Structural variation of coloured dissolved organic matter during summer and winter seasons in a tropical estuary: A case study

Marine Pollution Bulletin 149 (2019) 110563

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Structural variation of coloured dissolved organic matter during summer and winter seasons in a tropical estuary: A case study ⁎

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N.V.H.K. Charia,d, , Sudarsana Rao Pandie, Vishnu Vardhan Kanurib, , Charan Kumar Basuric a

Marine Chemistry Laboratory, Andhra University, Visakhapatnam 530003, India National Ganga River Basin Authority, Central Pollution Control Board, Kolkata 700107, India c Marine Biological Laboratory, Andhra University, Visakhapatnam 530003, India d Centre for Marine Living Resource and Ecology, Kochi 682037, India e National Centre for Polar and Ocean Research, Ministry of Earth Sciences, Goa, Government of India. b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coloured dissolved organic matter Spectral slope Excitation emission matrix spectra Parallel factor analysis

The diurnal variations in water quality and optical properties of organic matter were studied in the GautamiGodavari estuary during two contrasting seasons. Dissolved inorganic nitrogen (DIN) and silicate showed similar patterns with the tide during summer, whereas in winter contrasting trends were noticed. Three-folds higher N to P ratio was recorded in winter than in summer. The spectral slope ratio (SR) and specific ultra violet absorption coefficient (SUVA) peaked during summer (1.28 ± 0.09 and 3.95 ± 0.2) followed by winter (1.10 ± 0.18 and 1.91 ± 0.35). The parallel factor (PARAFAC) analysis of excitation emission matrix (EEM) fluorescence spectra was extracted three humic (C1, C2, C3) and one protein-like (tryptophan (C4)) fluorophore components. Humic like fluorophores inversely correlated with the tide in both the seasons, due to influence of seawater. In summer, the Chlorophyll a (Chl a) and dissolved organic carbon (DOC) showed positive correlations with humic like and C4 fluorophores, suggesting the insitu organic matter production.

Dissolved organic matter (DOM) plays a crucial role in the biogeochemical processes of marine and aquatic environments (Benner 2002). Major composition of DOM is carbohydrates, phenols, amino acids and other structural compounds. A very small fraction of the DOM is coloured and exhibits absorption and fluorescence characteristics (Coble, 2007), known as chromophoric dissolved organic matter (CDOM). The CDOM in the coastal and marine environments are mainly terrestrial in origin due to degradation of upland vegetation runoff through the riverine systems and autochthonous physical and biogeochemical processes, including phytoplankton exudates, and leachates from the degraded organic matter through microbial and photochemical activity (Sieburth and Jensen, 1970; Tranvik et al., 1993; Coble, 2007; Shank et al., 2009). The nature and characterization of CDOM can be explained by its absorbance and fluorescence properties and in association with the derived parameters such as absorption coefficient, spectral slope and ratio, specific ultra violet absorption (SUVA), humic and protein like fluoropores (Helms et al., 2008; Kowalczuk et al., 2005). Several studies have reported the spatio-temporal distribution and dynamics of CDOM in open oceans (Boehme et al., 2004; Conmy et al., 2004; Milbrandt et al., 2010), coastal and estuarine environments (Maie

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et al., 2006; Guo et al., 2007; Chari et al., 2012). In addition to the field studies, laboratory experiments have also reported on phytoplankton exudates (Romera-Castillo et al., 2010; Chari et al., 2013) and bacterial metabolites (Moran et al., 2000) for the quantification of their role and activities on the sources of CDOM. Optical properties of CDOM depend on the chemical composition of organic matter, which is considered a humic substance in aquatic environments. Spectral discrimination between natural and anthropogenic CDOM has been used to study untreated sewage, agricultural wastage and polycyclic aromatic hydrocarbon (PAH) distribution in riverine and estuarine systems. Absorption spectra derived parameters such as spectral slopes, SR, and SUVA can be used as indicators of the molecular size and aromatic content of CDOM (Chin et al., 1994; Del Vecchio and Blough, 2004). Moreover, fluorescence indices and their components of CDOM can be used to understand the dynamics of DOM in relation to the biological processes in the aquatic environments (Fellman et al., 2010). Estuaries are highly dynamic and productive environments (Hopkinson et al., 2005) due to the exchange of waters with adjacent coasts and availability of surplus nutrients. These environments receive significant amounts of organic matter during the

Correspondence to: N.V.H.K. Chari, Marine Chemistry Laboratory, Andhra University, Visakhapatnam 530003, India. Correspondence to: Vishnu Vardhan Kanuri, National Ganga River Basin Authority, Central Pollution Control Board, Kolkata 700107, India E-mail addresses: [email protected] (N.V.H.K. Chari), [email protected] (V.V. Kanuri).

