S. K. Baliarsingh

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.

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.

An Ephemeral Dinoflagellate Bloom during Summer Season in Nearshore Water of Puri, East Coast of India

The present paper reports on the phenomenon of pinkish-red discoloration of the nearshore water of Puri, Odisha on 12th May 2016. Many local newspapers covered this event, as Puri city is a major tourist and pilgrimage place on the east coast of India. Field observations were carried out in order to provide a scientific basis to the event and to elicit possible causes of this discoloration. Taxonomic analysis of the phytoplankton samples revealed the dominance of red colored dinoflagellate species Gonyaulax polygramma, contributing 90% to total phytoplankton population. The localized concentration of G. polygramma was responsible for the pinkish-red discoloration of nearshore water. The exact factor that lay behind the genesis of this bloom could not be delineated due to the short period of its persistence. But two factors - upwelling and anthropogenic nutrient influx - can be viewed as the main cause for this ephemeral bloom. Non-hypoxic conditions in the coastal water following the ephemeral bloom event indicated no significant risk of ecological deterioration to the ambient medium.

Response of phytoplankton community and size classes to green Noctiluca bloom in the northern Arabian Sea

A comprehensive analysis on the phytoplankton ecology with special reference to different phytoplankton size classes was carried out at green Noctiluca scintillans (hereafter Noctiluca) bloom and non-bloom locations in offshore waters of the northern Arabian Sea. At the bloom locations, green Noctiluca represented a dense mono-specific proliferation with average cell density of 10.16 ± 5.806 × 104 cells-L-1 and relative abundance share of 98.63%. Active photosynthesis through prasinophytic endosymbiont was depicted from net community production magnitude reaching 85.26 mgC/m3/Day under low prey abundance. Parallel swarming of Porpita porpita, a voracious copepod feeder signified the competitive advantage of Noctiluca to have the phytoplankton prey. Average concentration of picophytoplankton biomass was eleven times lower in surface waters of non-bloom stations in comparison to bloom. Higher N:P ratio in subsurface waters of non-bloom stations signified non-utilization of nitrogenous nutrients. Green Noctiluca bloom onset subsequent to diatom rich conditions was evident from spatio-temporal ocean colour satellite imageries.

Zooplankton Research in Indian Seas: A Review

To promote long-term studies on the distribution and diversity of marine zooplankton in Indian seas, a comprehensive review has been carried out based on the available literature. Zooplankton studies in Indian waters started in the early 1900s, and a plethora of literature has accumulated dealing with various aspects of zooplankton, especially from the Bay of Bengal, Arabian Sea and their associated estuaries and backwaters. From this review, a comprehensive description is offered on the species composition and distribution of zooplankton in the Indian Seas. Emphasis is given to reflect the existing knowledge on the variations in zooplankton species composition in the Bay of Bengal and Arabian Sea. Copepods emerge as the most dominant component in all of these marine waters, as is the case worldwide. Copepods are more diverse in the Bay of Bengal than in Arabian Sea.

Zooplankton Distribution in Coastal Water off Gopalpur, North-Western Bay of Bengal

An interannual study on zooplankton abundance, biomass, and species composition was carried out during different seasons in two local coastal water types off Gopalpur, north-western Bay of Bengal. Although, Type-1 was observed with higher zooplankton abundance in comparison to Type-2, pattern of variation followed similar seasonal trends in both water types during individual years. Well pronounced seasonality was observed in zooplankton distribution. Zooplankton community was composed of 217 holoplankton and 22 meroplankton. The holoplankton community was predominated by copepod in terms of species diversity and abundance followed by hydrozoa, tintinnida, malacostraca, gastropoda, chaetognatha and chordata. The meroplankton were represented by larval forms viz. bivalve veliger, brachyuran zoea larvae, caridean larvae, copepod nauplii, fish egg and gastropod veliger. Dominance of copepod species viz. Acrocalanus longicornis, Paracalanus aculeatus and Paracalanus parvus were observed frequently in both water types. The species richness was higher in Type-1 in comparison to Type-2 during both the years. Salinity regimes and availability of phytoplankton prey influenced the distribution and species composition of zooplankton assemblage.

Characterization of oceanic Noctiluca blooms not associated with hypoxia in the Northeastern Arabian Sea

Intense blooms of the heterotrophic dinoflagellate, green Noctiluca scintillans, have been reported annually in the Northern Arabian Sea since the early 2000s. Although not known to produce organic toxins, these blooms are still categorized as a harmful due to their association with massive fish mortalities. Recent work has attributed these blooms to the vertical expansion of the oxygen minimum zone, driven by cultural eutrophication from major coastal cities in western India. As diatoms are preferred prey of green Noctiluca scintillans, more frequent blooms of this mixotroph will likely impact the productivity of important fisheries in the region. The present study uses a satellite algorithm to determine the distribution of both diatom and green Noctiluca blooms in the Northeastern Arabian Sea from 2009 to 2016. The results from shipboard microscopy of phytoplankton community composition were used to validate the satellite estimates. The satellite algorithm showed 76% accuracy for detection of green Noctiluca and 92% for diatoms. Shipboard measurements and data from biogeochemical-Argo floats were used to assess the relationship between oxygen concentrations and green Noctiluca blooms in the Northeastern Arabian Sea. Regardless of the presence of a Noctiluca bloom, the dissolved oxygen in the photic zone was always >70% saturated, with an average oxygen saturation >90%. The variability in the relative abundance of diatoms and green Noctiluca is not correlated with changes in oxygen concentration. These findings provide no evidence that cultural eutrophication has contributed to the decadal scale shifts in plankton composition in the Northeastern Arabian Sea oceanic waters. Conversely, the climatic warming of surface waters would have intensified stratification, thereby reducing net nutrient flux to the photic zone and decreasing silicate to nitrate ratios (Si:N); both factors that could increase the competitive advantage of the mixotroph, green Noctiluca, over diatoms. If so, the decadal-scale trajectory of phytoplankton community composition in the Northeastern Arabian Sea may be a harbinger of future climate-driven change in other productive oceanic systems.

