Planktonic organisms are considered as the aquatic bioindicator in the open water system, and, especially in the riverine system (Parmar et al., 2016) and the blooming condition of any of the diatoms myself explain the eutrophic status of the water quality which signifies the lower water flow rate in the system (Yang et al., 2012). The study was conducted to monitor the blooming condition and distribution pattern of A. granulate in the Ganga River system.
Bacillariophyceae is widely recognized as a diatom and are predominantly unicellular consisting of rigid box-shaped silicified cell walls recognized as frustules(Cupp., 1943). The species is first reported by Simonsen in 197 (M.D. & Guiry,2021) whose Basionym is Gaillonella granulata (Ehrenberg) and the synonym is Melosira granulata (Ehrenberg) Ralfs (Mayama,1991). The morphological characteristics ensuring the confirmation of A. granulata consist of long linked cells with upright spines inserted on valves (Cox,1996). The variety is characterized by the presence of 1–4 separated pines (Bicudo et al., 2016), average total length was 418 ± 2.89 µm; mean valve length was 12.45 ± 0.06 µmcell length (Cox,1996). The species A. granulata exhibits a diverse range of morphological variations (Kilham et al., 1975). The structural variation of A. granulata in the different aquatic ecosystems has been observed diversely (Wang et al., 2020).
Ecology and distribution: This species has a worldwide distribution (Guiry&Guiry 2014) and occurs in a wide range of trophic conditions, but is mostly associated with eutrophic waters (Taylor et al. 2007).
4.2.Distribution and composition pattern
Many genera and species of phytoplankton communities belong to the Bacillariophyceae have the potential to enhance the degree of eutrophication in the river Ganga (Srivastava et al., 2016). Similarly, Roshith et al. (2018) have also observed that diatom, A.granulata, dominant species among the phytoplankton population in the summer and winter seasons. Lakshminarayana (1995) also reported that the diatoms had two peak points one is summer season (March) and another one is winter (December) in the river Ganga. The present investigation also agreed with the findings that the species A.granulata has shown a higher degree of abundance during pre-monsoon (905 cell l− 1) and winter season (53188 cell l− 1 ). The sudden plentiful rise of diatom A. granulate has been confronted from all the stations with the highest at Balagarh (285863 cell l− 1) followed by Tribeni (153608 cell l− 1) and Behrampore (20942 cell l− 1).
The Bray-Curtis cluster analysis was performed to know the similarity among the different stations based on total length, valve length, cell number, cell length, and density (Guinder et al.2020). The present study indicated a similar trend of the population in all the freshwater and saline water zones. The introduction of saline water in the lower sites like Fraserganj and Diamond Harbour has also created alteration in the morphological structure. The results obtained were found in agreement with previous studies (Blum et al. 1957; Kanaya and Kikuchi, 2008).
Among the different physicochemical parameters, the significantly influenceable parameters are water temperature (r = -0.402), depth ( r = 0.342), dissolved oxygen ( r = 0.507), and chlorophyll A (r = 0.564), which was similar to the other studies of different water bodies such as Danjiangkou Reservoir, China ( Zheng et al., 2020). Water temperature negatively affected the growth of A. granulata showed a higher growth rate during the winter season. The different nutrient parameters are dependent on water temperature (Thomas et al., 2017), which also affects the chlorophyll A composition and variation in the river system, by which the planktonic diversity and density are dependent (Moeller et al., 2019). Thus composition and variation of chlorophyll control the growth of plankton communities (Moeller et al., 2019). Similar observations i.e. the influence of water temperature on plankton growth specifically freshwater diatoms were found in several studies (Zhang et al., 2019). Similarly, the density has been positively influenced by depth during winter, higher depths were observed in the river as similar to the Gharraf river of Iraq (Al-Husseini et al., 2020). A significant positive correlation with Dissolved oxygen was also observed, during the daytime with sunlight, the photosynthetic activities of the phytoplankton get enhanced, and relatively more oxygen is released in the aquatic system when the numbers of Phytoplankton are higher and is also depending on water temperature (Sekerci et al., 2018). The higher dissolved oxygen in the riverine system has also been reported during the post-monsoon and winter season. Chlorophyll A greatly influences the process of photosynthesis in aquatic plants (Tsai et al., 2019). Similar observations were also found in the present findings as a significantly positive correlation with chlorophyll A was found with A. granulata.