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https://doi.org/10.1016/j.marpolbul.2019.110563 Received 2 May 2019; Received in revised form 30 August 2019; Accepted 2 September 2019 Available online 07 September 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.

Marine Pollution Bulletin 149 (2019) 110563

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Fig. 1. Station location map (•) represents the sampling point.

(~84%) is recorded during the south west monsoon (June to September) and almost ceases from December to May (Sarma et al., 2009). Before the river drains into the Bay of Bengal, a century old dam bifurcates the Godavari riverine system into two tributaries (i.e., Gautami and Vasishta Godavari). The Gautami Godavari merges in the Bay of Bengal at Yanam, where the average depth, tidal range and current speed around 8 m, 1.5 m and 1–2 m s−1, respectively. Several studies have reported on the biogeochemical dynamics of the Gautami Godavari estuary (Padmavathi and Satyanarayana, 1999; Bouillon et al., 2003; Tripathy et al., 2005; Sarma et al., 2009, 2010). Sarma et al. (2009) studied the inorganic and organic carbon dynamics of the estuary and revealed that the estuary becomes a significant source for carbon dioxide emissions during peak discharge and a sink during moderate to nil discharge periods. Also, studies on CDOM by Chari et al. (2012) reported that most of the organic matter in the Gautami Godavari estuary is dominated with high molecular weight compounds (humic like) during the monsoon and low molecular weight compounds (protein like) during the pre-monsoon. Yanam is located downstream of the Gautami Godavari estuary, where the tidal effect plays a significant role in the salinity levels where the river discharge from the upstream ceases. In addition, effluents from small scale industries and anthropogenic activities are released into this area. In this context, diurnal sampling was performed at 3 h intervals for 24 h to study the structural distribution of organic matter during spring tide of two seasons i.e., 19th (9 h) to 20th (9 h) April 2015 (summer) and 23rd (9 h) to 24th (9 h) January 2016 at Yanam. The sampling locations are shown in Fig. 1. Tidal amplitude with respect to time was collected from the Tides4fishing website (http://www. tides4fishing.com/as/india/kakinada). Sub-surface (< 1 m) and

Table 1 Summary of Hydro chemical parameters. Pre monsoon Min Salinity Temp (°C) pH DIN (μM) SiO4-Si (μM) DIP (μM) DO (μM) Chl a (mg/m3) DOC (μM)

Max

27.62 30.67 30.50 31.60 7.96 8.19 3.12 5.44 4.34 10.03 0.62 0.73 163 241 2.83 7.81 130.5 162

Post monsoon Avg ± SD

Min

Max

29.10 ± 1.11 31.14 ± 0.31 8.13 ± 0.07 4.14 ± 0.67 6.31 ± 1.71 0.68 ± 0.03 211 ± 18 5.00 ± 1.43 146 ± 9

21.20 26.03 24.50 26.50 7.84 8.05 7.85 11.96 10.41 26.38 0.24 0.83 145 281 3.30 8.32 190 321

Avg ± SD 24.34 ± 1.54 25.64 ± 0.48 7.96 ± 0.06 9.65 ± 1.32 14.61 ± 5.33 0.49 ± 0.19 197 ± 34 5.20 ± 1.71 237 ± 42

Where, Avg: average; SD: standard deviation; DO: dissolved oxygen; μM: micromoles; DIN (dissolved inorganic nitrogen) = (NO2−-N + NO3−-N + NH4+N); DIP (dissolved inorganic phosphate) = PO33−-P.

monsoon season through terrestrial catchments. On the other end, remineralization processes play an important role during summer and winter seasons in the distribution of organic matter. During these seasons, mixing of insitu and excite DOM leads to different structural characteristics. In this regard, diurnal observations were conducted here to study the variations in hydro-chemical parameters and optical properties (Absorbance and Fluorescence) of organic matter in tropical estuarine system (Godavari) during summer and winter seasons. Godavari is the second largest riverine system in India, originating from Trayambak, Maharashtra and draining into the Bay of Bengal. It receives an average rainfall of ~1512 mm y−1; maximum rainfall 2

Marine Pollution Bulletin 149 (2019) 110563

N.V.H.K. Chari, et al.

