The trophic resource continuum (TRC) in euphotic zones of the world's oceans is the spectrum of conditions from the richest runoff and upwelling areas to the most nutrient deficient subtropical seas. That spectral range has likely varied through geologic history as more efficient phytoplankton taxa evolved, as oceanic circulation rates changed, and in response to oceanic-mixing events. Reduced rates of oceanic turnover should produce regionally intensified eutrophy and oligotrophy, that is, TRC expansion. Because oceanic waters as a whole are relatively nutrient rich, increased rates of oceanic turnover should prevent extremely eutrophic and extremely oligotrophic conditions from developing, causing TRC contraction. Global biotic diversities in the oceans are strongly influenced by communities in oligotrophic environments. Nutrient deficient waters are more stable and more transparent than richer waters, permitting higher degrees of specialization. Sparse nutrient supplies necessitate smaller phytoplankton and longer, more complex food chains. Preservation potential of oligotrophic biotas is also greater because calcareous taxa are more prevalent and because calcium carbonate is less apt to be bioeroded or dissolved. Fluctuations in the TRC should profoundly affect diversity of habitats within the euphotic zone of the oceans. The evolutionary response to a TRC expansion, diversification, would occur in a geologic time frame of millions of years, while evolutionary responses to a TRC contraction, including extinction, could occur in a biological time frame of generations. A simple model indicates that a reduction in the trophic resource continuum, from expansion during a major marine transgression, down to modern levels could result in loss of 40% of the euphotic habitats. How fluctuations in the TRC control diversity of euphotic habitats in the oceans provides an hypothesis by which to interpret global taxonomic diversity cycles, as well as taxonomically selective extinctions, changes in species dominance, and some morphologic trends.