major factor in total global carbon fixation, carbon fixation in the deep sea deserves study because of its uniqueness. Characterization of the global extent of these systems, the rates at which their free-living and symbiotic bacteria fix carbon dioxide, and the extent to which organic materials at vents are distributed to other regions of the oceans will be key areas of research for the next decade. Beyond understanding the biogeochemical role of these communities, studies of vent communities will give insight into the evolution and functioning of nutritious and detoxifying mutualism among organisms. Support for this work has a broad international base, such as through the RIDGE program, which supports multidisciplinary investigations of the biology, geochemistry, and geophysics of mid-ocean ridge-crest systems.

Study of these diverse ecosystems in which chemosynthetic processes replace or complement photosynthetic productivity is necessary to understand the complex nature of marine food webs and the full suite of exchanges and transformations that constitute the global carbon cycle.

Effects of Climate Change on Populations of Marine Organisms

The characteristics of a region that determine its suitability for any given organism include not only the availability of food and the abundance of predators but also the dynamic physical features (mixing and circulation) of the local environment that influence the success of recruitment, efficiency of feeding, and susceptibility of organisms to predation. Global change could affect oceanic animal populations by changing physical processes of significance to the planktonic organisms. At present, it is not possible to predict definitively the impacts of global change on the physical parameters of the ocean and the atmosphere. However, the effects of climate change can be partly anticipated by examining similar effects on shorter time scales, such as seasonal freshwater pulses, El Niños, and other infrequent oceanographic phenomena. Three examples illustrate how global climate change could affect the physical features and processes of the sea that influence the abundance, distribution, and production of marine planktonic animals.

High-latitude marine ecosystems may be more susceptible to global change than low-latitude marine ecosystems. If precipitation patterns change as estimated and global warming triggers the rapid melting of previously persistent ice fields and the retreat of



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