Fig. 2. Variation of Salinity (a & b), DIN (c & d), SiO4-Si (e & f), DIN/DIP (g & h), DO (i & j), Chl a (k & l) during summer & winter respectively.

following standard spectrophotometric procedures (Grasshoff et al., 1999). A known volume of sample was filtered through glass fibre filters and then extracted in 90% acetone for 24 h at 4 °C and analysed for Chl a by the spectrophotometric method (Jeffrey and Humphrey, 1975). For the optical properties (absorbance and fluorescence), water samples were collected in amber coloured glass bottles and filtered immediately through pre-combusted (~450 °C) glass fibre filters. The filtrates obtained were scanned to obtain absorption spectra (200 to 800 nm with an interval of 1 nm) using Shimadzu UV–visible 1800 double-beam spectrophotometer with 10 cm path length quartz cuvettes (Milli-Q water was used as reference). The absorbance spectra were null corrected at the wavelength of 700 nm for all the samples

bottom (5 m) water samples were collected using a Niskin sampler from a small mechanized boat. For the dissolved oxygen (DO), the samples were fixed onboard with Winkler's reagents and analysed following the modified Winkler's titration method (Carritt and Carpenter, 1966). Water temperature was measured with a pre-calibrated thermometer (0.1 °C precision) suspended in the Niskin bottle. pH was determined on a bench top Thermo pH meter with accuracy of ± 0.01; salinity was determined by the following standard argentometric titration method. Water samples were collected in HDPE (High Density Poly Ethylene) bottles for the analysis of Chlorophyll-a (Chl a) and nutrients. The dissolved nutrients (ammonia, nitrite, nitrate, phosphate and silicate) were measured by 3

Marine Pollution Bulletin 149 (2019) 110563

N.V.H.K. Chari, et al.

Fig. 3. Variation of Spectral slopes (S275–295 and S350–400), and Spectral slope ratio (SR) during summer (a, b and c) & winter (d, e and f) seasons.

using MATLAB R2007a with the DOM FluorNway toolbox ver 3.1 for the extraction of the fluoropores as components and their concentrations (Stedmon and Markager, 2005). The hydro-chemical characteristics of Gautami Godavari estuary during the two seasons was shown in Table 1. No significant vertical distribution was found in hydro-chemical characteristics during the study period; this might be due to the low freshwater runoff from the upstream. Therefore, in the present study we considered average values to explain diurnal variability in hydro chemical parameters during the two seasons. Diurnal variation of salinity, temperature, DO, dissolved inorganic nitrogen (DIN = NH4+ + NO2− + NO3−), dissolved inorganic phosphate (DIP), silicate (SiO4−-Si) and Chl a during the two seasons are shown in Fig. 2. The average salinity was found to be high during summer (29.10 ± 1.11) when compared to that of winter (24.34 ± 1.54) season. During both seasons salinity showed significantly positive correlation (n = 5, p < 0.05, figure not shown) with tide height, which infers the salinity was highest at high tide and lowest at low tide (Fig. 2a, b, Table 1). Vertical salinity variation between the surface and bottom waters during the summer and winter seasons were 0.87 ± 0.69 and 0.43 ± 0.33 respectively, indicating that the prevalence of well-mixed waters in the estuary. The results were in accordance with the observations reported from Godavari estuarine system by Reddy and Ranga Rao (1994), Padmavathi and

