SIBER SPIS sept 2011.pdf - IMBER

SIBER
Science Plan and Implementation Strategy
Moreover, the degree to which Fe limitation is active over the entire basin, and any linkages
of its spatio-temporal variability to monsoonal forcing, are open questions (Behrenfeld et al.,
2009; Mackie et al., 2008; Wiggert et al., 2006; Piketh et al., 2000; Mc Gowan et al., 2000).
There are profound physical and biogeochemical differences between the AS and the BoB.
The AS is a globally important zone of open ocean denitrification (Naqvi et al., 2005), where
NO 3 and NO 2 are converted to N 2 O and N 2 gas, which are then released to the atmosphere
(Fig. 3). Thus, denitrification removes N from the ocean and generates N 2 O, which is a
prominent greenhouse gas (Ramaswamy et al., 2001). This process occurs in oxygendepleted
subsurface waters (200–800m) in the eastern-central AS and contributes ~20% to
global open ocean denitrification (Codispoti et al., 2001). In contrast, mesopelagic dissolved
oxygen concentrations in the BoB are slightly higher so it remains poised just above the
denitrification threshold. What are the relative roles of biological oxygen demand from surface
organic matter export versus circulation and ventilation in maintaining subtle differences in the
deep oxygen field along with the profound differences in biogeochemical cycling in these two
regions How have these differences been maintained over the last few decades and how are
they likely to change in response to global climate change Recent modeling studies suggest
that OMZs will expand in response to global warming (Doney, 2010; Stramma et al., 2008),
but uncertainties in model predictions are large, especially in the IO where global simulation
models fail to reproduce the observed oxygen distributions. It should also be noted that the
northern IO contains about two thirds of the global continental margin area in contact with
oxygen deficient (O 2 < 0.3 ml l-1) water (Codispoti et al., 2001), which is expected to expand
and significantly impact benthic biogeochemical and ecological processes. Yet currently, rather
little is known about these low oxygen impacts (Cowie, 2005).
The IO may also be a globally important zone of nitrogen fixation, where N 2 is split by
diazotrophic cyanobacteria and converted to ammonium that can be readily utilized by
phytoplankton. It has been estimated that 30-40% of euphotic zone nitrate in the AS is derived
from N 2 fixation (Brandes et al., 1998) and this region’s annual input of new N via this process
has been estimated to be 3.3 Tg N yr-1 (Bange et al., 2005). However, there are very few direct
N 2 fixation rate measurements from the IO. Thus, while it is agreed that the IO plays important
roles in the global N cycle and budget, there is still not enough information to quantify the net
atmosphere - ocean N flux in this basin.
Gl o b a l c h a n g e a n d a n t h r o p o g e n i c i m p a c t s
The AS and BoB differ markedly in terms of freshwater flux and terrestrial nutrient loading
(Seitzinger et al., 2005). The AS experiences net evaporation, whereas the BoB has large
freshwater inputs from direct rainfall and surrounding major river systems such as the Ganges-
Brahmaputra complex (Fig. 4). Coastal environments in the BoB are particularly vulnerable due
to high river nutrient loadings in surrounding countries that are experiencing rapid increases
in population density and economic growth (Millennium Ecosystem Assessment, 2005).
Cholera in Bangladesh has already been linked to changes in sea surface temperature and
height (Lobitz et al., 2000). To what extent are coastal environments in the BoB impacted by
anthropogenic effects How will they be impacted in the future What are the potential human
consequences
Overall the AS is a source of CO 2 to the atmosphere because of elevated pCO 2 within
the SWM-driven upwelling (Fig. 5). Whether the BoB is a CO 2 source or sink remains illdefined
due to sparse sampling in both space and time (Bates et al., 2006a). The southern IO
appears to be a strong net CO 2 sink, but the factors that maintain this sink are unclear; cold
temperatures certainly increase CO 2 solubility in the austral winter, but there is also evidence
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