Babula Jena

Occurrence of Nitrogen Fixing Cyanobacterium Trichodesmium under Elevated pCO2 Conditions in the Western Bay of Bengal

Recent studies on the diazotrophic cyanobacterium Trichodesmium showed that increasing CO2 partial pressure (pCO2) enhances N2 fixation and growth. We studied the in situ and satellite-derived environmental parameters within and outside a Trichodesmium bloom in the western coastal Bay of Bengal (BoB) during the spring intermonsoon 2009. Here we show that the single most important nitrogen fixer in today’s ocean, Trichodesmium erythraeum, is strongly abundant in high (≥300 μatm) pCO2 concentrations. N : P ratios almost doubled (~10) at high pCO2 region. This could enhance the productivity of N-limited BoB and increase the biological carbon sequestration. We also report presence of an oxygen minimum zone at Thamnapatnam. Earlier studies have been carried out using lab cultures, showing the increase in growth rate of T. erythraeum under elevated pCO2 conditions, but to our knowledge, this study is the first to report that in natural environment also T. erythraeum prefers blooming in high pCO2 concentrations. The observed CO2 sensitivity of T. erythraeum could thereby provide a strong negative feedback to rising atmospheric CO2 but would also drive towards phosphorus limitation in a future high CO2 world.

Application of Artificial Neural Networks for Sea-Surface Wind-Speed Retrieval From IRS-P4 (MSMR) Brightness Temperature

Indian Remote Sensing Satellite Multifrequency Scanning Microwave Radiometer (MSMR)-measured brightness temperatures (TB) in 6.6-, 10.65-, 18-, and 21-GHz channels with dual polarizations were utilized to retrieve sea-surface wind speed (SSWS). A concurrent and collocated database was constructed on MSMR TB- and deep-sea (DS)-buoy-recorded wind speeds for the period of June 1999-July 2001 over the north Indian Ocean. A radial-basis-function-based artificial-neural-network (ANN) algorithm was developed to estimate SSWS from MSMR TB values. Multiple ANNs were generated by the systematic variation of the architecture of input- and hidden-layer nodes. The performance of the most successful algorithm was evaluated based on statistical summary and network performance. The accuracy of the ANN-based wind-speed algorithm was compared with DS-buoy observations, and the result was then compared with the output of the regression analysis between buoy- and MSMR operational-global-retrieval-algorithm (OGRA)-derived SSWS values. On the average, 84% (92%) of ANN-estimated MSMR SSWS observations are within ±2 m/s ( ±3 m/s) when compared with DS-buoy observations. The correlation and root mean square error of 0.80 and 1.79 m/s, respectively, for ANN-predicted SSWS values are much better than that obtained from OGRA. The performance of the ANN algorithm was also evaluated during a super cyclone (October 1999) over the Bay of Bengal. The ANN algorithm could capture the high cyclonic winds, and the values match reasonably well with Special Sensor Microwave/Imager and SeaWinds Scatterometer (QuikSCAT) operational wind products.

Contrasting productivity and redox potential in Arabian Sea and Bay of Bengal

Understanding the past and present changes is critical for evaluating the future climatic changes. In order to understand the paleoproductivity and depositional environments of Northern Indian Ocean, two sediment cores were collected, one each from the Arabian Sea (lat. 16°51.40′N and long. 71°54.37′E, water depth 803 m) and the Bay of Bengal (lat. 13°05.35′N and long. 91°28.21′E, water depth 3 054 m). The surface seawater samples indicate higher pCO2 values in Arabian Sea as compared to the Bay of Bengal. The sediment organic carbon variations along with sedimentological and other geochemical parameters were studied. Sediment organic carbon varied from 0.5%–4.7% and 0.3%–1.22% in Arabian Sea and the Bay of Bengal, respectively. In Arabian Sea, low productivity, oxic conditions and less intense southwest monsoon prevailed during the deglacial period, whereas productivity has increased from last 16 kyr to the modern age. In the Bay of Bengal, organic carbon decreased from the Last Glacial Maxima (LGM) to the modern age, indicating higher productivity in the past as compared to modern age. Fe was associated with organic carbon in the Bay of Bengal and increased during LGM, showing similar trend to that of organic carbon, indicating that Fe may be the limiting factor for the growth of phytoplankton in the Bay of Bengal in the modern age. In the Bay of Bengal, Mn is enriched during modern age and is depleted during LGM, whereas chromium showed the opposite trend indicating anoxic conditions during the LGM, whereas in Arabian Sea the trends are opposite to the Bay of Bengal.

Shifting of phytoplankton community in the frontal regions of Indian Ocean sector of the Southern Ocean using in situ and satellite data

Abstract. The phytoplankton pigment indices were used to characterize the spatial succession of the community composition in the frontal regions of the subtropical front (STF), sub-Antarctic front (SAF), and polar front (PF) in the Indian Ocean sector of the Southern Ocean during austral summer 2013. Diagnostic indices revealed that the flagellates were dominant in STF (51%) and progressively declined toward SAF (39%) and PF (11%). Similarly, the prokaryotes were highest in STF (43%) and decreased to SAF (32%) and PF (28%). In contrast, the diatoms were gradually increased from STF (6%) to SAF (29%) and PF (61%). The variability of flagellates and diatoms from the STF to PF is attributed to the variability of photosynthetically available radiation, sea surface temperature, and sea surface wind speed. The in-situ pigment indices were then compared to the NASA Ocean Biogeochemical model that shows the similar patterns of frontal community distribution except their magnitude. Similarly, the satellite retrieved phytoplankton biomass (chlorophyll a) was checked for its consistency after comparing with the in-situ observations and the result shows underestimation of satellite measured values. The result suggests that the conjunctive analysis of in-situ, satellite, and model archive is suitable to study the impact of climate variability on the structure of marine ecosystems.