(Shank et al., 2009). The absorption coefficient (λ), spectral slopes (S275–295 and S350–400nm) and spectral slope ratio (SR) were calculated from absorbance spectra (Helms et al., 2008). About 25 ml of the filtered sample was collected in pre-combusted glass vials, acidified with 10% ortho-phosphoric acid (to remove the dissolved inorganic carbon), and used for the analysis of dissolved organic carbon (DOC). DOC of the samples was measured using a TOC analyser (Shimadzu, TOC-V) following the high-temperature catalytic oxidation method. The DOCspecific UV (254 nm) absorption coefficient [(SUVA) 254] was calculated as a ratio of a 254 (m−1) with DOC (g C m−3) (Helms et al., 2008; Neff et al., 2006). Three-dimensional Excitation Emission Matrix (EEM) spectra were taken for filtered samples on a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer equipped with 150 W ozone-free xenon arclamp and R928P detector. The excitation and emission wavelengths ranges were taken from 250 to 402 nm and 290 to 550 nm within 4 and 2 nm intervals, respectively. EEM spectra were collected as ratio mode (S1c/R1) using 5 nm band width on both excitation and emission monochromators with integration time 0.2 s. Spectra intensity was normalized with respect to the area under the Raman scatter peak (Ex: 350 nm, Em: 381–426 nm) (Murphy et al., 2010). The resultant spectra are in Raman units (RU), and then converted into milli Raman units (mRU) by multiplying with 1000 due to lower values. Parallel factor analysis (PARAFAC) was done for the EEM data set (36 samples) by 4

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Fig. 4. PARAFAC components obtained from the EEM spectra of two seasons. Table 2 Description of each PARAFAC component. Component

Ex/Em (nm)

Description and source

References

C1

C4

278/336

Red shifted UV humic associated with visible humic like (AR + C); Terrestrial humic substances, widespread UV humic associated with Visible humic like (A + C); Terrestrial humic like substances Marine humic like (M); insitu produced or microbially derived Tryptophan protein like (T); amino acid like peak

C4 (Luciani et al., 2008); C2 (Yamashita et al. 2011); C1 (Chari et al., 2012)

C3

274, 362/ 480 262, 354/ 460 310/388

C2

C4 (Stedmon and Markager, 2005); C3 (Murphy et al., 2008); C1 (Yamashita et al. 2008); C3 (Yao et al. 2011); C1 (Fellman et al., 2011); C4 (Chari et al., 2016) C1 (Ohno and Bro 2006); C6 (Yamashita et al. 2008); C4 (Chari et al., 2012); C1 (Fellman et al., 2011) C7 (Stedmon and Markager, 2005); C6 (Murphy et al., 2008); C5 (Yao et al., 2011); C5 (Yamashita et al. 2011); C5 (Chari et al., 2012); C2 (Fellman et al., 2011)

Fig. 5. Variation of PARAFAC components (C1, C2, C3 and C4) (a & b) during summer and winter respectively.

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because of the limitation of nitrogen and phosphorous for the phytoplankton growth in both the seasons respectively. Li et al. (2017) stated that the sedimentation of suspended particle matter during summer stratification may be one of the reasons for the DIP production. Similar observations were also reported from this estuary (Sarma et al., 2010) and other Indian estuaries e.g., Mandovi-Zuari (Ram et al., 2003), Cochin estuary (Gupta et al., 2009) and Hooghly estuary (Mukhopadhyay et al., 2006). Chl a did not show the linearity with the tide during both the seasons. The concentration of Chl a was found to be higher in winter than summer (Fig. 2k). This may be due to the insitu primary production which was higher on average in the later season. DO did not show much variation during summer but in winter a stable rise and fall was noticed (Fig. 2l). Sarma et al. (2009) suggested that the prevalence of autotrophic conditions or an increase in primary production could be existed in this estuary during the low to no freshwater discharge. The DOC concentrations varied significantly between the two seasons (p < 0.01, n = 36) and observed higher levels in winter. Irrespective of season, DOC showed a negative correlation (r = 0.632, n = 36) with tide during the diurnal observations. No significant tidal or seasonal variations were observed in absorption coefficient at 350 nm (aCDOM350) and are varied between 0.87 and 1.91 m−1 during the entire study period. aCDOM350 values are in agreement with the values reported from estuaries elsewhere (Green and Blough, 1994; Del Castillo et al., 1999; Hong et al., 2005). The spectral slope between 275 nm and 295 nm (S275–295) varied from 0.0120 to 0.0202 and exhibited remarkable variations between the two seasons (n = 35, p < 0.01) (Fig. 3a & d). The mean values of S275–295 were found to be high during summer (0.0194 ± 0.0007) and low during winter (0.0184 ± 0.0019). In contrast, the mean spectral slope between 350 nm and 400 nm (S350–400) was high during winter (0.0173 ± 0.0032) and low during summer (0.0153 ± 0.0008). Overall the spectral slopes of the present study were found to be < 0.02 and the spectral ratio is > 1.0, indicating that the CDOM is mostly due to marine sources (Del Vecchio and Blough, 2002; Fichot and Benner, 2012). Mostly the organic matter for the marine environment is autochthonous i.e. from the plankton, micro and macro algae of the associated marine environments (Thornton, 2014). The spectral slopes from the present study are in the range of the observations reported from Mississippi and Atchafalaya Rivers by Fichot and Benner (2012). The high DOC values, with the combination lower slopes during winter, reflects that the source of CDOM in the Godavari estuary might be due to insitu processes like greater primary production (Helms et al., 2008; Thornton, 2014) and it may be modified by the insitu biological activities (Fichot and Benner, 2012). S275–295 showed high values at low tide and low values at high tide during the summer (Fig. 3a). This infers that the lower molecular weight or highly degraded organic compounds observed during low tide when compared to high tide (Markager and Vincent, 2000; Twardowski et al., 2004) might be due to the excretion

