Investigating Indian oil sardine aggregation events in coastal waters of the southeastern Arabian sea.

The southwest coast of India experiences frequent Indian oil sardine (IOS) nearshore aggregation events, especially in the coastal waters off Kerala. These ephemeral dense IOS aggregation events are known as "Sardine Run". To investigate the reason and provide a scientific basis for these sporadic events, satellite/model-derived physical, meteorological, and biological parameters were analysed. Sea Surface Temperature during a majority of events was in the range of 26-29 °C, agreeing with the reported temperature conditions for IOS in the Arabian Sea. Additionally, a marginal lowering of SST as an effect of precipitation before most of the events might have attracted IOS towards the near-coastal waters in addition to the phytoplankton diet availability, resulting in the aggregation event. However, different scenarios also depicted coastal warming and probable hypoxic conditions in degrading IOS habitat and resulting in beach aggregation events. During most of the IOS aggregation events, the wind and surface current direction was alongshore/coastward, which complemented the propagation of live IOS shoals towards the beach.

Status and conservation challenges of the second-largest seagrass bed in India: Chilika lagoon.

Studies related to seagrass ecology, conservation, and management are sparse and highly biased in India. Even though the geographical distribution of seagrass is diverse in India, about 74% of the scientific publications have been made from Palk Bay and the Gulf of Mannar from Tamilnadu. Chilika, the largest lagoon in Asia, harbors the second largest seagrass meadow in India 22% of the total. The lagoon acts as a potential blue carbon stock and helps in thriving a rich floral and faunal biodiversity. However, the critical role of seagrass in this unique lagoon ecosystem is still poorly understood. This review is aimed at synthesizing the published literature about seagrass in Chilika. We believe this information would encourage more in-depth and diverse seagrass studies in the region and identify future priority areas for research. A total of seven species have been recorded from 169.2 sq. km of seagrass patch in Chilika. For the last two decades, no significant signs of decline in seagrass beds from this lagoon have been reported. Still, various natural and anthropogenic stressors could put this unique ecosystem under severe stress. Moreover, lax enforcement of existing legislation and a general lack of knowledge among the stakeholders about their ecosystem services can be significant impediments to their conservation. More targeted research on Chilika seagrass in changing climate regimes and their sustainable intensification is the need of the hour.

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.

Responses of phytoplankton community structure and association to variability in environmental drivers in a tropical coastal lagoon.

Spatial and seasonal heterogeneity in phytoplankton communities are governed by many biotic and abiotic drivers. However, the identification of long-term spatial and temporal trends in abiotic drivers, and their interdependencies with the phytoplankton communities' structure is understudied in tropical brackish coastal lagoons. We examined phytoplankton communities' spatiotemporal dynamics from a 5-year dataset (n = 780) collected from 13 sampling stations in Chilika Lagoon, India, where the salinity gradient defined the spatial patterns in environmental variables. Generalized additive models showed a declining trend in phytoplankton biomass, pH, and dissolved PO4 in the lagoon. Hierarchical modelling of species communities revealed that salinity (44.48 ± 28.19%), water temperature (4.37 ± 5.65%), and season (4.27 ± 0.96%) accounted for maximum variation in the phytoplankton composition. Bacillariophyta (Indicator Value (IV): 0.74) and Dinophyta (IV: 0.72) emerged as top indicators for polyhaline regime whereas, Cyanophyta (IV: 0.81), Euglenophyta (IV: 0.79), and Chlorophyta (IV: 0.75) were strong indicators for oligohaline regime. The responses of Dinophyta and Chrysophyta to environmental drivers were much more complex as random effects accounted for ~70-75% variation in their abundances. Prorocentrum minimum (IV: 0.52), Gonyaulax sp. (IV: 0.52), and Alexandrium sp. (IV: 0.51) were potential indicators of P-limitation. Diploneis weissflogii (IV: 0.43), a marine diatom, emerged as a potential indicator of N-limitation. Hierarchical modelling revealed the positive association between Cyanophyta, Chlorophyta, and Euglenophyta whereas, Dinophyta and Chrysophyta showed a negative association with Cyanophyta, Chlorophyta, and Euglenophyta. Landsat 8-Operational Land Imager satellite models predicted the highest and lowest Cyanophyta abundances in northern and southern sectors, respectively, which were in accordance with the near-coincident field-based measurements from the lagoon. This study highlighted the dynamics of phytoplankton communities and their relationships with environmental drivers by separating the signals of habitat filtering and biotic interactions in a monsoon-regulated tropical coastal lagoon.

