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-1 "x. _ Productivity inhibited

1 \ by intense light at surface

Photosynthesis \ rate \

Maximum productivity

Net productivity

Compensation point ' (marks bottom of euphotic zone)

Respiration rate

Figure 15.25 Productivity and respiration vs. depth, as would be measured using the light bottle/ dark bottle method. (Based on Garrison, 1993.)

If respiration is fairly uniform with depth, a point will occur where the rate of primary production becomes equal to the respiration rate, yielding a net primary production of zero. This depth is called the compensation depth. It tends to occur approximately at the depth where the light intensity has dropped to 1% of the surface intensity. This depth defines the bottom of the euphotic zone. Below the compensation depth respiration exceeds photosynthesis, and oxygen levels must be maintained by transport from surface waters by mixing and advection. Figure 15.25 shows these relationships, and Figure 15.26 shows how they can be measured.

There are some interesting seasonal and geographical patterns in productivity as well (Figure 15.27). One might expect that among open ocean waters, tropical oceans would have the highest productivity. However, nutrients are always being lost to the euphotic zone by the sedimentation of phytoplankton and zooplankton fecal pellets, and a permanent thermocline prevents the nutrients from being mixed back upward. At 20°N latitude the nitrate and phosphate concentrations are about 1% of the levels typical of temperate oceans during the winter. In the tropics the nutrients lie mostly below 150 m depth, and their concentrations peak at a depth of 500 to 1000 m. Productivity in most tropical waters is less than 30 g C/m2 per year. Exceptions to this situation are upwelling zones and reefs, as noted below.

Polar zones, on the other hand, compensate for their low temperatures and low insolation by a lack of stratification. Thus, nutrients mix freely at the surface, except when snowmelt forms a shallow layer of relatively fresh water. The shallow stratified layer traps phytoplankton close to nutrients at the pycnocline (location of high vertical density gradient), producing a very intense, though short-lived algal bloom. In the Antarctic waters a unique circulation pattern produces upwelling of nutrients that stimulate productivity. However, even with these factors, the polar region productivity as a whole averages less than 25 g C/m2 • yr.

Oxygen consumption in light (transparent) bottles includes the sum of respiration and photosynthesis.

Oxygen consumption in dark (opaque) bottles includes the respiration only.

Photosynthesis can be computed as the difference between the two.

Figure 15.26 Light bottle/dark bottle method of measuring primary and net productivity vs. depth. At the compensation point the dissolved oxygen (DO) concentration in the light bottle will not change with time. Above that point, photosynthesis exceeds respiration and DO will increase. At greater depth the DO will decrease with time. (Based on Garrison, 1993.)

Figure 15.26 Light bottle/dark bottle method of measuring primary and net productivity vs. depth. At the compensation point the dissolved oxygen (DO) concentration in the light bottle will not change with time. Above that point, photosynthesis exceeds respiration and DO will increase. At greater depth the DO will decrease with time. (Based on Garrison, 1993.)

In temperate and subpolar oceans, on the other hand, productivity is at a higher average level, due to more dependable insolation and the nutrient replenishment that comes with seasonal stratification. Productivity peaks with a spring algal bloom, but it is moderately high yearround. Typical productivity is around 120 g C/m2 • yr, but can be as high as 250 g C/m2 • yr. Thus, these regions provide most of the productivity of the world's oceans.

In all climates, productivity as high as 1000 g C/m2 • yr can be maintained where hydrodynamic factors bring nutrients to the surface. Places where this occurs are called upwelling zones. These are caused by wind-driven currents combined with the Coriolis effect. The Coriolis effect is the tendency caused by Earth's rotation for flows to be diverted to the right in the northern hemisphere and to the left in the southern. Thus, hemisphere, the trade winds around the equator cause a westerly current in the Pacific and Atlantic oceans. The Coriolis effect produces a divergence of flow, as the northern edge diverts toward the north and the southern toward the south. The net effect is that surface water flows away from the equator, and deeper waters are brought up to replace it. This phenomenon is called equatorial upwelling. The regions of high productivity that result from the upwelling nutrients can be seen in Figure 15.27. A similar divergence zone encircles the Antarctic, where ocean currents flow in opposite directions, due to

90° 110= 130° 150' 170° 170° 150= 130° 110° 90= 70° 50° 30= 10= 10° 30° 50° 70° 90= 110° 130° 150°

90° 110= 130° 150' 170° 170° 150= 130° 110° 90= 70° 50° 30= 10= 10° 30° 50° 70° 90= 110° 130° 150°

Figure 15.27 Global distribution of primary productivity in the world's oceans. (From Barnes and Mann, 1991. Used with permission.)

trade winds. The resulting enhancement to productivity partially compensates for the otherwise low productivity of the polar oceans.

Another important mechanism is coastal upwelling, in which winds parallel to the coast, in combination with the Coriolis effect, produce an offshore current. This brings the deeper waters to the surface with their nutrients. This occurs throughout the world but is particularly massive in four locations: the coasts of Peru, California and Oregon, northern Africa, and southwestern Africa. These upwelling zones form important food chains, with humans at the top. When climatic conditions interfere with their productivity, the human economy and food supply is severely affected. The most important example of this concerns the Peru upwelling, caused by a steady offshore wind at the equator, and is the basis of a huge commercial anchovy fishery.

Upwelling occurs on a smaller scale at the mouths of estuaries when relatively fresh water entrains deeper salt water. The mixture is still less dense than seawater and floats on the surface, carrying nutrient both from the river discharge and from deep entrained seawater.

Every two to 10 years, the winds responsible for the Peru upwelling reverse, in what is called the southern oscillation. Instead of the upwelling of cold, nutrient-rich water, warm, nutrient-depleted current appears, and the thermocline becomes deeper. The warm current is called the El Niño, and it creates havoc with the economy of Peru, as fishermen lose their anchovy harvest. (El Nino is also associated with climatic changes in regions far from Peru, such as in reducing the number of hurricanes in the Caribbean.) The phenomenon is referred to as ENSO, for El Nino-Southern Oscillation.

Phytoplankton extract carbon, nitrogen, and phosphorus from seawater in the ratio 116: 16 : 1, respectively. Carbon, of course, is readily available as carbonate. Nitrogen is more often limiting in marine environments than in freshwater environments, because nitrogen-fixing cyanobacter (blue-green bacteria) are less abundant in salt water. For this reason, and because phosphorus is recycled faster than nitrogen, phosphorus is less important than nitrogen as a limiting nutrient. Silicon can be a limiting nutrient for diatoms.

TABLE 15.22 Nutrient Enrichment Experiment Using Sargasso Sea Water

Nutrient Supplementation

Relative Uptake of 14C Compared to Control (%)

Control (no supplementation)

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