Fig. 6. Rainfall data in the East Godavari District during the year 2015–2016.

Satyanarayana (1999), Narasimha Rao (2001), and Sarma et al. (2009). Also, similar observations were reported from other estuaries of India including Mandovi estuary (Varma et al., 1975), Vellar estuary (Chandran and Ramamoorthi, 1984), Mahanadi estuary (Upadhyay, 1988), Bahuda estuary (Mishra et al., 1993) and Rushikulya estuary (Gouda and Panigrahy, 1993). The temperature was relatively higher in summer than winter season due to the intensity of solar radiation. Furthermore, higher pH values recorded during the summer period might be due to the high primary production which is in correlation with saturation of DO (%) = ([O2] / [OSaturated]) × 100) in the water column, which was averagely higher (101.7 ± 5.8%) during the summer than the winter season (82.7 ± 4.8%). This observation can be related with the low pCO2 levels reported during dry period in this estuary (Sarma et al., 2011). DIN and SiO4-Si concentrations were lower in summer (4.14 ± 0.67 and 6.31 ± 1.71 μM) than in winter (9.65 ± 1.32 and14.61 ± 5.33 μM) (Fig. 2c, d). DIN showed similar trend with tide as salinity in summer, whereas in winter, an inverse trend was observed. This may be because of the biotic uptake of nitrogen with increases in autotrophic and heterotrophic biomass (Bukaveckas et al., 2011; Fichez et al., 1992) during low tide. SiO4−-Si also showed a similar trend (Fig. 2e, f) with respect to tide during winter but not in summer. However, DIP did not show any trend with the tide during both the seasons and was observed to be slightly higher during summer (0.68 ± 0.03) than in winter (0.49 ± 0.19). The enrichment of DIP may be due to the microbial decomposition of organic matter produced during summer season (Satpathy et al., 2010). DIN/DIP ratio is a significant indicator of trophic status in the all aquatic environments. This ratio was remained below the red field ratio (16:1) during both the seasons (Fig. 2g, h) suggested the insitu production was significantly higher. However, this was ~3 times lower during summer (4.17 ± 1.10) than the winter season (15.52 ± 4.71). This may be

Fig. 7. a) Correlation of Humic like fluoropore components with Chl a. b) Tryptophan protein like fluoropore with DOC during summer season. 6

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humic like fluorophores and protein like fluorophores, which are associated with Chl a and DOC, respectively. The higher molecular weight and lower aromatic nature of organic compounds formed during the summer season were due to bacterial degradation organic matter in the estuarine region.

