Maps of Ocean Biology

Sunlight penetrating the ocean surface is depleted as it passes downward, creating what is called the euphotic zone where sunlight is sufficiently intense to promote photosynthesis (Figure 12). In the region below the euphotic zone, the net photosynthetic rate is negative due to lack of solar energy, resulting in very little biological activity below a certain level. The ingredients in addition to sunlight that are needed for biological production are nutrients, particularly nitrogen and phosphates. Figure 13 shows a typical nutrient deficiency in the surface layer due to consumption by micro-organisms. Deeper layers, where photosynthesis is suppressed due to lack of light, tend to have elevated levels of nutrients. If a mechanism were available to bring deep, nutrient-rich water into the euphotic zone, phytoplankton and algae would flourish, as would the marine life that live on these tiny organisms. Large-scale ocean circulation patterns in certain geographical regions and vertical motions in the ocean near continents circulate nutrient-rich water to provide the necessary nourishment for the euphotic zone.

The accompanying satellite picture (Figure 14) shows ocean biological activity in the vicinity of Antarctica during the Southern Hemisphere spring. The color coding indicates the level of biological activity, ranging from a magenta, which represents essentially no biological activity, to blue, yellow, green, and finally red, which represents the highest observed concentration of phytoplankton. The arrival of sunshine to the arctic region in spring and early summer leads to a rapid "bloom" of phytoplankton in this region. Notice that the regions of highest biological activity are along continental coastlines and around the Antarctic Continent. Equatorial ocean areas far from continents, on the other hand, are virtual biological deserts by comparison. Certainly sunlight is not lacking in these areas, so we must conclude that lack of nutrients prevents these regions from becoming biologically productive. Upwelling near continents creates rich biological regions, but closer examination reveals that not all coastal areas are equally productive. Again, we can conclude that differences in upwelling of nutrient-rich water must be the cause of these differences in ocean biology.

The next photograph of the Northern Hemisphere (Figure 15), centered on the North Pole shows that the whole North Atlantic Ocean is a very biologically productive region. In the North Pacific Ocean, coastal regions between Alaska and Russia also show high levels of phytoplankton. Considering that these small organisms cover such a large area and consume CO₂ , we must conclude that the polar oceans are tremendous sinks (removal mechanisms) for atmospheric carbon dioxide. In contrast to lower latitudes where lack of nutrients limits biological activity except near coasts, in the polar regions the supply of nutrients is persistent, but the lack of sunlight during winter periods shuts down phytoplankton production beginning in autumn. However, the large extent of the summer blooms means that a tremendous amount of carbon dioxide is used seasonally by these organisms. A closer look at coastal California in Figure 16 shows that northern California experiences a rich phytoplankton bloom, which is less pronounced in the ocean off southern California. Ocean currents (to be discussed later in the course) produce upwelling preferentially in the northern part of the state, a situation that also is responsible for ocean temperatures off San Francisco being much colder than those off Los Angeles.

A limiting factor for the growth of phytoplankton and algae around Antarctica, even during the spring bloom, is insufficient amounts of iron. Recognizing this, John Martin of the Moss Landing Marine Laboratories proclaimed, "Give me a half tanker of iron, and I will give you an ice age." His proposal was that by fertilizing the Antarctic ocean with iron, the growth in ocean marine plants would sequester large additional amounts of carbon dioxide from the atmosphere and counteract the anthropogenic increases, but numerous environmental concerns have been raised. More recent experiments by Boyd in the Southern Polar Ocean indicates artificially stimulated blooms may last longer than natural blooms, but in the sub-Arctic Ocean near Alaska some previously unreported limitations have been discovered.