Ajit Subramaniam

Causes and impacts of the 2005 Amazon drought

A rare drought in the Amazon culminated in 2005, leading to near record-low streamflows, small Amazon river plume, and greatly enhanced fire frequency. This episode was caused by the combination of 2002-03 El Nino and a dry spell in 2005 attributable to a warm subtropical North Atlantic Ocean. Analysis for 1979-2005 reveals that the Atlantic influence is comparable to the better-known Pacific linkage. While the Pacific influence is typically locked to the wet season, the 2005 Atlantic impact concentrated in the Amazon dry season when its hydroecosystem is most vulnerable. Such mechanisms may have wide-ranging implications for the future of the Amazon rainforest. S Supplementary data are available from stacks.iop.org/ERL/3/014002

Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N2 fixation far from the mouth and provides important pathways for sequestration of atmospheric CO2 in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr−1 of the new production supported by NO3 delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2.5 Tmol of C to the atmosphere [Mikaloff-Fletcher SE, et al. (2007) Inverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport. Global Biogeochem Cycles 21, doi:10.1029/2006GB002751]. The enhancement of N2 fixation and consequent C sequestration by tropical rivers appears to be a global phenomenon that is likely to be influenced by anthropogenic activity and climate change.

The United States' next generation of atmospheric composition and coastal ecosystem measurements : NASA's Geostationary Coastal and Air Pollution Events (GEO-CAPE) Mission

The Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission was recommended by the National Research Councils (NRCs) Earth Science Decadal Survey to measure tropospheric trace gases and aerosols and coastal ocean phytoplankton, water quality, and biogeochemistry from geostationary orbit, providing continuous observations within the field of view. To fulfill the mandate and address the challenge put forth by the NRC, two GEO-CAPE Science Working Groups (SWGs), representing the atmospheric composition and ocean color disciplines, have developed realistic science objectives using input drawn from several community workshops. The GEO-CAPE mission will take advantage of this revolutionary advance in temporal frequency for both of these disciplines. Multiple observations per day are required to explore the physical, chemical, and dynamical processes that determine tropospheric composition and air quality over spatial scales ranging from urban to continental, and over temporal scales ranging from diu...

Influence of the Amazon River on the surface optical properties of the western tropical North Atlantic Ocean

The distribution of chromophoric-dissolved-organic-matter (CDOM) was investigatedin the western tropical North Atlantic Ocean (WTNA) during the low and high flowperiods of the Amazon River. A strong correlation was observed between in situ CDOMabsorption at 490 nm and in situ diffuse attenuation coefficient at 490 nm (K490) duringthe high flow period (ratio of 0.7). SeaWiFS monthly composites of K490 showed (1) verylow values over most of the WTNA, with higher values restricted to the continental shelfduring the low flow season and (2) higher K490 over much of the WTNA extendingoffshore to >1000 km from the river mouth during the high flow season. The relativecontribution to light absorption by particulate (phytoplankton and detritus) and dissolved(CDOM) materials was investigated at selected wavelengths. At 440 nm during the highflow period (1) CDOM dominated ( 70%) total light absorption closer to the river mouth;(2) phytoplankton and detritus dominated ( 40% each) absorption farther offshore butstill within the Amazon plume; and (3) CDOM contributed 20%, phytoplankton >60%,and detritus 10% outside the plume. In situ data suggests a terrestrial source for theCDOM off the coast of South America during the high flow period with possible offshorelocal sinks (photodegradation) as well as sources (in situ production). The Amazon Riverstrongly influences the optical properties of the WTNA at distances over 1000 kmfrom the river mouth. Retrievals of phytoplankton biomass based on satellite ocean colordata can be largely overestimated in the WTNA, if the contribution to light absorptionby CDOM and detritus is not taken into account.

An improved bio‐optical model for the remote sensing of Trichodesmium spp. blooms

L � 1 . Using the in situ data set, this model is trained to successfully predict Trichodesmium blooms (� 92%) while minimizing false positive retrievals (� 16% of nonbloom observations). A validation of the approach applied to Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery shows that the model correctly predicts 76% of the bloom occurrences of an independent validation data set of in situ Trichodesmium observations. Ultimately, maps of Trichodesmium bloom occurrence will provide a means of addressing the ecology of Trichodesmium and its contribution to new production of the world oceans.

Evolution of the Macondo well blowout: Simulating the effects of the circulation and synthetic dispersants on the subsea oil transport

During the Deepwater Horizon incident, crude oil flowed into the Gulf of Mexico from 1522 m underwater. In an effort to prevent the oil from rising to the surface, synthetic dispersants were applied at the wellhead. However, uncertainties in the formation of oil droplets and difficulties in measuring their size in the water column, complicated further assessment of the potential effect of the dispersant on the subsea-to-surface oil partition. We adapted a coupled hydrodynamic and stochastic buoyant particle-tracking model to the transport and fate of hydrocarbon fractions and simulated the far-field transport of the oil from the intrusion depth. The evaluated model represented a baseline for numerical experiments where we varied the distributions of particle sizes and thus oil mass. The experiments allowed to quantify the relative effects of chemical dispersion, vertical currents, and inertial buoyancy motion on oil rise velocities. We present a plausible model scenario, where some oil is trapped at depth through shear emulsification due to the particular conditions of the Macondo blowout. Assuming effective mixing of the synthetic dispersants at the wellhead, the model indicates that the submerged oil mass is shifted deeper, decreasing only marginally the amount of oil surfacing. In this scenario, the oil rises slowly to the surface or stays immersed. This suggests that other mechanisms may have contributed to the rapid surfacing of oil-gas mixture observed initially. The study also reveals local topographic and hydrodynamic processes that influence the oil transport in eddies and multiple layers. This numerical approach provides novel insights on oil transport mechanisms from deep blowouts and on gauging the subsea use of synthetic dispersant in mitigating coastal damage.