Winter thermal inversion and Trichodesmium dominance in north-western Bay of Bengal

Clear thermal inversion was observed with cold surface waters (< 24°C) overlying the warm (> 26°C) subsurface water in the coastal waters of the northwestern Bay of Bengal during winter (January 2015). Simultaneously, preponderance of the cyanobacteria Trichodesmium erythraeum was observed dominating the phytoplankton community with > 90% of total population, reaching maximum density of 9.8 × 105 filaments/L. Further, the Trichodesmium predominance was associated with low water temperature (< 24°C).

Effect of physico-chemical regimes and tropical cyclones on seasonal distribution of chlorophyll-a in the Chilika Lagoon, east coast of India

A comprehensive analysis on spatiotemporal variation in physico-chemical variables and their control on chlorophyll-a during 2013–14 was carried out in the Chilika Lagoon. Spatiotemporal variation in physico-chemical regimes significantly controlled the phytoplankton biomass of the lagoon. Further, precipitation-induced river/terrestrial freshwater influx and marine influence controlled the physico-chemical regimes of the Chilika Lagoon, such as nutrients (NH4+, NO3−, NO2−, PO43− and Si(OH)4), temperature, salinity, total suspended matter and dissolved oxygen. This study revealed significant effects of tropical cyclones Phailin (2013) and Hudhud (2014) on physico-chemical regimes and in turn the phytoplankton biomass of the lagoon. Although both cyclones Phailin (2013) and Hudhud (2014) were intense, Phailin (2013) had a greater impact on the Chilika Lagoon due to the proximity of its landfall. Heavy precipitation caused an influx of nutrient-rich freshwater, both during each cyclone’s passage, through rainfall, and after, through increased river flow and terrestrial run-off. The increase in nutrients, carried by the run-off, promoted phytoplankton growth, albeit in lag phase. In general, phytoplankton growth was controlled by nitrogenous nutrients. However, the addition of SiO4 through terrigenous run-off fuelled preferential growth of diatoms. The salinity pattern (which can be considered a proxy for fresh and marine water influx) indicated injection of freshwater nutrients into the northern, southern and central sectors of the lagoon through riverine/terrestrial freshwater run-off; marine influx was restricted to the mouth of the lagoon. Present and past magnitudes of salinity and chlorophyll-a were also compared to better understand the pattern of variability. A significant change in salinity pattern was noticed after the opening of an artificial inlet, because of the resulting higher influx of marine water. The overall phytoplankton biomass (using chlorophyll-a concentration as a proxy) remained consistent in the lagoon pre- and post-restoration. Due to the wide range of salinity and temperature tolerance, diatoms remained dominant in both pre- and post-restoration periods, but the overall phytoplankton diversity increased after the artificial inlet was dredged.

Algal species dynamics in North Arabian Sea using long term ocean colour satellite data

North Arabian Sea experiences massive proliferation of variable algal species. The study presents variability of Noctiluca and its association with hydrographic parameters such as sea surface temperature (SST) and water column stability using ten years of satellite data. The area was categorized into three regions, North (23 to 26°N and 56 to 70°E), West (18 to 23°N and 56 to 62°E) and East (18 to 23°N and 62 to 74°E). The Noctiluca dominated area was extracted following approach of Dwivedi et. al. (2015) based on slope of Remote Sensing Reflectance (Rrs) between 488 to 443nm and 488 to 531nm. The data used in the present study depicted two distinct clusters based on regression between difference of Rrs(488) and Rrs(443) with Rrs(488) and Rrs(531). The major clusters representing Noctiluca falls within the range of 0.0004 to 0.0015 (Rrs488-Rrs443) and -0.0012 to -0.0004 (Rrs488-Rrs531). The occurrence of Noctiluca showed bi-modal distribution at an annual scale with the dominance in the northern region during winter monsoon (February- March). In western and eastern region higher frequency of Nuctiluca was during post monsoon having lag of one month from western (September) to eastern (October) region. The periodicity of Noctiluca, carried out using Fourier analysis, showed predominance at annual scale in Northern and semi-annual scale in Western and Eastern region. This indicates that the Noctiluca bloom in the northern region is primarily triggered by winter mixing whereas in western and eastern part of northern Arabian Sea it has combined effect of summer upwelling as well as winter mixing.