Indian Estuaries Research Articles

A review on variability of partial pressure of carbon dioxide in coastal waters of India

This review compiles scientific studies on the partial pressure of carbon dioxide (pCO2) in Indian waters (coastal and estuarine). Higher surface pCO2 level varying within 111–1808 µatm has been reported on the east coast of India (western Bay of Bengal) in comparison to the west coast (eastern Arabian Sea) with variability within 266–685 µatm. The estuarine ecosystems of Indian coast have been reported with higher surface pCO2 levels along the east coast (4–34,026 µatm) in comparison to the west coast (351–20,421 µatm). Indian estuaries can be considered as significant source of carbon dioxide (CO2). The seasonal variability pattern discerned higher magnitudes of pCO2 during monsoon period in comparison to pre-monsoon. This review summarizes the key findings of available literature and identifies spatio-temporally understudied pockets of the Indian coast on carbonate chemistry research.

Contrasting intra-annual inorganic carbon dynamics and air–water CO2 exchange in Dhamra and Mahanadi Estuaries of northern Bay of Bengal, India

Surface water partial pressure of carbon dioxide [pCO2 (water)], total alkalinity (TA), dissolved inorganic carbon (DIC), and air–water CO2 flux were measured in two estuaries of the Bay of Bengal namely Mahanadi and Dhamra. Though the annual average air–water CO2 fluxes at the Mahanadi and the Dhamra Estuaries were − 3.9 ± 21.4 (mean ± standard deviation) µmol m−2 h−1 and − 2.9 ± 11.6 µmol m−2 h−1, respectively, the intra-annual variation of air–water CO2 fluxes in the two estuaries was contrasting. Nonetheless, from the perspective of net primary productivity, the surface water of both the estuaries were found autotrophic throughout the study period with varying rates at different seasons and highest during summer months. Mahanadi Estuary acted as a CO2 source toward atmosphere during monsoon months, whereas, Dhamra Estuary acted as a source during pre-monsoon months. On the contrary, Mahanadi and Dhamra Estuaries acted as CO2 sink during pre-monsoon months and monsoon months, respectively. The salinity in Mahanadi Estuary was much lower compared to Dhamra, which indicated significant freshwater discharge rich in organic carbon, and the remineralization of this carbon to DIC during summer and monsoon months explained the CO2 source character. Whereas, in Dhamra, reduced freshwater flow and high turbidity were held accountable for net heterotrophic character of the water column during the post-monsoon months. The annual data set of air–water CO2 fluxes from these two estuaries produced from this study could be utilized in future to fill the data gap and upscale the Indian estuaries scenario from the perspective of blue carbon budgeting.

Impact of monsoon-induced discharge on phytoplankton community structure in the tropical Indian estuaries

To understand the influence of monsoon-induced river discharge and associated environmental changes on phytoplankton community structure in the Indian estuaries, physical and biogeochemical parameters were collected from 28 major and medium estuaries along the Indian coast during the peak discharge period. Relatively higher river discharge was recorded in the estuaries located in northern India associated with higher suspended particulate matter (SPM), which arrested light penetration, resulting in a decreased in phytoplankton abundance and biomass. In contrast, higher phytoplankton abundance and biomass were observed in the estuaries located in southern India due to lower discharge and SPM than northern India. Diatoms were the dominant phytoplankton group in the Indian estuaries, except in estuaries located in the southeast coast, where blue–green algae predominate due to the existence of brackish water and stagnant water conditions due to less discharge. Higher phytoplankton diversity was observed in the south Indian estuaries than north Indian estuaries due to the long water residence time in the former. A present study is revealing that the magnitude of river discharge and SPM load seems to be the primary factors controlling phytoplankton abundances and community composition in the Indian estuaries.

Screening of tropical estuarine water in south-west coast of India reveals emergence of ARGs-harboring hypervirulent Escherichia coli of global significance.

