SIBER SPIS sept 2011.pdf - IMBER

SIBER
Science Plan and Implementation Strategy
As a result of these differences in monsoonal forcing there are profound biogeochemical
differences between the AS and BoB as indicated, for example, by export fluxes from the
euphotic zone (Rixen et al., 2005; Rixen and Ittekkot, 2007). Contrary to the AS where
upwelling during the SWM and convective mixing during the NEM produce a clear monsoonsynced
signal in export flux, high freshwater inputs produce a rather diffused seasonal pattern
for export flux in the BoB (Fig. 16).
Furthermore, there are small but critical differences in intermediate water oxygen concentrations
(Fig. 3). In the AS, oxygen is almost completely depleted from a wide mesopelagic layer,
and bacteria begin to utilize nitrate as the terminal electron acceptor for respiration, finally
converting it to inert N 2 gas, which is then released to the atmosphere (Codispoti et al., 2001;
Naqvi, 1987; Ward et al., 2009). This process, known as denitrification, occurs at ~150 - 600m
in the eastern-central AS. The AS OMZ contributes ~20% to global water column denitrification
(Bange et al., 2000; Codispoti et al., 2001), although there is some debate about the relative
contributions of denitrification versus the chemosynthetic anammox reaction to N 2 gas
production in the AS (Ward et al., 2009). Upwelling of oxygen-depleted waters over the shelf
leads to formation of the world’s largest natural low-oxygen zone, over the continental shelf off
the Indian west coast. Evidence suggests that this oxygen deficiency has intensified in recent
years due to human activities, such as enhanced fertilizer loading from land. Anoxia (sulphate
reduction following complete denitrification) has been found to recur every year since 1998
during late summer/autumn in the inner-shelf region north of about 12°N (Naqvi et al., 2000).
By contrast, because the mesopelagic dissolved oxygen concentrations in the BoB are slightly
higher, it remains poised just above the denitrification threshold (Fig. 3), and studies of oxygen
variability and depletion on the Indian east coast are generally lacking and/or unpublished.
Nitrogen fixation carried out by cyanobacteria (diazotrophs) is the process that splits N 2
and converts it to “fixed” forms of N that can then be readily utilized by other phytoplankton.
Nitrogen fixation, favored by the mid-water nitrate losses and resulting nitrate deficits in the
upwelled water (Rixen et al., 2009), could in principle balance mid-water nitrate losses due
to denitrification. There is evidence from in situ measurements (Capone et al., 1998) as well
as satellite ocean color observations (Westberry and Siegel, 2006; Westberry et al., 2005)
of potentially high N 2 fixation rates in the AS, due to extensive blooms of the diazotrophic
cyanobacterium Trichodesmium (Fig. 17). The annual input of new N via this process has
been estimated to be 3.3 Tg N yr-1 (Bange et al., 2005; Bange et al., 2000). This could account
for 20–40% of the entire export flux from the euphotic zone into the deep sea (Brandes et al.,
1998; Rixen et al., 2002) but falls well below the estimated mid-water nitrate loss rates of ~
33 Tg N yr-1 due to denitrification. By contrast, although there have been anecdotal sightings
of Trichodesmium blooms in the coastal BoB (Gomes et al., 2000; Jyothibabu et al., 2003),
nitrogen fixation may be less extensive there than in the AS because of the absence of water
column denitrification and the resulting nitrate deficits that favor diazotroph growth.
Co r e q u e s t i o ns
1) How do natural and anthropogenic forcings influence the OMZs and therefore
ecosystem structure and biogeochemical processes in the AS and the BoB
Very small differences in the mid-water oxygen concentrations between the AS and the BoB
give rise to profound differences in biogeochemical cycling in these two basins, i.e. there is
open ocean denitrification in the former but not the latter. This suggests that small changes in
mid-water oxygen concentration of either basin could have global impacts. Further studies are
needed of the physical and biogeochemical mechanisms that set up and maintain the OMZs,
and, for example, control N cycling, in the AS and the BoB. How have these changed in the
past and how might they change in the future in response to global warming
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