Revisiting the &lt;i&gt;Noctiluca scintillans&lt;/i&gt; Paradox in Northern Arabian Sea

Prakash, Satya; Roy, Rajdeep; Lotliker, Aneesh A.

doi:10.18520/cs/v113/i07/1429-1434

Cited by 31 publications

(10 citation statements)

References 15 publications

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“…What is different, however, is that this increase in winter monsoon Chl a concentrations is not being fuelled by diatoms; the ubiquitous, trophically important, siliceous photosynthetic organisms, which dominated winter phytoplankton communities in the AS during the 1960s International Indian Ocean Expeditions (IIOE) and the mid-1990's Joint Global Ocean Flux Studies (JGOFS) 4,28 . Rather, this is due to blooms of the mixotrophic green dinoflagellate Noctiluca scintillans Suriray (synonym Noctiluca scintillans Macartney) [29][30][31][32][33][34] . Since they were first detected in the early 2000s 29,31,32,35,36 green Noctiluca blooms have become increasingly more pervasive over diatoms ( Fig.…”

Section: Resultsmentioning

confidence: 99%

“…In this study 35 , we showed using shipboard experiments in which natural populations were exposed to suboxic seawater 36 that endosymbionts in Noctiluca cells photosynthesized more efficiently under suboxic conditions. More recent studies 33,34,39 , have attributed the advent of Noctiluca blooms to acute "silicate stress", a situation that would prevent diatom populations from attaining bloom proportions, particularly in the northwestern AS where they have been the predominant algal group in the past 28,29,32,46 . Our examination of winter-time nutrient concentrations coincident with Noctiluca blooms provides no evidence of silicate stress 31 , but instead raises the intriguing possibility that autotrophic phytoplankton in the contemporary AS ecosystem may in fact be experiencing acute "nitrate stress".…”

Section: Resultsmentioning

confidence: 99%

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Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

Goés

Tian

Gomes

et al. 2020

Sci Rep

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The recent trend of global warming has exerted a disproportionately strong influence on the Eurasian land surface, causing a steady decline in snow cover extent over the Himalayan-tibetan plateau region. Here we show that this loss of snow is undermining winter convective mixing and causing stratification of the upper layer of the Arabian Sea at a much faster rate than predicted by global climate models. over the past four decades, the Arabian Sea has also experienced a profound loss of inorganic nitrate. in all probability, this is due to increased denitrification caused by the expansion of the permanent oxygen minimum zone and consequent changes in nutrient stoichiometries. these exceptional changes appear to be creating a niche particularly favorable to the mixotroph, Noctiluca scintillans which has recently replaced diatoms as the dominant winter, bloom forming organism. Although Noctiluca blooms are non-toxic, they can cause fish mortality by exacerbating oxygen deficiency and ammonification of seawater. As a consequence, their continued range expansion represents a significant and growing threat for regional fisheries and the welfare of coastal populations dependent on the Arabian Sea for sustenance. The Arabian Sea (AS) is a unique, low-latitude oceanic ecosystem because it is influenced by monsoonal winds that reverse their direction seasonally 1. These reversing winds cause dynamic shifts in surface currents and alterations in the pycnocline, which help fertilize its normally nutrient-depleted surface layers. However, the mechanisms that drive water-column mixing, the upward transport of nutrients and the consequent upsurge of phytoplankton biomass during the summer (Jun.-Sep.) and the winter (Nov.-Feb.) monsoons are different 2-6. During the summer monsoon, nutrient enrichment is via coastal upwelling, which begins when the adjacent land mass becomes warmer relative to the AS, and low pressure develops over the Arabian Peninsula 3,7,8. During this time of the year, strong topographically-steered southwesterly winds blowing over the AS form a low-level atmospheric jet called the Findlater Jet 9. This jet induces a northeastwardly flow of the surface currents, leading to strong upwelling of deep, nutrient-rich waters, causing large phytoplankton blooms along the coasts of Somalia, Yemen, and Oman 3,5,6,10,11. In contrast, during winter, when the Eurasian continent cools, nutrient enrichment is via deep penetrative convective mixing as a result of excessive heat loss from the AS surface layer caused by cold and dry northeasterly winds blowing from the snow-covered Himalayan-Tibetan Plateau (HTP) 4,5. The erosion of the thermocline, and entrainment of deep (100-150 m) nutrient-rich waters into the surface layer of the AS, begins as early as Dec., and has a major influence on winter phytoplankton blooms observable as far south as 14°N 4,11,12 .

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

Goés

Tian

Gomes

et al. 2020

Sci Rep

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“…These massive outbreaks of Noctiluca blooms were reported to be fueled by an unprecedented influx of oxygen-deficient waters into the euphotic zone (do Rosário Gomes et al, 2014). However, such claims have been refuted (Prakash et al, 2017) and proved that they are naturally driven by changes in nutrient stoichiometry (Lotliker et al, 2018;Sarma et al, 2018).…”

