Nitrogen and phosphorus phytoplankton growth limitation in the northern Gulf of Mexico

We conducted 158 bioassays to determine phytoplankton growth limitation in the northern Gulf of Mexico and made the following observations. Light limitation occurred where salinity was < 20; at higher salinities, phytoplankton biomass yield became mostly limited by N or by a co-limitation of N plus P (NP). The number of N-limited bioassays was 5 times greater than the P-limited bioassays. NP synergism occurred where salinity was > 20, and represented 59% of all samples that were not light-limited. The interaction of N and P co-limitation was frequently synergistically additive, i.e. the combined effects of N and P limitation together created a greater response than the sum of either separately. The dissolved inorganic nitrogen:phosphate ratio (DIN:Pi) and various concentrations of DIN and Pi did not offer reliable chemical boundaries describing likely areas of exclusive N or P limitation in these bioassays. We conclude that reducing N loading to the shelf is a prudent management action that will partially remediate eutrophic conditions, including those that lead to hypoxia, but the omission of a concurrent reduction in P loading would be shortsighted.