Exploring the Nitrogen Cycle in the Ganges-Brahmaputra Estuary
Valier Galy, Marine Chemistry & Geochemistry
Nitrogen (N) is one of the major nutrients in the ocean and plays a crucial role in biogeochemical
cycles. In the coastal ocean, the chemical speciation and availability of N control both water
quality and primary productivity. Yet, the input of organic N from rivers to the ocean has not
been precisely quantified and the behavior of the different chemical species of riverine N in the
coastal ocean is still poorly understood. The study of N cycling in estuaries and coastal waters is
therefore a major scientific and societal endeavor.
Here, I propose to study N cycling in the estuary of one of the largest suppliers of sediments and
water to the ocean, the Ganges-Brahmaputra (G-B) River system. Preliminary measurements of
particulate N concentration and isotopic composition indicate a strong increase in particulate
organic N loading and 15N depletion from the Ganges-Brahmaputra Rivers to the estuary and
shelf sediments, suggesting intense dissolved inorganic N assimilation in the estuary by
autochthonous heterotrophic organic matter production.
This project seeks to analyze sediments and water samples collected along a transect across the
estuary from the confluence of the Ganges and Brahmaputra Rivers to the Bengal Shelf. The
evolution of the concentration and isotopic composition of different N species across the salinity
gradient will be used to follow N assimilation in the estuary. The abundance and isotopic
composition of lipids derived either from vascular plants or autochthonous heterotrophic organic
matter will be used to infer changes in the composition of the organic matter pool.
The two major scientific outcomes of this study will be 1) a quantification of the discharge of
riverine N (dissolved inorganic N, particulate organic N) into the Bay of Bengal and, 2) a precise
characterization (speciation and isotopic composition) of the riverine N input to the coastal
ocean. These are important scientific goals with implications for 1) the understanding of the
global N cycle and, 2) the interpretation of the sedimentary record of particulate N, specifically
C/N and 15N variations.