3.1.1 Physicochemical parameters
The minimum, maximum, mean, and standard deviation values of the measured physicochemical parameters from the chosen ten sampling locations are presented in Table 2. Low mean pH values were recorded in all the sampling stations during SWM (Southwest Monsoon) when compared to PM (Pre-Monsoon) and NEM (Northeast Monsoon). CC (7.94 ± 0.27) and FB (7.86 ± 0.26) showed high values of pH during SWM. The least pH values were recorded from JB and MB, 7.68 ± 0.08 and 7.74 ± 0.11 respectively in SWM. The highest pH value (8.18 ± 0.19) was observed in HH and SB stations in the PM season. Moreover, Low values of pH during SWM season clearly suggest the influx of freshwater in the marine environment (Rajendran et al. 2018). The pH value is a critical factor for thermodynamically maintaining the carbonate and bicarbonate levels in the seawater (Borges and Gypens 2010; Gokul et al. 2018). While a decrease of pH in stations JB and MB might indicate a localized acidification process (Abbassi et al. 2017) due to the influx of heavy rainwater and poor oceanic circulation during SWM season (Jha et al. 2014).
Salinity during the SWM season was observed to be low when compared to PM season in all the stations except in CC and FB stations. Least salinity values (29.00 ± 2.45 PSU and 29.60 ± 2.41PSU) were measured from JB and MB respectively during the SWM season. On the contrary highest salinity was recorded from the same stations viz., JB (32.00 ± 0.82 PSU) and MB (32.25 ± 0.87 PSU) during PM season. The salinity gradually increased in all the stations in NEM and SWM seasons except in station BA. The distribution of life forms in any coastal ecosystem is vitally determined by the salinity (O'Conner and Lawler, 2004; Dasgupta et al. 2017).
Water temperature recorded during SWM season shows a low profile when compared to the PM season in almost all the stations except CH (26.90 ± 1.24°C) and FB (26.80 ± 0.91°C) station. The least water temperature was documented from MB (25.90 ± 0.99°C) station during SWM season and the highest was observed in HH station during PM season. Temperature plays a vital role in the efficient functioning of an ecosystem as it regulates the survival, growth, reproduction, and distribution of type of flora and fauna (Kinne 1970; Langford 1990; Roessig et al. 2004; Nordlie, 2006; Keller et al. 2009; Putnam and Edmunds, 2011; Muduli et al. 2011; Brothers and McClintock, 2015; Rajendran et al. 2018).
Generally the observed DO levels in all the stations were high during SWM season in comparison with PM and NEM seasons. The least value was recorded from MB (3.78 ± 0.67 mg/l) followed by JB (4.00 ± 0.06 mg/l) stations during SWM and the same trend continued in these stations during NEM seasons also. The increased level of DO during SWM season suggests the increased production of oxygen due to the photosynthetic activity of phytoplankton (Muduli et al. 2011; Begum et al. 2015). Also, it is noteworthy to mention that the DO level in the present investigation is deficient (< 6.0 mg/l) during all the seasons (Okbah et al. 2013; Boyle et al. 2013; Rajendran et al. 2018). Table 2 shows the inverse relationship between temperature and DO since seawater with warm temperature (PM season) becomes easily saturated thus can hold less DO (Wu et al. 2009, 2010; Jha et al. 2015b). Low concentration of DO is linked to effects such as juvenile and adult survival, growth, and larval recruitment (Osterman et al. 2009; Fuksi et al. 2018). Also, low values of DO in coastal ecosystems can be caused by two main processes, biological and physical. Biological processes include bacterial decomposition of organic matter in bottom waters, which could either be natural or induced by eutrophication (Kramer and Stein, 2003). The physical process that decreases the oxygen in coastal waters is vertical stratification, which could be due to multiple reasons, including low tidal energy, large freshwater inputs, deep channels, and the presence of structures impeding circulation (Nixon, 1988). In general low values of Salinity, pH, DO and temperature were observed during SWM season. On the contrary, variations were observed in the aforementioned parameters during NEM and PM at all the ten stations. The low values in Salinity, pH, DO and temperature can be attributed to copious rainfall during SWM and the variation in values articulate that the rains are sporadic during NEM and PM seasons. However lowest values of the physical parameters like salinity, pH, DO and temperature were recorded from JB and MB stations during SWM followed by NEM season when compared to other stations. On the other hand, high values were recorded from these two stations in the PM season.
