On entering water, light is both scattered and absorbed. The sum of these is attenuation. In the open ocean, the agent of most scattering and absorption is plankton, or plankton-derived products. The controls, therefore, governing the variability in light absorption and scattering are no less than the controls governing the distribution, abundance, and growth of plankton populations. Apart from being attenuated, light is also generated ubiquitously in the upper layers of the sea through mechanisms of bioluminescence. The overall and long-term goal of Biowatt is to establish causal links operating between the variability in light attenuation and light production in the ocean. The issues addressed range from behavioral relationships among macrozooplankton and micronekton, to the dynamics of absorbing and scattering populations, to the physical dynamics of the upper layers. A conceptual model is shown in Figure 1.