Sarma V. V. S. S.

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Sarma
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V. V. S. S.
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  • Article
    Effects of freshwater stratification on nutrients, dissolved oxygen, and phytoplankton in the Bay of Bengal
    (The Oceanography Society, 2016-06) Sarma, V. V. S. S. ; Rao, G. S. ; 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.
    The Bay of Bengal (BoB) is strongly density stratified due to large freshwater input from various rivers and heavy precipitation. This strong vertical stratification, along with physical processes, regulates the transport and vertical exchange of surface and subsurface water, concentrating nutrients and intensifying the oxygen minimum zone (OMZ). Here, we use basinwide measurements to describe the spatial distributions of nutrients, oxygen, and phytoplankton within the BoB during the 2013 northeast monsoon (November–December). By the time riverine water reaches the interior bay, it is depleted in the nutrients nitrate and phosphate, but not silicate. Layering of freshwater in the northern BoB depresses isopycnals, leading to a deepening of the nutricline and oxycline. Oxygen concentrations in the OMZ are lowest in the north (<5 µM). Weak along-isopycnal nutrient gradients reflect along-isopycnal stirring between ventilated surface water and deep nutrient-replenished water. Picoplankton dominate the phytoplankton population in the north, presumably outcompeting larger phytoplankton species due to their low nutrient requirements. Micro- and nanoplankton numbers are enhanced in regions with deeper mixed layers and weaker stratification, where nutrient replenishment from subsurface waters is more feasible. These are also the regions where marine mammals were sighted. Physical processes and the temperature-salinity structure in the BoB directly influence the OMZ and the depth of the oxycline and nutricline, thereby affecting the phytoplankton and marine mammal communities.
  • Article
    Sea–air CO2 fluxes in the Indian Ocean between 1990 and 2009
    (Copernicus Publications on behalf of the European Geosciences Union, 2013-11-06) Sarma, V. V. S. S. ; Lenton, Andrew ; Law, R. M. ; Metzl, Nicolas ; Patra, Prabir K. ; Doney, Scott C. ; Lima, Ivan D. ; Dlugokencky, Edward J. ; Ramonet, M. ; Valsala, V.
    The Indian Ocean (44° S–30° N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea–air CO2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea–air CO2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea–air CO2 uptake of −0.37 ± 0.06 PgC yr−1 is consistent with the −0.24 ± 0.12 PgC yr−1 calculated from observations. The fluxes from the southern Indian Ocean (18–44° S; −0.43 ± 0.07 PgC yr−1 are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea–air CO2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990–2009 of −0.01 PgC decade−1. This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO2 was faster than the observed atmospheric CO2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.
  • Article
    Newly discovered deep-branching marine plastid lineages are numerically rare but globally distributed
    (Elsevier, 2017-01-09) Choi, Chang Jae ; Bachy, Charles ; Spiro Jaeger, Gualtiero ; Poirier, Camille ; Sudek, Lisa ; Sarma, V. V. S. S. ; Mahadevan, Amala ; Giovannoni, Stephen J. ; Worden, Alexandra Z.
    Ocean surface warming is resulting in an expansion of stratified, low-nutrient environments, a process referred to as ocean desertification. A challenge for assessing the impact of these changes is the lack of robust baseline information on the biological communities that carry out marine photosynthesis. Phytoplankton perform half of global biological CO2 uptake, fuel marine food chains, and include diverse eukaryotic algae that have photosynthetic organelles (plastids) acquired through multiple evolutionary events. While amassing data from ocean ecosystems for the Baselines Initiative (6,177 near full-length 16S rRNA gene sequences and 9.4 million high-quality 16S V1-V2 amplicons) we identified two deep-branching plastid lineages based on 16S rRNA gene data. The two lineages have global distributions, but do not correspond to known phytoplankton. How the newly discovered phytoplankton lineages contribute to food chains and vertical carbon export to the deep sea remains unknown, but their prevalence in expanding, low nutrient surface waters suggests they will have a role in future oceans.
  • Article
    The Bay of Bengal exposes abundant photosynthetic picoplankton and newfound diversity along salinity‐driven gradients.
    (Wiley, 2023-06-13) Strauss, Jan ; Choi, Chang Jae ; Grone, Jonathan ; Wittmers, Fabian ; Jimenez, Valeria ; Makareviciute-Fichtner, Kriste ; Bachy, Charles ; Spiro Jaeger, Gualtiero ; Poirier, Camille ; Eckmann, Charlotte A. ; Spezzano, Rachele ; Loscher, Carolin R. ; Sarma, V. V. S. S. ; Mahadevan, Amala ; Worden, Alexandra Z.
    The Bay of Bengal (BoB) is a 2,600,000 km2 expanse in the Indian Ocean upon which many humans rely. However, the primary producers underpinning food chains here remain poorly characterized. We examined phytoplankton abundance and diversity along strong BoB latitudinal and vertical salinity gradients—which have low temperature variation (27–29°C) between the surface and subsurface chlorophyll maximum (SCM). In surface waters, Prochlorococcus averaged 11.7 ± 4.4 × 104 cells ml−1, predominantly HLII, whereas LLII and ‘rare’ ecotypes, HLVI and LLVII, dominated in the SCM. Synechococcus averaged 8.4 ± 2.3 × 104 cells ml−1 in the surface, declined rapidly with depth, and population structure of dominant Clade II differed between surface and SCM; Clade X was notable at both depths. Across all sites, Ostreococcus Clade OII dominated SCM eukaryotes whereas communities differentiated strongly moving from Arabian Sea-influenced high salinity (southerly; prasinophytes) to freshwater-influenced low salinity (northerly; stramenopiles, specifically, diatoms, pelagophytes, and dictyochophytes, plus the prasinophyte Micromonas) surface waters. Eukaryotic phytoplankton peaked in the south (1.9 × 104 cells ml−1, surface) where a novel Ostreococcus was revealed, named here Ostreococcus bengalensis. We expose dominance of a single picoeukaryote and hitherto ‘rare’ picocyanobacteria at depth in this complex ecosystem where studies suggest picoplankton are replacing larger phytoplankton due to climate change.