Cetacean-mediated vertical nitrogen transport in the oceanic realm

In natural systems, animal-mediated nutrient transport can be a major driver of primary productivity, but the role of marine megafauna such as cetaceans in mediating the transfer and recycling of nutrients has been overlooked. Here, we developed a spatially resolved, stochastic, nutrient-transport model for cetaceans in the oceanic Gulf of Mexico using species−specific foraging depths, distributions, and diets. An estimated 6.4 × 10⁸ mmol N d⁻¹, or 0.06 mt N yr⁻¹ ind⁻¹, is transported to the surface from depths below 100 m by the 19 cetacean species that occur in the oceanic Gulf of Mexico; 75% of this transport occurs seaward of the continental slope, but the per area transported nitrogen is greater on the continental slope (200–1000 m) than in the ocean basin. Benthos to surface transport comprised 6.0 × 10⁷ mmol N d⁻¹ and was much more common on the continental slope than the open basin. Compared to an existing physical-biogeochemical model, the transported nutrients add 8% N d⁻¹ to the estimated ammonium concentration above the nutricline and could add 16% N d⁻¹ to the surface ammonium concentration if expelled nutrients remain at the surface. Through feeding on diel vertical migrants, cetaceans retain an additional 2.7 × 10⁷ mmol N d⁻¹ in the surface waters that would otherwise return to depth via downward diel vertical migration. Cetaceans contribute to nutrient movements and recycling in the oceanic Gulf of Mexico, and may provide one of the few allochthonous sources of nutrients for primary producers in oligotrophic ecosystems.