PCA bi-plot is a well-known statistical tool for the analysis of specific variable and their interdependency on the other environmental variables (Singh et al., 2019). The principal component analysis is a widely accepted multivariate statistical tool used for the analysis of the wide number of environmental factors affecting the diversity and density of the planktonic species (Pratiwi et al., 2019; Herawati et al., 2020 and Matta et al., 2020). In this finding as the 1st PC is having positive loading with conductivity, total hardness, salinity, total suspended solids, calcium, chlorophyll-B, and chlorophyll-C while the negative loading was found with dissolved oxygen and silicate. The loading revealed the influence of the estuarine zone of the river. The same has been observed in other studies made in the estuarine stretch (Hooghly estuary) of river Ganga by Rakshit et al.(2017). The 2nd PC is having positive loading with free CO2, and Chlorophyll C shows the impact of monsoon due to Cloudy weather the free CO2 and higher chlorophyll content were observed in the riverine system and the observations were the same as that of the Erai river of Maharashtra, India and in the metropolitan city of Korea (Shende et al.,2019, and Hong et al., 2020). The 3rd PC is having positive loading with total alkalinity, total chlorophyll while negative loading was observed with water temperature the PC may also have denoted the contribution of organic matter loading and ion dissolution in the riverine system. The observations were similar to that of the upper stretch of river Ganga (Dimri et al., 2020). The 4th PC where the positive loading was observed with A.granualata and depth and negative loading was found with transparency and total phosphorus. The PC signifies the importance and influence of river depth. The negative loading with water transparency signifies the relatively higher riverine velocity which causes turbidity in the river (Ayoade et al., 2009). The negative loading with total phosphorus also signifies the higher riverine flow in which the nutrients such as phosphorus and nitrogen get dissolved. Similar observations were also observed with the Mississippi river where the maximum blooms of cyanotoxin-producing plankton were observed (Bargu et al., 2011). In the 5th and 6th PC, the negative loading was observed with biochemical oxygen demand and total nitrogen. This signifies the pollution status of the river and higher nutrient load which may cause eutrophication in the riverine system.
The use of WPI for the zonation and chemical characterization of the riverine system has been widely used in the studies of the large river such as in the Shanchong river of China (Wang et al., 2015), Sava River, Serbia (Popovic et al., 2016) and Damodar River, India (Chakraborty et al., 2021). The score of the WPI index was varied between 19–23. depicting fair water conditions throughout the stretch. The different WPI scores such as 19 were obtained from Buxar, Patna, Bhagalpur, Balagarh, and Fraserganj, while the score value of 21 was obtained at Farakka, Tribeni, and Diamond Harbour. However, comparatively higher scores of 23 were yielded at Jangipur, Behrampore, and Godakhali. The higher values signify the better quality of water and vice-versa (Atique et al., 2019 & 2020). As the structural differentiation of the Aulacoseira granulata has been used in many of the studies such as in the pearl river of China (Wang et al., 2017& 2020) for the assessment of aquatic ecosystems. The correlation was done between WPI and total length of the filament, valve length, total cell count of one filament, and cell length of one filament and abundance. A significantly negative correlation (r = − 0.663) with WPI was obtained with cell length. It is very much evident that the parameters related to the quantitative structure are very much influenced by the environmental changes (Bedoshvili et al. 2007; Poister et al. 2012). In the present study, the overall cell length varied significantly with the computed value of WPI. On the other hand, chemical parameters showed an intricate relationship with the morphological parameters also. Jewson et al. 1992 has opined that these environmental parameters have a strong association with its growth.
As in the earlier studies, it was also found that the blooming condition of the diatoms like A. granulata signifies the deteriorated environmental condition of the riverine system which hinders the growth and survival of the native aquatic organisms (Joung et al., 2013 and Poister et al., 2015) by adversely affecting their ecological niche. In the present study also the deteriorated environment has been shown with many of the environmental variables and morphological traits of the A. granualta which shows the significant correlation among each other..A. granulata is an indicator of water pollution because of its sustainability in a very stressful environmental condition (Grigoryeva et al., 2019).