher concentration of oxygen in the surface water during monsoon was always not in confirmation with the high plankton density whereas Das and Srivastava, (1956) reported that the phytoplankton peak corresponds to the high oxygen values while zooplankton peaks are associated with low oxygen values. Oxygen content observed to be poor during the period of the high temperature such as the summer season (Bhowmik, 1968; 1988; Sugunan et al., 2000) conforms with the present study. Kumar (1985) also reported similar observations. Dense aquatic vegetation, shallow water depth, and intense fishing activities can cause large fluctuations in the dissolved oxygen content of water in the beels. (Yadava et al., 1987).
Water reaction pH
In the present investigation, the hydrogen-ion concentration in the surface water of the beels was 8.0 and above excepting on few occasions. The observed pH as 8.0 and above has been recorded to be productive by various workers (Hutchinson, 1957; Banerjea, 1967). Michael (1969) observed that when pH ranged between 7.3 and 8.4 the water provided optimum conditions for the growth of plankton. The present study bears the agreement of alkaline pH with the study of Bhowmik (1988) where the pH value of the beels and baors of West Bengal was recorded between 6.8 and 9.1.
Since total alkalinity values are the resultant of the entire biological and chemical process taking place in the water body, as such it is also taken as a rough index of productivity of the water body (Laal, 1981). In the present investigation, the alkalinity of the beels waters was observed to be within the product range. The high alkalinity value was recorded in beels infested with a high density of macrophyte-associated fauna and benthic biomass. Sugunan et al., (2000) reported similar observations.
Such absence of free CO2 in water was found to be related to the presence of heavy phytoplankton populations (Michael, 1969). The pronounce absence of the free CO2 at the subsurface level of the water was in confirmation of the observation made by Reid (1961), who reported that at pH 8 and above the free CO2 is usually absent. Most fish species will survive waters containing up to 60 ppm (Hart, 1944).
The specific conductance of water is a measure of the resistance of a solution to electrical flow, which declines with increasing ion content (Wetzel, 2001). The specific conductivity values recorded from these lakes were in an acceptable range. It has been reported an optimum range as 250–400 µS/cm and opined that specific conductivity above 400 µS/cm does not limit or favor productivity. The total concentration of solid constituents in natural waters is measured by specific conductivity.
In the present investigation, the values of water-soluble nitrate varied from system to system and with seasonal changes. The level of the nutrient was 0.3–1.5 ppm in Kole, while, it was comparatively higher in the range of 0.8 to 1.3 ppm in Saguna (Tables − 1). The fluctuation trend in nitrate level indicated mesotrophic to the eutrophic condition of the beels (Goldman and Horne, 1983). The nitrate levels in the studied beels were within the productive range (Banerjea, 1967) and ideal for plankton growth, which is consistent with the findings of the study.
The phosphate cycle of the beels was in correlation with the dissolved oxygen and is known to play important role in controlling the rate of phosphorus release from the sediment to the photic zone (Munawar, 1970). A major distinguishing factor among the water bodies is the difference in frequency and length of river inputs or connectivity (Hamilton and Lewis, 1990; Amoros, 1991). The nutrient enrichment during low water was attributed by Hamilton and Lewis (1987) to turbulence from wind action and sediment resuspension. It was attributed by Boneto et al. (1984) to hypolimnetic anoxic conditions and the release of nutrients from the sediment.
Many workers (Bhowmick, 1968) have observed a direct relationship between silicate content and diatom population in the water body. Furch (1984) found a similar phenomenon in Amazonian ‘Varzea' (flood plain) lakes and a floodplain lake in Sao Paulo, Brazil.
During the study period, water temperature, pH, Dissolved Oxygen (DO), Free Carbon Dioxide (FCO2), Total Alkalinity were measured monthly for six months. The limnological parameters of the study site were found to be highest during October 2012. the pH of the Kole was found to be at par during the six months study period. DO and FCO2 were found to be highest during October and lowest during the winter months i.e, December 2011 – January 2013.
The water quality, nutrient level, and fish fauna of the Kole beel show change from time to time as it receives different types of water at the different parts of the year. During monsoon, the water from river Ganga enters the beel along with nutrients, fishes, and inundates the beel. Again the combined flow from the rivulet upstream along with nutrient and water from the catchment area reverts the flow to the Ganges that continues till the flood receded. Therefore, in both cases, many fishes enter the beel along with and against the water current giving a dual benefit to the beel.
