Primary production in the open ocean is thought to be due mostly to phytoplankton. Some 2 to 10% may be due to seaweeds, and less than 1% to chemoautotrophs. For all the oceans as a whole, the gross productivity is estimated to be 50 to 100 g C/m2 • yr. Due to surface scattering and light extinction, the efficiency of capturing incident sunlight is only about 0.1 to 0.2%.
The phytoplankton are dominated by diatoms, and secondarily by dinoflagellates. Although in the past most of the primary productivity of the oceans was attributed to the larger phytoplankton, evidence is growing that as much as 70% of all the photosyn-thetic activity in the oceans is due to smaller forms. These range from 2 to 20 mm in sizes and are termed nanoplankton. They include the small single-celled plants called cocco-lithophores. These are covered with plates of calcium carbonate. They can form deep beds of ooze on the ocean floor, which fossilize into chalk formations such as the White Cliffs of Dover in England. Another source of productivity is the phototrophic bacteria, such as cyanobacter (not necessarily nitrogen-fixing forms, however). Their abundance is of the order of 100,000 mL"1. They are from 0.2 to 2 mm in size and are termed picoplankton.
Productivity has an interesting relationship with depth in the epipelagic zone. Typically, the photosynthesis rate, and the concentration of chlorophyll, peaks some tens of meters below the surface (Figure 15.24). Several possible reasons are given for the surprising observation that photosynthesis is not a maximum at the surface. One is that, ironically, the light intensity may be inhibitory, causing bleaching of phytoplankton. It may also be due to the effects of grazing by zooplankton. Mathematical modeling of phy-toplankton growth dynamics suggests that nutrient limitations near the surface are responsible. Nutrients continue to increase with depth up to on the order of 1000 m, and then decrease modestly to the bottom. Oxygen reaches a minimum at similar depths, and then increases steadily toward the bottom.
Figure 15.24 Typical tropical structure of the water column. (Based on Mann and Lazier, 1991.)
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