Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth
of higher organisms. They also alter the marine nitrogen cycle, which is
strongly bound to the carbon cycle and climate. While higher organisms in
general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen
concentration below a threshold of about 20 µM (microbial hypoxia),
whereas anoxic processes dominate the nitrogen cycle at oxygen
concentrations of < ∼ 0.05 µM (functional
anoxia). The Arabian Sea and the Bay of Bengal are home to approximately
21 % of the total volume of ocean waters revealing microbial hypoxia.
While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally
anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even
though there are a few isolated reports on the occurrence of anoxia prior to
1960, anoxic events have so far not been reported from the open northern
Indian Ocean (i.e., other than on shelves) during the last 60 years.
Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen
concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the
surface ocean circulation twice a year and turns vast areas of the Arabian
Sea from an oligotrophic oceanic desert into one of the most productive
regions of the oceans within a few weeks. Thus, the comparably low
variability of oxygen concentration in the OMZ implies stable balances
between the physical oxygen supply and the biological oxygen consumption,
which includes negative feedback mechanisms such as reducing oxygen
consumption at decreasing oxygen concentrations (e.g., reduced respiration).
Lower biological oxygen consumption is also assumed to be responsible for a
less intense OMZ in the Bay of Bengal. According to numerical model results,
a decreasing physical oxygen supply via the inflow of water masses from the
south intensified the Arabian Sea OMZ during the last 6000 years, whereas a
reduced oxygen supply via the inflow of Persian Gulf Water from the north
intensifies the OMZ today in response to global warming. The first is
supported by data derived from the sedimentary records, and the latter
concurs with observations of decreasing oxygen concentrations and a
spreading of functional anoxia during the last decades in the Arabian Sea.
In the Arabian Sea decreasing oxygen concentrations seem to have initiated a
regime shift within the pelagic ecosystem structure, and this trend is also
seen in benthic ecosystems. Consequences for biogeochemical cycles are as
yet unknown, which, in addition to the poor representation of mesoscale
features in global Earth system models, reduces the reliability of estimates
of the future OMZ development in the northern Indian Ocean.
Dynamics of the elevated land plume over the Arabian Sea and the Northern Indian Ocean during northeasterly monsoons and during the Indian Ocean experiment (INDOEX)