The goal of this study was to investigate the involvement of a tropical Indian estuary in the emergence of antibiotic resistance genes (ARGs)-harboring hypervirulent E. coli of global significance. A total of 300 E. coli isolates was tested for antibiotic susceptibility to β-lactams, aminoglycosides, chloramphenicol, quinolones, sulphonamides, tetracyclines, and trimethoprim. The E. coli isolates were screened for the presence of antibiotic resistance genes (blaTEM, blaCTX-M, tetA, tetB, sul1, sul2, strA, aphA2, catI, dhfr1, and dhfr7), integrase (int1, int2, and int3), Shiga toxin genes (stx1 and stx2) and extraintestinal virulence genes (papAH, papC, sfa/focDE, kpsMT II, and iutA). The highest prevalence of antibiotic resistance was observed for ampicillin, followed by tetracycline, and nalidixic acid. Among E. coli isolates, 64% were resistant to at least one of the 15 antibiotics tested, and approximately 40% were multiple antibiotic-resistant (MAR). More than 40% (n = 122) of E. coli isolates had ARGs. Integrase 1 (int1) was found in 7.6% of E. coli isolates. Among E. coli isolates, 16.3% (n = 49) were extraintestinal pathogenic E. coli (ExPEC), and approximately 34.6% (n = 17) of ExPEC had ARGs. A hypervirulent ARGs-harboring STEC was isolated. The prevalence of Shiga toxin-producing E. coli (STEC) was low (n = 1). The prevalence of ARGs-harboring pathogenic E. coli isolates was higher in stations close to the City (urban area), than that of other stations. ERIC-PCR (enterobacterial repetitive intergenic consensus sequence polymerase chain reaction) analysis revealed a high degree of genetic diversity among the ARGs-harboring E. coli isolates. The results demonstrate a high prevalence of ARGs-harboring E. coli in estuarine water and confirm the need for a better wastewater treatment facility and proper control measures to reduce the discharge of sewage and wastewater into the aquatic environments.

A predictive model for salt intrusion in estuaries applied to the Muthupet estuary (India) and Bouregreg estuary (Morocco)

ABSTRACT In this paper, two 1D salt intrusion models, i.e. the steady-state and unsteady-state salinity intrusion model are used to assess the salt intrusion distribution in Muthupet estuary, a Ramsar site in Tamil Nadu, India; and Bouregreg estuary, in north-western part of Morocco. Results indicate that the steady-state salt intrusion model performs well to salinity observations especially during post-monsoon period (Indian estuary). Despite the fact that the high-salinity values observed during pre-monsoon period at a distance of 5 km to 7.5 km from the mouth are badly represented by the steady-state model it is a good indicator for the salt intrusion length. Additionally, this research shows that it is likely that an additional source of salt is causing the increase in salinity values at Lagoon reach and that the unsteady-state salt intrusion is able to represent this. On the other hand, a normal estuary is presented in this paper, for study comparison, i.e. Bouregreg estuary. The steady-state salt intrusion model outperformed the model results of the Indian estuary and is better to predict salt intrusion length and salinity variation along the Bouregreg estuary.

Variations in concentrations and sources of bioavailable organic compounds in the Indian estuaries and their fluxes to coastal waters

Estuarine organic matter (EOM) supports heterotrophic carbon demand in the estuaries. However, the fraction of bioavailable organic compounds, which are easily assimilated by biological organisms, in the EOM is unclear. We hypothesize that a significant fraction of dissolved EOM is bioavailable and supports heterotrophic activity in the Indian estuaries. In order to test this hypothesis, 26 major, medium and minor Indian estuaries were sampled during wet and dry periods (2011–2012) for dissolved bioavailable organic compounds and dissolved organic carbon (DOC). The mean concentration of total dissolved carbohydrates (TDCHO) and total dissolved proteins (TDPRO) were higher during wet than dry period in contrary to the observations of dissolved free Amino Acids (DFAA). TDPRO:TDCHO and statistical analysis revealed that terrestrial oganic matter contributed significantly to the EOM in the major and medium estuaries during wet period whereas fresh organic matter derived from in situ production contributed in all estuaries during dry period and minor estuaries during wet period. The lower concentrations of TDCHO and DFAA and their contribution to DOC were observed in the Indian than global estuaries suggesting that either efficient utilization of bioavailable organic compounds or less inputs (autochthonous/allochthonous) in the Indian estuaries. Indian estuaries transported an order of magnitude higher concentration of bioavailable organic compounds to the coastal Bay of Bengal (490 × 109 gC y−1) than to the Arabian Sea (50 × 109 gC y−1). The contribution of bioavailable organic compounds to the DOC export was 25% and 7% in the Bay of Bengal and Arabian Sea respectively. This study suggests that EOM contains a significant fraction of bioavailable organic compounds, as hypothesized, which might support heterotrophic carbon demand in the Indian estuaries as well as adjacent coastal waters.

