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.
Publication Source (Journal or Book title)
Aquatic Microbial Ecology
Turner, R., & Rabalais, N. N. (2013). Nitrogen And Phosphorus Phytoplankton Growth Limitation In The Northern Gulf Of Mexico. Aquatic Microbial Ecology, 68 (2), 159-169. https://doi.org/10.3354/ame01607