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process. With increasing depth, photosynthesis decreases while
decomposition and respiration increase, consuming oxygen and depressing
pH. A pH minimum generally coincides with the oxygen minimum. DOMES
results for dissolved oxygen are typical of oceanic conditions in the contract
areas, showing essentially saturated concentrations of dissolved oxygen
within the mixed layer and slight supersaturation [400-500 microns] just
below the mixed layer, resulting from a thin layer of enhanced
photosynthetic activity where phytoplankton biomass accumulates.
Below the thermocline, oxygen concentrations rapidly decrease to a
minimum. Between 300 and 500 m, concentrations as low as 1 µm have
been measured. Below the minimum, concentrations increased to about
350 µm near the bottom (5000 m) 50 .
Marine plants, or algae, require certain elements for their growth, as
do their terrestrial counterparts. Some of these elements, particularly
nitrogen, phosphorus and silicon, are required in relatively large amounts
and are termed macronutrients. Most of these elements are abundantly
available in seawater; however, in tropical and subtropical surface waters
iron, nitrogen and, to a lesser extent, phosphorus, can be present in
concentrations limiting to algal growth. Ambient nutrient concentrations
reflect a dynamic balance among the forces of water-mass advection,
diffusive mixing and biological cycling.
Nitrate concentrations in the DOMES study area were low in the
mixed layer (typically about 1-2 µm), reflecting active uptake by
phytoplankton 51 . In the thermocline, nitrate concentrations increased with
depth to about 35 µm. Occasionally a nitrate maximum was detected near
the base of the thermocline. Anderson 52 , in a summary of the DOMES
nutrient chemistry investigations, reported a resistant nitrate maximum
(approximately 45 µm) at the interface between the oxygen-minimum layer
and the "upper deep water" at depths of about 800-1000 m. Below this
layer, concentrations gradually decreased to about 41 µm and about 36 µm
at 4000 m. Within 20 m of the bottom, there was a further, abrupt drop to
less than 30 µm 53 .
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