Silicon cycle in Indian estuaries and its control by biogeochemical and anthropogenic processes

We study the silicon biogeochemical cycle and its associated parameters in 24 and 18 Indian estuaries during dry and wet periods respectively. We focus more specifically on dissolved Si (DSi), amorphous Si (ASi,) lithogenic Si (LSi), Particulate Organic Carbon (POC), Total Suspended Material (TSM), Dissolved Inorganic Nitrogen (DIN), salinity and fucoxanthin, a marker pigment for diatoms. Overall, we show that the estuaries have strong inter and intra variability of their biogeochemical parameters both seasonally and along salinity gradients. Based on Principal Component Analysis and clustering of categorised (upper and lower) estuaries, we discuss the four major processes controlling the Si variability of Indian estuaries: 1) lithogenic supply, 2) diatom uptake, 3) mixing of sea water and, 4) land use. The influence of lithogenic control is significantly higher during the wet period than during the dry period, due to a higher particle supply through monsoonal discharge. A significant diatom uptake is only identified in the estuaries during dry period. By taking into account the non-conservative nature of Si and by extrapolating our results, we estimate the fluxes from the Indian subcontinent of DSi, ASi, LSi to the Bay of Bengal (211 ± 32, 10 ± 4.7, 2028 ± 317Gmol) and Arabian Sea (80 ± 15, 7 ± 1.1, 1717 ± 932Gmol). We show the impact of land use in watersheds with higher levels of agricultural activity amplifies the supply of Si to the coastal Bay of Bengal during the wet season. In contrast, forest cover and steep slopes cause less Si supply to the Arabian Sea by restricting erosion when entering the estuary. Finally, Si:N ratios show that nitrogen is always in deficit relative to silicon for diatom growth, these high Si:N ratios likely contribute to the prevention of eutrophication in the Indian estuaries and coastal sea.

Variability in Concentrations and Fluxes of Methane in the Indian Estuaries

In order to examine the fluxes of methane (CH4) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH4 concentrations in the estuaries located along the west coast of India were significantly higher (113 ± 40 nM) compared to the east coast of India (27 ± 6 nM) during wet and dry periods (88 ± 15 and 63 ± 12 nM, respectively). Supersaturation of CH4 was observed in the Indian estuaries during both periods ((0.18 to 22.3 × 103 %). The concentrations of CH4 showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH4 saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH4 ranged from 0.01 to 298 μmol m−2 day−1 (mean 13.4 ± 5 μmol m−2 day−1) which is ~30 times lower compared to European estuaries (414 μmol m−2 day−1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39 × 1010 g CH4 year−1 with the surface area of 0.027 × 106 km2 which is significantly lower than that in European estuaries (2.7 ± 6.8 × 1010 g CH4 year−1 with the surface area of 0.03 × 106 km2). This study suggests that Indian estuaries are a weak source for atmospheric CH4 than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH4 fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01 × 1010 g CH4 year−1) but lower than those from Indian inland waters (210 × 1010 g CH4 year−1).

Variations in Concentrations and Fluxes of Dimethylsulfide (DMS) from the Indian Estuaries

In order to examine the variations in concentrations of dimethylsulfide (DMS) and its fluxes to the atmosphere, 25 major and medium estuaries from Indian subcontinent were sampled during wet and dry periods. River discharge brought substantial amount of nutrients and suspended particulate matter (SPM) to the Indian estuaries; however, the concentration of phytoplankton biomass was severely limited by latter due to shallowing of photic depth. Bacillariophyceae was the dominant phytoplankton group in the Indian estuaries followed by green algae, Cyanophyceae, and Dinophyceae. Relatively higher concentrations of DMS were observed in the estuaries located along the east (3.6 ± 5.7 nM) than the west coast of India (0.8 ± 0.3 nM) during wet period whereas no significant differences were observed during dry period. The concentrations of DMS were significantly lower during wet than dry period and it was consistent with the phytoplankton biomass. The slope of the relation between DMS and phytoplankton biomass displayed a significant spatial variation due to contribution of different groups of phytoplankton in the Indian estuaries. The concentrations of DMS in the Indian estuaries were higher than other estuaries in the world except some Chinese estuaries. The annual mean flux (1.95 ± 2.5 μmol m−2 day−1) from the Indian estuaries is lower than that of other estuaries in the world, except Pearl River estuary due to inhibition of phytoplankton growth by suspended load and low flushing rates.

