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Bivalves are benthic organisms inhabiting coastal marine habitats, especially estuarine tidal and mudflats. Due to their high stocking density and rich protein content, they form a major part of the artisanal fishery resource around the world. A clear partitioning in the population of bivalves, Paphia malabarica (Chemnitz, 1782), and Meretrix casta (Gmelin, 1791) in southern (Chicalim) and northern (Siridao) bank of a tropical Zuari estuary influenced by the monsoon along the Indian west coast, is evidenced. This study unravels the reasons for their partitioning in this estuary. Paphia malabarica is an exclusive inhabitant of Chicalim which has silty-sandy sediment, whereas M. casta is exclusive to Siridao, a sandy habitat. Observations showed that this segregation is facilitated by the semi-enclosed nature of habitat at Chicalim with the high amount of degraded and aged sediment organic carbon, high chlorophyll a, elemental, and biochemical components, whereas Siridao experiences the high impact of tidal currents, low sediment organic carbon, and high water column chlorophyll a. The habitat in Siridao gets exposed to UV radiation during low tide, reducing the photosynthetic oxygen production, turning the habitat to periodic anoxia indicated by differences in the TOC:TS ratio. However, such conditions may not influence M. casta, which can derive oxygen from the water column. The fatty acids specific to diatoms, dinoflagellates, higher plants, and partially degraded organic matter in the tissues of P. malabarica indicate their ability to source the food from the sediment and water column, whereas in tissues of M. casta, higher dinoflagellate-specific fatty acids followed by diatom and bacteria indicate water column-derived food. Chicalim can be considered an actively coupled benthic-pelagic habitat, and Siridao as an uncoupled habitat. Thus, the diverse flux of food particles, species-specific feeding ecology, and local hydrodynamics operating at these habitats could be the determining factors in the partitioning of the bivalves. Graphical abstract .

The influence of changing salinity on community composition and functional activity (Bacterial Production (BP) and ectoenzyme activity) of major bacterial taxa was evaluated using microcosm experiments in a tropical monsoon influenced estuary. Natural bacterial inocula at different salinities, representing marine, brackish, and freshwater, were inter-transferred and elucidated their response with an emphasis on community composition and β-Glucosidase (BGase) activity. The results revealed a significant decrease in the total bacterial count (TBC) and BP on the translocation of bacterial inocula to different salinity conditions in the case of freshwater bacteria. However, a significant increase in BGase activity coupled with shifts in the studied bacterial groups was evident in the case of marine as well as freshwater bacteria. Quantitative PCR (qPCR) revealed a shift in major bacterial taxa upon translocation to different waters, which was dependent on salinity and the source of inocula. Redundancy and qPCR analyses showed that members belonging to Gammaproteobacteria and Betaproteobacteria were higher, and possibly influenced BGase activity in marine and freshwater, respectively. Translocation of marine inocula to brackish and freshwater resulted in an emergence of Bacteroidetes, Actinobacteria, and Betaproteobacteria, respectively. Whereas, when freshwater inocula were translocated to marine or brackish water, Alphaproteobacteria and Gammaproteobacteria taxa emerged, and this was coupled with increased BGase activity. In contrast, brackish water bacteria showed a strong persistence in bacterial community composition when translocated to different salinities within this estuary. Such phylogenetic persistence or changes suggests species level shifts in specific bacterial taxa, and unravelling the same using different functional gene markers would ascertain their role in organic matter processing and is way forward., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The episodic monsoon condition in a tropical estuarine environment inputs high allochthonous materials that are rich in carbohydrates and are mainly broken down by enzymatic hydrolysis thus alter both organic matter and microbial community composition of an estuary. β-Glucosidases are one of the enzymes mediating the degradation of carbohydrates and are majorly produced by heterotrophic bacteria. The present study elucidated β-glucosidase-producing culturable bacterial diversity and measured their activity during non-monsoon and monsoon seasons in Zuari estuary. The results revealed that both bacterial abundance and β-glucosidase activity decreased significantly from non-monsoon to monsoon, whereas phylogenetic diversity increased. Majority of β-glucosidase producers during non-monsoon belonged to the members of Bacillales (53%), Pseudomonadales (26%), and Vibrionales (11%) which shifted to the members of Enterobacteriales (51%), Bacillales (14%), Alteromonadales (12%), Aeromonadales (9%), Xanthomonadales (7%), Pseudomonadales (5%), and Flavobacteriales (2%) during the monsoon. The shift in bacterial community structure points out the occurrence of different allochthonous forms with carbohydrate-metabolizing ability during the monsoon, and their relevance in ecology and health of this estuary can be elucidated by studying their functional diversity and is a step ahead.