of phytoplankton, photochemical degradation or bacterial respiration of organic matter (Fichot and Benner, 2012). In contrast, S350–400 values followed the tidal pattern (Fig. 3b & e), indicating that the presence of increasing aromaticity, humic-like DOM components (Blough and Green, 1995; Del Vecchio and Blough, 2004) in the estuary produced through microbial degradation (Chen et al., 2019). Similar values of optical properties were also reported from the Oubangui river (Bouillon et al., 2014). The spectral ratio SR is mostly used for the molecular weight characterization of CDOM. In particular, the SR ratio with high values is characterized as autochthonous contribution with lower molecular weight DOM compounds and vice-versa (Helms et al., 2008). The mean SR ratio however did not show any marked variability between the two seasons (Fig. 3c & f); the lowest mean value was found during the winter (1.10 ± 0.18) as compared to the summer (1.28 ± 0.09). Diurnal variations of SR were found to be similar with tidal observation during summer, whereas no such trend was observed during winter; this means that the higher molecular weight organic compounds may be high during winter and are mostly allochthonous (Helms et al., 2008; Bouillon et al., 2014). This is also evident from the SUVA values obtained in the present study. SUVA did not show any diurnal variations during both the seasons where as they were found to be higher (3.95 ± 0.21) during summer than winter (1.91 ± 0.35), indicating the presence of aromatic compounds with low molecular weight during summer which are mostly autochthonous (algal or microbial) or photodegraded DOM (McKnight and Aiken, 1998; Borsheim et al., 1999; Burdige et al., 2000; Cory et al., 2007). The PARAFAC analysis of EEM spectra extracted four primary fluorescent components from the data set; among these three are humic like (C1, C2 and C3) and one protein like fluoropore (C4) (Fig. 4). The excitation/emission maxima and the source characterization of each component were shown in Table 2. The mean intensity of these fluorophores were significantly higher in summer than winter and shown in the order of C2 > C1 > C4 > C3 in both the seasons. Humic like fluorophores (C1, C2 and C3) showed the inverse trend with the tide during summer season (Fig. 5a). However, during the winter season, the gradual increase in the intensities of the fluoropores (Fig. 5b) in the daytime (9 to 18 h) may be due to the influence of local and anthropogenic inputs during low tide period. This is because of dilution of organic matter with seawater during the tidal exchange, which was produced due to bacterial degradation during summer. All the fluoropores were formed from the insitu biological processes in the estuary, evident from the low rainfall (~37 mm/month) during and before the study period (Fig. 6). This is supported by a significant positive correlation (n = 18, p < 0.01) of humic like fluorophores (C1, C2 and C3) with Chl a during summer season (Fig. 7a), which infers that these are formed from phytoplankton. Similar observations also reported by field and laboratory studies (Chari et al., 2012, 2016; Romera-Castillo et al., 2010) associated to phytoplankton. The absence of tyrosine protein like fluorophore, which is masked by tryptophan (C4) like fluoropore, represents the unfolded protein (Mayer et al., 1999; Yamashita and Tanoue, 2004) formed from the freshly produced DOM by the photosynthesis process (Fellman et al., 2011). This is also supported by significant positive correlation (n = 18, p < 0.005) with DOC during summer season (Fig. 7b). Even though Chl a concentrations were high during winter, absence of correlations with fluoropores and their decrease in intensity leads to the biological degradation of organic matter. This is also evidenced by enhanced community respiration following the peak discharge (Sarma et al., 2009) in this estuary. From the present study, we conclude that insitu production was predominant during both the seasons (summer and winter) based on nutrients distribution and their ratios. From the results of spectral properties and spectral ratio of summer season, it was deduced that organic matter was enriched with lower molecular weight and aromatic organic compounds and is mostly autochthonous i.e. derived from phytoplankton exudates/excretes. This conclusion is also evidenced by

