Using chemical engineering theory, a marine ecologist at the University of California, Davis, has explained why the relationship between body size and metabolic rate is considerably different for most aquatic creatures than for other organisms. The flow of water around water-dwelling organisms significantly affects their metabolic rates, according to Mark R. Patterson, an assistant professor of environmental studies. His findings, which have theoretical and commercial importance, appear in the March 13 issue of the journal Science. Body size and metabolic rate have long been known to be indirectly connected -- the larger the body, the more slowly the organism takes in and processes substances necessary for creating energy and sustaining life. For example, the metabolic rate of a hummingbird is much faster than that of an elephant. Scientists have traditionally accepted that metabolic rate could be calculated using a rather simple equation. While that equation is amazingly predictable for most organisms, it does not hold true when applied to algae and aquatic invertebrates such as coral, sea anenome and sea urchins and algae. Recognizing that these aquatic organisms rely totally on their outer surfaces for taking in vital gases and nutrients, Patterson noted that water motion might reasonably be expected to affect metabolic rate. This is particularly true in light of a phenomenon called the boundary-layer effect. The boundary layer is a very thin layer of water directly surrounding an object, which is not actually in motion. Compounds such as oxygen that are important to the aquatic organism's metabolic processes must pass through this boundary layer. "The boundary layer is very important," said Patterson, "As the speed of water flow increases, the boundary layer gets thinner and thinner and affects the metabolic rate." Making adjustments for various shapes -- whether flat plate, spherical or cylindrical -- Patterson borrowed a series of chemical engineering formulas related to water flow to predict metabolic rates for various aquatic invertebrates and algae. Numerous computer-generated calculations indicated a remarkably consistent relationship between metabolic rate and body size for these organisms, providing water flow is taken into account. "These computations indicated that metabolic rate increases as water flow increases and the boundary layer thins," Patterson said. "I refer to it as 'flow-modulated metabolism.' " While the newly discovered relationship between body size and metabolic rate is intriguing from a theoretical standpoint, Patterson says it also should have practical applications in research and aquaculture. "Now, if we know the size of a given population of these organisms, we can calculate the energy requirements for that population," he said. "For example, someone cultivating algae can now calculate what photosynthetic requirements must be met." Patterson's findings should also help researchers better understand how water flow affects the population biology of a wide range of organisms including sea urchins and kelp. He intends to use the information in his own studies on the health of coral reefs. Patterson's research on metabolic rate and body size was financed by the National Science Foundation and by the National Oceanic and Atmospheric Administration.