Abstract Understanding the variability in organism's immunological response is crucial for predicting changes at population or community level. The present study investigated the immunoecology of a commercially valuable clam Paphia malabarica in a tropical monsoon-influenced estuary. Clams were collected monthly during a year cycle, which coincided with pre-monsoon (February–May), monsoon (June–September) and post-monsoon seasons (October–January). For assessment of immune functioning, selected hemocyte parameters (total hemocyte concentration, hemocyte mortality, lysosomal content, esterase activity, reactive oxygen species production, and phagocytic activity) were analyzed using flow cytometry. Simultaneously, clam's condition index, nutrients, chlorophyll a , dissolved oxygen, pH, temperature and bacterial density were also measured at the sampling site. Our results exhibited seasonal patterns in hemocyte functioning with the highest activity during the pre-monsoon season (suggestive of a suitable harvesting period) and lowest during monsoon (suggestive of a critical biological period). The critical biological period for P. malabarica was marked with compromised immune parameters inflicted by low salinity, food availability, and possibly high bacterial abundance. Also, the involvement of reproductive stress altering the hematological functioning in P. malabarica cannot be ruled out. Nutrients, dissolved oxygen, pH and temperature could not explain much of the hemocyte variability. The present study has further validated the usefulness of hemocyte as a suitable marker for understanding immunoecology of P. malabarica which is of prime importance, especially in a monsoon-influenced tropical estuarine environment. The findings of our research will be constructive in monitoring natural as well as cultivated bivalve populations of economic and ecological relevance. Highlights • Immunoecological investigation of a commercially valuable clam Paphia malabarica in a tropical monsoon-influenced estuary. • Seasonal pattern in hemocyte functioning influenced by environmental as well as physiological factors. • The compromised immune parameters during monsoon suggestive of a critical biological period. • Improved hematological activity during pre-monsoon season suggestive of a suitable harvesting period. [ABSTRACT FROM AUTHOR]

The changes that occur in biofilms reflect the influence of the environment. In a case study, the biofilms developed along a salinity gradient from marine to freshwater in a tropical monsoon influenced estuary (Zuari, west coast of India) were evaluated through in situ observations, and effect of translocation through microcosm experiments. The bacterial abundance was tide driven and high at the mouth of the estuary, whereas periphyton biomass, in terms of chlorophyll a was higher in the freshwater to brackish zone. These results point out decoupling of bacteria and phototrophic organisms in estuarine early stage biofilms. Vibrio spp. and faecal indicator bacteria were abundant in marine and brackish biofilms. The translocation experiments revealed that at times when marine biofilms with high total bacterial abundance were transferred to either brackish or freshwater, the abundance decreased. However, a significant increase in the bacterial abundance was observed when freshwater biofilms were transferred to either brackish or marine conditions. The microcosm experiments revealed that the influence of grazers differed with the type of biofilm and played an important role in modulating the composition of biofilms. The diagnostic pigments provided considerable insights into spatial and seasonal dynamics of periphyton groups in different biofilms and were determined by the surrounding water column communities and the availability of nutrients. Decay of freshwater periphyton in biofilms when translocated to marine and brackish conditions lead to remineralisation and higher bacterial population. [ABSTRACT FROM AUTHOR]

