primary production. PrasannaKumar et al. attribute that to high bacterial abundance
and associated high levels of microheterotrophs. It is likely that the DOC source to
support the bacterial stocks is terrestrial.
Indian Equatorial Zone
(^) Circulation along the Indian Ocean equatorial belt is markedly different from either
the Atlantic or Pacific. The trades blow in their usual east-to-west trajectory well to
the south of the equator, albeit with strong seasonal variability. Between Sri Lanka
(6°N) and the equator there is monsoonal oscillation of winds from both Africa and
Australia. These push surface waters both east and west. Headed east they are focused
on the equator and accelerated by Kelvin waves. Headed west, they generate Rossby
waves. Both wave modes reflect off the land masses at the ends of the long run,
creating a sloshing back and forth at remarkable velocities, with alternating periods at
some longitudes of a half year or less (Fig. 11.42). This keeps the same water
warming under the sun; suppresses most equatorial upwelling; and prevents elevation
of water at the western end from driving an undercurrent centered on the equator. The
system remains major-nutrient-limited most of the time, with low productivity.
Typical surface chlorophyll levels across the equatorial zone are 0.1–0.2 mg m−3,
comparable to those of the central equatorial Pacific, somewhat higher than the
Pacific warm pool (Antoine et al. 2005). There is a deep chlorophyll maximum, about
twice surface levels, in the nutricline, that is typically at 50 to 80 m. Primary
production runs in the vicinity of 150 to 200 mg C m−2 yr−1. There is no eastern
tropical tongue of higher phytoplankton stocks comparable to those occurring
seasonally in the Atlantic and more persistently, save for El Niño periods, in the
Pacific. Ecological relationships are much like those of a subtropical gyre. Nano- and
picoplankton are the dominant primary producers. Mesozooplankton and upper
trophic-level fauna resemble those of all subtropical gyres, including tropical–
subtropical tunas, dolphin fish (Coryphaena hippurus) and ommastrephid squid.
There are Arabian Sea endemic species, but few if any species are endemic to the
Indian equatorial zone. The southern Indian subtropical gyre is much like those in the
Pacific and Atlantic. Phytoplankton stocks are half those at the equator and just to its
north.
Fig. 11.42 East–west current velocity at 10 m depth at 80°E on the equator (due south
of Sri Lanka). The black line is extrapolated from acoustic Doppler current meter data
from 35 m using the correlation to 10 m Sontek (a different Doppler system less
affected by surface sound reflections) data from August 2006 to January 2007 (shown
in gray, overlying the black line and repeated 0.75 m s−1 above to make it more
visible). Velocity of 1 m s−1 is 2 knots, relatively rapid oceanic flow. Positive is
eastward flow, negative is westward.