The plankton population in the beels systems was diverse in respect of species and population density. From the 2 systems, 62 species of plankters belonging to 51 genera and 29 families were identified. (Table- 1). Sugunan et al., (2000) found that phyto and zooplankton populations in West Bengal flood plain wetlands were lower during the southwest monsoon, but increased after the ecosystem stabilized and the plankton population formed using inorganic nutrients and organic matter brought in by the incoming flood or run-off water. Bhowmik (1988) also found that phytoplankton dominated the maximum plankton population in the summer, while zooplankton dominated in the winter. In Bihar, Jha (1997) discovered a higher plankton population in closed flood plain lakes.
The beer's phytoplankton was low due to the use of nutrients by a thick growth of macro vegetation. (Yadava et al., 1987). The wetlands exhibited strong competition between the macrophytes and phytoplankton in respect to sunlight and nutrients (N: P), influencing the abundance and quality of phytoplankton from one system to another (Wetzel, 2001). Macrophytes being the dominant autotrophs might have used the available nutrients, sunlight more efficiently, as such grow rapidly, almost shadowing the proliferation of phytoplankton (Boyd, 1971).
Several researchers have documented increased phytoplankton production when dissolved oxygen levels are higher. (Alikunhi et al., 1955; Das and Srivastava, 1956; Moitra and Bhattacharjee, 1965; Saha et al., 1971). However, no such connection could be made during the ongoing investigation. According to Reid (1961), as the temperature is lowered, the solubility of oxygen in water increases.
Cairns (1965) investigated the optimum temperature range (15–30°C) as the most favorable for the growth of diatoms. Copepods were present throughout the year, but there was no discernible trend. Rotifers have a diverse ability to survive in a variety of habitats, as some feed on phytoplanktons, others on detritus and bacteria, and still others have been identified as predatory raptors (Singh, 2000). Rotifers predominate in Indian freshwaters, which is a natural occurrence. (Michael, 1969; Lahon, 1983). Singh (2000) recorded maximum production of rotifers during the summer season. Similar observations have been made by Michael (1966) and Singh (2000). Their growth seemed to be favored by a temperature range of 23.5 to 26.1°C, which is within the optimal temperature range for protozoan growth (Pennak, 1953).
During the investigation, a total of 15 taxa from three classes were discovered: 5 Cladocera taxa, 4 Copepoda taxa, and 6 Rotifera taxa. The site's highest Zooplankton abundance was in October 2012, and the lowest was in February 2013. A similar study in three different kinds of freshwater waterbodies in Penang Island, Ismail, and Zaidin, 2015 reported the highest Zooplankton abundance in November while the lowest in February. Among all the Zooplankton groups, Rotifera was reported to be dominant among other groups during the period of investigation consisting of 6 taxa of which Filinia sp. shows highest abundance and Keratella sp. shows the lowest abundance throughout the six months study. In the Cladocera group, Moina sp. shows highest and Bosmina sp. shows the lowest abundance while Copepoda group, Mesocyclops sp. shows highest abundance and Microcyclops sp. shows the lowest abundance throughout the study period. Compared to all other taxa, Filinia sp. of the Rotifera group were found to dominate the population of Zooplankton. The abundance of the Rotifera group may indicate the presence of dissolved solids in the study site, as they are known to increase the growth of minor phylum of Zooplankton in the water body (Goswami and Mankodi, 2012).
Figures show the percentage composition of Zooplankton in the study during the period of investigation. Cladocera contributes the highest percentage of 43% of the total Zooplankton composition followed by 42% of Rotifera and 15% of Copepoda. The presence of the highest percentage of Cladocera again depicts the better condition of the water body and can be used for the aquacultural program. Cladocera shows the highest abundance among other groups of Zooplankton present in the study site. Cladocera was found to be abundant during October 2012 whereas Copepoda and Rotifera were also found abundantly during October 2012. The lowest abundance of Zooplankton composition was found to be during February 2013.
