A global estimation of mesozooplankton ammonium excretion in the open ocean

Abstract

Mesozooplankton ammonium excretion rates in the ocean, based on published data, were studied across all latitudes in order to assess the amount of ammonium excretion by this community. Specific ammonium excretion rates were highest in equatorial waters and decreased rapidly pole-ward. Global community excretion in the upper 200 m of the oceans, integrated over all latitudes, accounted in general for 1.78 ± 0.60 Gt N·year−1. Excretion rates showed a decrease from tropical (0.65 ± 0.14 Gt N·year−1) to polar waters (0.05 ± 0.02 Gt N·year−1). The substrate metabolized by organisms as indicated by the O/N ratio showed a protein-based metabolism of mesozooplankton with the exception of the temperate and subpolar regions, which showed a more lipid-based metabolism. The largest contribution of ammonium to autotrophs was observed in the tropical and subtropical areas, decreasing to polar areas. On a global basis, nutrient regeneration by mesozooplankton in the oceanic realm was estimated to be in the range of 12–23% of the requirements for phytoplankton and bacterial production.