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The large fish we prize in the developed world, such as cod, salmon, and tuna, are usually apex predators that feed at or near the top of the food web. These fish and all other consumers in the food web would not exist without organisms at the bottom of the food web that convert solar energy into the chemical energy of sugars. The photosynthetic organisms of the oceans—including seaweeds, reef-building corals, and phytoplankton, the microscopic algae that drift with the ocean currents—account for roughly half of global primary production. Aquatic primary production varies widely across natural aquatic ecosystems, and it depends on climate and other forces that affect the global distribution of nutrients.

As wind blows across the surface of the ocean, it pushes and pulls water, creating ocean currents and influencing the nutrients available to fish stocks. On Earth, we have prevailing winds that blow consistently from one direction, but they do not end up moving directly north or south. Rather, the rotation of Earth creates a deflection in the winds called the Coriolis effect, which deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The result is the global pattern of prevailing winds: northeast trade winds, westerlies, and polar easterlies in the Northern Hemisphere; southeast trade winds, westerlies, and polar easterlies in the Southern Hemisphere (Figure 8.2). As the prevailing winds blow across the oceans, they set in motion oceanic currents. The Coriolis effect acts on these currents to create large-scale patterns of oceanic circulation that move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (see Figure 8.2). As a result, each hemisphere of each ocean basin has a large circular current called a gyre, which is centered under subtropical, high-pressure regions.

Oceanic currents exert major influences on regional climates by transporting heat or, in some situations, cooling waters from one region to another. The Gulf Stream in the Atlantic Ocean, for instance, transports heat from the tropics to higher latitudes, extending temperate climates much farther north in northwest Europe than would be the case otherwise (Figure 8.3). Meanwhile, the Labrador Current in the western Atlantic Ocean cools northeastern North America. The currents also modify the distribution of marine environments (Figure 8.4). For example, the currents extend cool marine waters northward along the southwest coast of Africa and southward along Africa’s northwest coast. This transport of cool surface waters significantly narrows the band of warm, tropical marine waters in the eastern Atlantic Ocean, compared with the western Atlantic.

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Although light penetrates water, it grows weaker with depth, extending to a maximum depth of 200 meters and restricting photosynthesis to a surface layer of the oceans and lakes known as the euphotic zone (Figure 8.5). As organic matter produced in the euphotic zone sinks through the water column, it carries with it various elements essential for photosynthesis, such as nitrogen, phosphorus, and iron. Warmer surface layers are less dense than deeper layers, which means there is little vertical mixing with deeper cool water. As a consequence, essential chemical nutrients depleted from warm surface waters build up in deeper cool-water layers as sinking organic matter decomposes, and primary production in the euphotic zone gradually declines. This means that any mechanism that promotes renewal of nutrients in surface waters, such as the vigorous mixing of deep and surface waters, will increase rates of primary production.

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The process of upwelling does just that. Driven by prevailing or seasonal winds, upwelling generally occurs where winds blow warmer surface water away from shore and replace it with colder, subsurface water. As shown in Figure 8.2, extensive areas of upwelling occur along the west coasts of North and South America, North and South Africa, southwestern Europe, and along shores of the northwest Indian Ocean, where upwelling is driven by seasonal monsoon winds.

Phosphorus is one of the critical elements that can be brought to the surface via upwelling. While the nitrogen and carbon cycles include a major atmospheric reservoir, another critical biogeochemical cycle—the phosphorus cycle—does not. As Figure 8.6 shows, phosphorus enters the cycle through the weathering of rock, so it begins its journey on land before becoming important in the ocean. Phosphorus released by weathering is taken up from the soil by plants and incorporated into plant tissues, where it is used to form cell membranes, nucleic acids, and the energy-bearing molecule ATP (adenosine triphosphate). Herbivores feeding on plant tissues ingest phosphorus, as do carnivores feeding on herbivores. Phosphorus is then released by these animals when feces or urine is returned to the soil; phosphorus is also released from dead and decaying organic matter by detritivores and decomposers. Once released into soils, this phosphorus can be taken up again by plants and recycled within the ecosystem or exported to the ocean by rivers or wind, where it participates in similar cycling patterns with algae, zooplankton, and fish.

When a fish dies and is incorporated into marine sediment, that sediment often ultimately becomes a rock that integrates phosphorus into a mineral, closing the cycle. Other minerals in the ecosystem that, like phosphorus, do not have gaseous forms either (e.g., iron, potassium) undergo similar cycles, with only minor variations in some of the details.

The highest levels of marine primary production lie along the margins of the continents, especially where upwelling brings nitrogen and phosphorus-rich deep waters to the surface euphotic zone (Figure 8.7). However, there are other coastal areas of high primary production, such as those along the eastern coasts of North America, South America, Africa, and Asia. Here, waters are shallow enough to renew nutrient levels in surface waters during periods of intense mixing by winds or storms. In these waters, nutrients are also elevated by runoff from land. Marine areas of highest primary production, such as the west coast of North America, support the highest biomass of fish and shellfish (Figure 8.8). Similarly, higher levels of phytoplankton production off the Faroe Islands in the northeast Atlantic are what sustained the historic production of Atlantic cod.

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The lowest levels of primary production tend to occur in deep, mid-ocean environments, particularly in tropical oceans where warm surface water rarely mixes with nutrient-rich deeper layers of cool water. Here, nutrients that could stimulate primary production are trapped below the euphotic zone.