Limnology and plankton community of floodplain wetlands
Floodplain lakes' chemistry and biology are strongly affected by their proximity to rivers. (Amoros and Roux, 1988; Van den Brink et al., 1992; Tockner et al., 2000b). One of the most striking features of plankton populations is the ongoing substitution of organisms (Hutchinson, 1967; Edmondson and Litt, 1982). The stagnant floodplain waters are one-of-a-kind environments in terms of water chemistry, phytoplankton and zooplankton composition, and dynamics, with exceptional spatial and temporal heterogeneity in the ecosystem, resulting in high species and population richness (Pethart, 1995). Changes in the prevailing environment over the wetland, as well as external factors, may cause changes in the physicochemical parameters (Abbasi, 1997). The phyto- and zooplankton community composition in floodplain lakes are influenced by hydrology, relevant nutritional resources, and habitat characteristics, primarily through N and P input from eutrophic main channels during floods. The complexity of ecosystems created by the presence of aquatic plants was linked to plankton species richness (Van den Brink et al., 1994). Plankton diversity in semi-isolated floodplain reservoirs, as well as natural river-floodplain systems, are known as biodiversity hotspots (Ward et al., 1999), due to the variability in hydrology and disturbances of the lentic, lotic and semi-aquatic habitat types present. When connectivity is considered the most important disruption of floodplain lakes, lakes with intermediate connectivity have the most diversity, while lakes that are permanently linked (highest connectivity) and isolated (lowest connectivity) have the least diversity. (Roozen, 2005).
The highest phytoplanktonic population is observed during the rising water cycle when limnological changes are most visible as river water reaches the floodplain for the first time. Water and plankton populations from the pools and depressions are replaced by the flood. Nitrate-nitrogen is abundant in river water, which can be used as a source of nutrients and oxygen. Diatoms and green flagellates, Volvocales, are carried in by the flood, and they will spread and take over after the flood. (Pethart, 1995). The river is a highly complex entity that has a significant impact on the limnology of stagnant waters in the floodplain. Even though plankton cannot swim against currents, rivers also have an abundance of it. (Hynes, 1970; Winner, 1975; Rzoska, 1978). Factors influencing the transport of species from the source region to the water, and factors affecting the growth and reproduction of organisms in the river, are the two types of factors that influence the abundance of plankton in rivers. (Hynes, 1970). Plankton can be supplied to the river by standing water in contact with the channel.
Many tropical rivers have a wide stretch of the natural floodplain that could be significant zooplankton sources (Saunders and William, 1988). Inundation of source areas is caused by changes in water levels, which increases zooplankton abundance.
The timing of a flood is thought to be important in assessing the effects on the water quality of flooded lakes. (Junk et al., 1989; Bayley, 1991; Hein et al., 1999; Tockner et al., 2000b). Phytoplankton is extremely sensitive to environmental changes, and major shifts in phytoplankton species composition are often a result of substantial changes in ecosystem ambient conditions. (Devassy and Goes, 1988, 1989). Floodplain lakes with a long annual flood cycle have cyanobacteria, Chlorophyta, and filter-feeding zooplankton taxa associated with open water. Bacillariophyceae and scraping zooplankton taxa associated with aquatic macrophytes, on the other hand, are popular in floodplain lakes with short annual flood durations. (Van den Brink et al., 1994). Physical properties of water, such as mixing and light availability, are two of the most important determinants of phytoplankton vertical distribution. (Reynolds, 1994). These properties are closely related to seasonal flood pulse changes in floodplain lakes. (Junk et al., 1989). Sept and Reynolds (1995) discovered that a water level's phytoplanktonic production was affected by temperature, light, and nutrients. Lentic phytoplankton is washed away during the inundation, while riverine species are taken in by floodwaters. Nonetheless, river water causes phytoplankton dilution and wash-out, resulting in a drop in abundance (Talling, 1986). Under insecure environmental conditions, small flagellate forms (cryptophytes or green algae) with rapid growth rates (r-selected) often dominate. (river water inflow can be considered a disturbance factor) (Reynolds, 1984). Another significant aspect is diatom input into river water, where they are often dominant. (Moss et al., 1989; Kasten, 2003). Zooplankton is an integral component of the marine ecosystem, performing a broad range of important functions. Water is purified by feeding on phytoplankton and microorganisms. The quality of water can be assessed based on the species dominance in the zooplankton population. The clay content of floodplain soils is higher than that of depressional wetland soils. (C.B. Craft, unpublished data). According to the few studies that have been conducted, open (floodplain) and closed (depressional) wetlands both sequester similar quantities of organic C and N.
