- 2018 Elsevier Ltd The -diversity in two large-scale, systematic macrobenthos surveys (19831985 and 20002001) in the deep northern Gulf of Mexico (GoM) has been compared under strong vertical and horizontal productivity gradients to examine underlying factors that control the distribution of deep-sea diversity. We calculated the effective numbers of equally abundant species (Hill numbers) from 100 randomly selected individuals to examine spatial patterns of species richness and evenness. Macrofauna -diversity was a negative parabolic function of depth; the minima occurred at high export POC flux on the upper continental slope and on the abyssal plain with extremely low input of POC; the maximum occurred at intermediate levels of macrofauna biomass and input of POC. The overall parabolic pattern of -diversity was stable over the 20-year period and not statistically different between meso-scale depressions (e.g., basins and canyons) and their adjacent slope habitats; however, the diversity was significantly higher in the NE than the NW slope habitats. The observed Mid-Depth-Maximum (MDM) in -diversity, we suggest, was likely a result of dynamic equilibrium between productivity and competitive exclusion. The higher diversity and productivity in the NE than the NW GoM at similar depth ranges suggests that, for the first time, the productivity-diversity relationship was observed independent of other depth-related factors. The findings provide a foundation for predicting the possible responses of diversity to climate change and anthropogenic impacts in the deep sea.