Isolation by environmental distance in mobile marine species: molecular ecology of franciscana dolphins at their southern range

The assessment of population structure is a valuable tool for studying the ecology of endangered species and drafting conservation strategies. As we enhance our understanding about the structuring of natural populations, it becomes important that we also understand the processes behind these patterns. However, there are few rigorous assessments of the influence of environmental factors on genetic patterns in mobile marine species. Given their dispersal capabilities and localized habitat preferences, coastal cetaceans are adequate study species for evaluating environmental effects on marine population structure. The franciscana dolphin, a rare coastal cetacean endemic to the Western South Atlantic, was studied to examine these issues. We analysed genetic data from the mitochondrial DNA and 12 microsatellite markers for 275 franciscana samples utilizing frequency‐based, maximum‐likelihood and Bayesian algorithms to assess population structure and migration patterns. This information was combined with 10 years of remote sensing environmental data (chlorophyll concentration, water turbidity and surface temperature). Our analyses show the occurrence of genetically isolated populations within Argentina, in areas that are environmentally distinct. Combined evidence of genetic and environmental structure suggests that isolation by distance and a process here termed isolation by environmental distance can explain the observed correlations. Our approach elucidated important ecological and conservation aspects of franciscana dolphins, and has the potential to increase our understanding of ecological processes influencing genetic patterns in other marine species.

Nitrogen fixation by Trichodesmium spp. and unicellular diazotrophs in the North Pacific Subtropical Gyre

unicellular diazotrophs were responsible for activity there. Our studies show a geographical variation in the dominant diazotroph in the North Pacific Subtropical Gyre in the summer with Trichodesmium being dominant around the Hawaiian Islands, Richelia associated with diatoms to be found in high numbers to the south of the islands while unicellular diazotrophs dominated to the west, away from the islands and evidence from the literature suggests iron may play a role.

Molecular ecology meets remote sensing: environmental drivers to population structure of humpback dolphins in the Western Indian Ocean

Genetic analyses of population structure can be placed in explicit environmental contexts if appropriate environmental data are available. Here, we use high-coverage and high-resolution oceanographic and genetic sequence data to assess population structure patterns and their potential environmental influences for humpback dolphins in the Western Indian Ocean. We analyzed mitochondrial DNA data from 94 dolphins from the coasts of South Africa, Mozambique, Tanzania and Oman, employing frequency-based and maximum-likelihood algorithms to assess population structure and migration patterns. The genetic data were combined with 13 years of remote sensing oceanographic data of variables known to influence cetacean dispersal and population structure. Our analyses show strong and highly significant genetic structure between all putative populations, except for those in South Africa and Mozambique. Interestingly, the oceanographic data display marked environmental heterogeneity between all sampling areas and a degree of overlap between South Africa and Mozambique. Our combined analyses therefore suggest the occurrence of genetically isolated populations of humpback dolphins in areas that are environmentally distinct. This study highlights the utility of molecular tools in combination with high-resolution and high-coverage environmental data to address questions not only pertaining to genetic population structure, but also to relevant ecological processes in marine species.

Decadal time‐series of SeaWiFS retrieved CDOM absorption and estimated CO2 photoproduction on the continental shelf of the eastern United States

[1] Published algorithms were employed to convert SeaWiFS images of normalized water-leaving-radiance to absorption images of CDOM (chromophoric dissolved organic matter). The best performing algorithm was employed to produce decadal time-series of CDOM monthly composites from 1998 through 2007. Deficits in CDOM absorption coefficient for surface waters across the shelf over the summer were then acquired relative to the uniformly mixed waters prior to and following stratification (spring and fall, respectively). Estimates were attained of the photochemical oxidation ofcarbontoCO2onandbeyondthe shelf of the Middle Atlantic Bight. Approximately 3–7 � 10 10 g C as CO2 were estimated to be produced via photooxidation of CDOM over the summertime, highlighting the significance of CDOM photochemistry and pointing out the importance of CO2 photoproduction at a global scale. In principle, this approach could be applied to global ocean color data. Citation: Del Vecchio, R., A. Subramaniam, S. Schollaert Uz, J. Ballabrera-Poy, C. W. Brown, and N. V. Blough (2009), Decadal time-series of SeaWiFS retrieved CDOM absorption and estimated CO2 photoproduction on the continental shelf of the eastern United States, Geophys. Res. Lett., 36, L02602,