The relatively high rate of soil organic C and N deposition in floodplain soils and possibly other floodplain wetlands is due to slower soil accretion and higher C and N concentrations in the subsoil. As demonstrated by higher P accumulation in floodplain wetlands, catchment size and connectivity to sources of fine-textured (clay) sediments influence P retention (Christopher and William, 2000). Wetland productivity, species diversity, and water quality are all influenced by nutrient accumulation and storage.
External nutrient loadings and accumulation are often linked to high primary and secondary productivity (Brinson et al., 1981; Hopkinson et al., 1992). The potential of freshwater wetlands to remediate water quality depletion by collecting sediment and sequestering nutrients is one of their advantages (C, N, and P). However, the efficacy of wetlands in nutrient and sediment retention is based on several factors, including the scale of the watershed, the land use within it, and the degree of wetland access to open water habitats. (Christopher and William, 2000). In infertile wetlands that receive little in the way of fertilizer subsidies or have low soil nutrient supplies, species diversity and the prevalence of rare species are typically higher. (Moore et al., 1989; Marrs, 1993). Long-term lake studies have provided clear evidence of the impact of increased major nutrients (nitrogen and phosphorus) on lake production and biota variations. (Abd El-Karim, 2009).
Living organisms and their abiotic surroundings are inextricably linked and interact with one another (Odum, 1983). Aquatic plants play an important role in the habitats of lakes, wetlands, rivers, and streams all over the world (Jamil, 1993). Sharma (1995) also recorded dominance by submerged and emergent vegetation in Kawar lake of Bihar. The high turbulence of water was perhaps the main constraint for lesser growth of macrophytes in Kole which conforms with the study of Saha et al. (1971); Sugunan et al., (2000). Submerged macrophytes regulate plankton density and primary productivity by providing optimal light quality and quantity, temperature, and total alkalinity (Yadava, 1987). Kole beel had a lower infestation and biomass of macrophytes than the parent river, which was partly due to management action and partly due to contact with the parent river.
The results show that both free-floating and submerged macrophytes have a wide seasonal variation in biomass. Camargo and Florentino (2000) found that the biomass of aquatic macrophytes in tropical water bodies varies greatly from season to season. Junk (1986), Junk and Piedade (1993), Da Silva and Esteves (1993), and Camargo and Esteves (1993) all made similar observations (1996). Saha et al. (1990) found a significant difference in water quality parameters during the planktonic and macrophytic phases in Kulia beels (closed beels) in West Bengal. During the macrophyte process, they discovered a lot of Secchi disc visibility (from top to bottom). In the current analysis, however, no direct correlation between macrophytic dominance and transparency could be found.
Munawar (1970) claimed that a dense macrophyte population would result in increased photosynthetic activity and, as a result, an increase in pH. The absorption of phosphorus by macrophytes both from the water and sediment is well known (Bristow, 1975; Denny, 1995; Chamber et al., 1989 and Gunnison and Barko, 1989). This suggests that nitrate is a more limiting nutrient for macrophyte growth in these lakes than phosphorus.
Aquatic macrophytes play an important role in the dynamics of the Beels' physicochemical and biological properties. Aquatic macrophytes provide nutrition to herbivores while also strengthening the Beel ecosystem's detritus food chain.
Marginal plants, especially Ipomoea fistulosa, were found in the Closed Beel. In the closed Beel, other aquatic plants such as Jussiaeadiffusa, Alternantheraphylloxeroides, and Paspalum scrobiculate were not present. Floating pants Eichhornia crassipes (Panimeteka) and Monochoria vaginalis (bhatmeteka) are the most common macrophytes, followed by Chara (submerged).
