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215 The Problem of Sustainable Yield Food for Fish: Primary Production Mapping Primary Production The Production Machine The Great Nutrient Cycles Food Chains and Food Webs Archetypes of Zooplankton Production varies widely in the sea; it is largely controlled by nutrient supply and the availability of sunlight. The crucially important nutrients are phosphate (fig. 8.1) and nitrate (which has a distribution similar to that of phosphate), as well as silicate (which tends to follow the same distribution patterns but with important exceptions). Silicate is the nutrient that allows diatoms to make shells of glass. Diatoms are an important group of primary producers sometimes referred to as “grass of the sea.” Primary production in the sea is carried out by both photosynthetic bacteria and microscopic algae, including diatoms and dinoflagellates. It is measured in terms of grams of carbon fixed per square meter per year (or per day). Typical values are around 100 production units (grams of carbon per year); somewhat lower than the equivalent photosynthetic fixation on land. The central questions of interest are the controls on production—largely the supply of nutrients wherever sunlight is adequate—and the manner of transfer of the matter generated in primary production into fish and consumers at the end of the food chain (apex consumers), including seabirds, whales and other marine mammals, and, prominently, people. This dual focus informs the discussions about the amount of food available for apex consumers in the context of fishery resources. One major topic concerns the amount of carbon fixed each year per unit area by chlorophyll-bearing organisms. An early global estimate of this production is by R.H. Fleming (the third author of the Oceans text).1 His map nicely shows the major features of the productivity of the sea (fig. 8.2). The other topic centers on the food chain. The “length of the food chain,” as emphasized in the 1960s by Woods Hole oceanographer John Ryther, turned out to be the most significant element of this concept.2 The ocean’s productivity is nothing like the one on land, where trees, brush, and grass EIGHT Meadows and Deserts of the Sea ON THE ELUSIVE CONCEPT OF OCEAN PRODUCTIVITY dominate. The fact is, the green primary producers in the sea, on the whole, are microscopically small and are unavailable for consumption by large animals. Only small creatures can eat the tiny algae. In turn, these small organisms feed small zooplankton and fishes, which serve as food for the larger animals that are of interest to fisheries. In other words, it takes several steps in the “food chain” till we come to the apex consumers . At each step, typically, only 10 to 20 percent of the matter at the lower step is transferred to the next-higher level. After two such transfers, a few percent of the original primary production is left to feed the apex consumer. After four transfers, less than one-thousandth is left. In the deserts of the sea—the central gyres and other warm-water regions—the chain is long. This is why the desert, in spite of having millions of green microbes in each liter of water, does not support large animals. In contrast, regions of deep mixing and upwelling, where the microbial algae are relatively large and food chains are much shorter therefore, support abundant marine mammals and seabirds, and fisheries. The question that moves to the foreground , then, is what controls the length of food chains. The answer holds the key to understanding the productivity patterns that matter to apex consumers. In the same manner, when asking how global warming will affect the productivity of the sea, we must ask how the warming of the planet affects the length of the food chain. To anticipate the answer: both the amount of nutrients and the particular mixture of nutrients that is locally available to support primary production help determine the length of the food chain. Specifically, a high supply of silicate shortens the food chain. THE PROBLEM OF SUSTAINABLE YIELD Just how large is the potential harvest of the ocean? Perhaps not surprisingly, the answer to this simple question depends on what is meant by “harvest.” If whales and large fishes are counted as “harvest,” as was true for the nineteenth century and much of the twentieth, there is one answer. If small herringlike fishes are counted, there is another, giving a much greater number. If we count krill (the food of baleen 216 M E...

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