SIBER SPIS sept 2011.pdf - IMBER

SIBER
Science Plan and Implementation Strategy
surface ocean or utilization within the food web. Basic questions regarding, for example, the
role of N 2 fixation for productivity and export flux have received very little attention. There are
very few direct measurements of N 2 fixation rates in the AS and no known measurements in
the BoB. Accordingly, N 2 fixation rates must be determined, the dominant N 2 -fixing organisms
need to be identified and their response to changing environmental conditions studied.
Like N 2 fixation, river and aeolian inputs are considered as external nutrient sources, but also
provide sediment and dust, which act as ballast for sinking particles. Ballast materials reduce
the decomposition of organic material by reducing its residence time in the water column
and could have an enormous impact on the biologically mediated uptake of CO 2 from the
atmosphere (Ittekkot, 1993; Kwon et al., 2009).
In contrast to the AS, the BoB is surrounded by some of the world’s most heavily populated
land (Fig. 4) and will be a “hot spot” for multiple nutrient inputs in the future (Harrison et
al., 2005; Seitzinger et al., 2005). Anoxic events are occurring in many parts of the world
due to eutrophication of the coastal zones (Diaz and Rosenberg, 2008). As pointed out
earlier, extensive hypoxia already develops seasonally over the shelf in the eastern AS, but
whether the same occurs in the BoB, and if so, how it varies seasonally is unknown, as are
the magnitudes of denitrification in shelf and slope waters in the BoB and the Andaman Sea.
Given the large freshwater and nutrient inputs onto the broad shelf in the northern BoB, the
questions are: What are the differences in drainage basin processes influencing the nature of
sediment and nutrient delivery to the two seas and what is the role of the coastal ecosystems
in controlling the fate of nutrients and their environmental impact on the coastal ocean and the
central basins
As opposed to river discharges which mainly affect the coastal ocean, atmospheric deposition,
which is also predicted to increase in the next few decades, mainly affects more offshore
sites (Duce et al., 2008). However, given the large loads of terrestrial particulate matter in
both the AS (e.g. from dust) and the BoB (e.g. from rivers), several questions arise: How do
these differences influence trace metal cycles and how do these cycles in turn impact on the
food web structure and the resulting spatial variability of export fluxes What roles do particle
adsorption/desorption processes play in carbon/nutrient cycles What role does terrestrial
organic matter play, what is the effect of mineral ballast and how does it affect export fluxes
and mid-water oxygen demands
4) What role do the marginal seas play in determining productivity and how do their
outflows affect mid-water oxygen concentrations in the AS and the BoB
In addition to the oxygen consumption caused by remineralization of exported organic material,
ventilation controls the mid-water oxygen concentrations in the OMZs of the AS and BoB,
which is strongly influenced by vertical mixing of different water masses. Boundary current
dynamics and the unique physical dynamics of the equatorial zone are addressed in Themes
1 and 2. The question is: How do marginal seas influence productivity, biogeochemistry and
the OMZs in the two basins In general marginal seas seem to be more important in the AS
compared with the BoB. The Andaman Sea in the BoB is very different from the marginal seas
of the AS in that it is only partially enclosed by the Andaman and Nicobar Island chains. The
ridge topography associated with these islands restricts deep water exchange, i.e. because
the sill depth is around 1.4 km, the deep water (depth > 4 km) in the Andaman Basin is warmer
and less oxygenated than the water at same depth in the BoB. Nevertheless, it is important to
know how these regions communicate with each other, which processes are responsible for
renewal of the Andaman waters and the importance of internal waves in the Andaman Sea. In
the AS, even though the outflows from the Red Sea and the Persian Gulf are < 0.4 Sv, they
exert a notable influence on the intermediate water composition due to their high salinity (> 40
psu in Persian Gulf waters exiting the Straits of Hormuz). What influences do these outflows
have on biogeochemical cycles and ecosystem dynamics in the AS How do these markedly
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