The effect of phytoplankton pigment composition and packaging on the retrieval of chlorophyll-a concentration from satellite observations in the Southern Ocean

ABSTRACT The Antarctic waters are known to be optically unique and the standard empirical ocean colour algorithms applied to these waters may not address the regional bio-optical characteristics. This article sheds light on the performance of current empirical algorithms and a regionally optimized algorithm (ROA) for the retrieval of chlorophyll-a (chl-a) concentration from Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) in the Indian Ocean Sector of Southern Ocean (IOSO). Analysis indicated that empirical algorithms used for the retrieval of chl-a concentration from Aqua-MODIS and SeaWiFS underestimate by a factor varying from 2 to 2.9, resulting in underestimation when in situ chl-a exceeds about 0.3 mg m−3. To explain these uncertainties, a study was carried out to understand the effect of phytoplankton pigment composition and pigment packaging on remote-sensing reflectance (Rrs,λ), from the analysis of phytoplankton-specific absorption coefficient (aph,*λ). The spatial variation of phytoplankton groups analysed using diagnostics pigments (DP) indicated shifting of the phytoplankton community structure from offshore to coastal Antarctic, with a significant increasing trend for diatoms and a decreasing trend for haptophytes population. The diatom-dominated population exhibits lower aph,*λ in the 405–510 nm region (with relative flattening in 443–489 nm) compared with the aph,*λ spectra of the haptophytes-dominated population that peaks near 443 nm. The flattening of aph,*λ spectra for the diatom-dominated population was attributed to its larger cell size, which leads to pigment packaging (intracellular shading) and in turn results in higher Rrs,λ. The relationship between pigment composition (normalized by chl-a) and blue:green absorption band ratios (aph,*443:aph,*555 and aph,*489:aph,*555) corresponding to the Aqua-MODIS and SeaWiFS bands showed in-phase associations with most of the pigments such as 19ʹ-hexanoyloxyfucoxanthin, 19ʹ-butanoyloxyfucoxanthin, peridinin, and zeaxanthin. In contrast, the out-of-phase association observed between the blue:green absorption ratios and fucoxanthin indicated apparent deviations from the general pigment retrieval algorithms, which assumes that blue:green ratios vary in a systematic form with chl-a. The out-of-phase correspondence suggests that the increasing trend of fucoxanthin pigments towards the Antarctic coast was associated with the decreasing trend of blue:green absorption ratios and in turn results in higher Rrs,λ. Therefore, an increase in Rrs,λ leads to underestimation of chl-a from Aqua-MODIS and SeaWiFS in the IOSO region.

Trends in sea ice extent as observed from microwave remote sensing

Changes in sea ice extent are strong modulators of climate change as well as indicator of the effect of global warming. The ocean-atmospheric heat budget is also affected by the sea ice dynamics owing to reflection of solar radiation back to the space by sea ice cover. In the present work, we have analysed about 30 years of sea ice data (1981–2013) over the Antarctic and Arctic regions using 25 km spatial resolution re-gridded data products obtained from National Snow and Ice Data Centre (NSIDC), USA. These sea ice data products result from combined observations by the Scanning Multi-channel Microwave Radiometer (SMMR) on Nimbus-7 platform and a series of Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) instruments on board the Defense Meteorological Satellite Program (DMSP) satellites.

A comparative assessment of IRS-P4 MSMR-derived sea surface temperature and sea surface wind speed over the north Indian Ocean

Indian Remote Sensing Satellite (IRS-P4) multi-frequency scanning microwave radiometer (MSMR) provides geophysical parameters like sea surface temperature (SST), sea surface wind speed (SSWS), integrated water vapour (IWV) and cloud liquid water (CLW). The retrieval procedure of these parameters given by Gohil et al. (2000, Geophysical parameter retrieval over global oceans from IRS-P4 (MSMR). In Preprints, Fifth Pacific Ocean Remote Sensing Conference, 5–8 December 2000, Goa, India (Goa: National Institute of Oceanography), pp. 207–211) was summarized by Sharma et al. (2002, Identification of large scale atmospheric and oceanic features from IRS-P4 multifrequency scanning microwave radiometer: preliminary results. Journal of Atmospheric and Oceanic Technology, 19, pp. 1127–1134) and Jena (2007, Studies on the retrieval, validation and applications of geophysical parameters from IRS-P4 (MSMR) data. PhD thesis, Berhampur University, Orissa). Demonstration of self-consistency of these parameters has primary scientific importance. This article deals with the validation of MSMR geophysical parameters such as SST and SSWS with in situ observations (buoy data) over the north Indian Ocean during 2000. Result shows that the MSMR-derived SST and SSWS can be utilized for several applications because of their reasonable accuracy and coverage even under cloudy condition.