12.8: Geographic Variation in Primary Production

All marine animals ultimately depend for food on phototrophic or chemosynthetic primary producers, which, in the water column, are mostly phytoplankton. Consequently, zooplankton, fishes, and other pelagic animals are most abundant in areas of high primary productivity. Most benthos rely on the rain of detritus from above. Exceptions are some species that live in limited areas where the seafloor is shallower than the photic-zone depth and in hydrothermal vents and other chemosynthetic communities. The quantity of detritus depends on the abundance of organisms in the overlying water, so benthic communities are also more abundant in areas of high primary productivity.

Phytoplankton biomass (standing stock) is determined by phytoplankton growth and reproduction rate versus consumption rate. If the productivity is high and the rate of grazing of phytoplankton by zooplankton is low, phytoplankton biomass increases, and vice versa.

Zooplankton populations and biomass adjust to changes in food supply, but the changes lag days or weeks behind changes in phytoplankton biomass. As a result, phytoplankton biomass initially increases when phytoplankton productivity increases, but it may fall when zooplankton biomass increases, even if there is no change in phytoplankton productivity. Thus, phytoplankton biomass does not vary in concert with phytoplankton productivity. Fortunately, averaged over several months, phytoplankton biomass is reasonably related to the average primary productivity. Figure 12-13 shows the seasonal variation in the distribution of chlorophyll (CC14) in ocean surface waters measured by satellites. The concentration of chlorophyll is a good indicator of the abundance or biomass of phytoplankton and also provides a reasonable approximation of the distribution of primary productivity.

The most productive parts of the ocean are coastal regions (Fig. 12-13), particularly along the western margins of continents, where coastal upwelling brings nutrients from deep waters into the photic zone (Chap. 13). Most of the open ocean has low productivity, the exceptions being certain high-latitude regions and the equatorial upwelling band across the eastern Pacific and, to a lesser degree, across other oceans (Fig. 12-13).

Figure 12-13. Seasonal variations in the distribution of chlorophyll in the surface waters of the world oceans for (a) September to November, (b) December to February, (c) March to May, and (d) June to August. Chlorophyll concentration is a reasonable measure of primary production. The data were obtained by a satellite sensor that measures ocean surface color within several wavelength bands. Black areas are land or where there were no data, because of cloud or ice cover or the limited orbital coverage of the satellite near the poles. Note the high productivity of many coastal regions and of the high-latitude seas in the Northern Hemisphere.

Throughout most of the tropical and subtropical open oceans, a permanent thermocline begins at a depth of about 100 to 200 m. Light intensity is relatively high, and the photic zone extends throughout most or all of the mixed layer above the thermocline. Nutrients are depleted in the photic zone, and the steep thermocline inhibits vertical mixing that would be necessary to resupply nutrients from the nutrient-rich water below the thermocline. High productivity in tropical and subtropical open oceans is limited to areas of upwelling. The band of high productivity across the equator, particularly in the eastern Pacific, coincides with upwelling at the tropical convergence along the equator (Fig. 8-3). During El Niño, primary productivity is dramatically reduced in this region because upwelling is inhibited (Chaps. 9, 10; Fig. 7-19).

Upwelling at the Antarctic Divergence (Fig. 8-3) is responsible for high productivity in the ocean around Antarctica. The Northern Hemisphere has no comparable divergence, primarily because of the presence of continents. At high latitudes in the North Atlantic and North Pacific Oceans, productivity is seasonally high because cooling of surface waters, strong westerly winds, and extratropical cyclones effectively mix surface and subsurface waters during winter, when light levels are low. Nutrients supplied by winter mixing support high productivity when light intensity increases in spring.

Productivity is lowest in the interior of subtropical gyres in each ocean. These regions are remote from nutrient inputs in runoff, have low rainfall (which can carry small amounts of nutrients), and are characterized by downwelling and deep thermoclines (Fig. 8-13). Light is plentiful, but lack of nutrients limits phytoplankton growth. Ocean waters are a brilliant blue in these regions because of the lack of suspended particles. However, areas of the Sargasso Sea (the interior of the North Atlantic subtropical gyre) are covered by vast rafts of Sargassum seaweed, despite the lack of nutrients (Chap. 15).