Chromophoric dissolved organic matter traces seasonally changing coastal processes in a river-influenced region of the western Bay of Bengal.

The optical characteristics of colored dissolved organic matter (CDOM) serve as a convenient tool for evaluating coastal processes, e.g., river runoff, anthropogenic inputs, primary production, and bacterial/photochemical processes. We conducted a study on the seasonal and spatial variability of absorbance and fluorescence characteristics of CDOM and nutrients in the coastal waters near the Gauthami estuary of River Godavari, the largest peninsular river of India, for a year. The surface aCDOM(350) showed a significant inverse relation with salinity in the coastal region, indicating a conservative mixing of marine and terrestrial end members. The aCDOM(350) was not conservative in the offshore (100 m isobath) waters due to enrichment by secondary sources. Seasonal variability in optical properties indicated diverse sources for CDOM, as revealed by principal component analysis. The excitation-emission matrix (EEM) spectra followed by parallel factor analysis (EEM-PARAFAC) revealed four distinct fluorophores. The tyrosine (B) fluorophore showed a predominant increase in the post-monsoon season (October to January), while tryptophan (T) was relatively more enriched, coincident with nutrient enrichment and transparency increase during the early monsoon phase (July). The biological index (BIX), which reflects recent photosynthetic activity, also displayed relatively higher values during the early monsoon. The humic fluorophores A and M, and humification index (HIX) were relatively enriched during the later phase of monsoon (July-October). HIX was > 4 in a few samples of the offshore region (100-m isobath) and indicated a probable contamination from drill-mud (bentonite) used in hydrocarbon exploration. During the monsoon, the relationship between T and B with CDOM was not evident due to the masking of B fluorescence in intact protein. However, during the post-monsoon (POM) and pre-monsoon (PRM) periods, this masking effect was not observed, likely due to protein degradation via bacterial and photochemical processes, respectively. Temporal variability in nutrients indicated that high ammonium levels were produced during POM (OM bacterial degradation), and high nitrite levels were observed during PRM (due to primary production). This study provides foundational insights into the use of CDOM for understanding the impact of diverse environmental, river discharge, and anthropogenic factors on coastal ecosystems.

Response of coastal phytoplankton pigment composition to tropical cyclone Fani.

Phytoplankton pigment composition was evaluated during the pre-cyclone phase (PRCP) and post-cyclone phase (POCP) of tropical cyclone Fani in the coastal waters of the northwestern Bay of Bengal. The chromatographic analysis revealed higher pigment diversity and an increase in individual pigment concentration during POCP. Chlorophyll-a (chl-a) was the dominant pigment during PRCP and POCP, followed by fucoxanthin. However, chl-a and fucoxanthin concentrations increased 18- and 14-folds, respectively, during the POCP, signifying Bacillariophyta bloom. Complementing microscopy confirmed the dominance of the toxic Bacillariophyta species Pseudo-nitzschia pungens (reaching 5.47 × 104 cells l-1) during the POCP. The cyclone-induced nutrient recharge of the ambient medium could have promoted phytoplankton growth, causing the reappearance of diatom bloom during the later phase of the pre-southwest monsoon. Small-sized Prymnesiophyta and Cryptophyta were not detected microscopically; however, they were identified by chromatographic analysis through pigment markers during POCP.

Unravelling tidal effect on zooplankton community structure in a tropical estuary.

The variability in zooplankton density and species composition in response to tidal oscillations were investigated in the lesser saline upper reaches (E1) and higher saline lower reaches (E2) of the Mahanadi Estuary, located at the mouth of the third largest river in Indian Peninsula. This is the first of its kind high frequency observation over the tidal cycle ranging from highest high tide to lowest low tide in this estuary revealing dynamic variability of zooplankton assemblages. Zooplankton abundance was higher during high tide in comparison to low tide, irrespective of salinity regimes. On the diurnal scale, it was higher at night in comparison to the day at both E1 and E2. The higher abundance of zooplankton groups such as Copepoda, Cladocera, and planktonic larvae during the night at E1 as well as E2 indicating an upward migration. Many of the zooplankton taxa (e.g., Pseudodiaptomus serricaudatus, Pseudodiaptomus sp., Acartia danae, Acrocalanus longicornis, Oithona sp., Corycaeus andrewsi) migrated towards E1 due to tidal effect during high tide and maintained their position even during low tide. In contrast, the prevalence of limnetic taxa (e.g., Brachionus rubens, Polyarthra vulgaris, Bosminopsis deitersi, Moina micrura, Heliodiaptomus sp.) at E2 during low tide indicated a predominant riverine source. The tidal variability of Brachyura (zoea and megalopa) revealed different emergence times that indicated dispersal of zoeas to the adjacent Bay of Bengal and the return of magalopa to the Mahanadi Estuary. Species diversity index was higher during high tide, and prominent at E2. At E1, marine, fresh, marine-brackish, and marine-brackish-fresh zooplankton species dominated during high tide, while brackish-fresh taxa dominated during low tide. Differently, E2 was enriched with marine, marine-brackish, and marine-brackish-fresh taxa during high tide, whereas fresh, brackish-fresh, and marine-fresh dominated during low tide. Salinity and suspended matter influenced the dominant zooplankton taxa at E2 and E1, respectively. Zooplankton assemblages exhibited a pattern of prominent diurnal-spatial variation in comparison to the tidal scale in the Mahanadi Estuary. Overall, this study documented a very high zooplankton diversity (92 taxa belonging to 13 groups) and significant variations in species abundance which highlighted the importance of carrying out sampling over the tidal cycle at contrasting salinity regimes.

Effect of Tidal Cycle on Escherichia coli Variability in a Tropical Estuary.