The open beel was devoid of floating plants such as Trapa natans var. bispinosa (cattail). Ipomoea fistulosa, Ipomoea aquatica (waterspinach), Ipomoea carnea, Alternantherasessilis, Alternantheraphylloxeroides, Jussicadiffusa, and Paspalumscorbiculatum are examples of marginal amphibious plants that can thrive on damp soil lands as well as float on the water surface. Monochoria vaginalis (water hyacinth), Eichhornia crassipes followed by Chara (submerged), and Ipomoea fistulosa (marginal) are all dominant macrophytes in the Open beel. Aquatic weeds are heavily infested in West Bengal's Beels. Eichhornia crassipes and Monochoria vaginalis (water hyacinths), Chara (submerged), Salvina, Lemna, Wolffia, Potamogeton, Hydrilla verticillate, vallisnereia, Ipomoea fistulosa, Nymphaea cristata (water lily), Eurylferox (Makhna), etc. are some of the common forms.
The primary productivity of the studied beels came from two sources: phytoplankton and macrophytes. The rate of energy transformation by phytoplankton is lower than that of macrophytes, according to Beels. It's a complicated process to transfer energy from the primary producer to fish (carnivores). The fish reflect a fraction of the energy trapped by primary producers as a secondary product of the beels. It's a complicated process to transfer energy from the primary producer to fish (carnivores). The fish reflect a fraction of the energy trapped by primary producers as a secondary product of the beels. In the Open Beel, the average Gross primary output (GPP) by phytoplankton is 1.82 g/m2 per day. The value is 1.68 g/m2 /day in the case of Closed Beel. The phytoplankton of Open Beel produced more gross primary production (GPP) than that of Closed Beel. The Open Beel's Net Primary Production (NPP) is 1 g/m2 per day, while the Closed Beel's NPP is 0.83 g/m2 per day. In the Beels, differences in phytoplankton net primary production (NPP) are 0.17g/m2/day.
Ranges of Gross Primary Production (GPP) 1206–4371 g/m2/yr. and 506 g/m2/yr. with an average value of 2655.80g/m2/yr and 2142.27 g/m2/yr in the Closed and Open Beels respectively. Lowest Gross Primary Productive (GPP) was observed in December (1456g/m2/yr.) in the Open Beel and 2160.33g/m2 /yr in the Closed Beel. In Closed Beel, the highest average value was observed in November (3174.33g/m2/yr) and in November (3174.33g/m2/yr). GPP falls during the winter and steadily rises during the summer. If water is accessible in the wetlands, Eichhorniacrassipes and Monochoria vaginalis (water hyacinth) are usually abundant throughout the year. Rainfall, high and high-temperature humidity all influence their development. During the monsoon and post-monsoon, the dominant period was observed.
Despite the drying up of some patches of the wetland beds, some weeds, such as Hydrilla verticillata, Vallisneria spiralis, and Potamageton octundrus, renew their seasonal cycle. Over the winter season, the macrophytes decompose. The availability of water in the wetlands' bed influences the seasonal variation of biomass.
Despite their immense ability, the Beels only use around 1% of the energy used by fish, and the rest is transformed into a detritus food chain at the bottom of the beels. Because of the two types of productivity sources, the primary productivity of the Beels is higher than the productivity of any other freshwater reservoir. Most of these macrophytes are not specifically grazed by herbivores, and the unused material is deposited at the bottom as detritus energy, which is generally high in both Beels. Detritus levels in the Open Beel ranged from 0.91 to 4.56 kg/m2/yr/m2, with an average of 2.55 kg/m2/yr. Detritus levels in Closed Beel ranged from 2.05 to 4.75 kg/m2/yr, with an average of 2.75 kg/m2/yr. In both the Beels, the value of detritus was higher in the winter season (4.45kg/m2/yr.) than in the summer season (1.3kg/m2/yr.). The rich growth of marginal and submerged vegetation in the Brahmaputra floodplain wetlands is a unique feature due to heavy nutrient loading from both allochthonous and autochthonous sources. These macrophytes often supplant the plankton population, hastening eutrophication by replacing the plankton community with macrophytes as the primary producer. This results in a higher rate of evapotranspiration and lake amplification. This method, however, can be reversed with good management. Open Beels, which have fewer macrophytes (on average 2142.27 g/m2/yr), are best suited for energy transformation by phytoplankton.
The productivity of West Bengal's Closed Beels (average 2655.80g/m2/yr) is hampered by floating (water hyacinth), submerged (Najas, Vallisnaria, Hydrilla, and Chara), and marginal (Typha) vegetation. As a result, both the Closed Beel (0.83 g/m2/day) and the Open Beel (1 gc/m2/day) have poor net productivity rates.