Estuaries are hotspots of anthropogenic activities. The deposition of pathogenic bacteria in the sediment and their re-suspension into the water column are influenced by riverine discharge and tides. The abundance of Escherichia coli O157:H7, Shigella spp., Salmonella spp., total coliforms (TC) and Vibrio spp. (Vibrio cholerae (VC), Vibrio parahaemolyticus (VP), Vibrio alginolyticus (VA)) was assessed along with the total bacterial count (TBC) and total viable count (TVC) in surface sediments along the banks of the Zuari estuary, Goa, India. The study was carried out fortnightly for a period of 17 months covering three seasons, i.e. pre-monsoon (PreM), monsoon (MON) and postmonsoon (POM). The spatial and temporal changes in the quality of organic matter were also assessed. The organic matter content was high and rich in carbohydrates and proteins towards upstream sites. The quality of organic matter was influenced by the seasons. E. coli O157:H7 was detected only during MON towards the upstream stations. A negative correlation between TC and TBC with salinity was evident indicating the influence of land run-off. The Shigella spp. and VA were high towards the mouth of the estuary during PreM. However, during POM, the TVC, TC and VP were abundant towards the upstream and VC were abundant at the mouth of the estuary. Among the Vibrios, VP and VA were the most frequently occurring bacteria whereas TC and Shigella spp. were dominant among allochthonous pathogens in the sediments irrespective of space and time. In addition to influence of seasons, the sampling time influenced by tidal condition also played an important role in the population dynamics of pathogenic bacteria in the sediments. Future studies should address the interaction of pathogenic bacteria with suspended particles, their transport and survival in the sediments. [ABSTRACT FROM AUTHOR]

Paphia malabarica is a predominant and commercially important bivalve in India, persistently challenged by wavering salinity in a monsoon-influenced estuary. To examine the organism's immunological response under such a condition we challenged P. malabarica with different salinities (0, 5, 15, 25 and 35) for varied periods using a two-way experimental approach (in vitro and in vivo). This is the first study to report the response of P. malabarica hemocytes to salinity stress from a monsoon-influenced estuary on the southwest coast of India. Evaluation of total hemocytes count, mortality, lysosomal content, reactive oxygen species production, phagocytic and esterase activity was carried out using flow cytometric analysis. In both the experimental conditions, hemocyte parameters were significantly compromised at lower salinities (0 and 5) with an evident immuno-salinity tolerance range of 15–35. The damaging impact of 0 and 5 salinities on hemocyte function intensified with a longer exposure period, indicating that prolonged exposure to low salinity could be detrimental to bivalve wellness if they are pushed beyond their tolerance range which is usually observed during the monsoon. Further studies should focus on the interactive effect of salinity tagged with different stressors influencing biology of P. malabarica . [ABSTRACT FROM AUTHOR]

The epibiotic communities (diatoms and metazoans) on the outer surfaces of the shell of the barnacle Balanus amphitrite ( BSh) and its opercular valves (the scutum and tergum; BST) were investigated on a monthly basis for 1 year in a tropical monsoon-influenced estuary and compared with that of the surrounding rock biofilm. BSh and BST were rich in the diatoms Achnanthes longipes and Melosira nummuloides and amongst the invertebrates, nematodes and tardigrades were abundant. To the best of our knowledge, this is the first report of epibiosis on the acorn barnacle. Diatom abundance was at its maximum during the monsoon season on all the inter-tidal substrata i.e. BSh, BST and rock. A significant correlation was found between the density of diatoms and invertebrates associated with the BSh and BST, possibly because of the trophic relationship between them or the suitability of the physical environment. The possible role of these epibionts on the settlement of this barnacle species is currently unknown and needs further investigation. [ABSTRACT FROM AUTHOR]

