The occurrence of a drought (d) in the diagram means less fresh water and fewer nutrients reaching the Gulf of Mexico. In 1988 (d), freshwater discharge and nutrient load were normal in spring, and hypoxia developed in May and June of that summer. The freshwater flow fell below the long-term average in July. The fresh water needed to maintain stratification (layering of the water column) was absent. Oxygen from the surface waters was then able to diffuse to the bottom and raise the oxygen concentrations. In 2000, there was a drought within the watershed in spring, fewer nutrients reached the Gulf, carbon fixation and flux were lower, and less hypoxia formed. There was sufficient fresh water in the summer to maintain stratification so that hypoxia occurred but over a smaller area.
Stratification (or layering of the water column) is a necessary condition for the maintenance of hypoxia once it has formed from decomposition of organic matter from the upper water column. A tropical storm or a hurricane in the summer will mix the water column and force oxygen to the bottom, especially in shallower waters. The mixed conditions and higher oxygen in the bottom waters persists for 4 to 7 days while the decomposition and depletion of oxygen overcomes the resupply of oxygen. Hurricanes (h) occurred just prior to the cruises that mapped the oxygen conditions in 2003, 2005, 2006, 2007, 2008, and 2010. Otherwise the size of the bottom area of hypoxia would have been larger.
Oceanographic currents (c) in response to winds (long-term weather patterns) will also affect the distribution of summer hypoxia. During the summers of 1998 and 2009 several weeks prior to the mapping cruises, the winds were stronger than usual from the southwest. This resulted in water masses being pushed towards the east. Thus hypoxia that may have been a feature of the upper Texas coast or the western Louisiana shore were piled up to the central and eastern coasts of Louisiana. Wind measurements and current measurements confirm this phenomenon. As a result the “footprint” of bottom-water hypoxia was smaller, but the total volume was consistent with other years. i.e., the hypoxia was thicker on the central and eastern portions of the area mapped.