Section: Biogeochemistrymentioning

confidence: 99%

Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

Vinayachandran¹,

Masumoto²,

Roberts³

et al. 2021

Preprint

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Abstract. The Indian Ocean presents two distinct climate regimes. The North Indian Ocean is dominated by the monsoons, whereas the seasonal variation is less pronounced in the south. The prevailing wind pattern produces upwelling along different parts of the coast in both hemispheres during different times of the year. Additionally, dynamical processes and eddies either cause or enhance upwelling. This paper reviews the phenomena of upwelling along the coast of the Indian Ocean extending from the tip of South Africa to the southern tip of the west coast of Australia. Observed features, underlying mechanisms, and the impact of upwelling on the ecosystem are presented. In the Agulhas Current region, cyclonic eddies associated with the Natal pulses drive slope upwelling and enhances chlorophyll concentration along the continental margin. The Durban break-away eddy spun-up by the Agulhas upwells cold nutrient-rich water. Besides, topographically induced upwelling occurs along the inshore edges of Agulhas Current. Wind-driven coastal upwelling occurs along the South coast of Africa and augments the dynamical upwelling in the Agulhas Current. Upwelling hotspots along Mozambique are present in the northern and southern sectors of the channel, and they are ascribed to dynamical effects of ocean circulation in addition to wind forcing. Interaction of mesoscale eddies with the western boundary, anticyclonic eddy pair interactions, and passage of cyclonic eddies cause upwelling. Upwelling along the southern coast of Madagascar is caused by Ekman wind-driven mechanism and by eddy generation and inhibited by Southwest Madagascar Coastal Current. The seasonal upwelling that occurs along the East African coast is primarily driven by the Northeast monsoon winds and enhanced by topographically induced shelf-breaking and shear instability between the East African Coastal Current and the island chains. Somali coast presents a strong case for the classical Ekman type of upwelling. This upwelling can be inhibited by the arrival of deeper thermocline signals generated in the offshore region by wind stress curl. The upwelling is nearly uniform along the coast of Arabia, it is caused by the alongshore component of the summer monsoon winds and modulated by the arrival of Rossby waves generated in the offshore region by cyclonic wind stress curl. Along the west coast of India, upwelling is driven by coastally trapped waves together with the alongshore component of the southwesterlies. Along the southern tip of India and Sri Lanka, the strong Ekman transport dives upwelling. Upwelling is feeble along the east coast of India and occurs during the summer, caused by alongshore winds. In addition, mesoscale eddies lead to upwelling but the arrival of river water plumes inhibits upwelling along this coast. Southeasterly winds drive upwelling along the coast of Sumatra and Java during summer. Kelvin wave propagation originating from the Equatorial Indian Ocean affects the magnitude and extent of Sumatra and Java upwelling. Both ENSO and IOD events cause large variability of upwelling here. Along the west coast of Australia, southerly winds can dominate over the Leeuwin Current, causing sporadic upwelling, which is prominent along the southwest, central, and Gascoyne coasts during summer. The open ocean upwelling in the southern tropical Indian Ocean and within the Sri Lanka Dome is driven primarily by the wind stress curl but also impacted by Rossby wave propagations. Upwelling is a key driver in enhancing biological productivity in all sectors of the coast, as indicated by enhanced sea surface chlorophyll concentrations. Additional knowledge at varying levels has been gained through in situ observations and model simulations. In the Mozambique channel, upwelling simulates new production, and circulation redistributes the production generated by upwelling and mesoscale eddies leading to observations of higher ecosystem impact along the edges of eddies. Similarly, along the southern Madagascar coast, biological connectivity is influenced by the transport of phytoplankton from upwelling zones. Along the coast of Kenya, both productivity rates and zooplankton biomass are higher during the upwelling season. Along the Somali coast, accumulation of upwelled nutrients in the northern part of the coast leads to spatial inhomogeneity in productivity. On the other hand, productivity is more uniform along the coasts of Yemen and Oman. Upwelling along the west coast of India has several biogeochemical implications, including oxygen depletion, denitrification, and high production of CH4 and dimethyl sulfide. Though feeble, wind-driven upwelling leads to significant enhancement of phytoplankton in the northwest Bay of Bengal during the summer monsoon. Along the Sumatra and Java coasts, upwelling affects the phytoplankton composition and assemblages. Dissimilarities in copepod assemblages occur during the upwelling periods along the west coast of Australia. Phytoplankton abundance characterizes inshore edges of the slope during upwelling season, and upwelling eddies are associated with abundance in Krill. The review identifies the northern coast of the Arabian Sea and eastern coasts of the Bay of Bengal as the least observed sectors. Further, sustained long-term observations with high temporal and spatial resolutions along with high-resolution modeling efforts are suggested for a proper description of upwelling, its variability, and its relationship to the ecosystem.

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“…By mid-February the activity of both cyclonic and anticyclonic eddies are responsible for the dispersal of this seed population of Noctiluca eastwards into the central and eastern Arabian Sea, ultimately engulfing the entire northern Arabian Sea (Gomes et al, 2009;Yan et al, 2019). First discovered in the early 2000's (Prakash et al, 2008;Prakash et al, 2017), these Noctiluca blooms have since become increasingly pervasive and widespread in the Arabian Sea, occurring with predictable regularity every year from December to mid of March (do Rosário Gomes et al, 2014;Goes et al, 2016;Lotliker et al, 2018;Prakash et al, 2017;Werdell et al, 2014). At the time the main sediment trap work and the JGOFS-India field studies were carried out (1989 -1997), cyanobacteria dominated the phytoplakton community in the Arabian Sea except during the peak of the upwelling seasons in the western Arabian Sea and during the winter bloom in the northern Arabain Sea (Garrison et al, 1998;Garrison et al, 2000).…”

Section: Noctiluca Bloomsmentioning

confidence: 99%

Present past and future of the OMZ in the northern Indian Ocean

Rixen¹,

Cowie²,

Gaye³

et al. 2020

Preprint

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Abstract. Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~ 59 % of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21 % of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N2) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.

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Revisiting the <i>Noctiluca scintillans</i> Paradox in Northern Arabian Sea

Cited by 31 publications

References 15 publications

Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

Present past and future of the OMZ in the northern Indian Ocean

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