ment (eutrophication) resulting from altered land use, and waste disposal as a reason for the low oxygen concentrations on continental shelves. An additional concern is the deliberate or accidental transport of species from one ocean to another by shipping or other activities, leading to outbreaks of the introduced species. Documenting the causes and effects of changes in marine populations is difficult, but new techniques and approaches will make this research possible in the future.

Effects of the Food Web on Global Biogeochemical Cycles

Phytoplankton, macroalgae, and sea grasses use energy derived from sunlight to incorporate inorganic carbon, nitrogen, phosphorus, and other elements into organic molecules that are the building blocks of life and sources of energy for nonphotosynthetic organisms that consume these plants. Some bacteria also synthesize organic matter using chemical energy (chemosynthesis) rather than sunlight as a primary energy source. In the ocean, most organic carbon produced by photosynthesis and chemosynthesis is ingested by zooplankton and higher animals, oxidized for energy, and ultimately respired as carbon dioxide in the upper few hundred meters of the water column. At the same time, nutrient elements, such as nitrogen, phosphorus, and some trace metals, are recycled and reused by phytoplankton. A variable fraction of organic matter is not recycled in surface waters; instead, it settles out of the upper ocean layers, thereby contributing to vertical fluxes. Thus fluxes of carbon, nitrogen, oxygen, phosphorus, sulfur, and other biologically important elements are controlled by food-web processes. A major research theme for the 1990s will be to describe the effects and possible controls that food-web structure and function have on fluxes from the euphotic zone to middepth and deep waters, to the ocean sediments, and into the geological record.

Episodic Export of Material from the Surface

The simplest description of the effects of marine food webs on vertical flux involves only the size and species of phytoplankton and whether the phytoplankton sink before being ingested. For example, the spring diatom bloom in the North Atlantic is thought to sink without significant predation, whereas where cyanobacteria are the dominant primary producers, sinking of organic material from the upper ocean is largely mediated by food-web processes.

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