• Fatty acid biomarkers utility in the evaluation of organic matter dynamics in tropical estuary. • Estuarine bank stations showed high content of phytoplankton-derived OM during non-monsoon seasons. • Terrestrial plant and bacteria-derived OM dominant during the monsoon. • Seasons, sites, and estuarine morphology influence the OM and benthic trophic status of Zuari estuary. Sedimentary organic matter (OM) composition serves as a biomarker and helps in hindcast analysis of the events in the habitat. The present study was carried out on a monthly basis to evaluate the contribution of different biological communities to the surface sediment in a monsoon-influenced tropical Zuari estuary, west coast of India, using source-specific fatty acid (FA) biomarkers and total organic carbon to nitrogen ratio (TOC/TN). Our observations revealed spatio-temporal variation in the OM composition in this estuary, which is influenced by in situ production, bacterial re-working, riverine inputs, and hydrodynamics. The content of TOC, TN, and total FAs in the sediment ranged from 0.13 to 4.27%, 0.01 to 0.31%, and 29 to 122 µg g−1 sediment dry weight, respectively. High content of TOC, TN, total FAs, terrestrial plants-specific FAs, and detrital derived FAs at the lower mid-estuarine (inner) stations when compared to the mouth of the estuary, points out lower mid-estuarine region as the major depositional site for the OM. Low amount of OM at the mouth of the estuary is attributed to high bacterial contribution, which seems to be maintained by repetitive cycles of resuspension-deposition of organic materials. Thus, it seems that the estuarine morphology influences the deposition of OM and plays an important role in the functioning of the estuarine ecosystem. The contribution of phytoplankton-specific FAs was high at the banks of the estuary, which form a suitable habitat for benthic organisms including artisanal fisheries. Diatom, dinoflagellate, and zooplankton-specific FAs were high during the post-monsoon season and decreased from the pre-monsoon to monsoon season. Terrestrial plants and bacteria-derived FAs were high during the monsoon suggesting the role of bacteria in the assimilation of terrestrial material. Elucidating the degradation pattern of in situ produced and terrestrial-derived organic matter through the laboratory designed microcosm experiments is a way forward. [ABSTRACT FROM AUTHOR]

Heterotrophic bacteria that differ in their nucleic acid content are frequently observed in aquatic environments. However, their relevance and roles in organic matter utilization in monsoon-influenced tropical estuaries have not been addressed. Transparent exopolymer particles (TEP) are one of the carbohydrate rich organic matter forms in aquatic environments, and their breakdown is mostly mediated by a heterotrophic bacterial ectoenzyme, i.e., β – glucosidase (BGase). The present study evaluated the influence of shifts in high nucleic acid content– (HNA) and low nucleic acid content – (LNA) bacteria on the degradation of TEP with emphasis on BGase activity. For this, monthly observations were carried out along the Zuari estuary, India, on the abundance of HNA, LNA bacteria, bacterial production (BP), and BGase activity. The results showed that the contribution of HNA bacteria to total bacterial count was high and positively influenced the BP and BGase activity during the non-monsoon seasons, which represents a well-mixed water column. During this time, HNA also influenced the active utilization of TEP. Whereas, LNA bacteria dominated during the monsoon, wherein the water column is strongly stratified, and showed decoupling with BGase activity, irrespective of the depth. Redundancy and response curve analyses revealed that the interactive effect of monsoon (rainfall) and stratification significantly altered the bacterial activity in this estuary. Besides, the variability in side scatter (SSC), and fluorescence intensity (FL 1) of HNA and LNA bacteria suggest that these two groups play distinct ecological roles in the tropical estuarine systems. Unravelling the links between the different nucleic acid content bacteria and their phylogenetic composition can provide insights into their functional roles in monsoonal estuaries. • Seasonal shifts in the bacterial physiological states were observed. • HNA bacteria influenced bacterial production and β-glucosidase activity during non-monsoon. • LNA bacteria dominated during monsoon, and decoupled with β – glucosidase activity. • Bacterial physiological states linked to distinct ecological roles. [ABSTRACT FROM AUTHOR]