Seasonal dynamics of turbidity maximum in the Muthupet estuary, India

Results are presented of the longitudinal and vertical profiling of salinity and suspended particulate matter (SPM) at the Muthupet estuary, India, during a one year period under widely varying freshwater flow conditions. Freshwater flow was available during post-monsoon and monsoon. An up-estuary shift in the location of estuarine turbidity maxima (ETM) was observed during the transition from post-monsoon to pre-monsoon and further it shifted downstream during the transition from pre-monsoon to monsoon, thereby exhibiting a pronounced seasonal cycle. The salinity intrusion was dependent on the freshwater discharge and was expressed as a power function of freshwater flow, explaining 97% of the variance. The formation of a salt plug in Muthupet estuary and its seasonal dynamics were observed, which is not an identified feature of any of the Indian estuaries studied so far. The geographical positions of salt plug and ETM core were more or less the same during their formation. The occurrence of two ETM during the LW of post-monsoon and the absence of ETM during monsoon explains the strong seasonal variation in the formation of ETM. The primary factor affecting the formation of ETM was identified as the freshwater flow over an annual cycle; the resuspension of sediments by tidal current affecting the formation on a flood/ebb cycle was secondary. The extent of shift of ETM was found to be an inverse logarithmic function of the freshwater discharge. The separation between ETM intrusion and salinity intrusion increased two fold with the increase in ETM intrusion.

Zooplankton characteristics in the Coringa Wild Life Sanctuary of Kakinada Bay

Zooplankton is an important biological component of any aquatic ecosystem. Its high rate of production influences enrichment of organic matter and it plays a vital role in secondary and tertiary productions. Zooplankton in the Indian estuaries had been widely studied. Though extensive studies were made on the distribution of zooplankton of estuarine and coastal waters, studies related to mangrove regions were meager. The mangrove ecosystem in the vicinity of Kakinada Bay is a complex environment since this happens to be a transition zone between limnetic and marine regions of the diverse aquatic regimes. Zooplankton form Gaderu and Coringa rivers of the mangrove area of the Kakinada were collected. The results reveal that during the months of November and December coinciding with the post monsoon period peak abundance of zooplankton was noticed. The post monsoons period is the period of re-colonisation. The meroplankton abundance varied between 25.04 % to 32.35 % in the Gaderu River and the similar observation was noticed in the Coringa River also. Since the mangrove habitat provides the natural nutrients through leaching activity, it supports higher percentages of meroplankton.

Depositional record of trace metals and degree of contamination in core sediments from the Mandovi estuarine mangrove ecosystem, west coast of India

The concentrations of seven trace metals (Fe, Mn, Cu, Cr, Co, Pb and Zn) in three sediment cores were analysed to assess the depositional trends of metals and their contamination level in the Mandovi estuary, west coast of India. All sediment cores showed enrichment of trace metals in the upper part of core sediments and decrease in concentration with depth, suggesting excess of anthropogenic loading (including mining activities) occurred during the recent past. Scanning electron microscope (SEM) images distinguished the shape, size and structure of particles derived from lithogenic and anthropogenic sources in core sediments. The geo-accumulation index (Igeo) values indicate that Mandovi estuary is ‘moderately polluted’ with Pb, whereas ‘unpolluted to moderately polluted’ with Fe, Mn, Cu, Cr, Co and Zn. The comparative analysis of trace metals revealed that Fe and Mn were highly enriched in the Mandovi estuary compared to all other Indian estuaries.

Genetic Diversity Analysis of <i>Elops machnata</i> (Forskal) Populations in South East and West Coasts of India Using RAPD Markers

The Random Amplified Polymorphic DNA (RAPD) technique was used to study the genetic diversity of four Elops machnata populations in South India. Elops machnata is considered as a least concern species (LC), categorized by the International Union for Conservation and Nature (IUCN). The population trends are currently stable in Indian Ocean, Eastern Africa, but are unknown throughout the rest of its expansive range, especially in Indian estuaries. Among the ten RAPD primers tested, eight primers got amplified and gave scorable bands. In total, 119 scorable bands were observed in all populations. The overall observed and effective number of alleles was found to be 2.000 ± 0.000 and 1.5307 ± 0.2503 respectively for the entire population. The overall polymorphic loci were 61.00% and the overall gene flow among the four populations was predicted to 0.1032. The genetic distance and geographic distance between the four populations showed a positive correlation. The highest genetic similarity (0.6824) was found between Parangipettai and Muthupettai population, which reflected the geographical relationship between them. Tow main clusters were obtained based on UPGMA dendrogram. This study proves that RAPD analysis has the ability to discriminate E. machnata populations in South Indian coastal waters.

