Effects of Freshwater Stratification on Nutrients, Dissolved Oxygen, and Phytoplankton in the Bay of Bengal

Sarma, V. V. S. S.; Rao, G. S. Prasada; Viswanadham, R.; Sherin, C. K.; Salisbury, Joseph E.; Omand, Melissa M.; Mahadevan, Amala; Murty, V. S. N.; Shroyer, Emily L.; Baumgartner, Mark F.; Stafford, Kathleen M.

doi:10.5670/oceanog.2016.54

Cited by 95 publications

(48 citation statements)

References 25 publications

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“…Recent primary production results from BoB (Anand et al, 2017 and this study) are comparable to that of the Arabian Sea (1,032 ± 260 mgC·m −2 ·d −1 , Barber et al, 2001; 924 ± 296 mgC·m −2 ·d −1 , Dickson et al, 2001; and 1752 ± 726 mgC·m −2 ·d −1 , Anand et al, 2018). In contrast, the present study suggests that higher primary production, supported by organic nutrients, is responsible for higher sinking carbon fluxes in the BoB; however, mineral ballast may be possible in the northern Bay where higher suspended load is observed (Ittekkot et al, 1991;Sarma et al, 2016). The present study further suggests that 40 to 70% of the primary production in the BoB occurs in the upper 25 m of water column where significant amount of organic nutrients is available indicating that organic nutrients are acting as an alternative source of nutrients, in the absence of inorganic nutrients.…”

Section: Possible Underestimation Of Primary Production In the Bobcontrasting

confidence: 76%

Organic Nutrients Support High Primary Production in the Bay of Bengal

Sridevi

Rao

Rajula

et al. 2019

Geophysical Research Letters

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The Bay of Bengal (BoB) is known to experience low productivity but high sinking carbon fluxes to the bottom, and this paradox is attributed to mineral ballast of organic matter. We found for the first time that primary production in the BoB is higher, and it is supported by dissolved organic nitrogen (DON) and phosphorus (DOP) transported from the shelf regions through eddies. Both DON and DOP contribute up to 70–99% to the total dissolved nutrients in the waters above the thermocline. DON and DOP displayed positive relationship with primary production in the upper 25 m of water column, suggesting that organic nutrients significantly support primary producers. Primary production and export production in the BoB is comparable to that in the Arabian Sea, in contrast to the earlier belief that it has low production due to lack of inorganic nutrients caused by strong salinity stratification.

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Section: Possible Underestimation Of Primary Production In the Bobcontrasting

confidence: 76%

Organic Nutrients Support High Primary Production in the Bay of Bengal

Sridevi

Rao

Rajula

et al. 2019

Geophysical Research Letters

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“…The episodic, high oxygen events in the BoB are likely driven by mesoscale eddies. Previous modeling (Lachkar et al, 2016;Resplandy et al, 2012) and observational (Kumar et al, 2004;Sardessai et al, 2007;Sarma et al, 2016;Sarma & Bhaskar, 2018) studies all point to the importance of eddies in controlling oxygen levels in these environments. For example, the depth of the oxycline in both the AS and BoB is strongly related to sea surface height variations driven by mesoscale eddies (Prakash et al, 2013).…”

Section: Resultsmentioning

confidence: 99%

Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

Johnson

Riser

Ravichandran

2019

Geophysical Research Letters

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Nitrate limits productivity in much of the ocean. Nitrate residence time is a few thousand years, and changes in nitrate loss could influence ocean productivity. Major sinks for nitrate are denitrification and anaerobic ammonia oxidation in the oxygen minimum zones (OMZs). The Bay of Bengal OMZ is anomalous because large amounts of nitrate loss do not occur there, while nitrate is removed in the nearby OMZ of the Arabian Sea. Observations of nitrate and oxygen made over 5 years by 20 profiling floats equipped with chemical sensors in the Bay of Bengal and the Arabian Sea are used to understand why nitrate is removed rapidly in the Arabian Sea but not in the Bay of Bengal. Our results confirm that nitrate is poised for removal in the Bay of Bengal. However, highly variable oxygen concentrations inhibit its loss. Nitrate loss is regulated by physical oceanographic processes that introduce oxygen.

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“…For example, based on a sediment trap study in the northwest Pacific Ocean, Hung et al () demonstrated that sinking particles contained abundant diatom cells, fecal pellets, detritus, and small particle aggregates mostly <50 μm that can account for a major fraction of the bulk sinking particles in open ocean regions and 234 Th was mainly carried by the smaller (<50 μm) sinking particles. A study by Sarma et al () suggested that stratification in the Bay of Bengal significantly influenced both (1) the input of nutrients to the sunlit zone, resulting in differences in phytoplankton biomass and (2) size structure in the picoplankton dominating the phytoplankton population in the Bay due to limited supply of nutrients and low nutrient requirements. Similarly in the Arabian Sea, the incoming short‐wave radiation was maximum during spring intermonsoon.…”

Section: Methodsmentioning

confidence: 99%

²³⁴Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean

Anand

Rengarajan

Sarma

2018

Global Biogeochemical Cycles

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234Th (t1/2 = 24.1 d), present in seawater, is a naturally occurring particle‐reactive radionuclide formed through the radioactive decay of its parent, 238U (t1/2 = 4.47 × 109 years). The 234Th:238U disequilibrium is exploited to quantify fluxes of elements moving out of the euphotic zone by attaching on to sinking particles. Under the Indian GEOTRACES programme, high‐resolution sampling in the upper 300 m depth was carried out at 11 stations in the Arabian Sea and the Indian Ocean during April–May 2014 from 17°N to 16°S along 65°E transect to estimate the 234Th‐based particulate organic carbon (POC) export flux from the upper ocean. Average 234Th fluxes integrated to 100 m depth were 2,612 and 1,968 dpm m−2 d−1 for the Arabian Sea and the Indian Ocean, respectively. The estimated POC export fluxes ranged from negligible to 9.0 mmol m−2 d−1, and the 234Th‐based POC export efficiencies were <2 to 5%. For the same season, the Arabian Sea and the Bay of Bengal showed highly contrasting carbon export trends (mean: 4.0 and 0.8 mmol C m−2 d−1, respectively). The modeled POC export fluxes from in situ and satellite‐derived primary production are higher than the 234Th‐based values for the Laws and Dunne models and are comparable for the Henson model. The modeled POC fluxes that depend on surface temperature and primary production could be further refined for the seasonal cycle in biological productivity and associated differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity, and associated dissolved organic carbon (DOC) export.

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Effects of Freshwater Stratification on Nutrients, Dissolved Oxygen, and Phytoplankton in the Bay of Bengal

Cited by 95 publications

References 25 publications

Organic Nutrients Support High Primary Production in the Bay of Bengal

Organic Nutrients Support High Primary Production in the Bay of Bengal

Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

²³⁴Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean

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Effects of Freshwater Stratification on Nutrients, Dissolved Oxygen, and Phytoplankton in the Bay of Bengal

Cited by 95 publications

References 25 publications

Organic Nutrients Support High Primary Production in the Bay of Bengal

Organic Nutrients Support High Primary Production in the Bay of Bengal

Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

234Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean

Contact Info

Product

Resources

About

²³⁴Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean