Tipping points may occur where the diatom-based food web collapses, HABs bloom or when organisms can no longer accumulate calcium-carbonate structures with decreasing pH. One recent tipping point is from the northern Gulf of Mexico hypoxic zone. The total area of its size in late summer has been predicted with great fidelity (R² = 0.85) using nitrogen loading from the Mississippi River 2-3 months earlier as the main driver. The prediction in 2022, however, severely over-estimated its actual size by 56% indicating that something was missing in the models Why? Thirty-seven years of data from monitoring the summer hypoxic zone size and water quality are reviewed. River water quality has not changed recently, but water temperature has been increasing (0.5 ^oC decade^-1) with significant declines in surface and bottom Chlorophyll a and hypoxic zone size per N loading. The hypoxic zone size is not decreasing because of nutrient reductions, but because of climate change. A tipping point is indicated that may be common to similar warm and eutrophic coastal waters. Hypoxia and food web models based on a stationary equipoise of nutrient loadings and ratios in surface and bottom waters will likely be deficient as coastal waters warm further.

Primary Presenter: R. Eugene Turner, Louisiana State University (euturne@lsu.edu)

Authors:

R. Eugene Turner, Louisiana State University  (euturne@lsu.edu)

Nancy Rabalais, Louisiana State University  (nrabalais@lumcon.edu)

Casandra Glaspie, Louisiana State University  (cassandra.glaspie@gmail.com)