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.

Seasonal dynamics of phytoplankton in response to environmental variables in contrasting coastal ecosystems.

Seasonal distribution of phytoplankton community and size structure was assessed in three different tropical ecosystems of the western Bay of Bengal viz. estuary (Mahanadi), lagoon (Chilika), and coastal waters (off Gopalpur) in response to ambient hydrobiology. Salinity regimes differentiated the study regions as contrasting ecosystems irrespective of seasons (pre-monsoon, monsoon, post-monsoon). Taxonomic account revealed a total no of 175, 65, and 101 phytoplankton species in the estuary, lagoon, and coastal waters, respectively. Prevalence of marine, brackish, and fresh water types in the coastal waters, lagoon, and estuary, respectively, characterized the contrasting nature of the study regions in hosting the phytoplankton community. In general, phytoplankton abundance was observed in increasing order of coastal waters > estuary > lagoon during post-monsoon and pre-monsoon, while lagoon > coastal waters > estuary during monsoon. Bacillariophyta dominated the phytoplankton community in the estuary and coastal waters during all the seasons. In contrast, the lagoon exhibited a diverse array of phytoplankton group such as cyanophyta, dinophyta, and bacillariophyta during monsoon, post-monsoon, and pre-monsoon, respectively. Over the seasons, microphytoplankton emerged as the dominant phytoplankton size class in the coastal waters. Diversely, nanophytoplankton contributed to major fraction of chlorophyll-a concentration in the estuary and lagoon. Interestingly, pre-monsoon dinophyta bloom (causative species: Noctiluca scintillans with cell density 9 × 104 cells·l-1) and monsoon bacillariophyta bloom (causative species: Asterionellopsis glacialis 5.02 × 104 cells·l-1) resulted decline in species diversity. Multivariate statistical analysis deciphered salinity as a major environmental player in determining the distribution, diversity, and composition of phytoplankton communities in the three contrasting environments. Trophic state indices signified the lagoon and estuary as hypereutrophic during all season. The coastal water was marked as highly eutrophic through trophic state index during monsoon and pre-monsoon.

Environmental dynamics of red Noctiluca scintillans bloom in tropical coastal waters.

An intense bloom of red Noctiluca scintillans (NS) occurred off the Rushikulya estuarine region along the east coast of India, an important site for mass nesting events of the vulnerable Olive Ridley sea turtle. At its peak, densities of NS were 3.3×10(5) cells-l(-1), with low relative abundance of other phytoplankton. The peak bloom coincided with high abundance of gelatinous planktivores which may have facilitated bloom development by their grazing on other zooplankton, particularly copepods. Ammonium concentrations increased by approximately 4-fold in the later stages of bloom, coincident with stable NS abundance and chlorophyll concentrations in the nano- and microplankton. This increase likely was attributable to release of intracellular ammonium accumulated through NS grazing. Dissolved oxygen concentrations decreased in sub-surface waters to near hypoxia. Micro-phytoplankton increasingly dominated chlorophyll-a biomass as the bloom declined, with diminishing picoplankton abundance likely the result of high predation by the ciliate Mesodinium rubrum. Together, these data illustrate factors that can disrupt ecosystem balance in this critically important Indian coastal region.

Distribution of trace metals in surface seawater and zooplankton of the Bay of Bengal, off Rushikulya estuary, East Coast of India.

Concentrations of trace metals such as iron (Fe), copper (Cu), zinc (Zn), cobalt (Co), nickel (Ni), manganese (Mn), lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), vanadium (V), and selenium (Se) were determined in seawater and zooplankton from the surface waters off Rushikulya estuary, north-western Bay of Bengal. During the study period, the concentration of trace metals in seawater and zooplankton showed significant spatio-temporal variation. Cu and Co levels in seawater mostly remained non-detectable. Other elements were found at higher concentrations and exhibited marked variations. The rank order distribution of trace metals in terms of their average concentration in seawater was observed as Fe>Ni>Mn>Pb>As>Zn>Cr>V>Se>Cd while in zooplankton it was Fe>Mn>Cd>As>Pb>Ni>Cr>Zn>V>Se. The bioaccumulation factor (BAF) of Fe was highest followed by Zn and the lowest value was observed with Ni. Results of correlation analysis discerned positive affinity and good relationship among the majority of the trace metals, both in seawater and zooplankton suggesting their strong affinity and coexistence.

Phytoplankton community structure in local water types at a coastal site in north-western Bay of Bengal.

