SIBER SPIS sept 2011.pdf - IMBER

SIBER
Science Plan and Implementation Strategy
shifting it offshore of the coastal upwelling area toward the eastern basin, which overlies the
OMZ (Buesseler et al., 1998). Elucidating mechanisms controlling diatom blooms in the AS is
thus central to understanding the unique characteristics of this system’s ecology and carbon
fluxes.
The winds of the NEM also give rise to elevated primary production and export in the AS, but
the mechanism is different, with cool dry winds inducing buoyancy mixing and entrainment
of nutrient-rich water from depth. As a result, light limitation due to relatively deep convective
mixing can be an important factor controlling primary production during the NEM. The
influence of the monsoon winds on phytoplankton production extends also into the BoB where
they induce pronounced seasonal reversals in the surface currents and “cryptic upwelling” in
offshore waters (Vinayachandran et al., 2005).
With the exception of chemosynthetic systems (e.g. hydrothermal vents) deep sea benthic
communities are supported by sunlight-driven surface primary production, i.e. export flux
from surface waters. In general, the rate and magnitude of phytoplankton production strongly
influences export fluxes and consequently benthic processes and the formation of coastal and
open ocean OMZs. However, it is important to note that in the AS, the geographic distribution
of the OMZ does not reflect that of surface production, the greater proportion of which occurs
on the western side of the basin. As discussed in the previous section, the extensive region of
hypoxic waters in the northern IO gives rise to globally significant biogeochemical processes
such as denitrification (benthic as well as pelagic). In addition to seasonal variability, longer
(millennial and higher) time scale changes in circulation, monsoons and hypoxia have been
inferred from sediment records. Such changes have major significance for C, N, and P cycling,
sequestration in the sediments, and ocean inventories and productivity. Yet the Indian margin
of the AS and almost all of the BoB have had little or no study to date with regard to benthic
processes and in particular how they relate to the fate of primary production in surface waters.
Investigations of nutrient flows and food chain dynamics (including the microbial loop) are
needed to understand mechanisms of transfer of primary production to higher trophic levels, to
interpret trends in C cycling, and to predict fluxes to benthic systems.
The little that is known about the controls and fate of primary production in the southern
hemisphere open ocean of the IO is derived from a small number of basinwide remote sensing
and modeling studies (e.g. Behrenfeld et al., 2009; Lévy et al., 2007; Resplandy et al., 2009;
Wiggert et al., 2006) which are informed by large-scale physical and chemical surveys (e.g.
the World Ocean Circulation Experiment and the ongoing global CO 2 surveys) and a few
process studies (e.g. Planquette et al., 2007; Pollard et al., 2007; Poulton et al., 2007; Poulton
et al., 2009). Although, as in the Atlantic and Pacific Oceans, the southern IO subtropical
gyre is oligotrophic, it is subjected to unique influences derived from the anomalous, warm
poleward-flowing LC in the east (Fi gs. 2 a n d 8), and the southward-flowing Agulhas and East
Madagascar Currents in the west (Fi gs. 2 a n d 9), and their associated eddies, as discussed
in Theme 1 above. Studies of sea surface height variability have revealed intense, westwardpropagating
eddy variability between 20° and 30°S in the eastern tropical and southern
subtropical IO basin (D.B. Chelton et al., unpublished data), but the biogeochemical and
ecological impacts of these open ocean eddies have not been investigated. Large-scale open
ocean phytoplankton blooms have been observed extending southeastward from the southern
tip of Madagascar in the southern hemisphere summer in satellite chlorophyll data (Uz, 2007).
It has been confirmed that these are associated with blooms of diazotrophic cyanobacteria and
diatom-diazotroph associations (Poulton et al., 2009), but the relative contributions of nitrogen
fixation versus other nutrient sources has not been quantified. Finally, questions about the
potential role of Fe limitation in controlling phytoplankton production, and also carbon export to
depth pertain to the entire IO basin (Hood et al., 2009).
33