Low values of DO in stations JB and MB can be attributed to induction of iron in these waters either by land runoff or harbor related activities (DiTullio et al. 1993; Klaas et al. 2001; Rose et al. 2009; Franklin et al. 2018), domestic wastewater rich in organic fillers (Abbassi et al. 2017), heavy rainwater influx into this sheltered Bay (JB and MB), and poor mixing and circulation of oceanic water (Jha et al. 2014). On the contrary, other stations like CH, HH, PB, SB, WA, CC, and BA are at the closest vicinity of the open sea for mixing and circulation (Fig. 1). Field investigation of these two stations (JB and MB) also confirms that copious amount of monsoonal runoff and domestic wastewater is flushed out at these bay through ‘nallahs’ (92° 43' 9.08" E, 11° 39' 9.82" N; 92° 43' 43.02" E, 11° 39' 23.97" N and 92° 43' 34.16" E, 11° 39' 43.23" N). Furthermore, Station JB is a major fish landing center and harbor.
Relatively high values of Nitrite was recorded during SWM followed by NEM and PM seasons (Nallathambi et al. 2002; Jha et al. 2015b) at stations MB (0.30 ± 0.14 µ mol/l) and JB (0.35 ± 0.18 µ mol/l). Consumption of nitrite by the phytoplankton could be the reason for its low concentration in these two stations during all seasons. The constant influx of domestic sewage through the nallahs could be another reason for the high concentration of nitrite in JB and MB stations during PM and NEM seasons. The nitrite concentration of coastal waters is usually elevated when there is the oxidation of organic matter and residues from fertilizers (Montaño and Robadue, 1995).
The concentration of nitrate was recorded high during SWM season and least during PM season (SWM > NEM > PM). It is very clear that the concentration of nitrates in the coastal waters of the study area follows the rainfall pattern. Whereas, the average nitrite + nitrate concentration is more than 1 µ mol/l in all the stations and in all the seasons thus, favoring the proliferation of phytoplankton in the presence of sunlight. Also, it is indicative that uninterrupted replenishment of nitrite + nitrate (> 1 µ mol/l) concentration may be due to oceanic circulation, rainfall, erosion, or natural deterioration of minerals, despite frequent uptake by phytoplankton (Begum et al. 2012; Jha et al. 2014).
JB and MB stations recorded a null concentration of ammonia during the PM season. Low concentration (0.01 ± 0.01 µ mol/l) was observed in BA and WA stations during SWM season. Also, WA (0.03 ± 0.06 µ mol/l) station showed the least concentration of ammonia during NEM. Increased values of ammonia concentration show the reciprocation to vertical salinity stratification and low dissolved oxygen that promote regeneration of ammonia from the bottom sediments (Jha et al. 2014). JB station recorded the highest concentration of silicate 6.38 ± 1.09 µ mol/l, 8.29 ± 3.76 µmol/l and 7.73 ± 5.79 µ mol/l during PM, SWM, and NEM seasons respectively. It is quite interesting to note that silicate concentration is high during NEM season when compared to SWM season. This high concentration of silicate during NEM could owe the fact that the water holding capacity of the upstream soil would have reached the saturation level due to SWM. Henceforth, high levels of silicate-rich sediments would have been supplemented to the coastal waters as runoff. Prevailing conditions like antecedent soil moisture, soil, and rolling topography warrants 50–90% rain runoff (Ganeshamurthy et al. 2000; Shiva Shankar et al. 2018). Furthermore, silicate originates due to the weathering of silicate rocks and not through anthropogenic influence (Jha et al. 2014). Also, the residence time of silicate is longer when compared to other nutrients like nitrite, nitrate, ammonia, and phosphate this is further corroborated by the high and frequent blooms of the diatom population when compared to dinoflagellate in the study area (Dharani et al. 2004; Begum et al. 2012; Karthik and Padmavathi 2014; Begum et al. 2015).
The concentration of phosphate was recorded high during NEM season and the least PM season (NEM > SWM > PM). JB station recorded the highest concentration of phosphate 0.14 ± 0.16 µ mol/l, 0.27 ± 0.19 µmol/l and 0.52 ± 0.07 µ mol/l when compared to other stations during PM, SWM, and NEM seasons respectively. The phosphate concentration of coastal waters is said to be linked to fertilizers and domestic wastewaters (Montaño and Robadue, 1995). Nutrients like nitrates and phosphates are brought into the coastal waters through river flow, agricultural and aquaculture runoff, industrial and household waste (Casali et al, 2007).