A comprehensive analysis on seasonal distribution of phytoplankton community structure and their interaction with environmental variables was carried out in two local water types (type 1 < 30 m isobath and Type 2 > 30 m isobath) at a coastal site in north-western Bay of Bengal. Phytoplankton community was represented by 211 taxa (146 marine, 37 fresh, 2 brackish, 20 marine-fresh, and 6 marine-brackish-fresh) belonging to seven major groups including 45 potential bloom forming and 22 potential toxin producing species. The seasonal variability depicted enrichment of phytoplankton during pre-monsoon in both water types. Total phytoplankton abundance pattern observed with inter-annual shift during monsoon and post-monsoon period at both water types. In both water types, diatom predominance was observed in terms of species richness and abundance comprising of centric (82 sp.) and pennate (58 sp.) forms. Pennate diatoms, Thalassiothrix longissima and Skeletonema costatum preponderated in both the water types. The diatom abundance was higher in type 1 in comparison to type 2. In general, SiO4 found to fuel growth of the dominant phytoplankton group, diatom in both the water types despite comparative lower concentration of other macronutrients in type 2.

A preliminary study on the microzooplankton of Chilika Lake, a brackish water lagoon on the east coast of India.

Abundance and composition of microzooplankton in Chilika Lake were studied covering three seasons during 2012-2013. Ciliates (19 species), rotifers (13 species), crustacean larvae (2 taxa) and heterotrophic dinoflagellates (2 species) were the four major groups. Ciliates, however, have emerged as the most dominant group throughout the lake in all the seasons except in the freshwater-influenced areas. The average contribution of ciliates was highest (avg. 55.3 ± 38.7 %) during the premonsoon season followed by post-monsoon (avg. 49.0 ± 32.5 %) and monsoon (avg. 47.8 ± 41.6 %) seasons. Crustacean larvae formed the second dominant group whose percentage contribution was marginally high in the premonsoon (avg. 41.2 ± 38.8 %) followed by monsoon (avg. 29.1 ± 27.0 %) and post-monsoon (avg. 28.7 ± 23.4 %). Results of cluster analysis and multidimensional scaling on abundance of microzooplankton depicted well-defined clusters. The stations with high salinity formed a different group indicating seasonal variation in species composition and abundance of microzooplankton is primarily governed by salinity.

Interannual and cyclone-driven variability in phytoplankton communities of a tropical coastal lagoon.

One of the main challenges in phytoplankton ecology is to understand their variability at different spatiotemporal scales. We investigated the interannual and cyclone-derived variability in phytoplankton communities of Chilika, the largest tropical coastal lagoon in Asia and the underlying mechanisms in relation to environmental forcing. Between July 2012 and June 2013, Cyanophyta were most prolific in freshwater northern region of the lagoon. A category-5 very severe cyclonic storm (VSCS) Phailin struck the lagoon on 12th October 2013 and introduced additional variability into the hydrology and phytoplankton communities. Freshwater Cyanophyta further expanded their territory and occupied the northern as well as central region of the lagoon. Satellite remote sensing imagery revealed that the phytoplankton biomass did not change much due to high turbidity prevailing in the lagoon after Phailin. Modeling analysis of species-salinity relationship identified specific responses of phytoplankton taxa to the different salinity regime of lagoon.

Spatiotemporal distribution and composition of phytoplankton assemblages in a coastal tropical lagoon: Chilika, India.

The Asia's largest lagoon, Chilika, is a shallow water estuary and a designated "Ramsar" site located in the east coast of India. The spatiotemporal diversity of phytoplankton based on the monthly sampling between July 2011 and June 2012 was investigated in relation to physicochemical variables of the surface water column from 13 stations. The salinity was minimum (average 9) during the monsoon which was primarily due to riverine discharge. As the season progressed towards post-monsoon, average salinity of the whole lagoon reached to 10 which further increased to 20 during pre-monsoon season. A total of 259 species of phytoplankton, mostly dominated by the Bacillariophyta (138 species) followed by Dinophyta (38 species), Chlorophyta (32 species), Cyanophyta (29 species), Euglenophyta (18 species), and Chrysophyta (4 species), were recorded in this study. Different ecological sectors of the lagoon (except the northern sector) were dominated by diatoms, while the northern sector due to its freshwater regime supported large population of euglenoids. Based on the multivariate ordination analysis, salinity regime and light availability played important role in determining the distribution, diversity, and composition of phytoplankton communities. Overall, this study documented a very high diversity of phytoplankton and highlighted the importance of taking extensive sampling in getting a clearer understanding of phytoplankton community structure in less-studied environments such as Chilika lagoon.