A faecal indicator bacteria, Escherichia coli (E. coli), is widely used in monitoring health of estuaries, where tidal amplitude plays a critical role in its variability. Mahanadi estuary, formed at the mouth of a major tropical river Mahanadi, has large socio-economic importance. This anthropogenically stressed estuary remains understudied with respect to E. coli. Hence, this study addressed E. coli variability in Mahanadi estuary with novel sampling strategy that can be implemented at other tropical estuaries. The sampling strategy includes simultaneous measurements, at lesser-saline upper-estuary and higher-saline lower-estuary, over a tidal cycle. Although no significant variability of E. coli was observed between upper and lower-estuary, overall average count was higher during low tide and lower during high tide attributed to salinity fluctuations. Intermittent surpass of E. coli counts above recommended limits in Mahanadi estuary, indicated potential health risk, thus demands for frequent water quality monitoring and management strategies.

A baseline investigation of phytoplankton pigment composition in contrasting coastal ecosystems of north-western Bay of Bengal.

A baseline investigation of the chromatographic characterization of phytoplankton pigments with complementing microscopy was conducted in Mahanadi estuary, Chilika lagoon, and coastal waters off Gopalpur along the east coast of India. Marker pigments specific to different phytoplankton groups have been discerned in these contrasting coastal ecosystems. A total of 16 phytoplankton pigments were identified. Irrespective of seasons, Chilika lagoon was characterized by a high concentration of zeaxanthin, indicating the predominance of picocyanobacteria. Zeaxanthin and fucoxanthin were the major diagnostic pigments in Mahanadi estuary during monsoon and other seasons, respectively. In coastal waters off Gopalpur, algal blooms resulted in a higher concentration of fucoxanthin during pre-monsoon and monsoon season. The pigment ratios were comparatively higher for Chilika lagoon than for Mahanadi estuary and off Gopalpur, irrespective of seasons. The present study highlights the advantages of the chromatography technique in identifying small-sized phytoplankton in coastal ecosystems in comparison to conventional microscopy.

Empirical relationships for remote sensing reflectance and Noctiluca scintillans cell density in the northeastern Arabian Sea.

A dinoflagellate under the ambit of Harmful Algal Blooms (HAB), the bioluminescent Noctiluca scintillans (NS), has been infesting the northern Arabian Sea increasingly over the last few decades during late winter. Their occurrence is found to be due to seasonal oscillations in the coastal currents. The physical and biogeochemical parameters associated with the seasonal blooms are reasonably well known. But accurate quantitative estimation capability using remote sensing sensors over the extensive oceanic regime is still lacking. This is especially due to a lack of information on bio-optical properties associated with cell density measurements. We attempted to show that remote sensing reflectance and chl-a show significant relationship e.g., Rrs(531)/Rrs(510) = 0.8261 + 6.06 × 10-6NS + 0.02323chl-a (N = 19, R2adj = 0.99, p = 2.5 × 10-17, RMSE = 0.1083) which is applicable over diverse areas of the northeastern Arabian Sea e.g., coastal, shelf and offshore regions. The model is supported by a second dataset with an RMSE of 0.022893 (N = 8) for the Rrs(531)/Rrs(510) ratio. The NS cell densities were derived from the Rrs(510)/Rrs(531) band ratio within reasonable error and accuracy limits. Including sensor capability at 510 nm is suggested in future satellite launches.

Long-term chlorophyll-a dynamics in tropical coastal waters of the western Bay of Bengal.

The long-term distribution of in situ optically active substances (OAS), accuracy assessment of satellite retrieved chlorophyll-a (chl-a) and its long-term trend has been carried out at a coastal site of the north-western Bay of Bengal. The temporal distribution of chl-a, total suspended matter (TSM) and absorption due to coloured dissolved organic matter at 440 nm (aCDOM440) discerned a common peak during southwest monsoon season (August-October). Chl-a also showed a prominent peak during pre-southwest monsoon period (March-April). The spatial variability of TSM and aCDOM440 was maximum during southwest monsoon, whereas in the case of chl-a, it was during pre-southwest monsoon. The accuracy assessment of chl-a retrieved from Moderate Resolution Imaging Spectroradiometer-Aqua (MODISA), Ocean Colour Monitor-2 (OCM-2) and Visible Infrared Imager Radiometer Suite (VIIRS) showed overestimation in nearshore waters. The error in satellite measurement of chl-a was within the range of 33 to 51%. The chl-a retrieved from MODISA was most accurate as indicated by statistical analysis. The long-term trend in satellite chl-a clearly indicated bi-modal distribution with a primary peak during pre-southwest monsoon attributed to recurrent phytoplankton bloom that was mostly confined to nearshore waters. Whereas, the secondary peak in chl-a, during the end of southwest monsoon, spreads far offshore.

Long-term trend and environmental determinants of phytoplankton biomass in coastal waters of northwestern Bay of Bengal.

The present study is focused on understanding long-term trend in distribution of chlorophyll-a and its association with water quality parameters in coastal waters of northwestern Bay of Bengal. In situ observations were carried out in monthly frequency during January 2009 to March 2017 at two ecologically important study sites, off Gopalpur and off Visakhapatnam. A bimodal distribution of chlorophyll-a was observed off Gopalpur with primary and secondary peak during pre- and post-monsoon, respectively. In contrast, maximum chlorophyll-a concentration was noticed in monsoon irrespective of sporadic higher magnitudes during pre-monsoon in coastal waters off Visakhapatnam. The pivotal role of river influx and coastal upwelling was discerned in enriching coastal waters off Gopalpur with macronutrients and in turn on chlorophyll-a distribution. Adequate availability of silicate signified favorable condition for diatom growth in coastal waters off Gopalpur leading to preponderance. Influence of coastal upwelling and nutrient-controlled growth of phytoplankton was observed in coastal waters off Visakhapatnam.