Genetic Diversity Analysis of <i>Elops machnata</i> (Forskal) Populations in South East and West Coasts of India Using RAPD Markers

The Random Amplified Polymorphic DNA (RAPD) technique was used to study the genetic diversity of four Elops machnata populations in South India. Elops machnata is considered as a least concern species (LC), categorized by the International Union for Conservation and Nature (IUCN). The population trends are currently stable in Indian Ocean, Eastern Africa, but are unknown throughout the rest of its expansive range, especially in Indian estuaries. Among the ten RAPD primers tested, eight primers got amplified and gave scorable bands. In total, 119 scorable bands were observed in all populations. The overall observed and effective number of alleles was found to be 2.000 ± 0.000 and 1.5307 ± 0.2503 respectively for the entire population. The overall polymorphic loci were 61.00% and the overall gene flow among the four populations was predicted to 0.1032. The genetic distance and geographic distance between the four populations showed a positive correlation. The highest genetic similarity (0.6824) was found between Parangipettai and Muthupettai population, which reflected the geographical relationship between them. Tow main clusters were obtained based on UPGMA dendrogram. This study proves that RAPD analysis has the ability to discriminate E. machnata populations in South Indian coastal waters.

Distribution and sources of particulate organic matter in the Indian monsoonal estuaries during monsoon

AbstractThe distribution and sources of particulate organic carbon (POC) and nitrogen (PN) in 27 Indian estuaries were examined during the monsoon using the content and isotopic composition of carbon and nitrogen. Higher phytoplankton biomass was noticed in estuaries with deeper photic zone than other estuaries receiving higher suspended matter. The δ13CPOC and δ15NPN data suggest that relatively higher δ13CPOC (−27.9 to −22.6‰) and lower δ15NPN (0.7 to 5.8‰) were noticed in the estuaries located in the northern India, north of 16°N, and lower δ13CPOC (−31.4 to −28.2‰) and higher δ15NPN (5 to 10.3‰) in the estuaries located in the southern India. This is associated with higher Chl a in the northern than southern estuaries suggesting that in situ production contributed significantly to the POC pool in the former, whereas terrestrial sources are important in the latter estuaries. The spatial distribution pattern of δ15NPN is consistent with fertilizer consumption in the Indian subcontinent, which is twice as much in the northern India as in the south whereas δ13CPOC suggests that in situ production is a dominant source in the southern and terrestrial sources are important in the northern estuaries. Based on the Stable Isotope Analysis in R model, 40–90% (70–90%) of organic matter is contributed by C3 plants (freshwater algae) in the estuaries located in the northern (southern) India.

Molybdenum isotopes in two Indian estuaries: Mixing characteristics and input to oceans

The distributions of dissolved and particulate Mo and their isotope composition (δ98Mo) have been measured in the Narmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the δ98Mo of dissolved Mo in the Narmada estuary ranges from 0.49‰ to 2.19‰ in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapi estuary, 0.99–2.36‰, in the salinity range 0–20.3psu. Mo concentration in suspended sediments of the Narmada estuary collected during monsoon average 512±44ng/g (range 459–602ng/g) similar to that measured in one sample from the Tapi estuary 560ng/g Mo. δ98Mo of particulate Mo in the Narmada ranges from −0.21‰ to 0.48‰ with an average −0.03±0.2‰. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mο compared to that in basalts, the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo rich sulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contribution from marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins.The distribution of δ98Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing line between river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and δ98Mo show concomitant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide. Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary there could be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. The results obtained in this study bring out the processes modifying riverine input of Mo and its δ98Mo in the estuaries, oxic sink in the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuary can modify the supply of dissolved Mo and its δ98Mo relative to riverine supply by up to 40%, this can significantly impact the Mo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relative to that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance of solute particle interactions and anthropogenic input in determining the Mo flux and its δ98Mo to the open Arabian Sea.

