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5.6. Pelagic biota
Within the existing contractor exploration claims, the environment
includes expanses of oligotrophic subtropical ocean. Data were not
available for the Indian Ocean claim area in particular, but the environment
is similar and can be expected to have the same and similar types of
biological resources as those found in the CCFZ.
5.6.1. Bacteria
Bacteria are found throughout the water column as well as in the
sediments. These microbial decomposers are concentrated at the sea
surface, in the oxygen-minimum zone and in the sediments. They are
associated, as organic aggregates, with all detritus, and their
remineralisation of organic matter provides a major source of nutrients for
use by algae. Sorokin 60 found that bacterial aggregates constituted 20-40%
of oceanic particles, with little depth variability other than a slight
concentration at the bottom of the thermocline. The size of the aggregates
(larger than some small phytoplankton) makes them easily available as
substrate for fine filter feeders, but also for coarse filter feeders and even
selective feeders such as larval fish. He estimates that "bacterioplankton"
are as important as algae as a primary food substrate.
5.6.2. Phytoplankton
El Sayed and Taguchi 61 examined water samples from the DOMES
area and identified 163 types of diatoms, 122 dinoflagellates, 48
coccolithophorids and 15 other types. The coccolithophorid Gephyrocapsa
huxleyi was the dominant species of phytoplankton at the depth of the
chlorophyll maximum. In subtropical surface waters, nutrient
concentrations are low and phytoplankton populations, measured by either
cell counts or chlorophyll a concentrations, are sparse. Daily primary
productivity is low, 100-200 mg of carbon per m 2 .
A subsurface maximum is seen in chlorophyll a concentrations
below the mixed layer. Concentrations of about 0.06 mg/m 3 are found in
the mixed layer but are three to four times those at 70-80 m. Typically,
production is limited by the low availability of nutrients in near-surface
waters and by the decreasing availability of light at greater depths. Surface
light at these latitudes is generally so intense that a thin layer at the surface
experiences photoinhibition. The highest values of gross primary production
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