Results and Discussion

3.1 Hydrological variations of lentic and lotic systems
Estuaries are dynamic ecosystems with diurnal and sea- sonal variations in hydrological and environmental parameters. However, in situ parameters of temperature and salinity did not reveal a drastic variation as the sam- pling was done for a season—pre-monsoon in both lentic and lotic systems. The average air temperature across dif- ferent habitats was 30.85 ± 2.60 °C. Water temperature is an important parameter that influences many abiotic chemical processes like dissolution–precipitation, oxida- tion–reduction, adsorption–desorption and physiology of biotic community in a habitat (van Aken 2008). It also influences the rate of photosynthesis of an aquatic system (Fatema et al. 2014; Yin 2002). All the eight lentic and lotic habitats had warm waters and high salinity, as sam- pling was done during summer months of the year. The changes in water temperature are influenced by solar insolation, fresh water influx, evaporation, cooling and mix up with ebb and flow from adjoining neritic waters (Madhu et al. 2007). The average water temperature recorded across the habitats was 31.21 ± 2.61 °C. In comparison, George et al. (2012) and Martin et al. (2011) reported 31–32 °C during March–April at the Tapi estuarine region, Gulf of Khambhat, Indian west coast (Cochin estuary). Godhantaraman (2002) reported similar surface water temperature with a variation range of 1–2 °C during pre- monsoon season at Parangipettai (south–east coast of India). The pH is a master variable that determines many chemical, biological and kinetic processes in natural waters (Millero 1986). The pH of unpolluted natural water bodies can broadly vary between 3 and 11; however, pH of those water bodies is in the range of 5–9, which generally sup- ports diverse assemblage of aquatic species (Alabaster et al. 1984). The pH remained alkaline at all the habitats 7.9 ± 0.09 except for salt marsh sedges (SMS) study sta- tion which showed an acidic pH due to hypoxic environ- ment with very low DO level due to dead and decaying organic matter. Satpathy et al. (2010) observed a pH range of 7.7–8.3 along the Kalpakkam estuarine waters of southeast coast of India. Madhu et al. (2007) observed a steady pre-monsoon pH of 7.1 ± 0.14 in Kochi backwaters corroborating well with the present study.
Salinity defines the relative proportions of fresh and saline waters found in different parts of the estuary. Salinity levels of a region are influenced by various factors such as the location of sampling station in the estuary, the daily tides and the volume of fresh water flowing into the estuary. Generally during pre-monsoon, salinity rises due to higher temperatures and increased evaporation rate (Mann 2000; Sumich and Morrissey 2004; Levinton 2017). As the sampling was done during the pre-monsoon season, salinity of both lentic and lotic habitats exhibited similar salinity levels (27.66 ± 1.50 ppt), except saltpan which had hypersaline condition of 58 ppt. Madhu et al. (2007) found salinity of 30 ppt during the pre-monsoon season in Cochin estuary, which matches well with the salinity ran- ges of the present study.
Dissolved oxygen is a critical parameter that determines the ability of an aquatic system to support aquatic biota. The average DO levels of lotic habitats (MF, KOSB, BRS, ERS) were 7.15 ± 0.18 mg/L, which are slightly higher when compared to lentic habitats. Among the lentic habi- tats, salt pan (SP) and salt marsh sedges (SMS) had the least DO levels of 2.70 ± 0.09 mg/L due to extreme salinity at the salt pan and hypoxic condition and very low water level at the salt marsh sedges, respectively. Increased salinity is known to reduce the dissolution of oxygen in water thus reducing the DO levels drastically. This could be the reason behind saltpan station exhibiting very low DO levels. The DO values in lotic habitats were higher than the DO values reported in many other Indian estuaries. George et al. (2012) observed a lower level of DO in Tapi estuary, at 5.8 ± 0.9 mg/L, during pre-monsoon season which was attributed to higher levels of domestic and industrial effluents. The DO levels of Aghanashini sites were higher than the DO level recorded in the Tuticorin estuary by Balakrishnan et al. (2017). However, lower values of DO were reported earlier for similar ecosystems in tropical estuaries of India and southeast Asia by Madhu et al. (2007), Martin et al. (2011), Ouyang et al. (2006) and Juahir et al. (2011), which could be attributed to the large influx of domestic and industrial effluents in those estuar- ies. In comparison, the Aghanashini estuary is more pris- tine with neither industrial establishments in the vicinity, nor any major township. Estuarine paddy field (APF) and Bargi gazni (BG) sites exhibited moderate DO levels than other two lentic habitats where DO averaged 7.08 ± 1.0 mg/L. This higher level of DO could be due to the inflow of more fresh water, since all the lotic habitat stations were near upstream of the estuary.
Nitrates (NO -–N), the oxidized form of nitrogen, are also an indicator of the level of anthropogenic stress.
Nutrients, especially nitrates (NO3- –N) and silicates (SiO44-), showed wider fluctuations in their ranges across different habitats, whereas the phosphate variations were comparatively marginal across habitats (Table 2). The phosphate values varied within a range of 0.21–2.38 mg/L. Mud flats (MF), egret roosting site (ERS) and bat roosting site (BRS) had very high nitrate levels of 8.41 ± 0.30 mg/ L, 9.82 ± 0.97 mg/L and 6.72 ± 0.20 mg/L, respectively, due to site-specific litter loadings from mangroves and sedges as well as leaf litter and fecal droppings of faunal species. The mudflats make a prominent intertidal region, constituting the main habitat of Paphia malabarica, the dominant bivalve which is harvested in bulk during pre- monsoon, in addition to six other bivalve species in lesser quantities (Boominathan et al. 2008). These bivalves are filter feeders that feed on phyto- and zooplankton which are higher in these regions, more in the pre-monsoon. Addi- tionally, large flocks of migratory and wintering shorebirds like sea gulls and egrets also visit these habitats. Thus, the organic debris loading from the dead bivalves, phyto- plankton and bird droppings and the excretes of the bivalves have contributed to higher nitrates in the mudflats. Bat and egret roosting sites also had higher nitrate levels due to droppings.
The level of silicates is a notable determinant of primary productivity, and the main sources of silicates in estuaries are through land-based runoffs during monsoon. Silicate (SiO44-) levels in estuarine and coastal regions are driven by factors such as physical mixing of fresh and saline waters, siltation in the upstream, rock weathering, adsorption from sedimentary particles, and nutrient upwelling that results in chemical interaction with deep clayey sediments and biological fixation by phytoplank- ton’s, especially by diatoms and silicoflagellates (Richardson et al. 2000; Shah et al. 2008; Prabu et al. 2008; Satpathy et al. 2010). The silicate in all the stations, cov- ering lentic and lotic habitats, ranged from 4.17 to 10.03 mg/L; mudflats exhibited the highest (10.03 ± 0.48 mg/L), whereas in all lotic habitats the sil- icates averaged 4.2 ± 0.06 mg/L. Lentic habitats of APF and BG showed silicate values averaging 4.4 ± 0.29 mg/ L, like BRS and ERS. Salt pan and salt marsh sedges had a silicate concentration of 7.34 ± 0.20 mg/L and 5.92 ± 0.13 mg/L, respectively. When the overall sea- sonality of silicate levels in other estuaries is considered, Martin et al. (2008), Satpathy et al. (2010), Balakrishnan et al. (2017) reported comparatively lesser silicate values during the pre-monsoon. These nutrient values of lentic and lotic systems in the Aghanashini estuary vary, as it is rel- atively an isolated system with wide variations in habitat conditions and trophic status having higher nutrient load- ings. The physicochemical parameters recorded at different stations are detailed in Table 2.