Acknowledgment This work was supported by the Department of Science and Technology, Science and Engineering Research Board to N.V.H.K. Chari under grant No. SR/FTP/ES-56/2013. The authors thank the Prof. Nittala S. Sarma (Emeritus Scientist, CSIR) for the technical support and for providing the instrumental facilities. Thanks are also due to Dr. Gregory Cooper, SOAS University of London, United Kingdom, for carrying out the necessary English modifications and the anonymous reviewer for the valuable suggestions to modify the earlier version of this manuscript. References Benner, R., 2002. Chapter 3 — chemical composition and reactivity. In: Hansell, Dennis A., Carlson, Craig A. (Eds.), Biogeochemistry of Marine Dissolved Organic Matter. Academic Press, San Diego 59–90. Blough, N., Green, S., 1995. In: Zepp, R.G., Sonntag, C. (Eds.), The Dahlem Conference Report—The Role of Nonliving Organic Matter in the Earth’s Carbon Cycle. Wiley, pp. 23–45 Spectroscopic characterization and remote sensing of non-living organic matter. In. Boehme, A.B., Shellenberger, G.G., Paytan, A., 2004. Groundwater discharge: potential association with fecal indicator bacteria in the surf zone. Environ Sci Techn 38, 3558–3566. Borsheim, K.Y., Myklestad, S.M., Sneli, J.A., 1999. Monthly profiles of DOC, mono and poly saccharides at two locations in the Trondheims fjords (Norway) during two years. Mar. Chem. 63, 255–272. Bouillon, S., Frankignoulle, M., Dehairs, F., Velimirov, B., Eiler, A., Abril, G., Etcheber, H., Borges, A.V., 2003. Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests. Glob. Biogeochem. Cycles. https://doi.org/10.1029/ 2002GB002026. Bouillon, S., Yambélé, A., Gillikin, D.P., Teodoru, C.R., Darchambeau, F., Lambert, T., Borges, A.V., 2014. Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin). Sci. Rep. 4, 1–10. Bukaveckas, P.A., Barry, L.E., Beckwith, M.J., David, V., Lederer, B., 2011. Factors determining the location of the chlorophyll maximum and the fate of algal production within the tidal fresh- water James River. Estuar Coast 34, 569–582. Burdige, D.J., Skoog, A., Gardner, K., 2000. Dissolved and particulate carbohydrates in contrasting marine sediments. GeochimCosmochim Act 64, 1029–1041. Carritt, D.E., Carpenter, J.H., 1966. Comparison and evaluation of currently employed modifications of Winkler method for determining dissolved oxygen in seawater – a Nasco report. J. Mar. Res. 24, 286. Chandran, R., Ramamoorthi, K., 1984. Hydrobiological studies in the gradient zone of Vellar Estuary 1 - physico-chemical parameters. Mahasagar –Bull. Natn.Inst. Occanogr 17, 69–77. Chari, N.V.H.K., Sarma, N.S., Pandi, S.R., Murthy, K.N., 2012. Seasonal and spatial constraints of fluorophores in the midwestern Bay of Bengal by PARAFAC analysis of excitation emission matrix spectra. Estuar. Coast. Shelf Sci. 100, 162–171. Chari, N.V.H.K., Keerthi, S., Sarma, N.S., Pandi, S.R., Chiranjeevulu, G., Kiran, R., Koduru, U., 2013. Fluorescence and absorption characteristics of dissolved organic matter excreted by phytoplankton species of western Bay of Bengal under axenic laboratory condition. J. Exp. Mar. Biol. Ecol. 445, 148–155. Chari, N.V.H.K., Nittala, S.S., Sudarsanarao, P., Chiranjeevulu, G., Kiran, R., Murty, K.N., Venkatesh, P., 2016. Fluorescent dissolved organic matter dynamics in the coastal waters off the central east Indian coast (Bay of Bengal). Environ. Ecol. Res. 4, 13–20. Chen, X., W., Wei, Wang, J., Li, H., Sun, J., Ma, R., Jiao, N., Zhang, R., 2019. Tide driven microbial dynamics through virus-host interactions in the estuarine ecosystem. Water Res. 160, 118–129. Chin, Y.P., Aiken, G., Oloughlin, E., 1994. Molecular-weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environ Sci Technol 28, 1853–1858. Coble, P.G., 2007. Marine optical biogeochemistry e the chemistry of ocean color. Chem. Rev. 107, 402–418. Conmy, R.N., Coble, P.G., Chen, R.F., Gardner, G.B., 2004. Optical properties of colored dissolved organic matter in the Northern Gulf of Mexico. Mar. Chem. 89, 127–144. Cory, R.M., McKnight, D.M., Chin, Y.P., Miller, P., Jaros, C.L., 2007. Chemical characteristics of fulvic acids from Arctic surface waters: microbial contributions and photochemical transformations. J. Geophys. Res. 112, G04S51. Del Castillo, C.E., Coble, P.G., Morell, J.M., López JM Corredor, J.E., 1999. Analysis of the optical properties of the Orinoco River plume by absorption and fluorescence spectroscopy. Mar. Chem. 66, 35–51. Del Vecchio, R., Blough, N.V., 2002. Photobleaching of chromophoric dissolved organic matter in natural waters: kinetics and modeling. Mar. Chem. 78, 231–253.

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Marine Pollution Bulletin 149 (2019) 110563

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