Contribution of N2O emissions to the atmosphere from Indian monsoonal estuaries

Estuaries are known to contribute a significant amount of nitrous oxide (N2O) to the atmosphere; however, the contribution from the Indian estuaries is unknown. We made an attempt to estimate emissions of N2O from the Indian estuaries by collecting samples from 28 major and minor estuaries along the Indian coast during the wet and dry periods. The N2O was mostly saturated in all measured Indian estuaries during the study period (72–631%), with exceptionally high saturation in the Ponniyaar estuary (5902%) during the wet period. The N2O saturation displayed a strong relation with dissolved inorganic nitrogen (DIN; nitrate +nitrite and ammonium), ammonium and dissolved oxygen saturation, suggesting that nitrification is the major source of N2O in the Indian estuaries. The negative relation between salinity and N2O saturation suggests inner estuaries are a strong source compared to outer estuaries. The annual mean N2O saturation (204±137%) and fluxes (1.3 µmol N2O m−2 d−1) in the Indian estuaries were significantly less than European estuaries (271% and ~2.7 µmol N2O m−2 d−1, respectively). The estimation of flux of N2O from the European estuaries was also biased due to the inclusion of an exceptionally high supersaturation value from a small UK estuary, Colne (2645%). However, low N2O saturation and fluxes in the Indian estuaries were related to mean low concentration of DIN that led to low nitrification rates compared to world estuaries. Despite India ranking second in artificial fertilizers use, high flushing rates during the wet period reduce residence time leading to less modification within the estuary.

Picophytoplankton community in a tropical estuary: Detection of Prochlorococcus-like populations

The influence of hydrography on the picophytoplankton (PP) abundance in estuaries was studied by sampling along a salinity gradient for the first time in an Indian estuary. Prochlorococcus-like cells were detected at salinities ranging from 0.06 to 35, which otherwise is reported from offshore regions, thereby showing that this group is capable of surviving in estuarine waters. PP also comprised picoeukaryotes and two groups of Synechococcus, one rich in phycoerythrin (SYN-PE) and other in phycocyanin (SYN-PC). Salinity played an important role in the picophytoplankton distribution. SYN-PE was represented by two sub-groups, one which was found only in saline waters (SYN-PEII) and the other throughout the salinity gradient (SYN-PEI). SYN-PEI and SYN-PC dominated downstream and upstream, respectively but were present throughout the salinity gradient unlike in other estuarine regions. Picoeukaryotes abundance showed an increasing trend from saline to brackish water and decreased in freshwater. The entry of seawater into the estuary regulated SYN-PE and Prochlorococcus-like cells downstream whereas their higher abundance in freshwater could be due to different strains of freshwater origin. The average contribution of PP to the total photosynthetic biomass during spring and neap tides was 43% and 29% respectively, which highlights the importance of PP in estuaries.

Carbon dioxide emissions from Indian monsoonal estuaries

Estuaries are known to be strong source for atmospheric CO2, however, little information is available from Indian estuaries. In order to quantify CO2 emissions from the Indian estuaries, samples were collected at 27 estuaries all along the Indian coast during discharge (wet) period. The emissions of CO2 to the atmosphere from Indian estuaries were 4–5 times higher during wet than dry period. The pCO2 ranged between ∼300 and 18492 μatm which are within the range of world estuaries. The mean pCO2 and particulate organic carbon (POC) showed positive relation with rate of discharge suggesting availability of high quantities of organic matter that led to enhanced microbial decomposition. The annual CO2 fluxes from the Indian estuaries, together with dry period data available in the literature, amounts to 1.92 TgC which is >10 times less than that from the European estuaries. The low CO2 fluxes from the Indian estuaries are attributed to low flushing rates and less human settlements along the banks of the Indian estuaries.

Reduced river discharge intensifies phytoplankton bloom in Godavari estuary, India

Changes in river discharge alter material load and the consequent estuarine and coastal biogeochemical process. Evidence for biogeochemical response to variable rainfall over catchment area or changes in river discharges due to dam regulations is sparse. Inter-annual variability in monsoon rainfall and the consequent river discharges, either dam regulated or otherwise, in India are the best suited to test the influence of altered discharges on estuarine biogeochemistry. Our experiments in Godavari river estuary over three years revealed that a decrease in precipitation over the Indian subcontinent from 2007 to 2009 resulted in the lowering of mean annual discharge from 748.63m3s-1 in 2007 to 218.40m3s-1 in 2009. The reduced water discharge, during the peak discharge period, slowed the flushing of the estuary from 1.2days to 6.3days, respectively. The consequent increase in stability of water column and reduced suspended material load gave rise to intense phytoplankton blooms (Chl a 18μgl-1 in 2007 to 28μgl-1 in 2009). Resilience towards the unwanted phytoplankton bloom and overall health of the Indian estuaries is thus tuned by the variability in monsoon rainfall and dam regulated discharge.